The first use of radar in combat was during the Battle of Britain, August-October 1940. Radar gave the Royal Air Force’s fighter command the margin of victory over the Luftwaffe.
The British Chain Home radars deployed on the eve of World War II had to cope with bombers flying at top speeds of about 250 mph. When the war ended, bomber and attack aircraft could reach speeds of some 350 mph, while the rocket-propelled Japanese "Baka”-piloted bomb could reach 570 mph in its final dive. The Baka was a harbinger of the postwar air threat—jet-propelled aircraft, as well as advanced piston-engine planes, and missiles.
Conventional radars that mechanically rotate or train in a vertical plane could not provide sufficiently rapid coverage to cope with advanced aircraft or missile threats. There also was a delay in coordinating azimuth from a two-dimension (2-D) search radar with altitude from a height-finding radar. On the other hand, electrically steered or focused radar beams could provide more rapid coverage and, possibly, combine the azimuth search and height-finding functions to permit rapid detection and tracking of highspeed air threats.
To “aim” the radar beam electronically the separate elements of the antenna are energized out of phase rather than in phase. For example, the phase of the top element lags the one below it, the next element lags the one below it, and so on down the face of the array. This lagging process points the radar beam upward, above the “boresight” axis of the array. The reverse scheme of lags in phasing the elements will aim the beam below the axis. By using a large number of elements, greater beam focusing is achieved, thus creating a very narrow, high-resolution beam. Because the aim can be changed quickly (i.e., faster than by mechanical means), large angular areas can be searched rapidly.
The first U.S. phased-array radar to be used operationally was the Mk-VIII (CXEM) main battery control radar developed by Bell Laboratories and Western Electric. It was widely used in U.S. Navy cruisers and battleships during World War II. The Mk-VIII, an S-band radar, had an array of 42 radiating elements on an antenna 10 feet wide and 3-1/3 feet high. The radar scanned by phase switching, and was credited with accuracies of 15 yards in range and 2 mils in bearing; ranges were described as 40,000 yards against a battleship, 31,000 against a destroyer, 10,000 for a submarine target, and 30,000 on a high-flying bomber. The set had a limited scan angle and scanned in azimuth only, with the antenna turning as the Mk-34 or -38 director rotated.
Phased-array technology was applied to several naval radars after the war, including the AN/SPS-26/39/42 series, and the improved AN/SPS-48 and AN/SPS-52. These were frequency scan (FRESCAN) radars that were rotated mechanically in azimuth, while pointing the beam in elevation was controlled by varying the frequency of the signal. The Hughes-built SPS-26 was laboratory tested in 1953, and the first set to go to sea went on board the test ship Norfolk (DL-1) in 1957. The SPS-39 was the production model, with the similar SPS-42 being configured for integration with naval tactical data system. The SPS-48 is a highly sophisticated, long-range FRESCAN.
Today, these phased-array, 3-D radars are in use on board U.S. aircraft carriers, Tarawa-class amphibious assault ships, guided-missile cruisers, destroyers, frigates, and the two large command ships.
As technology advanced in the late 1950s, Hughes began development of the massive “billboard” AN/SPS-32 and -33 frequency scan radars, so-called because their large, rectangular antennas resembled advertising billboards. Four fixed antennas for the SPS-32 and four for the SPS-33 provided 360° coverage (giving the ships that carried them a large squared, futuristic-looking superstructure).
The SPS-32 is a very-long-range, FRESCAN 2-D radar for scanning in azimuth. Detection ranges of 400 miles have been reported against large targets under ideal conditions. The billboard antenna of each SPS-32 face is 40 feet wide and 20 feet high. The SPS-33 provides 3-D multi-target tracking in azimuth and elevation. This radar uses FRESCAN in elevation and phase scan or energy switching in azimuth. Each of the four SPS-33 faces is 25 x 20 feet. In general the SPS-33 is used only for tracking while the SPS-32 searches and makes initial detections.
The SPS-32/33 combination was installed in the Navy’s first two nuclear-propelled surface warships, the missile cruiser Long Beach (CGN-9) and aircraft carrier Enterprise (then CVAN- 65), both commissioned in late 1961, although their radar installations were not completed until 1962.
No additional sets were produced. The SPS-32 radar, weighing about 48.5 tons, and the SPS-33 radar, about 120 tons, were too large for the nuclear- propelled “destroyer” Bainbridge (then DLGN-25), completed in 1962. Their size, very high cost, and considerable maintenance problems led to an early end of the program. Indeed, the Long Beach will have her SPS-32/33 radars removed during her forthcoming mid-life modernization, and will probably receive the SPS-48 for 3-D search and the SPS-49 for long-range air search.
Very large phased-array radars were also developed during the 1960s for use ashore. The Air Force-Bendix AN/ FPS-85 SPADAT (space detection and tracking) radar was built at Eglin Air Force Base in Florida, with a range of several thousand miles for use in identifying and tracking objects in space. This set has 5,184 transmitting elements on a 72 x 72 foot fixed antenna. An adjacent receiving antenna contains 19,500 elements in a circular pattern. The FPS-85, whose development began in 1962 and became operational in January 1969, was modified in 1973-1974 to additionally provide warning of submarine-launched missile attack from the direction of the Caribbean-Gulf of Mexico. Subsequently, the Air Force developed the large, phased-array AN/FPS-115 PAVE PAWS radars for warning of submarine-launched ballistic missile firings off the Atlantic or Pacific coasts, while the perimeter acquisition radars and missile site radars of the ill-fated Safeguard-Sentinel antiballistic missile systems are also large, phased-array radars.
