Page 40 - Fundamentals of Radar Signal Processing

individual pulse are frequently on the order of a few megahertz, and in some

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fine-resolution radars may reach several hundred megahertz and even as high as
1 GHz. This fact has several implications for digital signal processing. For
example, very fast analog-to-digital (A/D) converters are required. The
difficulty of designing good converters at multi-megahertz sample rates has
historically slowed the introduction of digital techniques into radar signal

processing. Even now, when digital techniques are common in new designs,
radar word lengths in high-bandwidth systems are usually a relatively short 8 to
12 bits, rather than the 16 bits common in many other areas. The high data rates
have also historically meant that it has often been necessary to design custom
hardware for the digital processor in order to obtain adequate throughput, that
is, to “keep up with” the onslaught of data. This same problem of providing
adequate throughput has resulted in radar signal processing algorithms being

relatively simple compared to, say, sonar processing techniques. Only in the late
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1990s has Moore’s Law provided enough computing power to host radar
algorithms for a wide range of systems on commercial hardware. Equally
important, this same technological progress has allowed the application of new,
more complex algorithms to radar signals, enabling major improvements in

detection, tracking, and imaging capability.

1.3 Elements of a Pulsed Radar

Figure 1.2 is one possible block diagram of a simple pulsed monostatic radar.
The waveform generator output is the desired pulse waveform. The transmitter
modulates this waveform to the desired radio frequency (RF) and amplifies it
to a useful power level. The transmitter output is routed to the antenna through a
duplexer, also called a circulator or T/R switch (for transmit/receive). The
returning echoes are routed, again by the duplexer, into the radar receiver. The

receiver is usually a superheterodyne design, and often the first stage is a low-
noise RF amplifier. This is followed by one or more stages of modulation of the
received signal to successively lower intermediate frequencies (IFs) and
ultimately to baseband, where the signal is not modulated onto any carrier
frequency. Each modulation is carried out with a mixer and a local oscillator
(LO). The baseband signal is next sent to the signal processor, which performs
some or all of a variety of functions such as pulse compression, matched

filtering, Doppler filtering, integration, and motion compensation. The output of
the signal processor takes various forms, depending on the radar purpose. A
tracking radar would output a stream of detections with measured range and
angle coordinates, while an imaging radar would output a two- or three-
dimensional image. The processor output is sent to the system display, the data

processor, or both as appropriate.