This information sheet is intended
to provide the technician with some of the background history and development
of radar, especially as it relates to the use of the technology in the
field of meteorology. The subjects covered in this sheet will follow
the sequence of topics listed below:
A Brief History Of Radar
Development Of Radar Meteorology
Frequencies In The Electromagnetic Spectrum
It is a common assumption that
radar is a by-product of World War II, brought about by military necessity.
According to Hiser (Radar Meteorology, Third Edition, 197Ø), this
is true only to the extent that the war produced radar equipment in great
quantities suitable for practical use. Actually, the fundamental
principle underlying all radars was first observed in 1886 by the physicist
Heinrich Hertz when he found that electromagnetic waves could be reflected
from various objects, and even focused into beams by appropriate reflectors.
In 19Ø4, a German engineer, Hulsmeyer, was granted a patent in several countries on a proposed method of using electromagnetic waves in an obstacle detector and navigation aid for ships. In 1922, Marconi urged the use of "short waves" for radio detection. From these beginnings, radar gradually evolved; it was not a science which was suddenly discovered. Various experimenters in electromagnetic waves during the following years reflected these waves from the upper atmosphere, and verified the existence of a series of ionized layers known as the ionosphere.
A suggestion for the practical utilization of reflected electromagnetic waves was proffered in 1922 by Dr. A. Hoyt Taylor and his associates in the U. S. Naval Research Laboratory, located at the confluence of the Potomac and Anacostia rivers in the District of Columbia. They were experimenting with relatively high-frequency radio communications from one side of the river to the other, when they discovered a significant loss of reception as ships passed between the transmitter and receiver. This situation led them to conclude that radio waves could be used to detect the presence of enemy ships.
Leading up to World War II, research
continued in both continuous wave and pulsed radio signals.
In 1925, the Carnegie Institution initiated ionospheric studies, using
pulsed radio waves. Further experiments and study by Lawrence Hyland
(U.S. Navy Research Labs) indicated (by 193Ø) that it was practical
to detect both ships and aircraft using continuous waves. Hyland
noticed that his radio "signal" meter would occasionally and mysteriously
fluctuate (display peaks and dips). He soon realized that the airplanes
taking off and landing at nearby Bolling Field were reflecting back a small
portion of his transmitted radio energy. In 1934, the Army Signal
Corps (using continuous waves) detected targets at very short ranges, but
as a result of their experiments, suggested the possibility of using pulsed
energy to observe targets at longer ranges. In Britain in 1935,
Robert Watson-Watt proposed a system nearly identical to the Army proposal,
and he went on to produce successful equipment in the same year.
The development of radar advanced rapidly, particularly in Great Britain,
where the need for an aircraft early-warning system was perceived as necessary
to the survival of the country. In 1937, the British commenced the
construction of a network of land-based radar early-warning stations.
This project placed the British far ahead of other nations in the development
of radar technology. One critical development by the British was
the magnetron, a high-power transmitting tube which they used in their
radar network. The British research, innovation, and necessity would
serve to help them maintain their out-front position in the field of radar
until around 1941, when the U.S. was forced to become involved in the war.
With the entry of the United States into World War II, the need for accelerated development of radar was fully realized on the west side of the Atlantic. The Armed Services' Laboratories had made some real progress in radar research and development, but their facilities and personnel were so severely limited that drastic measures were necessary if the U. S. was to adequately advance in the field of practical radar.
An arrangement was made with Great Britain in 1942 to pool all radar information, research, development, and even manufacturing. The United States Government sponsored and financed a Radiation Laboratory at Massachusetts Institute of Technology.
At M.I.T., hundreds of
scientists were gathered together to work on the task. This project, aided
materially by Bell Telephone Laboratories, succeeded quickly in placing
the U. S. and Britain far ahead of their enemies in the matter of radar
development and application. This position of superiority afforded
the Allies a military advantage which certainly resulted in bringing about
an earlier resolution in the conflict than would have otherwise been possible.
During World War II, it was quickly discovered that precipitation presented targets on radar displays, and frequently prevented the full utilization of the radar systems for the intended military purposes. Enemy aircraft, becoming aware of this limitation of radar, could plan their approaches on Allied radar systems from directions where precipitation was occurring. The enemy also used finely-cut strips of metal (called "window" or "chaff") to confuse radar operators with false signals, or to cover the approach of his aircraft. The effect was quite similar to that caused by precipitation, although it generally did not cover as large an area.
