Air and Spaceborne Radar Systems: An Introduction

by P. Lacomme, J.C. Marchais, J.P. Hardange, E. Normant, Jean-Claude Marchais

Overview

This new practical title provides a comprehensive account of the current state of the main types of airborne and spaceborne radar systems. Each system is covered in terms of the basic radar principles, functions and properties, as well as the radar's role within the overall system and mission. Given changing operational requirements, the authors also consider how radar may evolve into the future. The book benefits greatly from the unique knowledge of the author team, who are working in a company considered to be a center of excellence for commercial radar development.

Product Details

ISBN-13:	9780852969816
Publisher:	The Institution of Engineering and Technology
Publication date:	01/01/2001
Series:	Radar,Sonar,Navigation and Avionics Series
Pages:	504
Product dimensions:	6.12(w) x 9.25(h) x (d)

About the Author

Philippe Lacomme is a Senior Radar Designer with Thomson-CSF Detexis Company. He is the Technical Director of the Radar Unit, which is in charge of developing and producing airborne radar systems for Rafale aircraft, the Mirage 2000, and others. Professor Lacomme has taught radar theory at Thomson-CSF and in many universities and schools, and has lectured at numerous international conferences.

Jean-Claude Marchais was Technical Director of Thomson-CSF Radars & Contre-Mesures until his retirement. During his long career, he was involved in the development of radar systems for the Mirage aircraft family, a lecturer on radar at the ESME-Sudria engineering school, and the author of three books, including a highly successful one on operational amplifiers.

Jean-Philippe Hardange joined Thomson-CSF in 1982 and has worked there as a radar engineer on all types of airborne radar. In 1996 he was head of the Airborne Radar Engineering Department. Later he launched the SOSTAR project of ground surveillance for NATO. He is now leading the Airborne Systems Engineering Department at Thomson-CSF.

Eric Normant works as a research scientist at at Thomson-CSF Detexis and is head of the airborne reconnaissance radar team. He has worked on SAR processing and system engineering. He holds a dozen patents in the field of SAR and teaches general radar theory and SAR.

Read an Excerpt

1: The History and Basic Principles of Radar

1.1 History

In 1887 the German physicist Heinrich Hertz discovered electromagnetic waves and demonstrated that they share the same properties as light waves. These electromagnetic waves are often known as "Hertzian waves."

In the very early 1900s, Telsa in the US and Hülsmeyer in Germany proposed detection of targets by the use of radio waves.

The principle behind RADAR (Radio Detection And Ranging), based on the propagation of electromagnetic waves or, more precisely, that of radiofrequency (RF) waves, was described by the American Hugo Gernsback in 1911. In 1934 the French scientist Pierre David successfully used radar for the first time to detect aircraft. In 1935 Maurice Ponte and Henri Gutton, during trials carried out onboard the Orégon, part of the Compagnie Generale Transatlantique fleet, detected icebergs using waves with a 16 cm wavelength (lamda). In 1936 Professor Kunhold (Germany) detected aircraft.

Radar came into its own during the Second World War as the ideal technique for detecting the enemy, both day and night. As early as 1940 the British RAF, led by Watson Watt, developed a dense network of groundbased radars. This clinched their victory in the Battle of Britain, as it provided sufficient warning to deploy fighter planes under optimum conditions. The German army also set up its own ground-based radar network, which, from 1942 onward, they used to transmit the position of detected targets to the fighter control center. In order to intercept and shoot down the waves of allied bombers deployed at night, German fighter pilots used either daytime fighters to attack allied planes tracked by light from ground projectors, or night fighters equipped with radar.

The first ever operational warplane equipped with an airborne radar was the Messerschmitt Me 110 G-4 in 1941. Its Telefunken radar, the FUG 212, used a bulky antenna comprising a number of dipoles located outside the aircraft, on the nose. By June 1944 the German fighter unit possessed over 400 aircraft of this type with a radar range of approximately 5 km, this range being limited by the altitude at which the carrier was flying. By 1944 the American Naval Air Service was equipped with a Corsair with a radar pod on the right wing, while the American Air Force had a Northrop P-61A Black Widow fitted with a Western Electric radar system.

During the night of July 24-25, 1943, 800 RAF bombers carried out a raid on Hamburg. During this raid the bombers carried out the first ever operational chaff launch (metal strips whose dimensions vary depending on the wavelength of the radar they attempt to confuse). This operation rendered German ground-based and airborne radars totally nonoperational, blinded by an excess of objects to detect. It marked the beginning of electronic warfare.

