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The AN/APG-66 is a pulse-doppler radar designed specifically for the F-16 Fighting Falcon fighter aircraft. It was developed from Westinghouse's WX-200 radar and is designed for operation with the Sparrow and AMRAAM medium-range and the Sidewinder short- range missiles.
 APG-66 uses a slotted planar-array antenna located in the aircraft's nose and has four operating frequencies within the I/J band. The modular system is configured to six Line-Replaceable Units (LRUs), each with its own power supply. The LRUs consist of the antenna, transmitter, low-power Radio Frequency (RF) unit, digital signal processor, computer, and control panel.
The system has ten operating modes, which are divided into air-to-air, air-to-surface display, and sub-modes. The air-to- air modes are search and engagement. There are six air-to-surface display modes (real beam ground map, expanded real beam ground map, doppler beam- sharpening, beacon, and sea). APG-66 also has two sub-modes, which are engagement and freeze.
In the search mode APG-66 performs uplook and downlook scanning. The uplook mode uses a low Pulse Repetition Frequency (PRF) for medium- and high-altitude target detection in low clutter. Downlook uses medium PRF for target detection in heavy clutter environments. The search mode also performs search altitude display, which displays the relative altitude of targets specified by the pilot.
Once a target is located via the search mode, the engagement sub-mode can be used. Engagement allows the system to use the AMRAAM , Sidewinder , and Sparrow missiles. When engaging the Sidewinder , APG-66 sends slaving commands that slaves the missile's seeker head to the radar's line-of-sight for increased accuracy and missile lock-on speed. An Operational Capability Upgrade (OCU) was developed to modify the APG-66 to use the AMRAAM missile. The OCU is designed to provide the radar with the necessary data link to perform mid-course updates of the missile. The Sparrow 's semi-active homing seeker is facilitated in the engagement mode by a Continuous Wave Illuminator (CWI). The CWI also permits APG-66 to be compatible with Skyflash and other missiles with similar semi-active homing seekers.
Target acquisition can be manual or automatic in the track mode. There are two main manual acquisition modes, single-target track and situation awareness. The situation awareness mode performs Track-While-Scan (TWS), allowing the pilot to continue observing search targets while tracking a specific target. While in this mode, the search area does not need to include the tracked target's sector.
Four Air Combat Maneuvering (ACM) modes are available for automatic target acquisition and tracking. In the first ACM mode, a 20 x 20-deg Field Of View (FOV) is scanned. This FOV is equal to that of the Head Up Display (HUD). Once a target is detected, the radar performs automatic lock-on. The second ACM mode's FOV is 10- x 40-deg, offering a tall window that is perpendicular to the aircraft's longitudinal axis; this proves especially useful in high-G maneuvering situations. A boresight ACM mode is used for multiple aircraft engagement situations. The boresight uses a pencil beam positioned at 0-deg azimuth and minus 3-deg elevation to "spotlight" a target for acquisition. This is especially useful in preventing engagement of friendly aircraft. A slewable ACM mode allows the pilot to rotate the 60- x 20-deg FOV. The automatic scan pattern gives the pilot up to 4 sec of time. This mode is designed for use when the aircraft is operating in the vertical plane or during stern direction conversion.
The slant range measurement to a designated surface location is generated by the Air-to-Ground Ranging (AGR) mode. This real-time mode acts with the fire-control system to guide missiles in air-to-ground combat. AGR is automatically selected when the pilot selects the appropriate weapons deployment mode.
Terrain in the aircraft's heading is displayed via the real beam ground map mode. The radar provides the stabilized image mainly as a navigational aid in ground target detection and location. An extension of this mode is the expanded real beam ground map. The expanded real beam ground map provides a 4:1 map expansion of the range around a point designated by the pilot via the display screen's cursor.
Doppler Beam Sharpening (DBS) is available to further enhance the higher resolution of the expanded real beam ground map. This mode, which enhances the range and azimuth resolution by 8:1, is only available from the expanded real beam ground map mode.
In the Beacon mode the system performs navigational fixing. It also delivers weapons relative to ground beacons and can be used to locate friendly aircraft that are using air-to-air beacons.
The high-clutter environment of the ocean surface is countered in the sea mode. There are two sub modes in the sea mode. The first sub-mode, Sea-1 is frequency-agile and non- coherent to locate small targets in low sea states. The second sub-mode, Sea-2, is fully coherent, with doppler discrimination for the detection of moving surface crafts in high sea states.
The freeze sub-mode can only be accessed through the air- to-ground display modes. It pauses the display and halts all radar emissions as soon as the freeze command is received via the controls. The aircraft's current position continues to be shown on the frozen display. This mode is useful during penetration operations against stationary surface targets when the aircraft needs to prevent detection of its signals, yet continue to close in on the target.
The system's displays include the control panel, HUD, radar display, with all combat-critical controls integrated into the throttle grip and side stick controller.
The modularity of the LRUs allow for shortened Mean Time To Repair (MTTR) since they can simply be replaced, involving no special tools or equipment. The MTTR has been demonstrated to be 5 minutes, with 30 minutes for replacement of the antenna unit. APG-66 has also demonstrated a Mean Time Between Failure (MTBF) of 97 hours in service, but the manufacturers contend that it has achieved 115 hours. A cockpit continuous self-test system monitors for malfunctions. The manufacturers claim that the system's Built-In-Test (BIT) routine can isolate up to 98% of the faults to a particular LRU in the event of a malfunction.
A new version of the AN/APG-66, designated the AN/APG-66(V)2 is being installed in F-16A/B aircraft as they are modernized in the Midlife Update program. The equipment is lighter and provides greater detection range and reliability for the modernized F-16s.
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