The APS-42 Radar
April 1948 Radio News

April 1948 Radio News [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

The APS-42, as described in this 1948 issue of Radio News magazine, was truly a break-through x-band airborne search radar system born out of the lessons learned from its predecessor: the APS-10 search radar developed during World War II. This very compact radar system is contained within a volume of about 3 feet on a side (not including the cockpit controls and displays. The close proximity of the receiver front-end to the antenna made for a very low noise figure and, consequently, high sensitivity. Interestingly, there is not a whole lot of information available on the Internet for either radar. In fact, this article is probably the most information source available on the APS-42.

The APS-42 Radar

Design and operational features of a new type radar unit. It is compact, light in weight, and excels all other airborne equipment.

By L. W. Mallach

Project Eng., The Houston Corp.

This outfit doubles the range of the well-known wartime APS-10. It weighs only 115 pounds, involves only two units, and for flexibility of operation surpasses any radar yet airborne. It is an X-band navigational radar equipment complete within itself, and provides radar mapping, responder beacon operations, obstacle detection, and weather mapping. The Army Air Forces call it the AN/APS-42 (XA-2). It is intended for service in both military and commercial aircraft, preferably in a chin or belly location. In both positions it will furnish a nominal 360° scan.

For mapping, the equipment supplies an approximate cosecant-squared pattern from the antenna, for use in mapping areas adjacent to the path of flight, to show contours and type of terrain traversed.

In responder beacon operation, it transmits a 2.2 microsecond pulse and receives on a frequency of 9310 megacycles, with automatic frequency control for picking up signals from transponder type beacons operating in this band. The antenna utilizes a cosecant-squared pattern for beacon operation to minimize any differences in altitude of the aircraft or angle of flight while triggering and receiving a beacon.

For obstacle detection, this radar transmits a pencil beam approximately six degrees vertically and horizontally in the approximate plane of the aircraft. This is useful for detecting the position of any reflected object within the swept pattern, particularly mountains and other aircraft.

Of considerable importance, too, is the fact that the equipment also may be used for detecting areas of heavy moisture content and the accompanying turbulent areas with which they may be associated. For this service, a 2.2 microsecond pulse with a pencil beam is transmitted.

Of the two experimental units completed, one has gone to the All Weather Flying Squadron, the other to the Aircraft Radio Laboratory - both at Wright Field - for in-service testing. A total of 107 production units now are being manufactured for the Navy, for assignment to planes flown by NATS.

Tests so far indicate substantial fulfillment of the design objectives for transport-type radar navigation equipment. Problems involved consisted basically of evaluating the functions of wartime airborne radar which would seem more useful for air transport uses.

Among the functions studied were the bombing type radar's ability to map large land areas, the fighter radar's ability to detect other aircraft in the vicinity, and the paratrooper's radar equipment used for beacon navigation and more-or-less precise navigation of uncharted areas.

In looking over the field of wartime equipment, it became evident that the radar best suited for commercial use was the APS-10. But serious disadvantages were inherent. Accordingly, it was decided to utilize basic design, and add other desirable radar features, together with operational simplification and antenna stabilization.

Final design, as evolved by The Houston Corporation on AAF contract, includes these principal features; low over-all weight; compactness; flexibility of operation, as noted in the first paragraph; higher power, in order to achieve the results mentioned more easily; stabilization of the antenna to present a better picture regardless of aircraft attitude; simplicity of operation requiring a minimum of pilot-operated controls, and reliability of operation over long periods of time by simplification of adjustments and components.

To accomplish these features, the equipment was designed so that the radar transmitter-receiver and antenna were mounted in one spherical package measuring 33 inches in diameter and 36 inches over-all height. Such a package or unit is suitable for either belly or chin mounting with approximately 21 inches of the cylinder protruding below the aircraft skin with antenna enclosed. This unit is stabilized by an internal arrangement so that the antenna is mounted on gimbal rings with servos providing automatic stabilization for 25 degrees either direction in roll and 20 degrees in pitch.

