[Above/Below] The AIM-120 AMRAAM. The primary weapon for F-22, F-35, F-15, F-16 and F/A-18 Hornet and Super Hornet. It is a 330 lbs "medium-range" class weapon.
First Some background. There are three types of air-to-air missile guidance techniques:
- Semi-Active Homing.
- Active Homing.
- Passive Homing.
Semi-Active Homing Guidance:
Missile relies on an external pointed energy source to 'illuminate' the target. The energy reflected by this target is intercepted by a receiver on the missile. The weapon uses radio energy collected by its radar receiver to determine target trajectory and adjust control surfaces/nozzles to intercept.
The American AIM-7 Sparrow and British SkyFlash use this is homing technique.
Active Homing Guidance:
Active homing works like semi-active homing, except that tracking energy is both transmitted-received by the missile itself. No external source is needed. It is this reason that active homing missiles are called "fire-and-forget".
At longer ranges, called Beyond Visual Range (BVR), these weapons store target information downloaded internally from the launch aircraft - just prior to launch - however can also receive target position updates from launch platform via command data link (mid-course update) data pulse(s) after weapon release. In this BVR mode, the seeker head goes 'active’ (awakens) only for the final terminal phase - close to target. The AIM-54, AIM-120 and Vampel R-77 use this 'fire-and-forget' homing technique. Again most modern active homing air-to-air missiles that operate in the radio (radar) spectrum can delineate a noise jamming signal (from the target) from their own targeting transmission, and so (can) switch to home-on-jamming, coming off a target.
Passive Homing Guidance:
Passive missiles instead rely on some form of energy that is transmitted or emitted by the target. Weapon only receives signals and cannot transmit. This includes short-range heat-seeking Infrared (IR) class like American AIM-9 Sidewinder and Russian Vympel R-27 the medium range Vympel R-77T, and radio homing ‘anti-radiation’ missiles like AGM-88 HARM in the SEAD (Suppression of Enemy Air Defense) role - used against SAM radar systems.
Although anti-radiation typically is used against fixed enemy radar sites, other types of radio transmissions, including communication radios can also be targeted in this manner.
- Now enter DRFM Jamming -
Digital Radio Frequency Memory (DRFM) is an electronic method for digitally capturing and re-transmitting (reproducing) an RF signal. The DRFM technique ‘snoops’ then digitizes the received signals, stores it in memory, then if needed, replicates and retransmits.
The real twist with DRFM, is that a slight variations in frequency (phase) can be retransmitted (embedded) by the more powerful DRFM jam signal, to create Doppler (velocity) error in the attackers receiver (seeker) head.
The attacking weapon may not (or cannot) resolve these more powerful “false” DRFM signals (in time) - remember - only a fraction of a second of confusion is all that's required - the weapon will fly wide of the target – and so is defeated.
These type of DRFM signal reproduction could include snooping/creating/retransmitting distorted phase signals to confuse attacking aircraft main radar sets (including radar gunsights) as well?
Core issues for DRFM may be:
• Any radio-spectrum transmission can be snooped including beeps, squawks, data-links, battle networks and digitized radio communications.
• DRFM would not be effective in the Infrared (IR) EM spectrum.
• DRFM increases need for robust Within Visual Range (WVR) capability.
• DRFM may require offering aircrews more than one type of homing technique for BVR, similar to say Vympel R-77 plus Vympel R-77T usage model?
DFRM is used on new aircraft entering service, as well as being able to be fitted to existing legacy platforms (F-15, F-16, F-18) via pods.
|Photo: Dmitry Terekhov|
Ultra-long range air-breathing weapons, with fully passive wide-band EM-spectrum homing (anti-radiation class) seeker-heads might be the few options remaining for BVR?
One hypothetical (but probable?) scheme to attempt a counter to DRFM Doppler-shift deception, might be for attacking active-seekers to send out “dummy” pulses and use some form of anti-radiation homing to attempt to home in only on defender DRFM retransmission. The attacking weapon will need to do this quickly enough to fool DRFM into robust retransmission(s). DRFM would know the attacking weapon is closing and might counter this technique by going silent, at the same time the defender maneuvers with appropriate expendable(s). In towed decoy form, defender simply detaches DRFM as the attacking weapon homes in on its retransmission.
At the end of the day - all this will just as likely increase the probability of an infrared (infrared assisted) engagement - at close quarters. So again, we are back to helmet-sighting plus IRST+Radar cuing.