Graph
of global population living on under 1, 1.25 and 2 equivalent of 2005
US dollars daily (red) and as a proportion of world population (blue)
based on 1981–2008 World Bank data
Poverty Thresholds for 2013
The poverty threshold, poverty limit, poverty line or breadline is the minimum level of income deemed adequate in a particular country.
The poverty line is usually calculated by finding the total cost of all
the essential resources that an average human adult consumes in one
year. The largest of these expenses is typically the rent
required for accommodation, so historically, economists have paid
particular attention to the real estate market and housing prices as a
strong poverty line affect.
Individual factors are often used to account for various circumstances,
such as whether one is a parent, elderly, a child, married, etc. The
poverty threshold may be adjusted annually. In practice, like the
definition of poverty, the official or common understanding of the poverty line is significantly higher in developed countries than in developing countries.
In October 2015, the World Bank updated the International Poverty Line (IPL), a global absolute minimum, to $1.90 per day (in PPP), where it current stands (as of 2022),
and also as of 2022, $3.20 per day in PPP for lower-middle income
countries, and $5.50 per day in PPP for upper-middle income countries.
Per the $1.90/day standard, the percentage of the global population
living in absolute poverty fell from over 80% in 1800 to 10% by 2015,
according to United Nations estimates, which found roughly 734 million people remained in absolute poverty.
History
The poverty threshold was first developed by Mollie Orshansky
between 1963 and 1964. She attributed the poverty threshold as a
measure of income inadequacy by taking the cost of food plan per family
of three or four and multiplying it by a factor of three. In 1969 the
inter agency poverty level review committee adjusted the threshold for
only price changes.
Charles Booth, a pioneering investigator of poverty in London at the turn of the 20th century, popularised the idea of a poverty line, a concept originally conceived by the London School Board.
Booth set the line at 10 (50p) to 20 shillings (£1) per week, which he
considered to be the minimum amount necessary for a family of four or
five people to subsist on. Benjamin Seebohm Rowntree (1871–1954), a British sociological researcher, social reformer and industrialist, surveyed rich families in York,
and drew a poverty line in terms of a minimum weekly sum of money
"necessary to enable families … to secure the necessaries of a healthy
life", which included fuel and light, rent, food, clothing, and
household and personal items. Based on data from leading nutritionists of the period, he calculated the cheapest price for the minimum calorific
intake and nutritional balance necessary, before people get ill or lose
weight. He considered this amount to set his poverty line and concluded
that 27.84% of the total population of York lived below this poverty
line.
This result corresponded with that from Booth's study of poverty in
London and so challenged the view, commonly held at the time, that
abject poverty was a problem particular to London and was not widespread
in the rest of Britain. Rowntree distinguished between primary poverty, those lacking in income and secondary poverty, those who had enough income, but spent it elsewhere (1901:295–96).
Absolute poverty and the International Poverty Line
The term "absolute poverty" is also sometimes used as a synonym for extreme poverty. Absolute poverty is the absence of enough resources to secure basic life necessities.
To assist in measuring this, the World Bank has a daily per capita international poverty line (IPL), a global absolute minimum, of $1.90 a day as of October 2015.
The new IPL replaces the $1.25 per day figure, which used 2005 data. In 2008, the World Bank came out with a figure (revised largely due to inflation) of $1.25 a day at 2005 purchasing power parity (PPP).
The new figure of $1.90 is based on ICP PPP calculations and represents
the international equivalent of what $1.90 could buy in the US in 2011.
Most scholars agree that it better reflects today's reality,
particularly new price levels in developing countries. The common IPL has in the past been roughly $1 a day.
These figures are artificially low according to Peter Edward of Newcastle University. He believes the real number as of 2015 was $7.40 per day.
Using a single monetary poverty threshold is problematic when
applied worldwide, due to the difficulty of comparing prices between
countries.
Prices of the same goods vary dramatically from country to country;
while this is typically corrected for by using PPP exchange rates, the
basket of goods used to determine such rates is usually unrepresentative
of the poor, most of whose expenditure is on basic foodstuffs rather
than the relatively luxurious items (washing machines, air travel,
healthcare) often included in PPP baskets. The economist Robert C. Allen
has attempted to solve this by using standardized baskets of goods
typical of those bought by the poor across countries and historical
time, for example including a fixed calorific quantity of the cheapest
local grain (such as corn, rice, or oats).
The basic needs approach is one of the major approaches to the measurement of absolute poverty in developing countries. It attempts to define the absolute minimum resources necessary for long-term physical well-being, usually in terms of consumption goods. The poverty line is then defined as the amount of income
required to satisfy those needs. The 'basic needs' approach was
introduced by the International Labour Organization's World Employment
Conference in 1976.
"Perhaps the high point of the WEP was the World Employment Conference
of 1976, which proposed the satisfaction of basic human needs as the
overriding objective of national and international development policy.
The basic needs approach to development was endorsed by governments and
workers' and employers' organizations from all over the world. It
influenced the programs and policies of major multilateral and bilateral
development agencies, and was the precursor to the human development
approach."
A traditional list of immediate "basic needs" is food (including water), shelter, and clothing.
Many modern lists emphasize the minimum level of consumption of 'basic
needs' of not just food, water, and shelter, but also sanitation,
education, and health care. Different agencies use different lists.
According to a UN declaration that resulted from the World Summit on Social Development
in Copenhagen in 1995, absolute poverty is "a condition characterized
by severe deprivation of basic human needs, including food, safe
drinking water, sanitation facilities, health, shelter, education, and
information. It depends not only on income, but also on access to
services."
David Gordon's paper, "Indicators of Poverty and Hunger", for the
United Nations, further defines absolute poverty as the absence of any
two of the following eight basic needs:
Safe drinking water: Water must not come solely from rivers and
ponds, and must be available nearby (fewer than 15 minutes' walk each
way).
Sanitation facilities: Toilets or latrines must be accessible in or near the home.
Health: Treatment must be received for serious illnesses and pregnancy.
Shelter: Homes must have fewer than four people living in each room. Floors must not be made of soil, mud, or clay.
Education: Everyone must attend school or otherwise learn to read.
Information: Everyone must have access to newspapers, radios, televisions, computers, or telephones at home.
Access to services: This item is undefined by Gordon, but normally
is used to indicate the complete panoply of education, health, legal,
social, and financial (credit) services.
In 1978, Ghai investigated the literature that criticized the basic
needs approach. Critics argued that the basic needs approach lacked
scientific rigour; it was consumption-oriented and antigrowth. Some
considered it to be "a recipe for perpetuating economic backwardness"
and for giving the impression "that poverty elimination is all too
easy". Amartya Sen focused on 'capabilities' rather than consumption.
In the development discourse, the basic needs model focuses on the measurement of what is believed to be an eradicable level of poverty.
Relative poverty means low income relative to others in a country: for example, below 60% of the median income of people in that country.
Relative poverty measurements unlike absolute poverty
measurements take the social economic environment of the people observed
into consideration. It is based on the assumption that whether a person
is considered poor depends on her/his income share relative to the
income shares of other people who are living in the same economy. The threshold for relative poverty is considered to be at 50% of a country's median equivalised disposable income after social transfers. Thus, it can vary greatly from country to country even after adjusting for purchasing power standards (PPS).
A person can be poor in a relative terms but not in absolute
terms as the person might be able to meet her/his basic needs, but not
be able to enjoy the same standards of living that other people in the
same economy are enjoying.
Relative poverty is thus a form of social exclusion that can for
example affect peoples access to decent housing, education or job
opportunities.
"Relative poverty reflects better the cost of social inclusion and equality of opportunity in a specific time and space."
"Once economic development has progressed beyond a certain
minimum level, the rub of the poverty problem – from the point of view
of both the poor individual and of the societies in which they live – is
not so much the effects of poverty in any absolute form but the effects
of the contrast, daily perceived, between the lives of the poor and the
lives of those around them. For practical purposes, the problem of
poverty in the industrialized nations today is a problem of relative
poverty (page 9)."
However, some
have argued that as relative poverty is merely a measure of inequality,
using the term 'poverty' for it is misleading. For example, if everyone
in a country's income doubled, it would not reduce the amount of
'relative poverty' at all.
