Radar system

"I'll tell you what, it's got a fine radar inside, the APQ-120, you can get a skin paint on a skeeter 200 miles, or a stealth fighter."

A technician works on the AN/APQ-120(V) fire control radar in the nose of a 3rd Tactical Fighter Wing F-4E Phantom II aircraft

The Westinghouse AN/APQ-120 Fire Control Radar, a continuation of the F-4C's -100 and the 4D's 109, is a solid state pulse radar providing the F-4E with air to air intercept functionality, air to ground mapping, ground target reference provision to the LABS and WRCS bombing systems, as well as radar beacon capability. The APQ-120 also functions as the display system for TISEO and TV guided weapon imagery, and is integrated with the APX-80 interrogation system.

The primary conversion from the APQ-109 to APQ-120 included a reduction in the number of field replaceable units in the nose, as well as the reduction in their size; whereas the -109 had been a hybrid in its movement towards solid state hardware (primarily in the low voltage processing sections), the 120 was a fully solid-state system. While this update reduced the space overhead and weight for the system, the modification of the F-4 nose to include the M61 cannon required a reduction in antenna size, causing a marginal decrease in overall detection range. However, when taken with the increase in system reliability and reduced maintenance, the tradeoff was considered acceptable.

Starting with production of the F-4E block 60, and retrofitted to selected earlier block aircraft, was the addition of the Digital Scan Converter Group display. The inclusion of DSCG increased the overall ease of handling the APQ-120 by adding additional information on the display directly, including current radar range setting and the calculated range rate value against the current acquired target. Further, clarity of the display in all lighting conditions was improved by rendering the radar reference grids directly as part of the displayed image, rather than the previously used markings on the DVST glass. In addition, the DSCG provided the ability of the two crew members to utilize the displays independently; previously, the WSO had control over which mode both the DVST and front seat repeater display would show. With DSCG, the pilot could utilize TISEO or TV air to ground weapons while the WSO maintained a scan pattern or found a ground reference point and inserted it into the WRCS.

Principle

The APQ-120 is a pulse radar. The radar transmits a radio pulse focused in one direction via the antenna.

Any obstacle in that direction, for example another aircraft or simply ground, will reflect part of the energy back, which the radar then receives via the antenna to be processed and evaluated by the crew. Based on the time it took for the signal to travel back, the radar computes the distance to the obstacle.

All radar returns are then displayed on the screen based on their distance, resulting in a direct and unfiltered representation of the world in front of the aircraft.

However, since the emitter is not a perfect device, energy is transmitted into all directions, with the main focus being the direction in which the antenna is pointing at. This is referred to as the main lobe of the antenna, while any other unwanted returns from different directions are called side lobes.

The same situation from outside looks as follows.