Anomalous Echoes Captured by a B-52 Airborne Radarscope Camera
4. Radar Specifications and Mode of Operation
Reconstructing from scattered partial sources the radar system installed in the B-52H as of 1968 has required a degree of forensic ingenuity. The AN/ASB series variants used on the B-52 included the High Speed Bombing Radar (HSBR), Improved High Speed Bombing Radar (IHSBR), Advanced Capabilities Radar (ACR) and - latterly - the fully digital Offensive Avionics System (OAS). But changes in the earlier analogue variants are mainly associated with the introduction of Terrain Avoidance (TA) mode with the monopulse ACR in about 1960, responding to a strategic flight deck, but fortunately left the scopes used by navigator and radar-navigator (of concern to us here) essentially unaffected. In particular, the Station Keep mode of the radar is functionally and operationally independent of those evolving special functions.
The ACR remained basically unchanged until the conversion to digital OAS, which was still a decade or more in the future at the date of the incident. In 1968 the 'H' variant flown by this elite crew of instructors was the state-of-the-art B-52 variant. The radar AN designation is believed to have been either the ASB-9 or the ASB-16. It was part of the IBM-Raytheon AN/ASQ-38 bombing-navigation system, along with the AN/AJA-1 True Heading Computer and AN/APN-89 Doppler Radar Set (for accurate doppler-drift ranging). Most information here is extracted from a part copy of the ASB-4/9 Tech Order, Vol. 1, Sec. 4, and training manual Bomb Navigation Systems Mechanic CDC 32150K, Vol. 4 for ASB-9/16A, supplemented by expert consultation as indicated in Section 1. (Hereinafter the designation ASB-9 is used for convenience.)
The Raytheon transmitter for the ASB-9 put out a peak power of 250 kW tunable over a 1GHz range, between 8500 and 9500 MHz (or 8600 - 9600 MHz). An AUTO/MANUAL switch allows operator tuning or (usually) control by an Auto Frequency Control circuit. There are two displays, a 10" Topographical Comparator scope and a 5" Azimith Range Indicator scope, available respectively to the Navigator and to the Radar-Navigator sitting at adjacent consoles. Both displays are fed with identical signals from the radar receiver. The larger 10" scope has the camera mounted on it. The system can be used in several modes, combining full 360-degree rotation and sector scanning:
- Radar Mode - the PPI paints raw echoes with the antenna generally in continuous rotation
- Beacon Mode - mainly for tanker identification during refuelling (output fixed at 9285 MHz); the PPI displays only the coded signals from transponder beacon
- Radar-Beacon Mode - combined display showing raw paints and transponder codes
- Altitude Calibrate Mode - antenna pointed vertically down for auto updating of the aircraft altitude data stored by the computer
- Terrain Avoidance Mode - automatically plots a "clearance plane" by continually measuring ground elevations in a sector ahead of the a/c (antenna bore sight tilted down; sector width fixed at 90 degrees, 45 deg either side of a/c ground track; sector scan rate fixed at one cycle every 3 seconds; p.r.f. fixed 808 pps, p.w. fixed 1.0 microsec)
- Indirect Bomb Damage Assessment Mode - an automated sequence of groundscan radar modes designed to map and photograph the target area during and after bomb release
- Station Keep Mode - coverage elevated, as an air navigation aid, primarily for formation flying and for lining up with the docking boom of an air-refuelling tanker.
During the incident in question the radar was set in Station Keep mode (Fig. 3). The following combinations of pulse-length and prf (pulse repetition frequency, in pulses per second or pps) are listed in the Tech Order, in addition to the special Terrain Clearance setting mentioned above:
0.25 microsec @ 1617 pps
0.5 microsec @ 808 pps
1.0 microsec @ 323 pps
2.25 microsec @ 323 pps (for Beacon Mode)
Notice that the pulse length and the pulse rate tend to vary inversely, so that for a fixed peak power the average power-on-target in the first two radar modes is the same, and is only 20% lowger pulse is emitted and the average power-on-target remains constant. The trade off is range resolution, which begins to suffer as pulse length increases and so limits the use that can be made of this compensation (depending on design goals).