History of the concept of relative poverty
In 1776, Adam Smith
argued that poverty is the inability to afford "not only the
commodities which are indispensably necessary for the support of life,
but whatever the custom of the country renders it indecent for
creditable people, even of the lowest order, to be without."
In 1958, John Kenneth Galbraith
argued, "People are poverty stricken when their income, even if
adequate for survival, falls markedly behind that of their community."
In 1964, in a joint committee economic President's report in the
United States, Republicans endorsed the concept of relative poverty: "No
objective definition of poverty exists. ... The definition varies from
place to place and time to time. In America as our standard of living
rises, so does our idea of what is substandard."
In 1965, Rose Friedman
argued for the use of relative poverty claiming that the definition of
poverty changes with general living standards. Those labelled as poor in
1995, would have had "a higher standard of living than many labelled
not poor" in 1965.
In 1967, American economist Victor Fuchs proposed that "we define as poor any family whose income is less than one-half the median family income." This was the first introduction of the relative poverty rate as typically computed today.
In 1979, British sociologist, Peter Townsend
published his famous definition: "individuals... can be said to be in
poverty when they lack the resources to obtain the types of diet,
participate in the activities and have the living conditions and
amenities which are customary, or are at least widely encouraged or
approved, in the societies to which they belong (page 31)."
Brian Nolan and Christopher T. Whelan of the Economic and Social Research Institute (ESRI) in Ireland explained that "poverty has to be seen in terms of the standard of living of the society in question."
Relative poverty measures are used as official poverty rates by the European Union, UNICEF and the OECD.
The main poverty line used in the OECD and the European Union is based
on "economic distance", a level of income set at 60% of the median
household income.
Relative poverty compared with other standards
A measure of relative poverty
defines "poverty" as being below some relative poverty threshold. For
example, the statement that "those individuals who are employed and
whose household equivalised disposable income is below 60% of national
median equivalised income are poor" uses a relative measure to define
poverty.
The term relative poverty can also be used in a different sense to mean "moderate poverty" – for example, a standard of living or level of income that is high enough to satisfy basic needs (like water, food, clothing, housing, and basic health care),
but still significantly lower than that of the majority of the
population under consideration.
An example of this could be a person living in poor conditions or
squalid housing in a high crime area of a developed country and
struggling to pay their bills every month due to low wages, debt or
unemployment. While this person still benefits from the infrastructure
of the developed country, they still endure a less than ideal lifestyle
compared to their more affluent countrymen or even the more affluent
individuals in less developed countries who have lower living costs.
Living Income Concept
Living Income refers to the income needed to afford a decent standard of
living in the place one lives. The distinguishing feature between a
living income and the poverty line is the concept of decency, wherein
people thrive, not only survive. Based on years of stakeholder dialogue
and expert consultations, the Living Income Community of Practice, an
open learning community, established the formal definition of living
income drawing on the work of Richard and Martha Anker, who co-authored
"Living Wages Around the World: Manual for Measurement". They define a
living income as:
The
net annual income required for a household in a particular place to
afford a decent standard of living for all members of that household.
Elements of a decent standard of living include food, water, housing,
education, healthcare, transport, clothing, and other essential needs
including provision for unexpected events.
Like the
poverty line calculation, using a single global monetary calculation for
Living Income is problematic when applied worldwide.
Additionally, the Living Income should be adjusted quarterly due to
inflation and other significant changes such as currency adjustments.
The actual income or proxy income can be used when measuring the gap
between initial income and the living income benchmarks. The World Bank
notes that poverty and standard of living can be measured by social
perception as well, and found that in 2015, roughly one-third of the
world's population was considered poor in relation to their particular
society.
The Living Income Community of Practice (LICOP) was founded by
The Sustainable Food Lab, GIZ and ISEAL Alliance to measure the gap
between what people around the world earn versus what they need to have a
decent standard of living, and find ways to bridge this gap.
A variation on the LICOP's Living Income is the Massachusetts Institute of Technology's Living Wage Calculator, which compares the local minimum wage to the amount of money needed to cover expenses beyond what is needed to merely survive across the United States. The cost of living varies greatly if there are children or other dependents in the household.
Why poverty threshold matters
An
outdated or flawed poverty measure is an obstacle for policymakers,
researchers and academics trying to find solutions to the problem of
poverty. This has implications for people. The federal poverty line is
used by dozens of federal, state, and local agencies, as well as several
private organizations and charities, to decide who needs assistance.
The assistance can take many forms, but it is often difficult to put in
place any type of aid without measurements which provide data. In a
rapidly evolving economic climate, poverty assessment often aids
developed countries in determining the efficacy of their programs and
guiding their development strategy. In addition, by measuring poverty
one receives knowledge of which poverty reduction strategies work and
which do not,
helping to evaluate different projects, policies and institutions. To a
large extent, measuring the poor and having strategies to do so keep
the poor on the agenda, making the problem of political and moral
concern.
Threshold limitations
It
is hard to have exact number for poverty, as much data is collected
through interviews, meaning income that is reported to the interviewer
must be taken at face value.
As a result, data could not rightly represent the situations true
nature, nor fully represent the income earned illegally. In addition, if
the data were correct and accurate, it would still not mean serving as
an adequate measure of the living standards, the well-being or economic
position of a given family or household. Research done by Haughton and
Khandker
finds that there is no ideal measure of well-being, arguing that all
measures of poverty are imperfect. That is not to say that measuring
poverty should be avoided; rather, all indicators of poverty should be
approached with caution, and questions about how they are formulated
should be raised.
As a result, depending on the indicator of economic status used,
an estimate of who is disadvantaged, which groups have the highest
poverty rates, and the nation's progress against poverty varies
significantly. Hence, this can mean that defining poverty is not just a
matter of measuring things accurately, but it also necessitates
fundamental social judgments, many of which have moral implications.
National poverty lines
2008
CIA World Factbook-based map showing the percentage of population by
country living below that country's official poverty line
National estimates are based on population-weighted subgroup
estimates from household surveys. Definitions of the poverty line do
vary considerably among nations. For example, rich nations generally
employ more generous standards of poverty than poor nations. Even among
rich nations, the standards differ greatly. Thus, the numbers are not
comparable among countries. Even when nations do use the same method,
some issues may remain.
United Kingdom
In
the UK in 2006, "more than five million people – over a fifth (23
percent) of all employees – were paid less than £6.67 an hour". This
value is based on a low pay rate of 60 percent of full-time median
earnings, equivalent to a little over £12,000 a year for a 35-hour
working week. In April 2006, a 35-hour week would have earned someone
£9,191 a year – before tax or National Insurance".
In 2019, the Low Pay Commission estimated that about 7% of people employed in the UK were earning at or below the National Minimum Wage. In 2021, the Office for National Statistics
found that 3.8% of jobs were paid below the National Minimum Wage, a
decrease from 7.4% in 2020 but an increase from 1.4% in 2019. They note that this increase from 2019 to 2021 is connected to the COVID-19 pandemic in the United Kingdom. The Guardian reported in 2021 that "almost 5m jobs, or one in six nationally, pay below the real living wage".
India
India's official poverty level as of 2005
is split according to rural versus urban thresholds. For urban
dwellers, the poverty line is defined as living on less than 538.60
rupees (approximately US$12) per month, whereas for rural dwellers, it
is defined as living on less than 356.35 rupees per month (approximately
US$7.50).
In 2019, the Indian government stated that 6.7% of its population is
below its official poverty limit. As India is one of the fastest-growing
economies in 2018, poverty is on the decline in the country, with close
to 44 Indians escaping extreme poverty every minute, as per the World Poverty Clock. India lifted 271 million people out of poverty in a 10-year time period from 2005/06 to 2015/16.
Singapore
Singapore
has experienced strong economic growth over the last ten years and has
consistently ranked among the world's top countries in terms of GDP per
capita.
Inequality has however increased dramatically over the same time
span, yet there is no official poverty line in the country. Given
Singapore's high level of growth and prosperity, many believe that
poverty does not exist in the country, or that domestic poverty is not
comparable to global absolute poverty. Such a view persists for a
selection of reasons, and since there is no official poverty line, there
is no strong acknowledgement that it exists.
