Improvements to Stealthy Radar
Probably the most exciting news at the April 2001 Navy League show was new radar being developed bv Thales Nederland. For some years the company has marketed Scout, a stealthy surface-search radar. Sales are increasing: more navies, including the U.S. Navy, are buying it in small numbers for tests. Radar intercept receivers detect radars by their sheer signal strength. Scout defeats them by operating at a very low peak power: its peak and average powers are the same. A conventional radar detects targets by receiving the echoes of short high-- powered pulses. It is like an AM (amplitude modulation) radio. Information is reflected entirely in relative signal strengths. As radio listeners know, there is an alternative. In an FM (frequency modulation) radio, signal strength is constant, but frequency fluctuates. Information is transmitted by that fluctuation. The great advantage of FM is that the amplifier in the radio operates at a fixed signal strength. An AM amplifier may find it difficult to reproduce a great range of variations in signal strength. If the maximum signal is strong enough, the amplifier may not even notice weak but important signals. This weakness becomes a serious problem if the signal is being jammed. From a technical point of view, AM works because it is possible to detect minute variations in frequency.
Scout works like an FM radio. It sends out signals at constant strength, but it sweeps back and forth in frequency, on a cyclical basis. Within any one cycle, range is measured by the frequency of the returned echo—which indicates the point in the cycle corresponding to the echo. This technique is analogous to the time measurement used in a conventional radar. Not only is it easy to measure frequency, but the electronic noise surrounding the radar tends not to corrupt frequency measurements. The FM radar therefore can operate at a very low constant power. Another way to say this is that in a conventional pulse radar, average power is a very small fraction of peak power. For example, a typical air search radar might pulse 100 times per second, each pulse being 1.5 microseconds long. Each second, then, the radar is emitting only 1.5 ten-thousandths of a second. If peak power is a megawatt, it is putting out a total of only 150 watts. The mathematics of radar shows, moreover, that effectiveness depends not on peak power but on that 150 watts of average power.
A radar intercept receiver easily can pick up the megawatt of peak power (sharply reduced by transmission range) against background noise, but the signal that began as 150 watts is a very different proposition. In fact, Thales does better. Some of its Scout radars operate at an average power measured in thousandths of a watt. The company claims that since the radars are as good as conventional sets, they generally will pick up a target before the target can possibly pick up their radar emissions. Presumably the Thales concepts apply equally well to naval communications systems, such as data links. Indeed, sometimes the difference between a digital (pulsed) data link and a radar is not altogether clear. For example, the Russian Cold War pulsed underwater communications systems also measured distance.
This year, Thales announced something new: a Scout-like air-search radar. Air-search sets are essential to any ship trying to defend itself, but on the other hand they give away its identity. Most schemes for tracking warships in the open sea depend on the ship's unwitting cooperation. For example, during the Cold War, the U.S. Navy fingerprinted Soviet naval radars. Each major radar set had been hand-made. Just as each individual gun impresses specific markings on a bullet it fires, each such radar impresses unintentional modulations on each pulse it emits. A very sensitive receiver can detect and identify those variations. Ships can be tied to particular radar sets, so that each time that particular set is detected, so is the ship. Repeated detections, even from space, make it possible to track the ship.
The problem of air search is that Scout associates position with frequency. Any fast-moving target Doppler shifts the returning echo. For example, an airplane flying toward an X-band radar at about 1000 km/sec (about 620 miles per hour) shifts the returning echo by a noticeable 10 kHz (the radar operates at about a million times this frequency). How does the radar separate the shift due to speed from the shift due to position? Probably the key is that fast aircraft impose Doppler shifts outside the radar's usual range of frequency modulation. Given the right sort of receiver, the radar can detect a wide range of shifts, and thus can register one outside its usual range. It also registers a series of possible positions linked to possible target speeds. Given a series of detections, the radar can calculate the speeds associated with different possible positions and compare them with the speeds implicit in those position estimates. Target position and speed probably are resolved within a few tens of cycles.
