3rd Khordad: The RQ-4 downing

This post is about the Iranian 3rd Khordad surfance to air missile, a system that should not have been able to do it's first operational kill of a RQ-4 in summer 2019.

3rd Khordad SAM which shot down the RQ-4 (note the kill mark)

Understanding what was achieved

A RQ-4 Global Hawk is an expensive and vital asset in U.S airpower structure, that does the battlefield management of a whole front section.

It remains far behind the lines, employing its stand-off SAR/GMTI sensors and has no crew that is put to risk.

The first iteration of such a capability was to be protected by employing a survivable air platform, which resulted in the low-observable/stealth (LO) Tacit Blue design.

Technical hurdles forced the U.S to abandon this plan and the solution was the E-8 Joint STARS.

A large E-3 AWACS-like concept, where larger sensor size allowed for increased distance to the dangerous battlefield periphery.

It added up to the portfolio of U.S airpower force-multiplier support assets; systems that greatly enhance the U.S airpower concept, but are vulnerable.

Too vulnerable in confrontations with peer-level adversaries, yet still sufficient against opponents lacking "strategic assets".

Reconstructed RQ-4 wreckage

RQ-4's most vital sensor: SAR

The RQ-4 hence became what the Tacit Blue should have been; JSTARS capability within a survivable platform.

The aerodynamic penalty the Tacit Blue had to pay, due to its LO geometry with its heavy emphasis of deflecting radar waves could be reduced.

Instead the requirement for LO/stealth aspect of the design was achieved by material stealth.

Aerodynamic LO penalties were reduced so much that the RQ-4 became a kinematically high performance platform with vast endurance and high altitude performance.

Honeycomb RAS from the wreckage (Credits to members of military.ir)

Its radar absorbing materials and structures (RAM/RAS) are very expensive to manufacture and designed to disrupt a key element of the kill chain, of a system trying to kill it: The engagement phase, where normally X-band and higher radar wavelength are employed.

The X-band allows a small aperture seeker in the SAM and/or a sufficiently high precision ground/air tracking by the radar system.

The RAS of the RQ-4 BAMS-D that was shot down, is a complex double layer honeycomb design in critical areas. Its size suggest that its design perform best in the critical 8 Ghz X-band at which almost all SAM systems operate, including the 3rd Khordad.

Quantifying the LO performance of the RQ-4

The performance depends on the radar cross section of the geometry, overall size, to which an average RAS/RAM performance is added. It is best measured in dB.

RQ-4 geometric RCS estimate (credits to members of military.ir)

This is a crude simulation but gives a general idea what the RCS of a blank RQ-4 at 8-10 Ghz looks like.

It averages 0-5 dB at relevant aspects and would be higher if the emitter/radar is ground based, like in this case.

Another more professional analysis is available for the RAS, the key LO performance element of the RQ-4 design.

Different honeycomb RAS (credits to members of military.ir)

The highest reduction in X-band is between -25 to -20 dB. That's if a RQ-4 that applies similar double layer RAS all around the air frame. In practice this is not the case and here we must average the variations of geometric RCS and its RAS/RAM coverage. Ideal results are between -25 dB and -15 dB overall RCS of relevant aspects. A stable RCS for tracking is therefore estimated to be -10 dB to -20 dB, which gives an idea of the grade of low observability of the RQ-4. To be on the conservative side, a RCS of -20 dB or 0,01m² is selected for all frontal threat aspects and against a ground based X-band emitter. As the performance parameters of the 3rd Khordad have not been published, we can calculate the performance of other better known systems against a -20 dB class target. The highest performance system, of which the necessary data for the radar equation is known, is the export variant of the Russian S-400:

  • Tracking of a -20 dB class target by the 92N6E X-band engagement radar would be 56km.

  • Tracking of a -10 dB class target by the 92N6E X-band engagement radar would be 100km.

Iran claims that target was tracked or locked at 120km, missile was launched at 90km distance and hit the approaching RQ-4 at 75km distance. The 3rd Khordad however is not comparable to the S-400. In terms of aperture size, its about 7 times smaller and doesn't achieve as high power levels.

How was it done?

The inadequacy of the 3rd Khordad engagement radar and the comparatively high LO performance of the large conventional layout RQ-4 rise questions.

The AESA technology which the 3rd Khordad employs, compared to the PESA S-400, is no explanation either, as it could never make up for the performance level necessary.

Beyond the world of conventional techniques, and modes described in export rated system manuals, there are some interesting points.

A SARH seeker based SAM system with an IMU can fly an energy optimized pattern, where it climbs above the target and dives into it.

Taer-2 SAM terminal "dive" trajectory

This offers improved kinematic performance than a SAM doing just proportional navigation.

It also allows lock-on after launch in order to control emissions and remain passive.

Additional to all of that, it puts the ground based emitter and the approaching SAM's seeker into bi-static positions relative to each other.

