Early-Warning Radars (part 2) 

UHF Airborne Early-Warning Radars

Construction of the DEW Line resolved concerns about the security of the northern perimeter of the United States.  But, as was recognized both during the 1952 Summer Study and subsequently, the DEW Line did nothing to reduce the vulnerability of the east and west coasts to an attack over the ocean.

With no land to the east or west of the United States, the logical counterpart to the DEW Line was airborne radar. The members of the Summer Study discussed the need for airborne early-warning (AEW) radar and identified the most important requirements.

In particular, they observed that it was more important to alert SAGE of distant aircraft intrusion than to control interceptors. Range resolution, azimuth resolution, and height-finding capability were, therefore, less important characteristics for AEW radars than sheer range. The need for the greatest possible range drove the use of a relatively low operating frequency because the transmitter powers available were greater at low frequency and the impact of clutter returns from the ocean on detection performance was less.


Lincoln Laboratory's AEW radar program not only reopened the UHF spectrum for airborne radar applications, but highly effective airborne moving-target indicator systems were developed and techniques needed to integrate AEW aircraft with SAGE were demonstrated.

 

The Summer Study concluded that UHF AEW radar looked like a winner, and it proved to be just that. A program began at Lincoln Laboratory in the summer of 1952 to study existing radars and to test the feasibility of UHF radar. The first goal was to set up a UHF search radar to see if the hoped-for benefits were real. The frequency chosen for the first radar was 425 MHz, primarily because a few dozen war-surplus Western Electric 7C22 dual-cavity triodes were available. Their limited mechanical tuning range covered that frequency. The experiments were successful and 425 MHz became the frequency of choice for Lincoln Laboratory radars. In fact, Lincoln Laboratory's use of 425 MHz for numerous subsequent radars followed directly from the availability of 7C22s in 1952.

In 1953, recognizing the importance of flight-test support for the development of AEW radars, the Navy established a unit at South Weymouth Naval Air Station, Massachusetts, to support several Lincoln Laboratory programs. The Air Force based an RC-121D and a B-29 at Hanscom Air Force Base for the same purpose.

AEW radar on a blimpUHF early-warning radar was tested on a Navy blimp.

An early demonstration of UHF AEW radar was on a Navy blimp. Its operating altitude was limited to a few thousand feet, but its comparatively low velocity made detection of airborne moving targets easier.

Flight testing commenced in March 1954. Side-by-side tests with a low-power UHF AEW radar in one blimp and an AN/APS-20 S-band AEW radar (developed by the Rad Lab during WWII) in another proved the advantage of lower-frequency operation.

Despite some advantages, blimps failed as AEW-radar platforms because their operation was restricted to low altitudes. However, heartened by successful flight tests in the blimp, Lincoln Laboratory set out to install an AEW radar in a Super Constellation-class aircraft and to increase the transmitted power.

AN/APS-70 AEW radar mounted on a WV-2 aircraftAN/APS-70 AEW radar mounted on a WV-2 aircraft. A 17 × 4 ft antenna is mounted below the aircraft inside the black radome.

The new radar, the AN/APS-70, was fielded in three experimental development versions. Two radars were built by Lincoln Laboratory, two by Hazeltine Electronics, and two by General Electric (GE). The broadband 425 MHz antennas (including identification-friend-or-foe [IFF]) were supplied by Hughes. All three firms carried out production under contract to Lincoln Laboratory, and the technology was thus transferred to industry.

Lincoln Laboratory had demonstrated in 1954 that UHF AEW radar gave better results than S-band systems, but the Air Force felt that independent testing was warranted. Therefore, it carried out a series of flight-test comparisons of S-band and UHF AEW radars in 1956. In these tests, called Project Gray Wheel, an RC-121D aircraft was equipped with the AN/APS-20E (the most advanced configuration) S-band AEW radar, and another RC-121D aircraft was outfitted with Lincoln Laboratory's AN/APS-70 UHF AEW radar.

The tests proved the superiority of the UHF system in detecting bombers. Moreover, they demonstrated the capability of the UHF system to direct interceptors to the bombers. The success of the AN/APS-70-equipped aircraft helped convince the Air Force to outfit its fleet of RC-121 Super Constellations with UHF aircraft early-warning and control (AEW&C) radar.

AEW radar mounted on a WV-2E AN/APS-70 AEW radar mounted on a WV-2E aircraft. The 30 ft antenna was the largest airborne antenna of its day. Pictured in the center is Jerome Freedman of Lincoln Laboratory.

Following the prototyping of the AN/APS-70, the Laboratory produced an improved UHF AEW radar prototype of the AN/APS-95 that featured single-knob tuning and other features not included in the AN/APS-70. Hazeltine produced the production AN/APS-95 UHF AEW radar for the Air Force, and GE produced an advanced version, the AN/APS-96 UHF AEW radar, for the Navy.