Inasmuch as the transmitter-receiver is mounted directly over the antenna, the maximum electrical efficiency is achieved through the shortening of the connecting cables. All components required for the operation of the transmitter-receiver-antenna are stabilized and mounted within this package, which is also pressurized by an integral air pressure pump actuated by an automatic pressure switch. By this means, the entire assembly is maintained at sea level pressure up to 30,000 feet, thus providing a minimum of disturbances from high voltage discharges or other high altitude phenomena.

The indicator employed is a standard 7-inch type cathode-ray tube using a PPI type display. This display is, in effect, a polar coordinate map of surrounding area, with the plane's position being in the center of the tube and the relative angle to other objects being indicated by the angle in degrees from the top of the tube, the top being the straight-ahead direction.

In order to simplify the operation and make it suitable for the pilot to handle his own radar equipment with a minimum of effort, several controls are mounted on the indicator.

1. A combination "on-off" and "range" switch. In the maximum counter-clockwise position the equipment is "off," while moving the switch to any range position turns the equipment "on." Subsequent operation is automatic after an initial three-minute warm-up time-delay period. The ranges provided are 5, 15, 50, and 150 nautical miles, with five evenly spaced range marks on each range, together with "range-in-use" lights above the indicator tube.

2. A "function switch." This provides for the versatility of operation of the equipment. The first position is "mapping," which provides for a radiated beam known as cosecant-squared or equal energy; and, as its name implies, it distributes an equal amount of transmitted power to the ground in the immediate vicinity of the ship as well as at the horizon. Therefore, all of the ground contour from immediately ahead of the plane to the horizon is reproduced on the indicator, as a contour map of the surrounding area.

In the next position, "obstacle detection," the antenna pattern is a very narrow or pencil shape beam which is approximately 6 degrees in width in the vertical direction. The beam is useful for detecting any obstacle, either other aircraft in flight or high ground obstacles, such as a mountain range projecting into the path of the aircraft in flight. By eliminating the reflections from the ground in the vicinity of the aircraft, such as are obtained on the equal energy path, it. then becomes possible to see only those objects projecting into the flight path of the aircraft.

The third position is for "beacon operation" and it provides for the interrogation of ground radar beacons. These ground radar beacons have been established by the Army, Navy, and Coast Guard at various well-known geographical locations. It is possible for the radar to interrogate them so that they in turn show a signal on the indicator, which is suitably coded to indicate the beacon station identification and at the same time provide an accurate measure of azimuth and range to the particular beacon. Thus, precise navigation is possible through beacon operation.

The fourth position is for use in obtaining "weather information." Probably one of the most valuable assets of the transport radar is its ability to observe weather phenomena. By utilizing a pencil type of beam and tipping the antenna upwards to eliminate any reflection from the ground, it is possible to search the sky for heavy rain-bearing clouds, thunderstorms, and areas of super-cooled water. These manifest themselves by peculiar displays on the indicator which are easily recognized by an observer with practice. It is thus possible for transport aircraft to avoid such areas of possible danger. It is also possible, by mapping such areas by radar, to pick the narrowest or "softest" spot, if indeed it is not possible to avoid the weather altogether. Also, by searching in layers in a vertical direction, it is quite possible to find layers where a minimum of weather disturbances prevail so that flight altitudes may be changed to take advantage of such a situation.

The other controls to be operated by the pilot are (3) a receiver sensitivity control which, as its name implies, governs the sensitivity and hence the signal indication on the indicator. An intensity control (4) is used for governing the intensity of the display on the indicator tube primarily for adjustment under different cockpit lighting conditions. Lastly, there is a (5) trim control, which provides arbitrary displacement of the center of stabilization on the pitch axis over a range of plus or minus 7 1/2 degrees.

Operation of APS-42 is similar to all centimeter type radar equipment, in that it transmits a high power pulse of very short duration through a waveguide assembly to an antenna which focuses the energy to a very narrow beam and rotates the beam in azimuth. Timing of the transmitted pulse is very precise, and the time required for the transmitted pulse to be sent out and reflected from a target is so measured by the equipment that the distances are recorded with good accuracy. Received echoes are picked up by the same antenna assembly, and fed into the same waveguide assembly, separation taking place at the mixer unit so that the receiver is isolated from the transmitter, meaning that a common antenna-waveguide assembly is used for both transmission and reception.