In the Navy, the SPS-32/33 radars were plagued by maintenance and reliability problems. Still, they demonstrated the feasibility and value of fixed-antenna, phased-array radars for shipboard use. The next major Navy development in this area was the SPG- 59, intended as a smaller radar for use in large “frigates” for carrier screening. After cancellation of the Typhon air defense system in the late 1960s, which included the AN/SPG-59, a multi-face, phased-array radar was begun by RCA. By 1969, when the Navy awarded a contract to RCA for overall management of the Aegis air defense system as well as the SPY-1 radar, the threat to U.S. surface ships had increased significantly through the extensive deployment and foreign transfer of Soviet antiship cruise missiles. (Interestingly, in the late 1960s, when the Navy was developing the concept of a sea-based ballistic missile intercept system, the idea was put forward of employing older Polaris submarines with two-face, phased-array radars as picket ships in the Arctic to provide early warning and tracking of Soviet ICBMs. Two such submarines on station in the Arctic could have provided surveillance of all tracks of Soviet ICBMs fired against the United States.)
Aegis was developed as an integrated defensive system, including missiles, missile launchers, radars, and fire control equipment to defeat enemy air threats of the 1970s and 1980s. However, delays, which are now being experienced in virtually all U.S. naval systems, have slowed Aegis development for almost a decade. Improvements made during this period are expected to permit the basic Aegis to be effective through the 1990s against advanced antiship missiles.
In developing the AN/SPY-l, RCA went several steps in capability beyond the radar’s predecessors. For example, the SPY-1 combines the azimuth and height search, target acquisition, classification, and tracking functions, and can provide command guidance to ship-launched missiles as required. The replacement of several different radars with the single SPY-1 results in the reduction or elimination of several complex interfaces between specialized radars, speeds up all functions, and provides a very large target-handling capability.
The SPY-1 radar—consisting of the antenna, transmitter, signal processor, control groups, and auxiliary equipment—employs four fixed antennas and operates in the S-band. The antennas each contain 4,480 separate radiating elements in an octagonal face only some 12.5 feet across. This small size facilitates ship design, with Aegis warships having two antennas on a forward deckhouse (facing forward and to starboard) and two on an after deckhouse (facing aft and to port). These four antennas each cover a 90° quadrant from the horizon to zenith for total scanning around the ship.
Control of the radar is exercised by four AN/UYK-7 digital computers that schedule and direct the beams, necessary because the SPY-1 can in rapid sequence project hundreds of pencil-thin radar beams, far too many for manual control or coordination. Beam steering is a mathematical problem that requires the calculations of a computer system. Indeed, computer capacity is a practical limitation on the number of targets that the SPY-1 can handle at one time.
When a target is detected, the computers automatically schedule several more beams to “dwell” on the target within a second of the initial detection, thus initiating a track. Hundreds of targets can be thus identified and tracked simultaneously, out to ranges on the order of 200 miles.
The radar is also highly resistant to electronic countermeasures. It has frequency diversity and can “sense” jamming and automatically shift to different frequencies where less interference is present. Also, digital signal-processing techniques are employed to counter or suppress jamming as well as sea clutter. The latter feature is vital for an effective defense against sea-skimming missiles that are often lost to conventional radars because of sea clutter masking the target signal.
Still another feature of the SPY-1 is its ability to imitate other radars, both in frequency (within the S-band frequencies) and beam shape/movement.
The Aegis electronic system (described in the February 1978 Proceedings “Aegis,” W. E. Meyer and B. Dalla Mura, pp. 93-97) is not inexpensive, and the SPY-1 radars are a major contributor to the cost. However, in comparison with alternative radar systems, the SPY-1 offers overwhelming superiority in several performance categories. Further, in comparison with the earlier SPS-32/33, the SPY-1 appears to be more maintainable and more suitable for shipboard installation. The latter feature made it feasible to install the Aegis in the Spruance (DD-963)-class destroyer design, resulting in an increase of only some 1,100 tons for the entire Aegis system in the DDG-47 class.
The radar is slated for installation in nuclear-propelled cruisers—if and when they are constructed—and the SPY-1 is suitable for retrofitting in the Virginia (CGN-38)-class cruisers as well as the one-of-a-kind Long Beach. However, lack of a united position by the Navy’s leadership and Administration foot-dragging will prevent that ship from having the SPY-1 (or Aegis) during her remaining 15 years or more of fleet operations. Finally, the SPY-1 appears to offer significant benefits if installed in large-deck carriers during their modernization under the service life extension program.
The prototype SPY-1, with one radar face, began operation at a land-based test site in 1973, followed a year later by a set in the missile test ship Norton Sound (AVM-1), also with one face. The lead DDG-47, with SPY-1 installed, should be completed in January 1983.
RCA and other firms are looking into the possibility of a “miniaturized” Aegis, suitable for installation in smaller destroyer- and even frigate-sized warships. Second, the development of even more advanced antenna concepts is under way.
In present fixed-antenna, phased- array radars the angular coverage of each face is limited, with four faces required for 360° coverage. A micro- wave “lens” has been developed by the Sperry Rand Corporation that can “bend” a radar beam in the same manner that a glass lens bends a light beam. With this approach a single array can provide full hemispherical coverage. This technique, called a “dome” antenna, has been incorporated into a radar system under test since 1976. This radar, with multifunction and multi-target capabilities, would also result in a smaller shipboard installation than conventional phased-array radars.
Advanced phased-array radars, if deployed in sufficient numbers and in a timely manner, could enable the U.S. surface fleet to perform its missions in the face of the increasing Soviet and Third World air/missile threats of the 1980s and beyond. Unfortunately, like several other advanced naval systems, such modern phased-array radars will be put to sea only in small numbers and after considerable delays.