Yet, Singapore is not considering establishing an official poverty line,
with Minister for Social and Family Development Chan Chun Sing claiming
it would fail to represent the magnitude and scope of problems faced by
the poor. As a result, social benefits and aids aimed at the poor would
be a missed opportunity for those living right above such a line.
Poverty rate map of India by prevalence in 2012, among its states and union territories
A comparative map of poverty in the world in 2012, at national poverty line, according to the World Bank
United States
In
the United States, the poverty thresholds are updated every year by
Census Bureau. The threshold in the United States is updated and used
for statistical purposes. In 2020, in the United States, the poverty
threshold for a single person under 65 was an annual income of
US$12,760, or about $35 per day. The threshold for a family group of
four, including two children, was US$26,200, about $72 per day.
According to the US Census Bureau's American Community Survey 2018
One-year Estimates, 13.1% of Americans lived below the poverty line.
Women and children find themselves impacted by poverty more often when a part of single mother families. The poverty rate of women has increasingly exceeded that of men's.
While the overall poverty rate is 12.3%, women poverty rate is 13.8%
which is above the average and men are below the overall rate at 11.1%.
Women and children (as single mother families) find themselves as a
part of low class communities because they are 21.6% more likely to fall
into poverty. However, extreme poverty, such as homelessness, disproportionately affects males to a high degree.
Racial minorities
A
minority group is defined as "a category of people who experience
relative disadvantage as compared to members of a dominant social
group."
Minorities are traditionally separated into the following groups:
African Americans, American Indians, Alaska Natives, Asians, Pacific
Islanders, and Hispanics.
According to the current US Poverty statistics, Black Americans – 21%,
Foreign born non-citizens – 19%, Hispanic Americans – 18%, and adults
with a disability – 25%.
This does not include all minority groups, but these groups alone
account for 85% of people under the poverty line in the United States. Whites have a poverty rate of 8.7%; the poverty rate is more than double for Black and Hispanic Americans.
Impacts on education
Living below the poverty threshold can have a major impact on a child's education. The psychological stresses induced by poverty may affect a student's ability to perform well academically. In addition, the risk of poor health is more prevalent for those living in poverty. Health issues commonly affect the extent to which one can continue and fully take advantage of his or her education. Poor students in the United States are more likely to dropout of school at some point in their education. Research has also found that children living in poverty perform poorly academically and have lower graduation rates. Impoverished children also experience more disciplinary issues in school than others.
Schools in impoverished communities usually do not receive much
funding, which can also set their students apart from those living in
more affluent neighborhoods.
There is much dispute over whether upward mobility that brings a child
out of poverty may or may not have a significant positive impact on his
or her education; inadequate academic habits that form as early as
preschool typically are unknown to improve despite changes in
socioeconomic status.
Impacts on healthcare
The nation's poverty threshold is issued by the Census Bureau.
According to the Office of Assistant Secretary for Planning and
Evaluation the threshold is statistically relevant and can be a solid
predictor of people in poverty. The reasoning for using Federal Poverty Level
(FPL) is due to its action for distributive purposes under the
direction of Health and Human Services. So FPL is a tool derived from
the threshold but can be used to show eligibility for certain federal
programs.
Federal poverty levels have direct effects on individuals' healthcare.
In the past years and into the present government, the use of the
poverty threshold has consequences for such programs like Medicaid and the Children's Health Insurance Program. The benefits which different families are eligible for are contingent
on FPL. The FPL, in turn, is calculated based on federal numbers from
the previous year.
The benefits and qualifications for federal programs are
dependent on number of people on a plan and the income of the total
group.
For 2019, the U.S Department of health & Human Services enumerate
what the line is for different families. For a single person, the line
is $12,490 and up to $43,430 for a family of 8, in the lower 48 states.
Another issue is reduced-cost coverage. These reductions are based on
income relative to FPL, and work in connection with public health
services such as Medicaid. The divisions of FPL percentages are nominally, above 400%, below 138% and below 100% of the FPL. After the advent of the American Care Act, Medicaid was expanded on states bases. For example, enrolling in the ACA kept the benefits of Medicaid when the income was up to 138% of the FPL.
Department of Health & Human Services Seal
Poverty mobility and healthcare
Health
Affairs along with analysis by Georgetown found that public assistance
does counteract poverty threats between 2010 and 2015. In regards to Medicaid, child poverty is decreased by 5.3%, and Hispanic and Black poverty by 6.1% and 4.9% respectively. The reduction of family poverty also has the highest decrease with Medicaid over other public assistance programs.
Expanding state Medicaid decreased the amount individuals paid by an
average of $42, while it increased the costs to $326 for people not in
expanded states. The same study analyzed showed 2.6 million people were
kept out of poverty by the effects of Medicaid.
From a 2013–2015 study, expansion states showed a smaller gap in health
insurance between households making below $25,000 and above $75,000.
Expansion also significantly reduced the gap of having a primary care
physician between impoverished and higher income individuals. In terms of education level and employment, health insurance differences were also reduced. Non-expansion also showed poor residents went from a 22% chance of being uninsured to 66% from 2013 to 2015.
Poverty dynamics
Living above or below the poverty threshold is not necessarily a position in which an individual remains static.
As many as one in three impoverished people were not poor at birth;
rather, they descended into poverty over the course of their life.
Additionally, a study which analyzed data from the Panel Study of
Income Dynamics (PSID) found that nearly 40% of 20-year-olds received
food stamps at some point before they turned 65.
This indicates that many Americans will dip below the poverty line
sometime during adulthood, but will not necessarily remain there for the
rest of their life.
Furthermore, 44% of individuals who are given transfer benefits (other
than Social Security) in one year do not receive them the next.
Over 90% of Americans who receive transfers from the government stop
receiving them within 10 years, indicating that the population living
below the poverty threshold is in flux and does not remain constant.
Cutoff issues
Most
experts and the public agree that the official poverty line in the
United States is substantially lower than the actual cost of basic
needs. In particular, a 2017 Urban Institute
study found that 61% of non-elderly adults earning between 100 and 200%
of the poverty line reported at least one material hardship, not significantly
different from those below the poverty line. The cause of the
discrepancy is believed to be an outdated model of spending patterns
based on actual spending in the year 1955; the number and proportion of
material needs has risen substantially since then.
Variability
The
US Census Bureau calculates the poverty line the same throughout the US
regardless of the cost-of-living in a state or urban area. For
instance, the cost-of-living in California, the most populous state, was
42% greater than the US average in 2010, while the cost-of-living in
Texas, the second-most populous state, was 10% less than the US average.
In 2017, California had the highest poverty rate in the country when
housing costs are factored in, a measure calculated by the Census Bureau
known as "the supplemental poverty measure".
Government transfers to alleviate poverty
In addition to wage and salary income, investment income and government transfers such as SNAP
(Supplemental Nutrition Assistance Program, also known as food stamps)
and housing subsidies are included in a household's income. Studies
measuring the differences between income before and after taxes and
government transfers, have found that without social support programs,
poverty would be roughly 30% to 40% higher than the official poverty
line indicates.
Long-range radar antenna, used to track space objects and ballistic missiles.
Radar of the type used for detection of aircraft. It rotates steadily, sweeping the airspace with a narrow beam.
Radar (radio detection and ranging) is a detection system that uses radio waves to determine the distance (ranging), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor
to determine properties of the object(s). Radio waves (pulsed or
continuous) from the transmitter reflect off the object and return to
the receiver, giving information about the object's location and speed.
Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is LIDAR, which uses predominantly infrared light from lasers
rather than radio waves. With the emergence of driverless vehicles,
radar is expected to assist the automated platform to monitor its
environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat
caused by the passage of a third vessel. In his report, Popov wrote
that this phenomenon might be used for detecting objects, but he did
nothing more with this observation.
The German inventor Christian Hülsmeyer
was the first to use radio waves to detect "the presence of distant
metallic objects". In 1904, he demonstrated the feasibility of detecting
a ship in dense fog, but not its distance from the transmitter. He obtained a patent for his detection device in April 1904 and later a patent for a related amendment for estimating the distance to the ship. He also obtained a British patent on 23 September 1904 for a full radar system, that he called a telemobiloscope.