Signals and other radar indications appear on a circular tube face in the conventional PPI pattern such that the zero azimuth position on top of the tube represents straight ahead, and the other azimuth positions are calibrated in a 360-degree interval around the top. Since the antenna rotates constantly, it furnishes a complete azimuth picture at all times, which continues to change as the aircraft moves. The various functions of the equipment are obtained by combinations of changes to the radar characteristics, particularly the transmitting characteristics. For instance, in switching from "search" to "weather," the transmitter pulse is lengthened by a factor of approximately three. In switching from "search" to "beacon," the transmitter pulse is likewise increased, and at the same time, the transmitted antenna pattern as well as the received antenna pattern is changed from a narrow vertical beam: to abroad vertical beam, and the receiver frequency is shifted approximately 50 megacycles

The antenna system consists of an 18-inch diameter parabolic reflector with a double dipole radiator located at the focal point of the parabola and fed from a standard X-band waveguide. In addition, a double parasitic dipole is located on top of the antenna feed, and immediately behind the normal dipole radiators. This parasitic dipole is arranged in such a manner that it can be actuated so that its dipoles are parallel to, and therefore parasitic to the radiating dipoles. The parasitic dipoles are free to rotate 90 degrees, so that they are then 90 degrees with respect to the radiating dipoles and their effect is negligible. It is by the use of such an arrangement at the frequency of 9375 megacycles that it is possible to focus nearly all the energy radiated into a very narrow beam. The beam will be approximately six degrees wide in the horizontal and vertical dimensions at the half-power points. This energy, by reason of the dipoles' horizontal position, is horizontally polarized - the most effective type of polarization on search type radar for use in detecting land and sea targets. It is also necessary that the energy be horizontally polarized for beacon operation, as the beacon antennas themselves are horizontally polarized.

When the parasitic dipole arrangement is used, a certain amount of the radiated energy is deflected downward toward the earth's surface so that there is, in effect, what amounts to a cosecant-square pattern of radiated energy. The parasitic dipoles are tilted slightly with respect to the axis so that they can radiate in a downward direction. Being located in the path of the energy being reflected to the parabolic reflector, they absorb and reradiate a certain amount of the distributed energy, and are, therefore, actually spoilers in the radiated field of the antenna.

Because it is necessary to place the parasitic dipoles in two positions, they may be turned 90 degrees against a spring by gears connecting to the antenna drive motor. The spring re-turns them to the non-radiating position when the power is turned off.

The antenna is mounted directly below the chassis containing the transmitter-receiver components, and is supported by a fork-type casting. Since the transmitter is fixed and the antenna rotates, a rotating joint has been arranged to transfer the energy from the magnetron transmitter to the antenna, with no change in the transmission characteristics. The antenna is driven by a 400-cycle induction motor which operates at a normal speed of 7200 r.p.m., and which is reduced by a gear train to a normal antenna rotation of 30 r.p.m.

Since not everyone in the aviation industry thinks alike it is possible that the transport industry may require modifications of this equipment, one of which might be the substitution of a 5-inch indicator, due to severe cockpit space limitations. This modification is easily accomplished and, in fact, the equipment can be operated with such an indicator, with an external control box located for easy access by the pilot. It is also probable that many of the later design aircraft are better suited to a nose installation, rather than to tolerate the drag occasioned by a belly installation, even though the nose installation reduces the radar coverage in the aft direction. The physical size of this particular unit would prohibit its installation in most of the nose areas available, but suitable modifications can be made to accomplish this. The equipment is normally able to operate with two indicators and a third could be provided, should it be required, for navigator's use.

Combining the several achievements in a single lightweight assembly has been no easy task. Now that the services have underwritten its early development, I believe the design can be produced within the economic range of the air transport industry. Coming tests by the storm-searchers, radio experts, and transport fliers should demonstrate the value of such many-in-one equipment.

Posted May 31, 2022