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book reviews
Radar and electronic warfare booksreviewed by T. Nelson |
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book reviews
Radar and electronic warfare booksreviewed by T. Nelson |
Reviewed by T. Nelson
This is an introductory book on modern radar from a course on defense technology. The coverage is pretty much the same as in Fundamentals of Radar Signal Processing, 3e (reviewed at right), but in much less detail, especially on topics like clutter and waveforms. The formulas are given rather than derived and use only simple math and a few matrices.
The discussion of electronic warfare (EW) doesn't start until page 373 (Chapter 16), with short chapters on EA, EP, and ES (electronic attack, protection, and support). It makes up for its more superficial coverage by providing excellent diagrams and drawings (all in grayscale) and by Matlab/Simulink programs that can be downloaded from the book's website. This allows students to gain hands-on experience with how the parameters affect the radar signal.
The most interesting and unique chapter is on monopulse processing (Chapter 9), which is a clever way of increasing the accuracy of AoA (angle of arrival) measurements. Also interesting is Chapter 17, Electronic Protection, which describes how digital beamforming (DBF) is used to block interference from a jammer by cancelling the signal or slapping it into the null between the antenna sidelobes (see image). There's also more coverage of antennas than in most other radar books.
In general, if you read Richards's book, you already know most of this stuff. If your interest is in EW, read James Genova's book (reviewed below) instead. For a first book on radar, I'd recommend this one, especially if you're mathophobic. But I'd never call anyone a MERF (math exclusionary reader of fiction).
nov 16, 2023
Reviewed by T. Nelson
Who would have thought that all those years playing Battleship would some day come in handy? Well, it turns out that computers in anti-ship missiles represent their targets in much the same way as we did all those years ago (see Fig. 1).
According to Dr. James Genova, the biggest threat to an aircraft carrier is a swarm of supersonic missiles using computerized radar to identify the target. The old defenses no longer work. Some missiles now use low probability of intercept (LPI) radar instead of pulsed Doppler: instead of a strong pulse train they use phase-coded or frequency-coded waveforms and coherent radar receivers. This allows them to use much lower ERP without losing resolution, and it makes the radar stealthy and difficult to detect.
The missiles fly close to the waves and pop up for a few seconds only 20 km or so from the target to get a final bearing. If they can't identify the target at that point, they initiate home on jam (HOJ) and try to ‘burn through’ the jamming closer to the target. Typically in the last few seconds they jump up and hit the ship from above. Or they might aim for the waterline instead.
There are other tricks as well: Genova says the British lost a ship in the Falklands war because their defensive systems were programmed to treat Exocet missiles as being launched by NATO and therefore not a threat. The Americans had the same programming. If that war had not occurred, the US Navy might never have fixed that gaping, Gulf of Oman-sized hole in their security. But the threat of missiles to high-value targets, namely our ships (and, it goes without saying, to our sailors), remains.
This means that after the decoy is struck, the probability of the next missile hitting something hideously expensive is greatly increased. In the future, these missiles will use optical imaging along with pattern recognition to avoid being deceived by radar jamming and countermeasures. And jamming doesn't just mean transmitting noise; the DRFM unit records and plays back the incoming radar signal with a fake Doppler or range signal to make the missile think, for example, that the ship just launched itself into orbit at Mach 1. Or it might transmit signals to make the missile think it successfully burned through and is now seeing the high-value unit.
But missiles are getting more sophisticated. If the jamming signal is too weak or too strong, the missile will ignore it as being outside its RCS (radar cross-section) window for the pre-programmed target. It's so easy for a missile to identify and avoid chaff that, according to Genova, a viable strategy is to trick it by electronically simulating a cloud of chaff on top of the ship (something which strikes me as having a little too much faith in Chinese technology). Defenders also have to adjust the polarization and other RF properties of the decoy to counteract the missile's complicated mathematical algorithms.
The need for up-to-date intercepts of the radar waveforms used by the adversary (i.e., the PRC) is evident. Most of the illustrations here are geometric diagrams, but Genova has some images of actual radar data arrays that show just how convincing these decoys can be.
Genova's conclusions are chilling: noise jamming is “transparent” to the PRC's anti-ship missiles, and neither kinetic defenses nor current EA is adequate. With more modern missiles using AI and passive IR, and (soon) cloaking, the challenges facing the Navy are formidable.
Though there are numerous equations, they're reasonably simple, mostly variations of the well known radar range equation, sine wave formulas, SNR calculations, FFTs and the like. The technology is ten years old, but the fundamentals are still valid. A great book on an important topic.
apr 21, 2018
Reviewed by T. Nelson
At the risk of stating the obvious, the main difference between processing radar signals and processing other kinds of time-dependent signals is the radar stuff. That makes this book sort of a cross between a signal processing text and a basic radar text. It's intended to fill a gap between traditional radar books and signal-processing books, and it does a pretty good job. After introducing basic concepts about radar, it talks about sampling, coherent processing interval, waveforms, Doppler processing, detection, tracking, and synthetic aperture radar.