It operated on a 50 cm wavelength and the pulsed radar signal was
created via a spark-gap. His system already used the classic antenna
setup of horn antenna with parabolic reflector and was presented to
German military officials in practical tests in Cologne and Rotterdam harbour but was rejected.
In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen
and during the 1920s went on to lead the U.K. research establishment to
make many advances using radio techniques, including the probing of the
ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office
model after noting its manual's description of a "fading" effect (the
common term for interference at the time) when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U.S. Navy researchers A. Hoyt Taylor and Leo C. Young
discovered that ships passing through the beam path caused the received
signal to fade in and out. Taylor submitted a report, suggesting that
this phenomenon might be used to detect the presence of ships in low
visibility, but the Navy did not immediately continue the work. Eight
years later, Lawrence A. Hyland at the Naval Research Laboratory (NRL) observed similar fading effects from passing aircraft; this revelation led to a patent application
as well as a proposal for further intensive research on radio-echo
signals from moving targets to take place at NRL, where Taylor and Young
were based at the time.
Similarly, in the UK, L. S. Alder took out a secret provisional patent for Naval radar in 1928. W.A.S. Butement
and P. E. Pollard developed a breadboard test unit, operating at 50 cm
(600 MHz) and using pulsed modulation which gave successful laboratory
results. In January 1931, a writeup on the apparatus was entered in the Inventions Book
maintained by the Royal Engineers. This is the first official record in
Great Britain of the technology that was used in Coastal Defence and
was incorporated into Chain Home as Chain Home (low).
In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil
(CSF) headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M.
Hugon, began developing an obstacle-locating radio apparatus, aspects
of which were installed on the ocean liner Normandie in 1935.
During the same period, Soviet military engineer P.K. Oshchepkov, in collaboration with the Leningrad Electrotechnical Institute, produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver.
The Soviets produced their first mass production radars RUS-1 and RUS-2
Redut in 1939 but further development was slowed following the arrest
of Oshchepkov and his subsequent gulag sentence. In total, only 607 Redut stations were produced during the war. The first Russian airborne radar, Gneiss-2, entered into service in June 1943 on Pe-2 dive bombers. More than 230 Gneiss-2 stations were produced by the end of 1944.
The French and Soviet systems, however, featured continuous-wave
operation that did not provide the full performance ultimately
synonymous with modern radar systems.
Full radar evolved as a pulsed system, and the first such
elementary apparatus was demonstrated in December 1934 by the American Robert M. Page, working at the Naval Research Laboratory. The following year, the United States Army successfully tested a primitive surface-to-surface radar to aim coastal batterysearchlights at night. This design was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and the firm GEMA [de] in Germany and then another in June 1935 by an Air Ministry team led by Robert Watson-Watt in Great Britain.
In 1935, Watson-Watt was asked to judge recent reports of a German radio-based death ray
and turned the request over to Wilkins. Wilkins returned a set of
calculations demonstrating the system was basically impossible. When
Watson-Watt then asked what such a system might do, Wilkins recalled the
earlier report about aircraft causing radio interference. This
revelation led to the Daventry Experiment of 26 February 1935, using a powerful BBC
shortwave transmitter as the source and their GPO receiver setup in a
field while a bomber flew around the site. When the plane was clearly
detected, Hugh Dowding, the Air Member for Supply and Research was very impressed with their system's potential and funds were immediately provided for further operational development. Watson-Watt's team patented the device in GB593017.
A Chain Home tower in Great Baddow, Essex, United Kingdom
Memorial plaque commemorating Robert Watson-Watt and Arnold Wilkins
Development of radar greatly expanded on 1 September 1936 when
Watson-Watt became Superintendent of a new establishment under the
British Air Ministry, Bawdsey Research Station located in Bawdsey Manor,
near Felixstowe, Suffolk. Work there resulted in the design and
installation of aircraft detection and tracking stations called "Chain Home"
along the East and South coasts of England in time for the outbreak of
World War II in 1939. This system provided the vital advance information
that helped the Royal Air Force win the Battle of Britain;
without it, significant numbers of fighter aircraft, which Great
Britain did not have available, would always need to be in the air to
respond quickly. If enemy aircraft detection had relied solely on the
observations of ground-based individuals, Great Britain might have lost
the Battle of Britain. The radar formed part of the "Dowding system" for collecting reports of enemy aircraft and coordinating the response.
Given all required funding and development support, the team
produced working radar systems in 1935 and began deployment. By 1936,
the first five Chain Home
(CH) systems were operational and by 1940 stretched across the entire
UK including Northern Ireland. Even by standards of the era, CH was
crude; instead of broadcasting and receiving from an aimed antenna, CH
broadcast a signal floodlighting the entire area in front of it, and
then used one of Watson-Watt's own radio direction finders to determine
the direction of the returned echoes. This fact meant CH transmitters
had to be much more powerful and have better antennas than competing
systems but allowed its rapid introduction using existing technologies.
A key development was the cavity magnetron
in the UK, which allowed the creation of relatively small systems with
sub-meter resolution. Britain shared the technology with the U.S. during
the 1940 Tizard Mission.
In April 1940, Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defence. Also, in late 1941 Popular Mechanics
had an article in which a U.S. scientist speculated about the British
early warning system on the English east coast and came close to what it
was and how it worked. Watson-Watt was sent to the U.S. in 1941 to advise on air defense after Japan's attack on Pearl Harbor. Alfred Lee Loomis organized the secret MIT Radiation Laboratory at Massachusetts Institute of Technology,
Cambridge, Massachusetts which developed microwave radar technology in
the years 1941–45. Later, in 1943, Page greatly improved radar with the monopulse technique that was used for many years in most radar applications.
The war precipitated research to find better resolution, more
portability, and more features for radar, including complementary
navigation systems like Oboe used by the RAF's Pathfinder.
Applications
Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.
The information provided by radar includes the bearing and range (and
therefore position) of the object from the radar scanner. It is thus
used in many different fields where the need for such positioning is
crucial. The first use of radar was for military purposes: to locate
air, ground and sea targets. This evolved in the civilian field into
applications for aircraft, ships, and automobiles.
In aviation,
aircraft can be equipped with radar devices that warn of aircraft or
other obstacles in or approaching their path, display weather
information, and give accurate altitude readings. The first commercial
device fitted to aircraft was a 1938 Bell Lab unit on some United Air Lines aircraft. Aircraft can land in fog at airports equipped with radar-assisted ground-controlled approach systems in which the plane's position is observed on precision approach radar
screens by operators who thereby give radio landing instructions to the
pilot, maintaining the aircraft on a defined approach path to the
runway. Military fighter aircraft are usually fitted with air-to-air
targeting radars, to detect and target enemy aircraft. In addition,
larger specialized military aircraft carry powerful airborne radars to
observe air traffic over a wide region and direct fighter aircraft
towards targets.
Marine radars
are used to measure the bearing and distance of ships to prevent
collision with other ships, to navigate, and to fix their position at
sea when within range of shore or other fixed references such as
islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters.
A radar system has a transmitter that emits radio waves known as radar signals in predetermined directions. When these signals contact an object they are usually reflected or scattered
in many directions, although some of them will be absorbed and
penetrate into the target. Radar signals are reflected especially well
by materials of considerable electrical conductivity—such as most metals, seawater, and wet ground. This makes the use of radar altimeters
possible in certain cases. The radar signals that are reflected back
towards the radar receiver are the desirable ones that make radar
detection work. If the object is moving either toward or away from the transmitter, there will be a slight change in the frequency of the radio waves due to the Doppler effect.
Radar receivers are usually, but not always, in the same location
as the transmitter. The reflected radar signals captured by the
receiving antenna are usually very weak. They can be strengthened by electronic amplifiers. More sophisticated methods of signal processing are also used in order to recover useful radar signals.