Apart from Beacon Mode, the applications of these settings are not identified. Specifically, the setting for Station Keep is not identified, but function and operation dictate that it will be the setting giving the finest range resolution at the shortest range.
We can infer unambiguous range. The reciprocal of the prf gives the maximum out-and-back path length for unambiguous range, so half that length gives the maximum design range.
The corresponding ranges would be:
1617 pps = 67.5 miles
808 pps = 115 miles
323 pps = 288 miles
The short range-scale requirement of Station Keep suggests that the appropriate setting is: 0.25 microsec @ 1617 pps., unambiguous range 67.5 miles. The theoretical range resolution of this beam (1/2 pulse length) would be 123 ft, or four times as good as the 490 ft resolution of a 1 microsecond pulse at 323 pps.
In continuous scan or sector scan modes the antenna rotation rate is variable. There appear to be only "slow" and "fast" settings. The fast scan is given as 17.5 - 22.5 RPM, which would be nominally 20 RPM consistent with the scope camera being triggered once per scan about every 3 seconds, confirming that the photos capture all the video data there was.
The antenna produces a beam with a cosecant-squared vertical profile like an ATC or surveillance pattern turned upside down. Such a profile causes the antenna gain to vary inversely with depression angle in such a way that the echo intensity from the ground below the aircraft is reduced relatively to the echo intensity from longer slant ranges, and the brilliance of the coverage on the display tends thereby to be evened out. Another electronic circuit called a Sensitivity/Time Control or STC is available to amplify this swept gain when the effect of the cosecant-squared shaping is less effective in certain conditions.
The overall beam shape is the usual broad vertical fan, narrow in azimuth. This is actually a bi-lobe monopulse pattern produced by a 4-feedhorn antenna assembly. In ACR Terrain Avoidance mode the two lobes are squinted in elevation, effectively a "binocular" radar allowing sum-and-difference computer to compare echoes from the two lobes and so calculate accurate heights directly from range and antenna elevation data. For the purposes of PPI presentation in a surveillance mode, however, this computer function is to a large extent irrelevant. An Anti Jamming mode then becomes available by switching the pairing of the antenna feeds, so that the two lobes of the beam are now squinted in azimuth to allow Monopulse Sidelobe Reduction. MSR rejects signals that might otherwise be injected from emitters away from the bore-sight azimuth. But otherwise in Station Keep the PPI display is that of a simple pulse radar.
Manual CDC 32150K, Vol.4 gives the nominal horizontal beam width as 1.59 degrees; the overall vertical beamwidth is variable between about 54 and 60 degrees depending on antenna boresight elevation. As shown in Fig. 3, in Station Keep the boresight angle is elevated, with the top edge of the "usable field pattern" set at +8 degrees above horizontal flight level, and it follows that the bottom edge of the vertical pattern can be taken as being between -46 and -52 degrees below flight level. This is only a guesstimate, however. The "usable field pattern" is evidently meant to be indicative of a contour of equiprobability of detection for a large jet (such as a B-52 or KC-135 tanker) out to the maximum display range of 5 NM. But no detailed gain figures are available, neither is the contour of the "usable field pattern" defined in terms of any specific operational criterion such as a probability of detection of a target of known RCS.
We can see (Fig. 4) that the Station Keep mode selector switch is coupled to the range scale selector switch and so enforces the special 5-mile PPI display range scale with 1/2- NM range rings. The ranges of these markers are shown in Fig. 3 above (Note 2).
Other range-scale options available, in various full-scan PPI and off-centre sector-scan radar modes, are 10, 15, 20, 30 and 50 NM. Longer ranges are available by introducing a sweep delay. This circuit effectively regauges the zero-point of range by making the start of the PPI trace correspond to a non-zero echo time.
As mentioned, the radar system status is identified in these other modes by the sequence of lamps illuminated on the camera data plaque (as seen on frame 784) which replaces the clock and frame counter to the right of the PPI (seen on frames 771-783). The lamp codes are reproduced in Fig.5