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"F/A-18C/D (APG-73)
The APG-73 is an X-band pulse-doppler radar for the twin-engine F/A-18C/D fighter/attack aircraft. Armed with AMRAAM and AIM-7 Sparrow missiles, the F/A-18 is tailored to carrier-based navy and marine applications.
Implementation. The radar consists of five flight-line replaceable units (LRUs) - antenna, transmitter, receiver/exciter, processor, and power supply.
The antenna is a monopulse planar array. Mounted in azimuth and elevation gimbals, it is directly positioned by electric torque motors, controlled by a servo electronics unit which plugs into the gimbal base. Aircraft roll rotation is accomodated by suitably resolving the azimuth and elevation drive and position indicating signals. To maximize ground coverage at steep look-down angles, the feed can be switched from producing a 3.3 degree-wide pencil beam to producing a wide fan beam. Included in the array are horns for a guard channel, for reducing the nulls in the radiation pattern during AIM-7 missile launches, and for providing flood illumination for AIM-7F visual launches.
The transmitter employs a liquid-cooled, gridded TWT, has 4% bandwidth, and is capable of 13:1 Barker-code pulse compression.
The exciter provides a coherent transmitter drive signal of controllable amplitude for sensitivity time control and LPI power management. It provides local oscillator and reference signals for the receiver, and is capable of coherent PRI-to-PRI or noncoherent pulse-to-pulse frequency agility.
The receiver has two, triple down-conversion channels. During search, they carry the radar and guard signals; during monopulse operation, they carry the sum and difference signals and are time-shared for azimuth and elevation tracking. Zero to 45 dB of coarse AGC (automatic gain control) is provided in 15-dB increments; and 0 to 63 dB of fine AGC, in 1-dB increments. A/D converters sample the received signals at the following rates and resolutions.
A/D Sampling Rate - Resolution
10 MHz - 11-Bit, Single
5 MHz - 11-Bit, Dual
58 MHz - 6-Bit, Dual or Single
The processor includes five mesh-connected processing elements (PEs): three identical pipe-line signal-processing elements (for range gating, doppler filtering and related functions) and two identical data processing elements (for loading programs for the selected modes of operation into the signal processing elements and performing overall control of the radar). Data word length is 32 bits; instruction word length, 64 bits; FFT (fast Fourier transform) filter banks, 2048-point.
The pipelines incorporate dedicated multiple intermediate memories and multiple high-speed parallel arithmetic units and are programmed so that no cycle time is devoted to nonproductive tasks, such as waiting for data or instructions from memory or for the incrementing of addresses.
All circuits are mounted on multi-layer circuit boards, packaged in standard 5- by 9-inch, flow-through air-cooled plug-in modules.
The power supply LRU rectifies the aircraft's prime power, converts it to the desired voltage levels, and conditions it. The unit is notable for having an overall efficiency of 82% and using programmable gate-array technology for control.
Air-To-Air Operation. A complete set of air-to-air search and track modes is provided.
For detecting high-closing-rate targets at maximum range, high-PRF velocity search is provided. For all aspect target detection, high-PRF with FM (frequency-modulated) ranging is interleaved on alternate bars of the search scan with medium PRF employing 13:1 pulse compression and a guard channel for rejecting return from large-RCS (radar cross section) point targets in the sidelobes.
In both modes, the high-PRF waveform alternates between two PRFs to minimize eclipsing, and a spotlight option provides high update rate in a restricted pilot selected volume. Background tracks are initiated for all target 'hits'.
For target tracking, both track-while-scan (TWS) and single-target tracking modes are available. In TWS 10 targets can be tracked, up to eight of which, as prioritized by the pilot can be displayed. At the pilot's request, to facilitate multiple AMRAAM launches the radar automatically keeps the scan centered on the high-priority targets.
In single-target tracking, performance is optimized by automatically switching between high and medium PRF. To provide target illumination for AIM-7 launches, high PRF alone is used.
For situation awareness, TWS may be interleaved with single-target tracking. To break out suspected multiple targets, finer than normal range and doppler resolution may be selected.
Four air-combat modes are provided. All employ medium PRFs, scan out in range, and automatically lock onto the first target detected.
Mode - Antenna
Gun - Scans HUD field of view
Vertical - Scans two vertical bars
Boresight - Fixed in boresight position
Wide Angle - Scans wide azimuth sector
At close ranges, a special medium-PRF track mode, employing CPI-to-CPI frequency agility to minimize glint, provides the high accuracy needed for the aircraft's gun-director.
Air-To=Ground Operation. The pilot has a wide choice of ground mapping modes.
- Real-beam, with sensitivity time control, 13:1 pulse compression in the longer range scales for increased clutter-to-noise ratio, and automatic switching from pencil to fan beam at steep depression angles.
- For navigation: wide-field-of-view, 8:1 DBS (doppler beam sharpening), with real beam mapping filling the +-5 degrees forward blind sector.
- For finer resolution, a 45 degree-sector mode, with 19:1 DBS, 13:1 pulse compression, and four-look summing to reduce speckle.
- For still finer resolution, a similar mode maps a 12.6 degree wide patch with 67:1 beam sharpening.
- A SAR mode which maps a similar patch with the same medium resolution at all ranges.
- For detecting ships in high sea states, a noncoherent mode with pitch and roll compensation, 13:1 pulse compression, and pulse-to-pulse frequency agility for speckle supression.
Ground-moving-target detection and tracking with coherent enhancement of slow moving targets may be interleaved with the mapping modes.
Air-to-ground navigation modes include fixed target tracking, coherent low-PRF air-to-ground ranging and its inverse, precision velocity update, and terrain avoidance for low-altitude penetration.
Reliability and Maintainability. The radar has a predicted mean-time-between failures of 208 hours. Through extensive built-in tests (BIT), it can detect 98% of all possible failures and isolate 99% of them to a single WRA.
Growth. With the addition of a stretch generator module in a spare slot provided for it in the receiver/exciter LRU, a very high resolution SAR mode can be added. This LRU also contains all of the additional circuitry and inputs and outputs to enable replacement of the transmitter and antenna LRUs with a next-generation active ESA (electronically steered array)."