The AN/APQ-159 was a I-band/J band radar which was developed by Emmerson Electric from the APQ-153. The AN/APQ-159 was a fire control air-to-air radar system. It had four primary modes of operation, two search modes with different ranges using a simple B-Scope display, a boresight gunnery display with ranging and automatic lock-on "dogfight mode", and a similar mode used with the AIM-9 Sidewinder that calculated the missile's engagement envelope and provided cues to the pilot to fly into the envelope. Nevertheless, the radar offered no air-to-ground modes at all, nor was it capable of firing the AIM-7 Sparrow in spite of its BVR-capable range.
The AN/APQ-159 was equipped with a new planar phased array antenna, replacing the APQ-153's parabolic dish. This made the antenna smaller front-to-back and allowed it to be pointed to higher angles within the nose. It also greatly reduced the sidelobes, which improved gain and allowed the range to be greatly increased from roughly 10 nm to 20 nm. The APQ-159 had also upgraded electronics, offering increased frequency agility and dramatically improving mean time between failure (MTBF) from about 62 hours in the -153 to 125 in the latest models of the AN/APQ-159, which have actually demonstrated 150 hours MTBF in the field.
The AN/APQ-159 was manufactured in four separate models with the same radar electronics, but different displays. The APQ-159-1 and -2 models used a display that could operate in television mode to operate the AGM-65 Maverick air-to-ground missile, while the -3 and -4 lacked this capability. The difference between the odd and even numbered versions was the inclusion of a second set of displays and controls for the even-numbed versions, for use in the two seater F-5F. The APQ-159-5 version was a product improvement that further improved reliability and reduced weight to the same as the original APQ-153, making in-field upgrades much simpler. The final version was the APQ-159-7.