Radar and electronic warfare booksreviewed by T. Nelson
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
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