This can offer improvements against systems that employ techniques of shape/geometry LO/stealth, which deflect radar waves away from the emitter line of sight.

The RQ-4 is not a sophisticated representative of shape LO and concentrates on high performance radar absorption as a subsonic design.

Simplistic graphic on bi-static effect of SARH SAM

Another reality of such next generation systems are their inherent data fusion and multi-band architecture.

Here the 3rd Khordad's own AESA radar would do textbook tracking engagements against non-LO/stealth conventional targets.

Against LO/VLO targets, its main task would become that of high-power missile up-link, missile tracker and terminal illuminator.

The long range tracking in such cases could be done by other radar systems that perform better against X-band optimized LO assets.

Lower band radars in VHF-band nearly completely neutralize the benefits of RAM and RAS and to a lesser extend also help against LO shaping techniques.

However they often fail to provide accurate coordinates for the SAM to get sufficiently close to the target to kill it.

At some point sooner or later, a portion of the illuminated engagement radar frequency energy would get picked up by the SAM's SARH seeker, even if a large portion is absorbed or deflected by the LO target.

In this case the 3rd Khordad would do a blind illumination based on the coordinate data of the lower band radar that tracks the LO target. A key requirement for such a operation mode, would be knowledge on the accurate positions of both radars relative to each other.

Najm-802B S-band AESA radar

Illuminating a spot in airspace, blindly requires a sufficiently high accuracy. Irans family of S-band AESA's offer such accuracy levels at relative long ranges.

This could be sufficient to allow the 3rd Khordad's X-band radar, to illuminate the LO target without own tracking.

The 3-3,5 Ghz operating frequency of these radars reduces the -20-25 dB performance of the double layer honeycomb RAS, effectively down to around -7 dB.

The result is that the lower-band of this S-band AESA, allows it to confront the RQ-4 as a -7 dB (0,2m²) class LO asset, not -20 dB as in X-band.

Simulated models of the Najm-802B with conservative, very low power TRMs show that it would offer sufficient capability to do a 80-100km tracking of a -5-7 dB class LO target.

Thermal imaging camera of the 3rd Khordad

Thermal imaging systems able to detect air targets at extended ranges are a quite recent development. The performance class needed to detect a air targets at 70-100km distance was only in the hands of few advanced nations in the 2000's.

Today Iran employs such high performance TI optics on its tactical 3rd Khordad SAM system.

They allow angle tracking of targets that either can't be radar tracked due to electronic warfare, or by employing LO/stealth techniques like in the case of the RQ-4.

The TI system could either help to compensate radar positional errors of the multi-band engagement solution as mentioned above, or assist in autonomous engagements.

In the latter scenario, a low-band radar would do the coarse tracking, such as the widespread Matla-ol-Fajr-2.

Matla-ol-Fajr-2 VHF-band planar array

The low coordinate accuracy would then be compensated by angular tracking via the TI camera system.

This combined employment of multi-band radar data and corrections from the TI sensor, could then be sufficient for the blind illumination concept via the 3rd Khordad own X-band radar.

Active radar homing SAM could be another explanation.

Cost and robustness are main reasons why Iran stays with the SARH principle, just like Russia.

Russia's newest SAM system, the S-350 employs ARH SAM to counter terrain masking cruise missile, that are not protected by electronic warfare.

In such scenarios, loss of LOS illumination is no issue anymore because the SAM is illuminating its own target from above.

Ambush scenarios are other cases where ARH may be employed, such as long-range shots by the S-400 40N6 missile against physically large U.S airpower support assets.

However the reason even the newest S-350 retains the expensive X-band PESA engagement radar, is that under severe electronic warfare conditions, the dual-mode SAM seeker can switch to SARH/SAGG mode, which is the most robust mode.

Iran claims that there is a 105km range variant of the Taer SAM for the 3rd Khordad. This maybe a special variant, that uses ARH seeker at ranges where the 3rd Khordad's own engagement radar can't effectively illuminate. However, this could also be the maximum range of the newest Taer-2 variant against kinematically low performance targets.

Irans view on ARH tactical SAM seems to be that they are too fragile on one side and too expensive, if to be made robust, on the other.

After the shot down of an Ukrainian airliner by an Iranian Tor-M1, Iran acknowledged that U.S electronic warfare assets are taken very seriously.

The concept of an imaging infrared seeker SAM variant sounds attractive, just like the ARH concept initially. However here, its not the cost for the seeker but the robustness and redundancy factors. Beside an aerodynamic penalty for a blunt IIR seeker on SAM kinematics, SARH or even SAGG is simply regarded as very mature and robust. However there exists a Taer-2 variant with a IIR seeker, as an option for engagments under adverse conditions.

3rd Khordad TELAR transiting

There are more techniques that could be applied to allow for an extended-range engagement of LO targets in the class of the RQ-4, but it would be too theoretical at this point.

