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本帖最后由 Ragnarok 于 2011-11-20 23:18 编辑
http://www.sukhoi.ru/forum/showthread.php?t=21709&page=10
02.04.2004 08:57#242
SwingKid
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"F-16C/D (APG-68)
The APG-68 pulse-doppler radar meets the all-weather air superiority and air-to-ground strike requirements of the F-16C/D fighter. Employing both head-up (HUD) and cockpit displays, it provides the easy hands-on, head-up operation essential for situation awareness in a one-man fighter.
Implementation. Consisting of four air-cooled line-replaceable units - antenna plus low-power RF unit, transmitter, and processor - it weighs 379 lbs., has a predicted mean time between failures of 250 hours, and a mean time to repair on the flight line of 30 minutes.
The antenna is a planar array, mounted in azimuth and elevation gimbals. Rotation about the roll axis is handled by suitably resolving the azimuth and elevation drive and position indicating signals.
A key feature of the transmitter is use of a dual mode TWT to meet the conflicting requirements of low peak power for high-PRFs (pulse repetition frequencies) and high peak power for medium PRFs.
The processor consists of a programmable signal processor and a radar data processor in a single unit.
Operation. A complete set of air-to-air, air-to-ground, and air-combat modes is provided.
The principal air-to-air search modes are a high-peak-power medium-PRF, and, an alert/confirm mode in which velocity search is used for alert. When a target is detected, it is confirmed on the next scan with an optimized medium-PRF waveform. If the target proves valid, it is presented in a range versus azimuth display. In both modes, the pilot can optionally restrict the search to a particular region of interest or request altitude data on a given target.
Also provided are track-while-scan for up to 10 targets, single-target-track, and a situation awareness mode in which one or two pilot-selected targets are tracked continuously while the radar searches a pilot-selected volume. The radar also has a raid mode, which analyses possible multiple targets for differential velocities; a long-range up-look medium-PRF mode, optimized for low to moderate clutter environments; and a track retention capability for coasting through periods of single-target tracking when the signal drops below the clutter.
Air-to-ground modes include real-beam mapping, in which "hard" targets are sharpened with a monopulse technique; an expanded version of this mode optimized for maritime surveillance; and two doppler beam sharpening modes, providing 6:1 and 64:1 azimuth resolution improvement, respectively. Supplementing these are fixed target tracking, ground-moving-target detection and tracking, and beacon modes.
Air-combat modes are selected automatically by pressing a "dog fight" switch on the throttle. Initially, the radar scans 20 degress by 30 degrees body-stabilized field of view and locks onto the first target detected within 10 nmi. The pilot also has the options of (a) selecting a 10 degree by 60 degree vertical scan, (b) steering to place the cursor of the HUD on the target and locking onto it by releasing a designate switch on the side stick, or (c) automatically acquiring a target anywhere within the antenna scan limits.
Growth. The APG-68 has sufficient throughput to support the addition of SAR (synthetic aperture radar), terrain following, terrain avoidance, PVU, PPU and other advanced modes."
--George W. Stimson, Introduction to Airborne Radar, Second Edition
So the F-16 antenna is not mechanically roll-stabilized. (It's obvious because the antenna is oval, and can't roll inside the nose of F-16.) Instead, the rotation of the scan pattern is done by software controlling the azimuth and elevation drives.
A technician who worked with F-15 told me the APG-63 radar uses the same method: no roll-stabilization, but rather software control of elevation and azimuth motors to compensate for the roll. He said that one of the tests for the radar antenna on the ground was to see if it could scan in a continuous diagonal line.
And, in some modes, the antenna is body-stabilized. This is a perfect stabilization to use not only for close combat, but for any single-target-tracking mode. Switching the MiG-29/Su-27 from horizon-stabilized scan coordinates to body-stabilized coordinates, and rotating the antenna from the horizon-stabilized position to the body-stabilized position, could be a believeable reason why the lock requires 2-7 seconds to accomplish in Russian fighters, but not in western fighters. But in the body-stabilized position, there should be no roll limits, because like in the Western fighters, the antenna wouldn't roll!
...
02.04.2004 08:58#243
SwingKid
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...
Except for the comments of F-15 technician, the book doesn't give the same detail about APG-63. However, it describes the APG-73 radar of F/A-18C, which has a round antenna:
"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)."
Conclusion:
Most western radar antennas don't roll. They move only in azimuth and elevation. Therefore, they should not have any mechanical roll limit.
Hope this helps,
-SK
02.04.2004 16:38#249
SwingKid
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From what I know, American fighter radars never used Cassegrain reflector antennas - only parabolic dish or phased array.
Cassegrain antennas were used in Tornado ADV fighter variant for United Kingdom, and also in the Swedish Viggen fighter.
I don't know the details of their radar modes, but one Swedish Lock On tester spoke to a Swedish fighter pilot, who said it was very unlikely for Russian fighter radars to be roll-stabilized during a single-target lock. According to him, roll-stabilization is useful only in scan mode. I don't know if he was a Viggen pilot (with cassegrain reflector experience) or a Gripen pilot (using phased array). When I learn more, I'll write.
Удачи!
-SK
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发表于 2011-11-20 19:18:09
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