From another perspective, radar
observations of precipitation areas were frequently of significant value
in military operations, and meteorological officers soon discovered a new
tool with which they could increase the accuracy of their short-term forecasts.
Unfortunately, due to the scarcity of equipment, the security of the new
technology, and the dedication of existing equipment to direct military
applications, radar meteorology development was slow to occur during the
war years. At the conclusion of the war, and after the security restrictions
were relaxed, the new science of radar meteorology began to emerge.
Much of the equipment which became
available after the war had been designed exclusively for military applications.
Scientists and technicians in the meteorology field gradually realized
the strengths and limitations of these old radar systems in weather-detection,
and began to incorporate modifications which allowed for better application
to meteorological requirements. A number of concepts and specifications
were developed which ultimately resulted in a
rather thorough description of a radar system which would be designed exclusively for detection of meteorological phenomena. Early National Weather Service and Air Force weather radars were converted airborne systems designed for military applications, and did not meet the needs of a weather oriented community.
A pair of examples of these older, military systems are:
NWS WSR-1 "S" band
USAF APQ-13 "X" band
Due to the high cost of equipment and a shortage of funds, only a few types of radars have been built solely for meteorological use. The first of these was the AN/CPS-9 "X" band radar, developed for the U.S. Air Force. This system was installed at many strategic locations, both in the U.S. and around the world. Although there were some reservations within the meteorological community about the characteristics of the CPS-9 for weather use, the system was instrumental in pointing out the importance of radar meteorology in both wartime and peacetime use.
The second weather radar system was the WSR-57, an "S" band system developed for the United States Weather Bureau and the U.S. Navy. Placed in service beginning in 1959, the WSR-57 has served the National Weather Service for over 3Ø years, and still comprises a major portion of the nation's weather radar network.
During the early 1960s, the AN/FPS-68
and AN/FPS-81 "C" band weather radar systems were developed for the Navy.
The FPS-81 was an improved version of the FSP-68. The U.S. Air Force
also acquired some of the FPS-68 systems, and from them, the AN/FPS-77
"C" band system was developed. In 1974, the National Weather Service
and the U.S. Air Force began the purchase and installation of the WSR-74
"C" band system (called FPQ-21 in the military nomenclature).
These radars serve the NWS as local warning radar systems, while the Air Force used the FPQ-21 to replace many of their older "front-line" systems. Also in the 1970s, an "S" band version of the WSR-74 came into being. This radar has been used by the NWS for replacement of a number of older, difficult-to-maintain WSR-57 systems. During the same period, there have also been notable developments of weather radar equipment in Europe, Japan, Russia, and Australia.
Today, the NWS, Department of
Defense, and the FAA have jointly purchased, and are installing, the WSR-88D
"S" band Doppler radar system. The WSR-88D represents the continuing
advances in the technologies of meteorological radar, computer processing
and display, and high-speed dissemination of radar information. The
WSR-88D is planned to serve the meteorological community for a period which
will extend well into the next century.
The positions of the most-used
radar frequencies in the electromagnetic spectrum are depicted in the table
below. Note that the "radar" region of the spectrum extends from
about 25 MHz to 7Ø,ØØØ MHz (7Ø GHz).
The Audio and Video frequencies in the low end of the spectrum are used in radar system display and control signals. The VHF range of the spectrum contains frequencies which are used in radar receiving systems (intermediate frequencies, etc.). The Radar Frequency range depicted in the table indicates the area of the spectrum where radar transmitters operate. The upper end of the microwave region (above 3ØØ GHz) is not normally discussed in terms of radar systems. At wavelengths shorter than 1 millimeter, microwave techniques are normally replaced by optical transmission, control, and reception techniques.
Early in the development of radar, the system of letter codes (L, C, X, and S) were adopted to designate the bands of radar frequencies. The original purpose of the letter designators was for military security during World War II, but the codes have continued to be utilized as a convenient means of classifying radars in their groups of operating frequencies. Unfortunately, the letter codes do not have any official status, and there is not complete agreement as to the limits associated with each band. The table below provides a generally acceptable list of the letter designations, along with their respective frequency and wavelength boundaries.
The wavelengths given in the table are based upon the equation:
Wavelength (cm) = --
where.. "c" is the speed of light in centimeters
and "f" is the frequency of the transmitter (in Hertz).
Radar Meteorology, (H.W. Hiser, 197Ø)
Radar Principles, (NWSTC MRRAD41Ø, 1988)