The new radar is an exciting experimental project. In a way, it is the ultimate extension of pulse compression. As its name implies, pulse compression was conceived as a kind of pulsed (AM) radar. In the late 1950s, it was pointed out that a pulse need not be very short. If frequency varied within the pulse, then the radar receiver could compress it, so that a long "chirp" could be made equivalent to a short pulse. Just how well the pulse could be compressed depended on how well frequency variation could be controlled within it. Fortunately the necessary radar tubes appeared as they were needed. At first, pulse compression was a way to increase effective pulse power without causing electrical breakdown in a radar. Then someone realized that it was a way of reducing peak power without really affecting overall performance. Given low enough peak power, it actually might be impossible for someone listening to the radar to detect such characteristics as its pulse rate, let alone intimate details of each pulse for fingerprinting. Several modern air-search radars use this pulse compression stealth technique. They can be recognized by the data sheets their manufacturers release, which give average radar power but not pulse details.
Thales believes this step makes radar detection virtually impossible. For a dominant navy such as ours, it is difficult to say whether this is good or bad news. If it enters service in quantity, it certainly will complicate any attempt we may make to detect and track ships. Perhaps we will have to look at other options, such as whatever signature may reside in a surface ship's wake. On the other hand, we face increasing numbers of enemies bent on denying us free use of nearby seas. In the current debate in Washington, much is being made of the vulnerability of large visible warships. In fact, such ships are almost impossible to detect and track except by picking up their characteristic electronic emissions. The Thales radar actually may be a more potent form of stealth than the careful shaping and construction techniques proposed for the new destroyer. After all, given sufficiently good signal processing, a radar can probably always overcome stealth, whether airborne or seaborne. But it is not so clear that a new receiver can overcome the sort of stealthy signal Thales is offering.
Space Operations Get Attention
Early in May, the Bush administration announced that it was appointing a four-star Air Force general as chief of space operations, a step intended to symbolize the administration's strong awareness of the importance of maintaining and protecting U.S. space assets. In one sense, such a step is long overdue. U.S. military forces increasingly rely on the products of space-based sensors, space-based communications links, and the space-based global positioning system (GPS). One reason why awareness is limited is that much of the space-based sensing was developed to gather intelligence, and kept extremely secret during the Cold War. Many officers whose lives literally would have depended on the fruits of such systems had little or no interest in their details, possibly because of the secrecy involved. The U.S. Navy probably remains the primary tactical user of space-based sensors, but it did not pay for most of them because they were built for Cold War strategic reasons. Now some of these systems are wearing out, and there is a real question whether they will be replaced.
There is, unfortunately, another side to this particular coin. The announcement stressed the need to defend U.S. space assets against all comers. The assets certainly are valuable, but right now there are no comers to speak of. The Russians retain an antisatellite system nominally effective against craft in fairly low orbits. It can knock out optical and radar reconnaissance satellites but neither GPS nor high-orbiting electronic intelligence and communications craft. It is possible that directed-- energy weapons mounted either on the earth or in space could damage or destroy satellites in high orbits, but no one has demonstrated anything approaching the necessary technology. In these terms, defense presumably would mean some way of destroying a Russian-type interceptor satellite before it reached its target, hence also the ability to knock out other satellites in space. There must be a question as to just how effective such a weapon would be. Much of the world's satellite capability is now commercial. A country denied dedicated military communications satellites might find itself well served by commercial ones. Somewhat similar considerations might apply to imagery, now that commercial imaging satellites are in service. One might ask whether the ability to destroy some objects in space is a declining asset, since in any conflict short of total war it would be difficult for us to justify destroying valuable commercial assets merely to ensure ourselves a monopoly of space systems.
One way to read the announcement, then, would be that it plays to a long-standing Air Force view that the most important role of space operations is to dominate space militarily. Given finite resources, that necessarily means that communications, navigation, and sensing will have to pay for offensive and defensive weapons. Perhaps there is a historical analogy.
In 1914, the warring powers discovered that aircraft offered them extremely valuable means of reconnaissance. Throughout World War I, reconnaissance probably was the most decisive contribution that air power made. Yet it was frustrating: fliers wanted to dominate the air, to shoot down rival aircraft. The question, which has never really been answered, is just how valuable that attempt to gain air superiority was. If combat aircraft really could deny an enemy aerial reconnaissance, surely that was well worth the effort. If not, then one might argue that it would have been better to spend resources on better reconnaissance platforms and systems. Clearly it is possible to invent antisatellite weapons. But is anyone actually building them? Can our likely adversaries afford to develop them? The Chinese have written a great deal recently about "asymmetric attacks" intended specifically to exploit U.S. weaknesses, and such writing might well refer to our space systems. However, much such writing is empty or academic bluster. During the Cold War, there was always considerable tension between those who based estimates of Soviet development on their writings (including classified writings), and those who relied instead on what actually could be seen—which often was not written about at all.