The weak absorption of radio waves by the medium through which
they pass is what enables radar sets to detect objects at relatively
long ranges—ranges at which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light,
are too strongly attenuated. Weather phenomena, such as fog, clouds,
rain, falling snow, and sleet, that block visible light are usually
transparent to radio waves. Certain radio frequencies that are absorbed
or scattered by water vapour, raindrops, or atmospheric gases
(especially oxygen) are avoided when designing radars, except when their
detection is intended.
Illumination
Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves
emitted by the target objects themselves, such as infrared radiation
(heat). This process of directing artificial radio waves towards objects
is called illumination, although radio waves are invisible to the human eye as well as optical cameras.
Brightness can indicate reflectivity as in this 1960 weather radar image (of Hurricane Abby). The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.
If electromagnetic waves travelling through one material meet another material, having a different dielectric constant or diamagnetic constant from the first,
the waves will reflect or scatter from the boundary between the materials. This means that a solid object in air or in a vacuum,
or a significant change in atomic density between the object and what
is surrounding it, will usually scatter radar (radio) waves from its
surface. This is particularly true for electrically conductive materials such as metal and carbon fibre, making radar well-suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night.
Radar waves scatter in a variety of ways depending on the size
(wavelength) of the radio wave and the shape of the target. If the
wavelength is much shorter than the target's size, the wave will bounce
off in a way similar to the way light is reflected by a mirror.
If the wavelength is much longer than the size of the target, the
target may not be visible because of poor reflection. Low-frequency
radar technology is dependent on resonances for detection, but not
identification, of targets. This is described by Rayleigh scattering, an effect that creates Earth's blue sky and red sunsets. When the two length scales are comparable, there may be resonances.
Early radars used very long wavelengths that were larger than the
targets and thus received a vague signal, whereas many modern systems
use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread.
Short radio waves reflect from curves and corners in a way
similar to glint from a rounded piece of glass. The most reflective
targets for short wavelengths have 90° angles between the reflective surfaces. A corner reflector
consists of three flat surfaces meeting like the inside corner of a
box. The structure will reflect waves entering its opening directly back
to the source. They are commonly used as radar reflectors to make
otherwise difficult-to-detect objects easier to detect. Corner
reflectors on boats, for example, make them more detectable to avoid
collision or during a rescue. For similar reasons, objects intended to
avoid detection will not have inside corners or surfaces and edges
perpendicular to likely detection directions, which leads to "odd"
looking stealth aircraft. These precautions do not totally eliminate reflection because of diffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff,
are very reflective but do not direct the scattered energy back toward
the source. The extent to which an object reflects or scatters radio
waves is called its radar cross section.
Radar range equation
The power Pr returning to the receiving antenna is given by the equation:
In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range.
This yields:
This shows that the received power declines as the fourth power of
the range, which means that the received power from distant targets is
relatively very small.
Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler radar performance, which can be used to increase detection range and reduce transmit power.
The equation above with F = 1 is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects should also be considered.
Change of wavelength caused by motion of the source.
Frequency shift is caused by motion that changes the number of
wavelengths between the reflector and the radar. This can degrade or
enhance radar performance depending upon how it affects the detection
process. As an example, Moving Target Indication can interact with Doppler to produce signal cancellation at certain radial velocities, which degrades performance.
Sea-based radar systems, semi-active radar homing, active radar homing, weather radar, military aircraft, and radar astronomy
rely on the Doppler effect to enhance performance. This produces
information about target velocity during the detection process. This
also allows small objects to be detected in an environment containing
much larger nearby slow moving objects.
Doppler shift depends upon whether the radar configuration is
active or passive. Active radar transmits a signal that is reflected
back to the receiver. Passive radar depends upon the object sending a
signal to the receiver.
The Doppler frequency shift for active radar is as follows, where is Doppler frequency, is transmit frequency, is radial velocity, and is the speed of light:
Only the radial component of the velocity is relevant. When the
reflector is moving at right angle to the radar beam, it has no relative
velocity. Vehicles and weather moving parallel to the radar beam
produce the maximum Doppler frequency shift.
When the transmit frequency () is pulsed, using a pulse repeat frequency of , the resulting frequency spectrum will contain harmonic frequencies above and below with a distance of . As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of , called the Nyquist frequency,
since the returned frequency otherwise cannot be distinguished from
shifting of a harmonic frequency above or below, thus requiring:
Or when substituting with :
As an example, a Doppler weather radar with a pulse rate of 2 kHz and
transmit frequency of 1 GHz can reliably measure weather speed up to at
most 150 m/s (340 mph), thus cannot reliably determine radial velocity
of aircraft moving 1,000 m/s (2,200 mph).
In all electromagnetic radiation, the electric field is perpendicular to the direction of propagation, and the electric field direction is the polarization
of the wave. For a transmitted radar signal, the polarization can be
controlled to yield different effects. Radars use horizontal, vertical,
linear, and circular polarization to detect different types of
reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used by navigation radars.
Echo heights above ground Where : r : distance radar-target ke : 4/3 ae : Earth radius θe : elevation angle above the radar horizon ha : height of the feedhorn above ground
A radar beam follows a linear path in vacuum but follows a somewhat curved path in atmosphere due to variation in the refractive index of air, which is called the radar horizon. Even when the beam is emitted parallel to the ground, the beam rises above the ground as the curvature of the Earth sinks below the horizon. Furthermore, the signal is attenuated by the medium the beam crosses, and the beam disperses.
The maximum range of conventional radar can be limited by a number of factors:
Line of sight, which depends on the height above the ground. Without a direct line of sight, the path of the beam is blocked.
The maximum non-ambiguous range, which is determined by the pulse repetition frequency. The maximum non-ambiguous range is the distance the pulse can travel to and return from before the next pulse is emitted.
Radar sensitivity and the power of the return signal as computed in
the radar equation. This component includes factors such as the
environmental conditions and the size (or radar cross section) of the
target.
Signal noise is an internal source of random variations in the signal, which is generated by all electronic components.
Reflected signals decline rapidly as distance increases, so noise introduces a radar range limitation. The noise floor and signal to noise ratio are two different measures of performance
that affect range performance. Reflectors that are too far away produce
too little signal to exceed the noise floor and cannot be detected. Detection requires a signal that exceeds the noise floor by at least the signal to noise ratio.
Noise typically appears as random variations superimposed on the
desired echo signal received in the radar receiver. The lower the power
of the desired signal, the more difficult it is to discern it from the
noise. Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized.
Shot noise
is produced by electrons in transit across a discontinuity, which
occurs in all detectors. Shot noise is the dominant source in most
receivers. There will also be flicker noise caused by electron transit through amplification devices, which is reduced using heterodyne
amplification. Another reason for heterodyne processing is that for
fixed fractional bandwidth, the instantaneous bandwidth increases
linearly in frequency. This allows improved range resolution. The one
notable exception to heterodyne (downconversion) radar systems is ultra-wideband radar. Here a single cycle, or transient wave, is used similar to UWB communications, see List of UWB channels.
Noise is also generated by external sources, most importantly the
natural thermal radiation of the background surrounding the target of
interest. In modern radar systems, the internal noise is typically about
equal to or lower than the external noise. An exception is if the radar
is aimed upwards at clear sky, where the scene is so "cold" that it
generates very little thermal noise. The thermal noise is given by kB T B, where T is temperature, B is bandwidth (post matched filter) and kB is Boltzmann's constant.
There is an appealing intuitive interpretation of this relationship in a
radar. Matched filtering allows the entire energy received from a
target to be compressed into a single bin (be it a range, Doppler,
elevation, or azimuth bin). On the surface it would appear that then
within a fixed interval of time one could obtain perfect, error free,
detection. To do this one simply compresses all energy into an
infinitesimal time slice. What limits this approach in the real world is
that, while time is arbitrarily divisible, current is not. The quantum
of electrical energy is an electron, and so the best one can do is match
filter all energy into a single electron. Since the electron is moving
at a certain temperature (Planck spectrum)
this noise source cannot be further eroded. We see then that radar,
like all macro-scale entities, is profoundly impacted by quantum theory.
Noise is random and target signals are not. Signal processing can
take advantage of this phenomenon to reduce the noise floor using two
strategies. The kind of signal integration used with moving target indication can improve noise up to for each stage. The signal can also be split among multiple filters for pulse-Doppler signal processing, which reduces the noise floor by the number of filters. These improvements depend upon coherence.
