Conventional surface radars would seem to lack any appropriate reflecting background, unless the stealthy aircraft or missile must pass mountains which the radar can see. In the past, hopes for using surface radars against stealthy platforms have concentrated on longer-wave systems, secondary detection, and bistatics.
A longer-wave system, e.g., a high-frequency (HF) surface wave at sea, presumably will ignore those subtleties of shape that stealthy platforms use to send radar echoes away from the transmitter. Its signals probably will reflect from radar-absorbing materials, which actually cover somewhat limited frequency bands. On the other hand, HF radar will not give anything like the precision enjoyed by microwave radar. Metric-wave radar, which at least one navy—the Chinese—still uses, is sometimes suggested as a compromise. The Russians, for example, have publicized a modification kit that they claim permits their metric-wave ground radar to detect stealthy targets. In the absence of test flights by F-117s, however, skepticism seems reasonable.
Another possibility is that a surface radar unable to detect an airplane or missile might detect a turbulent wake. Some years ago, for example, the Australians claimed that their Jindalee over-the-horizon radar had tracked B-2s successfully by detecting their wakes. When the U.S. Air Force protested that it had never flown B-2s anywhere near Australia, the Australians pointedly replied that the B-2s they had seen were over Texas (their radar could see that far by bouncing its beam off the ionosphere and the sea).
Recent interest in clear-air turbulence—a threat to aircraft that can apparently be detected by appropriately used microwave radar, suggests that conventional equipment can do as well, but probably at short range only. It is virtually impossible to suppress an airplane's turbulent wake, which will extend at least a few lengths behind it, because the creation of the wake (e.g., of wingtip vortices) is closely bound up with the mechanism which makes flight possible. Skeptics may care to fly light aircraft immediately behind large airliners, exiting as their airplanes are flipped over by the wake.
Ship wakes, far more intense because they occur at the interface between two media, are extremely prominent signatures, even to a space-based radar. They extend well abaft a moving ship, as users of wake-following torpedoes know and exploit. As in the case of aircraft wakes, they are probably impossible to suppress; quite exotic hull forms are needed even to halve the wake.
Then there is bistatics. Stealthy platforms, such as the F-17, cannot absorb most radar illumination. Instead, they reflect most of the signal away from the transmitter and the collocated receiver—but the signal still goes somewhere. If the transmitter is linked to several receivers, and all are timed together, then presumably the reflected signal can be picked up and exploited. The U.S. Navy's new cooperative engagement capability (CEC) actually does link multiple radars. It would not seem too difficult to enhance it to the point that it could exploit bistatics.
The most important bistatic radars currently in service are large over-the-horizon sets, such as the Australian Jindalee, in which the receiver must be isolated from the transmitter. Readers may be amused, however, by another example named Brigand. A Brigand receiver picks up the radar pulses produced by another radar as they are scattered by the targets that radar illuminates. It turns out that a properly tuned set can, in effect, pick up roughly the same radar picture that the exploited radar sees. Brigand has been a staple of U.S. electronic intelligence for about 35 years, initially as a way of locating enemy radars (by mapping the ground clutter surrounding them).
It is by no means clear how widely, if at all, Brigand has spread since then, but the technique has interesting implications. For example, a group of ships might try to avoid detection by electronic support measures (ESM) as such by limiting radar use to only one of them. Brigand analysis of that radar's scattered signals would tend to reveal the other ships in the group, in which case radar silence might be less than golden.
Bistatics can be pushed farther. Radars generally see a target by illuminating it and detecting reflected radiation. A human eye, on the other hand, sees objects illuminated by available light; it is a fully passive sensor. The commercial use of the electromagnetic spectrum is exploding, to the point where there is serious competition between the military and civilian sectors for important frequency bands. In some cases, compromise has been possible only because radars were likely to be used only well out to sea. Clearly that cannot apply to a littoral situation. Too, the burgeoning commercial use of bands formerly limited to military equipment has enormously complicated ESM, since there are more and more signals to sort out.
There is another way to look at the problem. All of that commercial activity provides a sort of radar-frequency backlighting. Objects in the sky either reflect or diffuse this light. Radars can detect that sort of radiation, with reasonable pointing precision (though not as good as eye performance in visible light). In the past, radars have been active and the main passive devices have been low-gain and relatively low-precision ESM systems.
