Radio detection and ranging (radar) involves the transmission of an electromagnetic wave to a potential object of interest, scattering of the wave by the object, receipt of the scattered energy at the receive site, and signal processing applied to the received signal to generate the desired information product. Originally developed to detect enemy aircraft during World War II, radar has through the years shown diverse application, not just for military consumers, but also for commercial customers. Radar systems are still used to detect enemy aircraft, but they also keep commercial air routes safe, detect speeding vehicles on highways, image polar ice caps, assess deforestation in rain forests from satellite platforms, and image objects under foliage or behind walls. A number of other radar applications abound.

Millimeter wave (MMW) radar does not represent a specific radar application but a class of radars that operates over the range of frequencies from 30 GHz to 300 GHz. This corresponds to wavelengths measured in millimeters, hence the name. While the first MMW systems were large and bulky, today's systems have higher effective output powers and greater efficiencies in extremely small packages. This paper presents the unique capabilities and challenges presented by the use of MMW for target detection, tracking, and identification in the current environment for both the defense and commercial market sectors. Implementations and examples are provided along with key operating considerations.

The main objective of this chapter is to describe how an airborne pulse-Doppler radar isolates targets of interest from clutter and measures target range and velocity. Related topics such as pulse compression, automatic detection, target tracking, and target and clutter statistics are described. The intent is to present basic principles and concepts. Although some of the approximations used sacrifice generality and rigor for the sake of clarity, they are otherwise fairly accurate for most conditions of interest.

The principal U.S. radars composing this class are listed. While this chapter is focused on BMD, the MW and SSA missions are also addressed in the introduction because a given sensor may perform aspects of all three missions. In the United States, the Integrated Tactical Warning and Attack Assessment (ITWAA) system detects and tracks strategic ballistic missile threats to the United States. The current generation of early warning radars (EWRs) supporting the ITWAA mission employs active electronically scanned arrays (AESAs) operating in the ultrahigh frequency (UHF) band. While the United States has deployed geosynchronous satellites to detect missile launches from the short-wave infrared (SWIR) signature of the boost phase, EWRs are critical to verifying and characterizing the attack as well as estimating the impact points of the individual reentry vehicles (RVs). The first generation of missile warning radars employed fixed-beam search radars and mechanically scanned dish radars and was one of the first billion-dollar development projects undertaken by the U.S. defense establishment. The current generation of AESA EWRs provides superior traffic handling and reliability.

Surface moving target indication (SMTI) involves searching Earth's surface for moving objects using a dedicated radar mode. Ground-moving target indication (GMTI) is a subordinate, commonly referenced mode implying the detection, location, and discrimination of vehicles and personnel (dismounts) against rural, suburban, and urban land settings. The general class of SMTI radar modes also includes searching for vessels against sea, lake, and riverine backgrounds. An SMTI platform can be an aircraft, an unmanned aerial system (UAS), a satellite, an aerostat, or a tower.

We can conclude that passive bistatic radar has come a long way since the first experiments in the early 1980s - and certainly since the first radar experiments using broadcast signals more than 50 years before that. Potentially, it offers covert operation with simple and low-cost equipment without the need for a transmitting license, and the ability to use parts of the elecromagnetic spectrum not normally available for radar use. The wide variety of broadcast, communications, and radionavigation sources and their excellent spatial coverage gives great scope for PBR. In common with all bistatic radars, it may allow mechanisms such as forward scatter, which enhances the radar signature of targets, to be exploited. The fact that PBR systems can be simple and low cost has meant that they have been very suitable for research by university groups, and there have been numerous publications on the subject.

The rest of this chapter begins by describing the general characteristics, specifications, and end uses of some of the common types of weather radar in use today. These radars include the NWS NEXRAD and terminal Doppler weather radar (TDWR) systems, as well as a description of airborne radars. Next, a special form of the radar-range equation will be developed that is suited for describing the volume scattering found in weather-related echoes. The reflectivity factor, Z, will be introduced along with the more common measurement quantity, dBZ. This will be followed by a description of the application of Doppler processing for weather measurements, including several potential pitfalls in this processing, such as range folding and the so-called Doppler dilemma. The methods used to measure important meteorological phenomena will be discussed with an emphasis on the development of the relationship for measurement of rain. The characteristics of hail and snow, as well as clear air and nonmeteorological targets, will also be discussed. Some basic radar characteristics of weather systems such as supercells, tornadoes, and hurricanes will be shown. The chapter concludes with a short discussion of some of the most recent advances in weather-radar systems and processing, including dual-Doppler processing, various mobile weather-radar systems, and some radar systems and concepts currently still in development or on the drawing board.

This chapter discusses the design of a pulsed-ground penetrating radar (GPR) system, radar transmitters, radar receivers and radar antennas.