Radar systems must overcome unwanted signals in order to focus on the
targets of interest. These unwanted signals may originate from internal
and external sources, both passive and active. The ability of the radar
system to overcome these unwanted signals defines its signal-to-noise ratio
(SNR). SNR is defined as the ratio of the signal power to the noise
power within the desired signal; it compares the level of a desired
target signal to the level of background noise (atmospheric noise and
noise generated within the receiver). The higher a system's SNR the
better it is at discriminating actual targets from noise signals.
Clutter refers to radio frequency (RF) echoes returned from targets
which are uninteresting to the radar operators. Such targets include
natural objects such as ground, sea, and when not being tasked for
meteorological purposes, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections, meteor trails, and Hail spike. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.
Some clutter may also be caused by a long radar waveguide between the radar transceiver and the antenna. In a typical plan position indicator
(PPI) radar with a rotating antenna, this will usually be seen as a
"sun" or "sunburst" in the center of the display as the receiver
responds to echoes from dust particles and misguided RF in the
waveguide. Adjusting the timing between when the transmitter sends a
pulse and when the receiver stage is enabled will generally reduce the
sunburst without affecting the accuracy of the range, since most
sunburst is caused by a diffused transmit pulse reflected before it
leaves the antenna. Clutter is considered a passive interference source,
since it only appears in response to radar signals sent by the radar.
Clutter is detected and neutralized in several ways. Clutter
tends to appear static between radar scans; on subsequent scan echoes,
desirable targets will appear to move, and all stationary echoes can be
eliminated. Sea clutter can be reduced by using horizontal polarization,
while rain is reduced with circular polarization (meteorological radars wish for the opposite effect, and therefore use linear polarization to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.
Clutter moves with the wind or is stationary. Two common strategies to improve measure or performance in a clutter environment are:
Moving target indication, which integrates successive pulses and
Doppler processing, which uses filters to separate clutter from desirable signals.
The most effective clutter reduction technique is pulse-Doppler radar. Doppler separates clutter from aircraft and spacecraft using a frequency spectrum,
so individual signals can be separated from multiple reflectors located
in the same volume using velocity differences. This requires a coherent
transmitter. Another technique uses a moving target indicator
that subtracts the receive signal from two successive pulses using
phase to reduce signals from slow moving objects. This can be adapted
for systems that lack a coherent transmitter, such as time-domain pulse-amplitude radar.
Constant false alarm rate, a form of automatic gain control
(AGC), is a method that relies on clutter returns far outnumbering
echoes from targets of interest. The receiver's gain is automatically
adjusted to maintain a constant level of overall visible clutter. While
this does not help detect targets masked by stronger surrounding
clutter, it does help to distinguish strong target sources. In the past,
radar AGC was electronically controlled and affected the gain of the
entire radar receiver. As radars evolved, AGC became computer-software
controlled and affected the gain with greater granularity in specific
detection cells.
Radar multipath echoes from a target cause ghosts to appear.
Clutter may also originate from multipath echoes from valid targets caused by ground reflection, atmospheric ducting or ionospheric reflection/refraction (e.g., anomalous propagation).
This clutter type is especially bothersome since it appears to move and
behave like other normal (point) targets of interest. In a typical
scenario, an aircraft echo is reflected from the ground below, appearing
to the receiver as an identical target below the correct one. The radar
may try to unify the targets, reporting the target at an incorrect
height, or eliminating it on the basis of jitter
or a physical impossibility. Terrain bounce jamming exploits this
response by amplifying the radar signal and directing it downward.
These problems can be overcome by incorporating a ground map of the
radar's surroundings and eliminating all echoes which appear to
originate below ground or above a certain height. Monopulse can be
improved by altering the elevation algorithm used at low elevation. In
newer air traffic control radar equipment, algorithms are used to
identify the false targets by comparing the current pulse returns to
those adjacent, as well as calculating return improbabilities.
Radar jamming refers to radio frequency signals originating from
sources outside the radar, transmitting in the radar's frequency and
thereby masking targets of interest. Jamming may be intentional, as with
an electronic warfare
tactic, or unintentional, as with friendly forces operating equipment
that transmits using the same frequency range. Jamming is considered an
active interference source, since it is initiated by elements outside
the radar and in general unrelated to the radar signals.
Jamming is problematic to radar since the jamming signal only
needs to travel one way (from the jammer to the radar receiver) whereas
the radar echoes travel two ways (radar-target-radar) and are therefore
significantly reduced in power by the time they return to the radar
receiver in accordance with inverse-square law.. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the line of sight from the jammer to the radar (mainlobe jamming). Jammers have an added effect of affecting radars along other lines of sight through the radar receiver's sidelobes (sidelobe jamming).
Mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle
and cannot fully be eliminated when directly facing a jammer which uses
the same frequency and polarization as the radar. Sidelobe jamming can
be overcome by reducing receiving sidelobes in the radar antenna design
and by using an omnidirectional antenna to detect and disregard non-mainlobe signals. Other anti-jamming techniques are frequency hopping and polarization.
Pulse
radar: The round-trip time for the radar pulse to get to the target and
return is measured. The distance is proportional to this time.
One way to obtain a distance measurement is based on the time-of-flight:
transmit a short pulse of radio signal (electromagnetic radiation) and
measure the time it takes for the reflection to return. The distance is
one-half the round trip time multiplied by the speed of the signal. The
factor of one-half comes from the fact that the signal has to travel to
the object and back again. Since radio waves travel at the speed of light,
accurate distance measurement requires high-speed electronics.
In most cases, the receiver does not detect the return while the signal
is being transmitted. Through the use of a duplexer, the radar switches
between transmitting and receiving at a predetermined rate.
A similar effect imposes a maximum range as well. In order to maximize
range, longer times between pulses should be used, referred to as a
pulse repetition time, or its reciprocal, pulse repetition frequency.
These two effects tend to be at odds with each other, and it is
not easy to combine both good short range and good long range in a
single radar. This is because the short pulses needed for a good minimum
range broadcast have less total energy, making the returns much smaller
and the target harder to detect. This could be offset by using more
pulses, but this would shorten the maximum range. So each radar uses a
particular type of signal. Long-range radars tend to use long pulses
with long delays between them, and short range radars use smaller pulses
with less time between them. As electronics have improved many radars
now can change their pulse repetition frequency, thereby changing their
range. The newest radars fire two pulses during one cell, one for short
range (about 10 km (6.2 mi)) and a separate signal for longer ranges
(about 100 km (62 mi)).
Distance may also be measured as a function of time. The radar mile is the time it takes for a radar pulse to travel one nautical mile,
reflect off a target, and return to the radar antenna. Since a nautical
mile is defined as 1,852 m, then dividing this distance by the speed of
light (299,792,458 m/s), and then multiplying the result by 2 yields a
result of 12.36 μs in duration.
Frequency modulation
Continuous wave (CW) radar. Using frequency modulation allows range to be extracted.
Another form of distance measuring radar is based on frequency
modulation. In these systems, the frequency of the transmitted signal is
changed over time. Since the signal takes a finite time to travel to
and from the target, the received signal is a different frequency than
what the transmitter is broadcasting at the time the reflected signal
arrives back at the radar. By comparing the frequency of the two signals
the difference can be easily measured. This is easily accomplished with
very high accuracy even in 1940s electronics. A further advantage is
that the radar can operate effectively at relatively low frequencies.
This was important in the early development of this type when
high-frequency signal generation was difficult or expensive.
This technique can be used in continuous wave radar and is often found in aircraft radar altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave
or sawtooth pattern at audio frequencies. The signal is then sent out
from one antenna and received on another, typically located on the
bottom of the aircraft, and the signal can be continuously compared
using a simple beat frequency modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.
The modulation index
riding on the receive signal is proportional to the time delay between
the radar and the reflector. The frequency shift becomes greater with
greater time delay. The frequency shift is directly proportional to the
distance travelled. That distance can be displayed on an instrument, and
it may also be available via the transponder. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are AZUSA, MISTRAM, and UDOP.