I found the most interesting section to be Chapter 4, Radar Waveforms. For instance, dechirping is actually not as great a mystery as I thought. The diagram below of a dechirp mixer, also known as a stretch processor, is pretty much all there is to it.
The dechirp mixer takes the input signal from the antenna and mixes it with a reference signal that contains two terms. The first term removes the carrier. The second term is a replica of the transmitted chirp containing a time delay. Then the FFT does a spectral analysis. That's it. (x and y are input and output signals; τ=pulse length, β=bandwidth, Ω=carrier frequency, t=time).
Why is chirping needed at all? The reason is that detectability increases with transmit energy, and range resolution increases with bandwidth. So a fixed-frequency pulse radar would need enormously long pulses to raise the energy enough to see a target, but this decreases the bandwidth, so it would have trouble measuring distance. So radar engineers use “pulse compression,” the most widely used one being chirping, aka linear FM or LFM modulation. Chirping can improve the resolution by a factor of 100 or more compared to a simple pulsed radar. This is called processing gain, but it also creates problems: the output is a sinc function instead of a triangle function, so you get sidelobes that must be removed by filtering. One trick to reduce them is nonlinear FM, where the frequency doesn't increase in a straight line, but this is technically more challenging.
There are many other challenges that come up: blind zones, where the return pulse happens to coincide with the next transmitted pulse; ground clutter, which decreases by R3 (compared to the targets, which decrease by R4, where R is range); blind speeds, where targets of a specific velocity can't be detected; range-Doppler coupling, where fast objects appear at the wrong distance; and a blizzard of technical challenges, such as amplitude jitter and phase jitter, which are outside the scope of the book.
Another problem occurs with phase-steered arrays, where the look direction can change during a chirp. One solution for this is to use stepped frequency pulses, but this too creates problems: you need to demodulate each pulse in a different way. There's a whole science to varying the pulse repetition rate to avoid blind zones and blind speeds.
In the chapter on Detection, the focus isn't on radar detectors or warning systems, but on classifying radar returns. The author says that detection is a problem in statistical hypothesis testing. That makes it something of general interest. Well, not <airquotes> interest </airquotes> interest, but you know what I mean. The chapter gets into the gory details of incomplete gamma functions and equations for probability of detection in various models, known as Swerling cases, and then discusses constant false alarm rates or CFAR. This is a big deal because an increase in noise of only 6 dB can increase the probability of a false detection by a million-fold. CFAR may be tedious, but it's a big topic in many other fields.
If any of that makes sense to you, you'll have no trouble understanding the many graphs and equations in this interesting book. There's no gee-whiz stuff here: coding goes only up to Barker codes; there's no discussion of missile radars, LPI, advanced algorithms, or propagation, and certainly nothing on AI. On the plus side, there's also not a single sentence on the Maxwell equations. It says ‘fundamentals’ right in the title, and the author means it. He talks mainly about pulse Doppler radar and regards FMCW radar as an inferior type, useful maybe in a police car for catching speeders but not much else. Even so, both types use chirping. The difference is that in FMCW, range is derived from the beat frequency and velocity is derived from comparison of multiple sweeps, while in pulse Doppler they're both derived by analyzing the pulses.
A familiarity with Fourier transforms, autocorrelation, and radio waves is recommended. Appendices on these topics in the back will help beginners, but the most helpful sections are the extensive lists of math symbols and acronyms.
jul 18, 2023
Reviewed by T. Nelson
This is a high-level overview of the process of commissioning, designing, and developing an anti-missile system for The Navy. That's an inherently exciting and important topic: whether this technology works could well determine what kind of world we live in and, indeed, maybe whether we live in it at all. But it's written in a style that can only be described as Pentagonese.
Here's a typical sentence:
Interceptor missile terminal homing modeling must include all of the error sources associated with handover (heading, cross-range, seeker pointing angle), terminal sensor range-dependent and range-independent noise, parasitic noise, and guidance and navigation instrument (inertial reference unit [IRU], inertial measurement unit [IMU], inertial navigation system [INS], global positioning system [GPS]) noise. [p.106]
The audience seems to be engineers who already know how to design a functioning missile and radar system but are unfamiliar with DoD terminology and performance requirements, and planners who don't care about the technical details but need to know how the overall system is supposed to work. It's essentially one of those 2000-slide PowerPoint presentations that the government gives that goes on for two whole days and teaches technical people what they already know, only with many, many acronyms.
Anti-missile missiles rely heavily on radar, so some of the more interesting sections are on radar. But here again, it's mostly stuff that even us laymen already know. For example, in Chapter 5 it compares some of the general design parameters of active and passive phased-array radar: transmit antenna gain, antenna diameter, peak power per element, and so forth. (Come on, everybody knows this stuff, man!) In the chapter on missiles, there are a few equations, a bunch of PowerPoint diagrams (all in grayscale), and many graphs showing the missiles' trajectory, propellant weight flow rate, homing time as a function of required acquisition range for different terminal velocities, and other parameters.
The last chapter has some basic mathematical equations for aerodynamics, geodetic models, and radar clutter.
For some reason I got a craving for fruit salad after reading this book.
aug 04, 2018