Even though UHF operation helped remove some sea clutter, a way to remove more of it without losing low-flying targets was badly needed. By 1952, long-range ground-based air-surveillance radars could discriminate between targets that were moving radially and those that were not by pulse-to-pulse subtraction of successive received signals after detection. However, the radar transmitter could not be counted on to produce the exact same frequency and starting phase each time it was pulsed, so the reference signal had to be coherently locked to the transmitted signal for every pulse.

Lincoln Laboratory developed a two-part solution to the detection of airborne targets in a clutter background, referred to as airborne moving-target indication (AMTI). First, the reference signal was locked to a sample of the clutter return from surface scatterers at close range. This technique was called time-averaged clutter-coherent airborne radar (TACCAR).

For moderate levels of sea clutter, TACCAR worked well. As the radar antenna scanned through 360° in azimuth, TACCAR automatically took care of the problem of when the radar was looking forward or backward. The implementation of TACCAR at a radar's intermediate frequency (IF) was an early application of the phase-locked loop.

The second part of the solution was the displaced phase-center antenna (DPCA), first suggested by engineers at GE. DPCA compensated for the translation of an aircraft by comparing successive received pulses for AMTI and adjusting the phase center of the antenna to offset the phase difference between pulses caused by motion.

The Laboratory's existing UHF AEW-radar antennas were easily adapted to DPCA operation, and the integration of DPCA techniques with IF TACCAR was demonstrated by Lincoln Laboratory and was then implemented in the AN/APS-95.

Lincoln Laboratory subsequently demonstrated an RF version of TACCAR, which was made compatible with antijam circuitry. Because an airborne radar could be vulnerable to jamming, a tool kit was developed to strengthen the AN/APS-95 in this regard.

To improve target-detection performance and at the same time to narrow the beamwidth of the UHF radar, the Navy's Bureau of Aeronautics sponsored the installation of a large rotating radome high above the fuselage of a Super Constellation. One of Lincoln Laboratory's AN/APS-70 AEW radars was installed in the fuselage. Although the combination proved to be very effective, tests of the aircraft showed it was often on the verge of stalling.

By late 1957, the UHF AEW radars (with improved AMTI systems) had become accurate enough to be considered for incorporation into the SAGE system. To test the compatibility of the radars with SAGE, Lincoln Laboratory began the airborne long-range input (ALRI) test program.

The ALRI tests were conducted by flying an AN/APS-70–equipped AEW aircraft within line of sight of the Experimental SAGE Subsector installation at South Truro, Massachusetts. The video output from the radar's AMTI receiver was quantized and relayed to the ground over a wideband UHF data link. At the Experimental SAGE Subsector site, the data were fed into a fine-grain data system as if they were coming from a radar nearby. ALRI was a complex improvisation, but it worked.

The AMTI-radar technology that Lincoln Laboratory developed and demonstrated in the AN/APS-70 series of radars provided the foundation for the AN/APS-96. This radar used a high-power UHF vacuum-tube amplifier for the transmission of linear-FM pulse-compression signals. The finer-grained range resolution afforded by the compressed pulse after reception improved the target-to-sea-clutter ratio, making the AMTI job easier. The radar's sharper discrimination in range between closely spaced targets made the job of a combat information center easier. Another important feature was a height-finding capability for every target on every scan.

The Air Force retrofitted its RC-121C/Ds with Hazeltine AN/APS-95 UHF AEW radars, and the Navy installed General Electric AN/APS-96 UHF radars in Grumman W2F-1 Hawkeye turboprop aircraft.

Lincoln Laboratory's AEW radar program came to an end in the middle of 1959. Not only had the seven-year effort reopened the UHF spectrum for airborne radar applications, but highly effective AMTI systems had been developed and techniques needed to integrate AEW aircraft with SAGE had been demonstrated. Contractors were hard at work building radars that could apply these advances to Air Force and Navy aircraft. The Laboratory's assignment was complete.

The technologies developed as part of the Laboratory's AEW program continued to evolve and eventually became the bases of the Air Force's Airborne Warning and Control System (AWACS) and the Navy's E-2C airborne early-warning platforms.  Parenthetically, years later the Laboratory re-engaged with the Air Force and Navy, and developed and prototyped key technologies for the next generations of these two systems. Airborne early warning is an active research area in the Laboratory today.

Part 3: Jug Handle, Boston Hill, and Texas Tower radars

Back to part 1: Early-warning radars and the DEW Line


Adapted from E.C. Freeman, ed., Technology in the National Interest, Lexington, Mass.: MIT Lincoln Laboratory, 1995.

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