Fire and Forgotten





[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.
Because it’s a copy of the original signal, the attacking transmitting radar will not be able to distinguish its legitimate original return signal from the DRFM ‘copy’. Neither does DRFM generate and transmit radio 'noise' class jamming, so the ‘home-on-noise-jamming' used by current weapons - is useless.











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.



[Above] Su-27M: note wingtips pods. Pods reportedly can “receive” as well as “transmit.”

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.
 


[Below] a new DRFM based "Richag-AV" system on Mi-8MTPR1.
Photo: Dmitry Terekhov
This could be one reason that stealth is effectively absent on Gripen, Typhoon and Rafale

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.


Below: Expendable DRFM decoy/jamming.



Below a Russian expendable active decoy. If DRFM based is unknown. 

LPI (Low Probability of Intercept) radars might give a new lease-on-life to traditional semi-active guidance techniques? [google supplanted link]


However, if DFRM has indeed turned the radio-spectrum portion of the battlefield upside down, then even gun-sights using IRST rather than radar - might soon be essential?


Your thoughts?

[Be advised that Google has deleted this original writing dated 2009 - scholarship purposes require the original publish date to be retained]

#aim120, #amraam, #missiles, #raytheon

- All media found here is for scholarship and research purposes and protected under U.S. Internet ‘Fair Use’ Law -

Comments

  1. Hi. I first read about DFRM on this blog of yours, and after doing further research, it seems there is a way to counter it. In fact, I would say that the technology seems to favor the transmitter as algorithms can be used to vary its transmission properties so it knows that the signal is coming from it. Here is an interesting post from the "Pakistan Defence" Forum:

    http://www.defence.pk/forums/air-warfare/66507-how-counter-bvr-missiles-drfm.html#post1019767

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  2. Hi rhk111,

    Checkout what the United States Air Force Association says in their video about DRFM jamming on our post here: http://theboresight.blogspot.com/2009/07/threats-to-air-supremacy-usaf.html

    Yes DRFM can be countered by air-intercept radars that process signals at higher speeds and/or frequency hop. However DRFM engineers will counter this with higher speeds as well – so effectively we have an arms race of higher and higher speed microwave (radio wave) processing.

    Brute force CPU power can be mitigated by designing wider bit widths. Typically military electronics are behind consumer electronic capabilities because of the special operations conditions required for MIL spec. A consumer laptop, desktop or smart phone (CPU, motherboard, and memory) doesn’t need to function from say -51C (-60F) to 426C (800F) at 7G.

    Combat aircraft electronic do.

    Assuming most military-spec possessing runs 8-bit or 16 bit, DRFM using a 32-bit, 64-bit or 128-bit would be very fast without increased MHz CPU speeds.

    At the end of the day – it is an arms race.

    Who has the advantage depends on who has fielded the better system (and in enough quantities) at the time of hostilities.

    - The Boresight

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  3. Where DRFM causes its mischief is during the attackers radar listening-period. Because the timing of the return pulse must by definition be “an unknown” DRFM has a widow to produce a false “return pulse” that arrives close to (inside the window with) the authentic return pulse.

    The attacking radar (or missile seeker head radar) know receives two (similar or perhaps identical) return pulses for every one transmit pulse.

    So which one is the real distance to target? Which one to ignore? The first pulse or the second pulse? Modern radar sets likely ignore both and try again - only to have the scenario repeat. This might be why the USAF says friendly radars go “blind” with defenders using DRFM.

    Even if you send 5 pulses in any combination of freq/duration - before listening – its same problem – five additional pulses (10 total) return.

    So how can the set delineate the difference between the authentic and the decoy pulse(s) – they all look the same?

    AESA radar uses pulses of different pulse frequencies and duration nearly simultaneously - to overwhelmed typical jamming with too many things to cope with.

    Again if DRFM can keep up with AESA class radars – I don’t see any escape for this problem.

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    Replies
    1. Hi,

      First congrats for the great blog. I first read about DRFM in this entry and I found it very interesting. I have some thoughts about the subject:

      I guess there could be two types of replication:

      1st is the near-real time replication when the defender's jammer will replicate and resent signals on-the-fly as it receives. This means the fake signal must be received by the attacker slightly later than the original signal.

      2nd is the "on demand" replication where a legitimate signal is stored in long term memory and replicated where deemed necessary. So if the defender receives and stores the “first” radar signal it will be able to send a fake signal to the attacker before the “second” legitimate signal’s echo reaches the attacker. This will enable the defender’s fake signal to be ahead of the legitimate echo.