What happend most likely on that night

The RQ-4 BAMS-D started from an airbase the UAE and climbed to altitude. During that climb it should have appeared at some point on the screens of the Iranian Ghadir/Rezonans long range early warning radar. With its strict emission control, no active IFF, and flight pattern Iranians would realize at some point that it is a Global Hawk leaving UAE airspace towards Iran.

Later a manned P-8 maritime surveillance aircraft added up and they formed a group, typical for a long range reconnaissance mission along Iranian borders. A slower MQ-9 drone added to the group on their way back towards the straight of Hormuz.

Once near Iranian borders, the old, static early warning radar network and mobile VHF Matla-ol-Fajr-2 early warning radars would have detected and tracked the LO/stealth RQ-4.

When warnings were ignored and Irans airspace violated, the decision was made to engage the RQ-4.

At the remote missile site where the 3rd Khordad system was, likely also a widespreadly used Matla-ol-Fajr-2 was present, directly linked to the 3rd Khordad.

The 3rd Khordad remained passive, as illumination of the RQ-4 could trigger its powerful towed electronic warfare system.

Coarse coordinated were continuously provided to the 3rd Khordad which allowed angle tracking by its TI camera. Coarse range data came via the Matla-ol-Fajr-2 and this was sufficient to launch a Taer-2 towards the estimated interception point.

The Taer-2 would then have received mid-course updates via the short range data-link of the 3rd Khordad.

3rd Khordad operators

Approaching the target the 3rd Khordad would have started emissions of its LPI AESA radar to track the SAM and do the later stage, long range, course updates.

This was not perceived as a threat by the ESM suite of the RQ-4.

Continuous command guidance would not be necessary as the IMU of the Taer-2 can work with one update every several seconds without wasting too much kinetic energy.

It appears that, if at all, only the missile approach warning system detected the inbound Taer-2 and this was to late to deploy the towed jammer, since the Iranian operators claim that there was no ECM as they had expected.

Seconds away from impact the 3rd Khordad radar would have started blind illumination of the target based on the external VHF-radar data, corrected by the angular data from its own TI camera. This would then have been precise enough to concentrate the electronically steered, illumination pencil beam on the RQ-4.

The now very close distance Taer-2 SARH seeker would then have picked up the portion of RF energy that had not been absorbed by RAS or deflected by shape.

Once locked it would have corrected the trajectory uncertainties caused by the VHF and TI sensors, got the exact range to the target and brought itself into a parallel position for the highest directed fragmentation effect of its warhead.

In the last phase redundant seeker and proximity fuses would determine the right timing and direction for the detonation of the warhead.

Taer-2 approaching a low RCS target drone


Iranians claim that Russians called the 3rd Khordad's missile "mythical" after information was exchanged.

The reason should be the fact, that as per conventional engagement techniques, even large brute-force SAM systems such as the S-400 can't track the RQ-4 at extended ranges with their engagement radar alone. Certainly Russians apply similar unconventional techniques for engagement of LO assets, at least in their domestic federation standard systems.

3rd Khordad would not have been able to track a RQ-4 class LO target at beyond 25-30km, but was able to engage at 90km and kill at 75km.

Its a display of what is possible if you are the owner and creator of the system design, it can be continuously updated and integrated with other systems.

It must also be understood that the RQ-4 is a genuine LO/stealth asset which capabilities in this field should not be downplayed after the shot down. Its AESA radar is said to have high LPI capabilities and its emissions very difficult for ESM systems to pick-up and finding the source of it.

The same defeat tactics could be employed against more capable LO/stealth assets such as F-22 and F-35, even if their LO performance is higher than that of Global Hawk.

The 3rd Khordad is the final result of IRGC-ASF's quest for a:

  • cost-efficient (low cost SAM, low cost TEL, low cost radar, COTS components)

  • highly mobile and off-road capable

  • highly autonomous (target search/acquisition and engagement done by single radar on TEL)

  • low footprint (small size and easily disguised)

  • shoot and scoot (fast change of position)

  • multi-target engagement capable

  • low maintenance and support footprint

SAM system which roots go back to the SA-6, which was highly regarded by the IRGC.

It creates a sphere of 150km diameter under protection against tactical fighters and 210km against kinematically low performance assets such as U.S air power support platforms.

It can suddenly appear near a contested front sector or areas where the SEAD/DEAD mission is seen as accomplished.

It can also operate within a battery of other 3rd Khordad TELAR and TELs if such structures are intact.

Its high performance TI camera allows to avoid decoys and only engage targets that are worth it.

The load of just 3 Taer-2 may appear low but the missile is regarded as highly sophisticated, indicated by the single shot against the RQ-4.

Overall it's asymmetric approaches which the 3rd Khordad employs to be successful against an air power machinery, as that of the U.S.

It's the flexible backbone of IRGC-ASF's air defense, designed to remain a threat down to the last phase of any potential conflict.

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Enthusiast-level open source (-only) military technology and missile analysist