HR2S-1W
The US Navy evaluated an AEW version, called HR2S-1W for weather tracking [to be stationed in Alaska]. Two aircraft (called HR2S-1W) fitted with a large diameter radar scanner in the nose for early-warning duties were procured by the Navy in 1957. The HR2S-1W Airborne Early Warning (registration 57-1646, cn 56-015) were delivered to the US Navy in 1958. The most obvious visual difference between the CH-37 and the Navy HR2S- 1W prototype was the latter's huge chin radome covering the AN/APS-20E search radar. This version of the S-56, built in duplicate and intended for patrol and detection missions, was equipped with a large AN/APS-20 radar mounted under the nose.
Three world records for helicopters were established during the period 9-11 November 1956 when a HR2S with a Marine pilot at the controls carried a 11,050 lb payload to an altitude of 12,000 ft, carried 13,250 lbs to over 7,000 ft, and set a three kilometer speed record of 162.7 mph. It was ugly and didn`t look like it was air worthy; cumberson seems like a good word.
As early as1940, British radars were also designed compact enough to fit into combat aircraft,as well as mast-mounted on ships, for detecting surfaced German U-boats. However, none of these WWII radars had the appropriate design and performancecharacteristics to detect small radar-cross-section targets such as exposed periscopes.
One of the earliest U.S. Navy airborne radar suites with good surface surveillancecapability was the AN/APS-20, which was initially developed during WWII but didnot enter operational service until 1946. By 1947, in evaluating the performance of the AN/APS-20 airborne radar in search for a given target, the Navy was interested in estimating the chance of detecting the target with the equipment in question in each of the tactical situations of operational interest. Those tactical search situations were usually treated as one of two types, direct approach or area search. On direct approach, 50% probability of detecting a single fighter is attained at 45 miles; the Sweep Width for a passing course is about 130 miles. The two factors which limit the probability of detection and, therefore, both warning radius and Sweep Width, are sea return and relative speed.
The APS-20 operated at L-band, S-band, and X-band, had selectable Pulse Repetition Frequencies (PRFs) in each band, as well as a wide selection of pulsewidths in each band. It also provided a host of other features and operator tools such as automatic target indicator, plan position indicator, three choices of heading reference and stabilization, and selectable azimuth and elevation beam widths.
Perhaps there were too many operator-selectable controls that could be set non-optimally. It included selectable output-radiated gain, selectable receiver-radiated gain, selectable antenna gain, automatic gain control low and high settings, plus awide assortment of display and strobe-light control selections, all of which could be employed by a highly-trained operator.
It was a powerful radar thatcould radiate up to 1 megawatt in L-band, and could detect large surface ships beyond 200 nmi on a good refracting day. On a good day, a highly trained operator could determine approximate target size, heading and speed within three or four sweeps. The S-band was significantly better at discrimination and resolution of targets at 100 to 150 nmi. The X-band was even better at detection ranges of 75 to 100 nmi, and was especially effective at detecting low-flying aircraft. But like all of these early radars, the APS-20 was very large, heavy, power-hungry, and operator-intensive, and was ineffective at detecting small RCS surface targets such as exposed periscopes and masts.
During the Cold War, ASW consisted primarily of open-ocean operations using mostly passive acoustic sensor systems that, at the time, were deemed more than adequate. Therefore, radar detection of periscopes was not a priority. The Navy ultimately abandoned further development of the HR2S-1W because severe airframe vibration severely degraded the radar's performance. The result of the Navy's tests was that it was better to use a fixed-wing, faster and less cost-effective aircraft for these missions. A similar experiment in which an Army CH-37 was fitted with a bulbous, dielectric radome for use as Marine Corps radar patrol aircraft proved unsuccessful.
The APS-20 was deployed operationally in 1953 on Lockheed’s land-based maritime patrol aircraft, the P-2 Neptune. It was not until the early 1970s that the first tactical radar designed specifically for periscope detection, the US Navy’s AN/APS-116 radar on S-3A ASW aircraft, arrived on the scene. Furthermore, it was not until the early 1990s that the US Navy established a formal requirement for automatic periscope detection andclassification, and subsequently initiated the technology development for detecting and classifying periscope targets automatically.
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