      The first method will be easy to counter (simply ignoring the second signal) for the attacker if it could “decide” that the defender is using an on-the-fly DRFM technique.
      For the second method the problem resembles the "Message Authentication" problem from "Network Security" perspective. (http://en.wikipedia.org/wiki/Message_authentication_code). Message Authentication problem deals with both integrity and authenticity of the message, in our case authenticity is distinguishing the original signal echo from the fake transmission(s) and integrity is understanding weather the signal phase is distorted or not. Had the radar signal were like network messages one could easily implement a DRFM proof system using currently available technologies like “Message Authentication Code” (mentioned above) and more importantly “Cryptographic nonce” (http://en.wikipedia.org/wiki/Cryptographic_nonce) which is basically a use once number which makes “replay attacks” virtually impossible. Since the radar signals are not “network messages” I guess one cannot embed a “Code” into a radar signal.
      But maybe existing ECCM techniques like “frequency hopping” and “polarization” can be utilized as an “Authentication Code”. If a very advanced AESA radar sends each successive signal to the same target with a different polarization or frequency determined by an algorithm (a time dependent algorithm which chooses a frequency/polarization combination according to an atomic clock or randomly selects a combination and stores it to memory in a sequence to compare the received echoes later) the DRFM jammer will only be able to send a fake signal with a wrong polarization/frequency combination which could be discarded by the attacker.

      Of course it would require immerse amounts of CPU power to track each transmission and echo pair and check them against the authentication algorithm. Also there will always be lost signals which may confuse the attacker and causes it to discard its original signal’s echo.

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    3. Hi Oytun,

      Thanks for writing!

      The problem here is that Doppler-shift is the primary radio mechanism and [perhaps] also the strength (the radio energy) of the return echo. The Doppler-shift is translated directly to a distance or "barring" of the target in relation to the attacker, then translated to flight control movement of the attacker missile round - the goal being to reduce the Doppler-shift to zero (with the strength of the return echo also increasing). Radar must transmit and then wait for the echo. Because it can't know when the return echo will arrive and at what strength... this is where DRFM causes its mischief.

      So for every transmission pulse (regardless of complexity or scheme) under DRFM employment - the attacker get 2 return echo pulses: the real one and the DRFM one.

      [Remember how this works exactly would be classified] As the attackers’ missile gets closer to DRFM source the DRFMs fake "return echoes" become stronger and drowns out the stronger authentic return echo(s). Because the DRFM signal contain a velocity error (i.e. the "Message Authentication Code" will be valid - but not when it arrives) the attacking weapon using Doppler will listen to the stronger ‘fake’ DRFM signal. Any miscalculation (even only a fraction of a second) and the missile will go wide of target - and is defeated.

      In order to home in on a target, the missile seeker head must allow:

      1) Stronger valid radar signals over weaker ones.
      2) An unknown time from the valid transmit pulse to receiving a valid return echo.
      3) An unknown strength of a valid return echo.

      - The Boresight

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    4. (i.e. the "Message Authentication Code" will be valid - but not when it arrives) 'When' meaning in what time frame it will arrive.

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    5. Yes I believe you are correct. Our understanding of DRFM is that it can store any received signal and then replicate and retransmit at the time of its choosing.

      It is conceivable that DRFM would even give F-22A LPI (pseudo-random scan) radars trouble.

      No matter the complexity, frequency, power, or duration of the each original signal pulse – an exact copy return echo of higher strength should be accepted by the attacker radar-set receiver as valid. It has to.

      My guess is - if the attackers radar set gets 2 valid return signals of varying strengths it will ignore both – and retransmit a new sequence (to try again).

      This might be why the USAF association says DRFM causes attacker air-intercept radars to go blind as targets (w/DRFM) approach.

      - Boresight

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    6. Thanks a lot for the response. From your point of view any semi-active or active homing missile could be evaded by an efficient DRFM jammer. Which makes not only those types of missiles but also any radar guided anti-air defence (S-300, S-400 patriot vs.) obsolete.

      Do you have any information weather those types of jammers are used on ballistic or cruise missiles ? I believe it would be a lot more effective since the speed of those weapons are far beyond airplanes and the time window to intercept is smaller. So even a tiny deception will surely result in targeting error for the anti-air defence system. Will this also force defence systems to rely on infra-red spectrum ?

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    7. It’s difficult to say. Take a look at the video in:
      http://theboresight.blogspot.com/2009/07/threats-to-air-supremacy-usaf.html
      I know that some IR tracking is used on the US-Navy Aegis SM2 anti-missile system.

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  4. Nice blog, good read for anyone. But I wanted to ask, what is the name of that russian expendable DRFM jammer?

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    1. I put some feelers out to see if we can get a name. If I remember this was on display at MAKS 2009. Thanks for you for your kind words. We’ve but a lot of work (and especially thought) into the material.

      - Boresight

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