The capability of microwaves to penetrate cloud and rain has placed the weather radar in an unchallenged position for remotely surveying the atmosphere. Although visible and infrared cameras on satellites can detect and track storms, the radiation sensed by these cameras cannot probe inside the storm’s shield of clouds to reveal, as microwave radar does, the storm’s internal structure and the hazardous phenomena that might be harbored therein. The Doppler radar is the only remote sensing instrument that can detect tracers of wind and measure their radial velocities, both in the clear air and inside heavy rainfall regions veiled by clouds—clouds that disable lidars (i.e., radars that use radiation at optical or near optical wavelengths) because optical radiation can be completely extinguished in several meters of propagation distance. This unique capability supports the Doppler radar as an instrument of choice to survey the wind and water fields of storms and the environment in which they form. Pulsed-Doppler radar techniques have been applied with remarkable success to map wind and rain within severe storms showing in real time the development of incipient tornado cyclones, microbursts, and other storm hazards. Such observations should enable weather forecasters to provide better warnings and researchers to understand the life cycle and dynamics of storms.

1.1 Historical Background

The term radar was suggested by S. M. Taylor and F. R. Furth of the U.S. Navy and became in November 1940 the official acronym of equipment built for radio detecting and ranging of objects. The acronym was by agreement adopted in 1943 by the Allied powers of World War II and thereafter received general international acceptance. The term radio is a generic term applied to all electromagnetic radiation at wavelengths ranging from about 20 km (i.e., a frequency of 15,000 Hz—Hz or hertz is a unit of frequency in cycles per second that commemorates the pioneering work of Heinrich Hertz, who in 1886–1889 experimentally proved James Clerk Maxwell’s thesis that electrical waves are identical except in length to optical waves) to fractions of a millimeter.

Perhaps the earliest documented mention of the radar concept was made by Nikola Tesla in 1900 when he wrote in Century Magazine (June 1900, LX, p. 208): “When we raise the voice and hear an echo in reply, we know that the sound of the voice must have reached a distant wall, or boundary, and must have been reflected from the same. Exactly as the sound, so an electrical wave is reflected … we may determine the relative position or course of a moving object such as a vessel at sea, the distance traveled by the same, or its speed….”

The first recorded demonstration of the detection of objects by radio is in a patent issued in both Germany and England to Christian Hulsmeyer for a method to detect distant metallic objects by means of electromagnetic waves. The first public demonstration of his apparatus took place on 18 May 1904 at the Hohenzollern Bridge, Cologne, Germany, where river boats were detected when in the beam of generated radio waves (not pulsed) of wavelength about 40 to 50 cm (Swords, 1986).

Although objects were detected by radio waves as early as 1904, ranging by pulse techniques was not possible until the development of pulsed transmitters and wideband receivers. The essential criteria for the design of transmitters and receivers for pulsed oscillations were known in the early 1900s (e.g., pulsed techniques for the acoustical detection of submarines were vigorously developed during World War I), but the implementation of these principles into the design of practical radio equipment first required considerable effort in the generation of short waves.

The first successful demonstration of radio detection and ranging was accomplished using continuous waves (cw). On 11 December 1924, E. V. Appleton of King’s College, London, and M. A. F. Barnett of Cambridge University in England used frequency modulation (FM) of a radio transmitter to observe the beat frequency due to interference of waves returned from the ionosphere (i.e., a region in the upper atmosphere that has large densities of free electrons that interact with radio waves) and those propagated along the ground to the distant receiver. The frequency of the beat gives a direct measure of the difference of distance traveled along the two paths and thus the height or range of the reflecting layer. This technique is based on exactly the same principles used in the FM-cw radars that are comprehensively described in Section 7.10.3 of the first edition (1984) of this book.

Pulse techniques are commonly associated with radar and in July 1925 G. Breit and M. A. Tuve (1926) in their laboratory of the Department of Terrestrial Magnetism of the Carnegie Institution obtained the first ranging with pulsed radio waves. They cooperated with radio engineers of the United States Naval Research Laboratory (NRL) and pulsed a 71.3-m wavelength NRL transmitter (Station NKF, Bellevue, Anacostia, D. C.) located about 10 km southeast of their laboratory, and detected echoes from a reflecting layer about 150 km above the earth. The equipment of Appleton and Barnett can be considered perhaps the first FM-cw radar, and that of Breit and Tuve the first pulsed radar. On the other hand, because the height of ionospheric reflection is a function of the radio wavelength, these radio systems might not be considered radars because they did not locate an object well defined in space as an aircraft (Watson-Watt, 1957, Chap. 21). Nevertheless, these radar-like systems were assembled for atmospheric studies and not for the location of aircraft, which was the impetus for the explosive growth of radars in the late 1930s and early 1940s.

It is likely that the first attempt to use pulsed radar principles to measure ionospheric heights came from a British physicist, W. F. G. Swann, who during the years 1918 to 1923 joined the University of Minnesota in Minneapolis where Breit was an assistant professor (1923–1924) and Tuve was a research fellow (1921–1923) (Hill, 1990). It was at the University of Minnesota that Swann and J. G. Frayne made unsuccessful attempts to measure the height of the ionosphere using radar techniques. Although many have contributed to the development of radar as we know it today, the earliest radars were developed by men interested in research of the upper atmosphere and methods to study it (Guerlac, 1987, p. 53).1

The role of atmospheric scientists in the early development of radar is also evident from the British experience. It was in January 1935 that the Committee for the Scientific Survey of Air Defense (CSSAD) approached Robert A. Watson-Watt to inquire about the use of radio waves in the defense against enemy aircraft. Sir Watson-Watt graduated as an electrical engineer, and in 1915 joined the Meteorological Office to work on a system to provide timely thunderstorm warnings to World War I aviators. After this wartime effort, it was realized that meteorological science was an essential part of aviation. He therefore was able to continue his research on direction finding of storms using radio emissions generated by lightning.

The CSSAD inquiry triggered Watson-Watt and his colleague A. F. Wilkins to propose, in a memo dated 27 February 1935, a radar system to detect and locate aircraft in three dimensions. The feasibility of their proposal was based on their calculation of echo power scattered by an aircraft, and was supported by earlier published reports by British Post Office engineers who detected aircraft that flew into the beam of Postal radio transmitters (Swords, 1986, p. 175). It was in July 1935, less than five months after their proposal, that Watson-Watt and his colleagues successfully demonstrated the radio detection and ranging of aircraft. This radar system, after considerable modifications and improvements, led to the Chain Home radar network that provided British avaitors with early warning of approaching German aircraft.

Although many have contributed to the development of radar, Watson-Watt credits many of his remarkable achievements to the earlier nonmilitary work of atmospheric scientists. To quote Watson-Watt (1957, p. 92): “… without Breit and Tuve and that bloodstream of the living organism of international science, open literature, I might not have been privileged to become … the Father of Radar.”

Throughout the 1930s, independent parallel efforts in radar development took place in the United States, Germany, Italy, Japan, France, Holland, and Hungary. The almost simultaneous and similar radar developments in all these countries should not be surprising because the ideas basic to radar principles had been repeatedly presented for many years preceding its development. It was during this period that the threat of faster and more lethal military aircraft, and the looming of global conflict, gave tremendous impetus to the development of equipment for the early detection and location of aircraft. On 28 April 1936, scientists at NRL obtained the first definitive detection and ranging of aircraft, and on 14 December the U.S. Army’s Signal Corps, in an independent work, succeeded in locating an airplane by the pulse method. For a detailed description of these efforts and those in other countries, the reader is referred to the...