For the F/A-18, the small radar of the YF-17 had to be replaced with a more powerful installation that could handle BVR missiles. This in turn required an enlarged nose shape to accommodate the 28-inch radar dish needed to meet the Navy's weapons system search range requirement of over 30 nautical miles.
At the end of 1977, the Hughes AN/APG-65 digital multi-mode pulse-Doppler radar was selected as the F-18 radar over its Westinghouse competitor. The APG-65 operates in the I/J-band (8-12.5 GHz). The radar is provided with built-in test equipment (BITE), which assists in identifying and isolating failures. There some two dozen on-board computers which operate in conjunction with the radar and weapons delivery systems. The ones operating in conjunction with the radar convert data generated by onboard sensors to a readily comprehensible display for the pilot. At the same time, they relieve the pilot's work load by performing rapid calculations of ballistics, windage, velocity and altitude for accurate weapons delivery, with release cues being given to the pilot by means of the HUD and CRT displays.
The radar can operated in several different modes, depending on what the pilot wants to accomplish.
AIR-TO-AIR RADAR MODES:
The velocity search mode is used for maximum range encounters, sacrificing detail for range and giving velocity and azimuth information only. In this mode, targets can be detected at ranges in excess of 80 nautical miles, and the software controlling the radar is programmed to pay attention only to these returns which are approaching the F-18.
In the range-while-search mode, the APG-65 provides information on all contacts occuping the portion of the sky ahead of the Hornet at ranges of between 40 and 80 nautical miles.
In the track-while-scan mode, which is used for ranges of less than 40 nautical miles, the system can track up to ten targets simultaneously and display eight of them at a time. The computer will present additional data on the contact deemed to be the greatest threat, this data typically consisting of aspect, altitude, and velocity.
If a specific target comes within range while the radar is operating in the range-while-search mode, a single target track mode can be selected by the pilot for display on the HUD, with steering commands and weapons launch data also being simultaneously displayed to the pilot. The system also provides a "shoot" cue to the pilot when a firing solution is obtained.
The system also has a raid assessment mode, which uses Doppler beam sharpening to examine a specific return more closely to see if it comes from a single target or from a group of aircraft flying in close formation.
Once a target has been selected for attack, the system will use the boresight mode if the Hornet is in a traditional tail-chase encounter with an enemy plane. In this mode, a very narrow 3.3-degree beam scans a small area of sky directly ahead of the aircraft. When both the target aircraft and the Hornet are maneuvering heavily, the vertical acquisition mode is used. In this mode, the radar scans an arc 5.3 degrees wide and extending 60 degrees above boresight axis to 14 degrees below. In order to achieve automatic lock-on, the pilot rolls his aircraft into the same plane of motion as that of his target, ideally positioning the enemy plane just above the canopy bow and aligned vertically with the HUD. The system can also do a head-up display acquisition mode, in which the radar antenna scans a box corresponding to the field of view of the HUD itself. This typically extends 10 degrees left and right of centerline, 14 degrees above and 6 degrees below.
These combat modes are effective from ranges varying from 500 feet to five nautical miles. When in any one of these modes, the radar automatically locks onto the first target that is acquired, and an indication of a lock-on is displayed on the cockpit CRTs and on the HUD. However, the pilot can override the system and reject specific targets until he acquires the one most desired. Alternatively, the pilot can use a moveable cursor to designate the target.
The gun director mode is employed at ranges of less than 5 nautical miles. The radar provides data pertaining to target position, range, and velocity to drive the gun aiming point on the HUD. The pilot then positions the pipper on the selected target and squeezes the trigger.
AIR-TO-SURFACE RADAR MODES:
There are also several air-to-surface modes available.
The real beam ground mapping mode is used for identifying substantial geographical features at long ranges. A small-scale radar map is displayed of the terrain ahead. The computer automatically adjusts the display so that it appears as a vertical "God's view" image, rather than the oblique view that the radar actually sees.
There are more detailed mapping modes which employ Doppler beam sharpening to provide higher resolution. These are used to provide better resolution for navigation and for target location. Once a ground target is identified, the air-to-surface ranging mode provides information on the distance to the target, and fixed and moving ground target track modes use two-channel monopulse angle tracking to provide precise information on ground targets.
The Hornet does not have automatic terrain following capability, but the radar can be used for terrain avoidance, warning the pilot if he is about to fly into anything hard, leaving it up to the pilot himself to figure out how to avoid the collision.
The APG-65 also had a sea-surface mode in which a computer filters out the clutter coming from reflections off sea waves, which makes it easier to identify, track, and attack enemy surface vessels.
When the Hornet is operating in the ground attack mode, the Ford Aerospace AN/AAS-38 Forward-Looking Infra-Red (FLIR) pod and the Martin-Marietta AN/ASQ-173 laser spot tracker/strike camera (LST/SCAM) pod can be carried on the external fuselage corner stores stations that are ordinarily occupied by AIM-7 Sparrow missiles when the aircraft is operating in the fighter mode. These pods are mounted to port and starboard respectively. The FLIR unit is used to enhance night attack capability by providing real-time thermal imagery which is displayed on one of the cockpit CRTs The FLIR can be fully integrated with the other avionics of the F/A-18, and data from it can be used in the calculation of weapons release solutions. The LST/SCAM is used for accurate bombing in bad weather, and it uses a tracking device to lock onto a laser beam reflected from a pre-designated target and provides information on target location to cockpit displays and mission computers.
The earlier versions of the LST/SCAM pod did not have their own laser illuminators, so the Hornet was dependent on target illumination from other aircraft for the delivery of laser-guided weapons. However, this omission has been corrected on the very latest pods, which makes the Hornet completely autonomous in the delivery of laser-guided weapons.
The Hornet carries the Itek AN/ALR-67 radar warning receiver set, which has the ability to detect, isolate, classify, and initiate countermeasures against a variety of electronic threats. The pilot is informed of these threats by means of cockpit displays, and their location is indicated. The pilot can then elect to carry out more active countermeasures such as the release of chaff or the dropping of decoy flares.
Two blade antennae are located on the dorsal spine. The forward antenna is for the Collins AN/ARN-118 TACAN, and the rear antenna is for UHF communications.