Terrestrial radar uses low-power FM signals that cover a larger
frequency range. The multiple reflections are analyzed mathematically
for pattern changes with multiple passes creating a computerized
synthetic image. Doppler effects are used which allows slow moving
objects to be detected as well as largely eliminating "noise" from the
surfaces of bodies of water.
The two techniques outlined above both have their disadvantages. The
pulse timing technique has an inherent tradeoff in that the accuracy of
the distance measurement is inversely related to the length of the
pulse, while the energy, and thus direction range, is directly related.
Increasing power for longer range while maintaining accuracy demands
extremely high peak power, with 1960s early warning radars
often operating in the tens of megawatts. The continuous wave methods
spread this energy out in time and thus require much lower peak power
compared to pulse techniques, but requires some method of allowing the
sent and received signals to operate at the same time, often demanding
two separate antennas.
The introduction of new electronics in the 1960s allowed the two
techniques to be combined. It starts with a longer pulse that is also
frequency modulated. Spreading the broadcast energy out in time means
lower peak energies can be used, with modern examples typically on the
order of tens of kilowatts. On reception, the signal is sent into a
system that delays different frequencies by different times. The
resulting output is a much shorter pulse that is suitable for accurate
distance measurement, while also compressing the received energy into a
much higher energy peak and thus reducing the signal to noise ratio. The technique is largely universal on modern large radars.
Speed measurement
Speed
is the change in distance to an object with respect to time. Thus the
existing system for measuring distance, combined with a memory capacity
to see where the target last was, is enough to measure speed. At one
time the memory consisted of a user making grease pencil marks on the radar screen and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers.
If the transmitter's output is coherent (phase synchronized),
there is another effect that can be used to make almost instant speed
measurements (no memory is required), known as the Doppler effect. Most modern radar systems use this principle into Doppler radar and pulse-Doppler radar systems (weather radar,
military radar). The Doppler effect is only able to determine the
relative speed of the target along the line of sight from the radar to
the target. Any component of target velocity perpendicular to the line
of sight cannot be determined by using the Doppler effect alone, but it
can be determined by tracking the target's azimuth over time.
It is possible to make a Doppler radar without any pulsing, known as a continuous-wave radar
(CW radar), by sending out a very pure signal of a known frequency. CW
radar is ideal for determining the radial component of a target's
velocity. CW radar is typically used by traffic enforcement to measure
vehicle speed quickly and accurately where the range is not important.
When using a pulsed radar, the variation between the phase of
successive returns gives the distance the target has moved between
pulses, and thus its speed can be calculated.
Other mathematical developments in radar signal processing include time-frequency analysis (Weyl Heisenberg or wavelet), as well as the chirplet transform which makes use of the change of frequency of returns from moving targets ("chirp").
Pulse-Doppler signal processing. The Range Sample axis represents individual samples taken in between each transmit pulse. The Range Interval
axis represents each successive transmit pulse interval during which
samples are taken. The Fast Fourier Transform process converts
time-domain samples into frequency domain spectra. This is sometimes
called the bed of nails.
Pulse-Doppler signal processing includes frequency filtering in the
detection process. The space between each transmit pulse is divided into
range cells or range gates. Each cell is filtered independently much
like the process used by a spectrum analyzer
to produce the display showing different frequencies. Each different
distance produces a different spectrum. These spectra are used to
perform the detection process. This is required to achieve acceptable
performance in hostile environments involving weather, terrain, and
electronic countermeasures.
The primary purpose is to measure both the amplitude and
frequency of the aggregate reflected signal from multiple distances.
This is used with weather radar
to measure radial wind velocity and precipitation rate in each
different volume of air. This is linked with computing systems to
produce a real-time electronic weather map. Aircraft safety depends upon
continuous access to accurate weather radar information that is used to
prevent injuries and accidents. Weather radar uses a low PRF.
Coherency requirements are not as strict as those for military systems
because individual signals ordinarily do not need to be separated. Less
sophisticated filtering is required, and range ambiguity processing is
not normally needed with weather radar in comparison with military radar
intended to track air vehicles.
The alternate purpose is "look-down/shoot-down"
capability required to improve military air combat survivability.
Pulse-Doppler is also used for ground based surveillance radar required
to defend personnel and vehicles.
Pulse-Doppler signal processing increases the maximum detection
distance using less radiation in close proximity to aircraft pilots,
shipboard personnel, infantry, and artillery. Reflections from terrain,
water, and weather produce signals much larger than aircraft and
missiles, which allows fast moving vehicles to hide using nap-of-the-earth flying techniques and stealth technology
to avoid detection until an attack vehicle is too close to destroy.
Pulse-Doppler signal processing incorporates more sophisticated
electronic filtering that safely eliminates this kind of weakness. This
requires the use of medium pulse-repetition frequency with phase
coherent hardware that has a large dynamic range. Military applications
require medium PRF which prevents range from being determined directly, and range ambiguity resolution
processing is required to identify the true range of all reflected
signals. Radial movement is usually linked with Doppler frequency to
produce a lock signal that cannot be produced by radar jamming signals.
Pulse-Doppler signal processing also produces audible signals that can
be used for threat identification.
A Track algorithm is a radar performance enhancement strategy.
Tracking algorithms provide the ability to predict future position of
multiple moving objects based on the history of the individual positions
being reported by sensor systems.
Historical information is accumulated and used to predict future
position for use with air traffic control, threat estimation, combat
system doctrine, gun aiming, and missile guidance. Position data is
accumulated by radar sensors over the span of a few minutes.
Radar video returns from aircraft can be subjected to a plot
extraction process whereby spurious and interfering signals are
discarded. A sequence of target returns can be monitored through a
device known as a plot extractor.
The non-relevant real time returns can be removed from the
displayed information and a single plot displayed. In some radar
systems, or alternatively in the command and control system to which the
radar is connected, a radar tracker is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.
Radio signals broadcast from a single antenna will spread out in all
directions, and likewise a single antenna will receive signals equally
from all directions. This leaves the radar with the problem of deciding
where the target object is located.
Early systems tended to use omnidirectional broadcast antennas,
with directional receiver antennas which were pointed in various
directions. For instance, the first system to be deployed, Chain Home,
used two straight antennas at right angles
for reception, each on a different display. The maximum return would be
detected with an antenna at right angles to the target, and a minimum
with the antenna pointed directly at it (end on). The operator could
determine the direction to a target by rotating
the antenna so one display showed a maximum while the other showed a
minimum.
One serious limitation with this type of solution is that the broadcast
is sent out in all directions, so the amount of energy in the direction
being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.
More modern systems use a steerable parabolic
"dish" to create a tight broadcast beam, typically using the same dish
as the receiver. Such systems often combine two radar frequencies in the
same antenna in order to allow automatic steering, or radar lock.
Parabolic reflectors can be either symmetric parabolas or spoiled
parabolas:
Symmetric parabolic antennas produce a narrow "pencil" beam in both the X
and Y dimensions and consequently have a higher gain. The NEXRADPulse-Doppler
weather radar uses a symmetric antenna to perform detailed volumetric
scans of the atmosphere. Spoiled parabolic antennas produce a narrow
beam in one dimension and a relatively wide beam in the other. This
feature is useful if target detection over a wide range of angles is
more important than target location in three dimensions. Most 2D
surveillance radars use a spoiled parabolic antenna with a narrow
azimuthal beamwidth and wide vertical beamwidth. This beam configuration
allows the radar operator to detect an aircraft at a specific azimuth
but at an indeterminate height. Conversely, so-called "nodder" height
finding radars use a dish with a narrow vertical beamwidth and wide
azimuthal beamwidth to detect an aircraft at a specific height but with
low azimuthal precision.
Types of scan
Primary
Scan: A scanning technique where the main antenna aerial is moved to
produce a scanning beam, examples include circular scan, sector scan,
etc.
Secondary Scan: A scanning technique where the antenna feed is moved
to produce a scanning beam, examples include conical scan,
unidirectional sector scan, lobe switching, etc.
Palmer Scan: A scanning technique that produces a scanning beam by
moving the main antenna and its feed. A Palmer Scan is a combination of a
Primary Scan and a Secondary Scan.