Radar operators noted that the British Mosquito fighter planes and the Japanese Zero fighter planes, both wooden constructions, were particularly difficult to detect; they were the original stealth aircraft.

In 1943 Allied surface ships fitted with radar were used to detect German submarine snorkels, causing the German navy to suffer heavy losses.

Later the main steps in radar technological evolutions were

• pulse compression (in the early `60s)
• pulse Doppler radar (late `60s)
• digital radars (`70s)
• medium PRF radar (late `70s, early `80s)
• multimode programmable radar (mid-`80s)
• airborne electronically scanned antenna radar (`90s)

The first radar images of the Earth were obtained in 1978 using Synthetic Aperture Radar (SAR), operating in the L-band (X = 30 cm) and mounted on the American satellite Seasat. Resolution of the images obtained, both day and night, was close to 25 m.

1.2 Basic Principles

Radar is a system that transmits an electromagnetic wave in a given direction and then detects this same wave reflected back by an obstacle in its path.

1.2.1 Basic Configuration

Figure 1.1 illustrates the first basic radar design. The various components of radar include: for transmission, a transmitter sending a continuous sinusoidal wave to a transmitting antenna and, for reception, an antenna plus a high-gain receiver and a detector whose output signal is displayed using a radar display such as a CRT.

The role of the transmitting antenna is to concentrate the energy transmitted in a chosen direction in space (beam center). The transmitting antenna gain, G_t, is maximum along the axis and varies depending on the direction (see Chapter 3).

The receiving antenna collects the transmitted energy backscattered by the target in the same chosen direction. This receiving antenna has a gain G_r. Supposing the two antennas are identical, G_t = G_r.

The wave transmitted (in this case continuously) is propagated to and from the target at the speed of light, c. In a non-magnetic medium, the following is true:

c = 299.7925.10⁶/ (K_e)^1/2 m/s

In a vacuum, the dielectric constant K_e is equal to one. In air, its value varies slightly depending on temperature, composition, and pressure. At sea level it equals 1.000 536. In practice, the speed of light for radars is taken to be 300 000 km/s....

...To ensure that the receiving channel only detects the signal backscattered by the obstacle or target, it must be decoupled from the transmision channel. An antenna, whatever technology it uses, has a radiation pattern composed of a main lobe and sind and far lobes (see Figure 1.2)....

...For the radar shown in Figure 1.1, despite the fact that both antennas are operating in the same direction, they have a leakage, in this case due to the far lobes. For example, if the far lobes of both antennas are 40 decibels below the maximum level of the main lobe (along the beam center), the isolation of the two channels is equal to 80 dB. Under such conditions, if the signal backscattered by the obstacle and received by the receiving channel is stronger than that caused by spurious coupling, the obstacle will be detected. In practice, numerous other factors come into play. These will be dealt with in turn, and in particular in Chapter 3.

The radar shown in Figure 1.1 is a bistatic system. Although transmission and reception are adjacent, they do not physically overlap. This frequently used concept (e.g., for launching semi-active missiles) will be examined in a later section.

1.2.1.1 Range Measurement

If the radar transmission is a pure continuous wave with frequency fo, the backscattered wave will have the same frequency (if the relative velocity between radar and target is equal to zero), whatever the range. However, the greater the target range, and the lower the Radar Cross Section (RCS) of the target, the weaker the received signal. The RCS characterizes the backscattering coefficient of the target.

The target range can be obtained using one of several methods:

• by calculating the time between the detected target echo and the transmitted wave
• by calculating the difference in frequency between the received echo and the transmitted wave in the case of linear frequency modulation
• by calculating the differential phase of the double detection of an echo obtained using two transmissions of different frequencies (Chapter 8.6)

The following sections give a rapid overview of the first two methods....

Table of Contents

1: General principles: history and basic principles of radar; initial statements of operational requirements; the radar equation; propagation; noise and spurious signals; detection of point targets.

2: Target detection and tracking: clutter cancellation; air-to-air detection; air target tracking; ground target detection and tracking; maritime target detection and tracking; electromagnetic pollution.

3: Ground mapping and imagery: ground mapping; radar imagery; synthetic aperture radar (SAR); SAR specific aspects; inverse SAR; other observation radars .

4: Principal applications: radar applications and role; design overview; multifunction radar; technological aspects.

5: Radars of the future: the changing target; operational aspects; principal limitations of present day radars; electronically steered antennas; airborne and spaceborne radar enhancement; conclusions.