A passive device exploiting radio-frequency "light" would need very high gain as well as a wide dynamic range, since energy levels will often be quite small. To achieve the necessary gain, it might look like an array of radio telescopes, staring out continuously over a wide angle and forming many beams simultaneously. The main problem, in both acoustic and radiofrequency "daylighting," is probably how to handle the dynamic range, which in visual terms is the range of gray scales. The wider the dynamic range, the more numbers any digital processor must handle. That is why computer-aided animation (i.e., production of a usable visual image) is so difficult and so expensive in terms of computer power (it is also why so many digital devices simply convert data into accepted and rejected detections, 1s and 0s, depending on whether a signal does or does not cross a threshold).
Using "daylight," however, is surely by no means an immense technological challenge, just one whose time has come.
Revisiting the Two-Ocean Navy
Recent discussions about a replacement for the Panama Canal may carry important implications for the U.S. Navy of the next century. As merchant ships, particularly container ships, grow, fewer and fewer of them can navigate the existing Canal. Nor, for that matter, can U.S. aircraft carriers. Yet the volume of shipping from the Far East to the Atlantic expands yearly. When the U.S. government (under President Jimmy Carter) signed the current treaty, it not only agreed to hand over the Canal to Panama—it also agreed not to build any competitor suited to the newer ships. For that matter, the sheer cost of building such a waterway might well be prohibitive. The existing Canal cannot be enlarged easily, because its massive locks would have to be replaced (a project for a second, wider, set of locks to accommodate a new generation of capital ships was suspended in 1942 and permanently canceled in 1946).
One solution, of increasing interest in East Asia, is a land bridge—a railway connecting the two oceans across, say, Nicaragua. That was how travelers went across the Isthmus before the canal was built, but 19th-century railways could not compete with ships. That was mainly because break-bulk ships were so laborious to unload at one end and then to reload at the other. Containers are an entirely different story, as a generation of longshoremen around the world learned to its enormous cost.
Of course, for President Theodore Roosevelt, building the Canal was never a commercial proposition; it was the key to building an effective U.S. Navy. As a keen student of Alfred Thayer Mahan, President Roosevelt knew that unless the battle fleet were concentrated, it could be defeated in detail. The United States had potential enemies in both oceans and a fleet concentrated on one coast might well have to move to the other. Although a U.S. battleship (the USS Oregon ) was able to dash from Pacific to Atlantic in time for the war with Spain in 1898, and although the battle fleet did manage a passage of the Straits of Magellan in 1907, clearly it would have been far better to cut that journey dramatically. Of course, President Roosevelt's military asset was largely paid for by commercial traffic.
During World War II, the Canal made it possible for the industrial might of the U.S. East Coast to be exerted against Japan. Conversely, during the early 1980s it was recognized that in any protracted crisis U.S. forces in Europe would have to be augmented from the West Coast. Incidentally, this need for safe crisis or wartime transit helps to explain the Reagan administration's determination to prevent Soviet proxies from taking over Central America.
The Canal treaty was signed at an unusual time during the Cold War, when the President badly wanted to demonstrate U.S. warmth toward the Third World (including Panama) and when a hot war seemed remote. Also, the President was assured that the Canal no longer had much military significance because U.S. capital ships could not use it. That left only the commercial rationale, and President Carter could very reasonably see any U.S. insistence on maintaining control as an insult to Panama; surely (as in the case of the Suez Canal) the locals could and should be trusted to operate a vital national asset.
If the land bridge is created, the logic may be turned on its head. We will still need the Canal for our warships. Indeed, as the fleet shrinks, we may find ourselves relying on it more rather than less. Commercial support for the Canal is likely to shrink dramatically, however, as shipping firms build expensive ships that cannot pass through. As income from the Canal shrinks, can we really expect the Panamanians to spend more and more to maintain what amounts to a U.S., rather than a national, asset?
We may not be able to save the Panama Canal over the next three or four decades; the logic of the land bridge may be inescapable, and the crash in local revenues inevitable. In that case it may be wise to consider far more carefully current policies. For example, some years ago the Todd yard at Tacoma tried to stay in business by arguing that it was the sole major private warship repair yard on the West Coast. It lost; there were two big naval shipyards (Puget Sound and Mare Island) on the same coast, not to mention Long Beach. Now that Mare Island and Long Beach are gone, it can still be argued that ships can transit the Canal to the East Coast. And if the Canal dies?