Radar is defined as 'a device for transmitting electromagnetic (EM) signals and receiving echoes from objects of interest (targets) within its volume of coverage'. Radar was originally an acronym for radio detection and ranging.

This introductory discussion of radar is divided into three broad topics: How radar waves are generated and transmitted; How radar waves interact with external objects and return to the radar; How (fundamentally) the returned signals are processed to yield interesting information.

After the EM radiation leaves the radar antenna, it propagates into the external environment. Some of the radiation scatters from one or more objects in the environment, returns to the antenna, and enters the radar receiver. The radar may also receive some external noise simultaneously. This chapter discusses those effects.

Having examined how a radar signal is generated, is emitted from the antenna, scatters from external objects, reenters the radar, and is recorded as a voltage by an A/D converter, we now begin consideration of how to interpret the resulting measurement. Throughout this chapter, the returned echo is considered as a signal voltage plus a noise voltage, both analog (continuous). Effects of AID quantization noise can be considered part of the noise voltage.

This chapter investigates techniques for measuring the angular location of a target by the use of multiple subapertures and by comparing the signals received from them, and examined the potential accuracy of target angle measurement using a single-aperture radar.

The preceding chapters have devoted some detail to the fundamental aspects of radar: transmission/reception; antennas; waveforms; propagation; RCS; SNR; detection; and accuracy of measurement of range, velocity, and angular position. This chapter combines those fundamentals and considers the particular advantage of applying those techniques to the case of a rotating target, which leads us to the important domain of imaging radar.

So far, most of our discussion has concerned RAR, in which the antenna is a physical object that first emits, then collects the radiation. We now turn our attention to the case in which the antenna moves to cover a synthetic aperture (L_SA), thus producing SAR. SAR generally refers to the case of a moving radar and a stationary target - usually an extended scene, such as the surface of the Earth; ISAR refers to the case in which the radar is relatively stationary and a rotating target provides all (or most) of the motion to create the synthetic aperture. Obviously, those distinctions are not fundamental, because they depend on the user's coordinate system. Furthermore, the two concepts are not mathematical inverses, and there are gray areas where they merge. This chapter assumes an LFM SAR waveform. It also assumes that the Earth's surface is stationary and (except as noted) flat. For a discussion of SAR imaging of the ocean, which is moving.

This chapter investigates more carefully this step of image formation from digitized focused k-space data and discusses techniques for obtaining imagery with improved resolution and reduced sidelobes compared with imagery obtained simply using the weighted FT. The authors also introduce two other definitions: Signal processing, which is the processing of the k-space signal to form an image; Image processing, which is additional processing performed on the image to improve the contrast, resolution, and so on.

Chapter 4 summarized the theory of detection of a target in noise and/or clutter, assuming that the result of a single measurement is a single voltage or test statistic to be compared with a threshold to determine whether the target should be declared present or absent. This chapter discusses an extension of that theory to the case in which several complex voltages are considered simultaneously, in the presence of noise or clutter, and a decision is made regarding whether that set of complex voltages represents a target. The set of complex voltages (often called cells) can represent pixels in a complex SAR/ISAR image. More generally, it can represent any set of complex voltages collected by a radar, including range versus azimuth for a real-beam radar, doppler bin versus angle, polarimetric or interferometric dimensions (Chapter 7), and so forth.

A Doppler radar is defined by as 'a radar which uses the Doppler effect to determine the radial component of relative radar target velocity or to select targets having particular radial velocities.' A pulse-Doppler (or pulsed-Doppler) (PD) radar is defined as 'a Doppler radar that uses pulsed transmissions.' PD radar is used extensively for detecting and characterizing moving targets.

Airborne Moving-Target Indication (AMTI) radar: An MTI radar flown in an aircraft or other moving platform with corrections applied for the effects of platform motion, which include the changing clutter Doppler frequency and the spread of the clutter Doppler spectrum.

This chapter considers a radar with an antenna consisting of several subapertures, or antenna elements; the echoes received by each subaperture are processed using the adaptive matched filter. Such a radar is capable of a certain degree of suppression of jammers, as well as (for an airborne radar) more effective endoclutter MTI. A single-aperture radar receives a series of echoes, each characterized by the time at which it is received. A multiple-subaperture radar receives a set of echoes, each characterized not only by the reception time but also by the subaperture on which it is received. Because the subapertures are characterized by their location in space, the processing of the combined set of echoes is referred to as space-time processing. If spatial and temporal weights are adaptively calculated (i.e., using the adaptive matched filter), the technique is called space-time adaptive processing (STAP). This discussion of STAP follows that of Ward; Klemm has also developed a detailed exposition. This chapter summarizes the results; more complete derivations of equations can be found. Although the discussion here is in the context of an airborne radar, it can also be applied to a stationary surface-based radar by setting the platform velocity equal to zero. That is particularly applicable to jammer suppression.

This discussion of the fundamentals of bistatic radar follows that of Willis, who provides a history of many bistatic radar proposals and programs and a summary of many technical analyses. Figure 13.1 introduces some basic terminology. Instead of two locations of interest (the radar and the target), we now have three (transmitter, Tx; receiver, Rx; and target) forming the vertices of the bistatic triangle, which defines the bistatic plane. All three vertices may vary with time, as in an air warfare scenario. The line between Tx and Rx isthe baseline. From the target location, the angle between the vector to the Tx and the vector to the Rx is the bistatic angle.

This chapter examines two topics of modern radar: meteorological radar and ground-penetrating radar.