Conical scanning: The radar beam is rotated in a small circle around the "boresight" axis, which is pointed at the target.
Applied similarly to the parabolic reflector, the slotted waveguide
is moved mechanically to scan and is particularly suitable for
non-tracking surface scan systems, where the vertical pattern may remain
constant. Owing to its lower cost and less wind exposure, shipboard,
airport surface, and harbour surveillance radars now use this approach
in preference to a parabolic antenna.
Phased array
Phased array: Not all radar antennas must rotate to scan the sky.
Another method of steering is used in a phased array radar.
Phased array antennas are composed of evenly spaced similar
antenna elements, such as aerials or rows of slotted waveguide. Each
antenna element or group of antenna elements incorporates a discrete
phase shift that produces a phase gradient across the array. For
example, array elements producing a 5 degree phase shift for each
wavelength across the array face will produce a beam pointed 5 degrees
away from the centerline perpendicular to the array face. Signals
travelling along that beam will be reinforced. Signals offset from that
beam will be cancelled. The amount of reinforcement is antenna gain. The amount of cancellation is side-lobe suppression.
Phased array radars have been in use since the earliest years of radar in World War II (Mammut radar),
but electronic device limitations led to poor performance. Phased array
radars were originally used for missile defence (see for example Safeguard Program). They are the heart of the ship-borne Aegis Combat System and the Patriot Missile System.
The massive redundancy associated with having a large number of array
elements increases reliability at the expense of gradual performance
degradation that occurs as individual phase elements fail. To a lesser
extent, Phased array radars have been used in weathersurveillance.
As of 2017, NOAA plans to implement a national network of
Multi-Function Phased array radars throughout the United States within
10 years, for meteorological studies and flight monitoring.
Phased array antennas can be built to conform to specific shapes, like missiles, infantry support vehicles, ships, and aircraft.
As the price of electronics has fallen, phased array radars have
become more common. Almost all modern military radar systems are based
on phased arrays, where the small additional cost is offset by the
improved reliability of a system with no moving parts. Traditional
moving-antenna designs are still widely used in roles where cost is a
significant factor such as air traffic surveillance and similar systems.
Phased-array interferometry or aperture synthesis
techniques, using an array of separate dishes that are phased into a
single effective aperture, are not typical for radar applications,
although they are widely used in radio astronomy. Because of the thinned array curse,
such multiple aperture arrays, when used in transmitters, result in
narrow beams at the expense of reducing the total power transmitted to
the target. In principle, such techniques could increase spatial
resolution, but the lower power means that this is generally not
effective.
Antennas generally have to be sized similar to the wavelength of the operational frequency, normally within an order of magnitude.
This provides a strong incentive to use shorter wavelengths as this
will result in smaller antennas. Shorter wavelengths also result in
higher resolution due to diffraction, meaning the shaped reflector seen
on most radars can also be made smaller for any desired beamwidth.
Opposing the move to smaller wavelengths are a number of
practical issues. For one, the electronics needed to produce high power
very short wavelengths were generally more complex and expensive than
the electronics needed for longer wavelengths or didn't exist at all.
Another issue is that the radar equation's
effective aperture figure means that for any given antenna (or
reflector) size will be more efficient at longer wavelengths.
Additionally, shorter wavelengths may interact with molecules or
raindrops in the air, scattering the signal. Very long wavelengths also
have additional diffraction effects that make them suitable for over the horizon radars. For this reason, a wide variety of wavelengths are used in different roles.
The traditional band names originated as code-names during World
War II and are still in military and aviation use throughout the world.
They have been adopted in the United States by the Institute of Electrical and Electronics Engineers and internationally by the International Telecommunication Union. Most countries have additional regulations to control which parts of each band are available for civilian or military use.
Other users of the radio spectrum, such as the broadcasting and electronic countermeasures industries, have replaced the traditional military designations with their own systems.
Very long range, ground penetrating; 'very high frequency'. Early
radar systems generally operated in VHF as suitable electronics had
already been developed for broadcast radio. Today this band is heavily
congested and no longer suitable for radar due to interference.
P
< 300 MHz
> 1 m
'P' for 'previous', applied retrospectively to early radar systems;
essentially HF + VHF. Often used for remote sensing because of good
vegetation penetration.
Very long range (e.g. ballistic missile early warning),
ground penetrating, foliage penetrating; 'ultra high frequency'.
Efficiently produced and received at very high energy levels, and also
reduces the effects of nuclear blackout, making them useful in the missile detection role.
Long range air traffic control and surveillance; 'L' for 'long'. Widely used for long range early warning radars as they combine good reception qualities with reasonable resolution.
Moderate range surveillance, Terminal air traffic control,
long-range weather, marine radar; 'S' for 'sentimetric', its code-name
during WWII. Less efficient than L, but offering higher resolution,
making them especially suitable for long-range ground controlled interception tasks.
Missile guidance, marine radar,
weather, medium-resolution mapping and ground surveillance; in the
United States the narrow range 10.525 GHz ±25 MHz is used for airport
radar; short-range tracking. Named X band because the frequency was a
secret during WW2. Diffraction off raindrops during heavy rain limits
the range in the detection role and makes this suitable only for
short-range roles or those that deliberately detect rain.
From Germankurz, meaning 'short'. Limited use due to absorption by water vapour at 22 GHz, so Ku and Ka
on either side used instead for surveillance. K-band is used for
detecting clouds by meteorologists, and by police for detecting speeding
motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
Mapping, short range, airport surveillance; frequency just above K
band (hence 'a') Photo radar, used to trigger cameras which take
pictures of license plates of cars running red lights, operates at
34.300 ± 0.100 GHz.
Millimetre band,
subdivided as below. Oxygen in the air is an extremely effective
attenuator around 60 GHz, as are other molecules at other frequencies,
leading to the so-called propagation window at 94 GHz. Even in this
window the attenuation is higher than that due to water at 22.2 GHz.
This makes these frequencies generally useful only for short-range
highly specific radars, like power line avoidance systems for helicopters
or use in space where attenuation is not a problem. Multiple letters
are assigned to these bands by different groups. These are from Baytron,
a now defunct company that made test equipment.
Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
Modulators
Modulators act to provide the waveform of the RF-pulse. There are two different radar modulator designs:
High voltage switch for non-coherent keyed power-oscillators These modulators consist of a high voltage pulse generator formed from a high voltage supply, a pulse forming network, and a high voltage switch such as a thyratron. They generate short pulses of power to feed, e.g., the magnetron, a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as pulsed power. In this way, the transmitted pulse of RF radiation is kept to a defined and usually very short duration.
Hybrid mixers,[51] fed by a waveform generator and an exciter for a complex but coherent
waveform. This waveform can be generated by low power/low-voltage input
signals. In this case the radar transmitter must be a power-amplifier,
e.g., a klystron or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use pulse compression techniques.
Coherent microwave amplifiers operating above 1,000 watts microwave output, like travelling wave tubes and klystrons,
require liquid coolant. The electron beam must contain 5 to 10 times
more power than the microwave output, which can produce enough heat to
generate plasma. This plasma flows from the collector toward the
cathode. The same magnetic focusing that guides the electron beam forces
the plasma into the path of the electron beam but flowing in the
opposite direction. This introduces FM modulation which degrades Doppler
performance. To prevent this, liquid coolant with minimum pressure and
flow rate is required, and deionized water is normally used in most high
power surface radar systems that utilize Doppler processing.
Coolanol (silicateester) was used in several military radars in the 1970s. However, it is hygroscopic, leading to hydrolysis and formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire. Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named Pollution Prevention
(P2) to eliminate or reduce the volume and toxicity of waste, air
emissions, and effluent discharges. Because of this, Coolanol is used
less often today.
A radiodetermination system
based on the comparison of reference signals with radio signals
reflected, or retransmitted, from the position to be determined. Each radiodetermination system shall be classified by the radiocommunication service in which it operates permanently or temporarily. Typical radar utilizations are primary radar and secondary radar, these might operate in the radiolocation service or the radiolocation-satellite service.
Configurations
Radar come in a variety of configurations in the emitter, the receiver, the antenna, wavelength, scan strategies, etc.
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