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F-111 Upgrade Options Part 3 Attack Radar, GPS Guided Bombs and Pave Tack
by Carlo Kopp
In the preceding part of this series, we explored a number of upgrade options which could enhance the long term survivability of the F-111. We continue in this part with a discussion of attack radar upgrade options, GPS guided bombs and Pave Tack.
SAR/GMTI Attack Radar Issues
The Attack Radar is a major issue within itself, as noted earlier. Clearly a high resolution SAR/GMTI radar (Synthetic Aperture Radar/Ground Moving Target Indicator) is a necessary supporting capability for the weapons package to be acquired under AIR 5398. In the simplest of terms, such a radar is an "enabling" capability to support the emerging generation of smart weapons, be they types currently planned for, or the GPS guided bomb/glidebomb. Many of these weapons have been designed with such radars in mind, and the AGM-142 is the prime example thereof. In some respects it is almost curious that the AIR 5398 weapons program was initiated without provisions for such a radar, since the operational difficulties associated with using long range weapons without such a supporting radar are formidable.
The existing reconnaissance capability provided by four camera and IR line-scanner equipped RF-111Cs is simply no longer viable, and the idea of trying to overfly defended targets with these aircraft to collect targeting imagery for the next day's AGM-142 strike simply does not bear up to scrutiny.
Until the RAAF acquires a suitable high resolution SAR/GMTI attack radar for portions of if not all of the F-111 fleet, it will be unable to genuinely get its worth from the weapons package planned under AIR 5398, due to a lack of supporting recce assets for prebriefed missions, and the inability to target these weapons accurately on sorties where robust prebriefing intel is not available. The luxury of sneaking up close to a target, eyeballing it with the Pave Tack, and then putting the weapon into a window is basically incompatible with the tactical model of a standoff weapon. Technology has evolved, but we have arguably not adapted our tactical and operational paradigm to match.
The existing F-111C/G APQ-169 real beam analogue radar is ill matched to the task of targeting the AGM-142 SOW and the follow-on munitions to be acquired under AIR 5398, due to poor resolution at typical weapon launch ranges. This and the need for supporting targeting reconnaissance suggests that a modern SAR/GMTI capable attack radar should be very high on the list of priorities for the F-111. A number of off-the-shelf fighter radars now have this capability in various measures. The most capable radar in this class deployed operationally at this time is the Norden APG-76 MMRS, used by the Israelis to target the AGM-142 from the F-4E. The upper left pictures illustrate the radar and its installation in an IAF F-4E, the upper right image is a convoy crossing a bridge at 37.8 NMI range and 18 metre resolution, with the GMTI mode painting slow moving vehicles as white rectangles. The lower three images show F/A-18s at NAS Cecil Field taxiing from about 40 NMI range (Norden).
Fitting a modern SAR/GMTI radar addresses the issues of targeting standoff weapons and providing strike support reconnaissance and Bomb Damage Assessment (BDA), if done properly. An aircraft returning from a strike can map areas of interest outbound from the target, from well outside the range of defences, and a GMTI capability adds additional intelligence into the deal. Indeed, comparing the cost of recce capable SAR/GMTI attack radars on the 35 F-111s vs the cost of a package of long range UAVs and/or recce satellites, the radars on the F-111 look very good indeed.
A state of the art SAR/GMTI radar will provide SAR spot mapping with resolutions down to 1 ft or better, at ranges of about 30-40 NMI, with absolute positioning accuracies as low as 20 ft, if GPS is used on the F-111. The GMTI capability allows the detection and tracking of multiple low speed surface targets, and many types incorporate Non-Cooperative Target Recognition (NCTR) capabilities, allowing the identification of armour, soft-skinned vehicles, helicopters and even rotating radar antennas. This provides the capability to attack all surface targets in any weather, regardless of the cloudbase, using standoff weapons, GPS guided bombs, and even dumb bombs. The weather and cloudbase imposed limitations of the thermal imaging Pave Tack (or other such pods) become irrelevant. This is well and truly giving the F-111 a "Knowledge Edge" over the surface bound opponent, which are denied the sanctuary of inclement weather. If it moves, it is found and killed. Needless to say, SAR/GMTI revolutionises both strike/interdiction operations, as well as providing unprecedented capability to support Army operations with precision all weather battlefield strike and close air support,
An important point to make here is that there are distinct technological differences between SAR/GMTI attack radars, and SAR/GMTI reconnaissance and surveillance radars. This appears to be an ongoing source of confusion in lay defence circles, who seem to be unable to distinguish the two categories despite their obvious differences in design and operation.
A SAR/GMTI attack radar (eg APG-70, APG-73, APG-76, APQ-164, APQ-181) is optimised to produce small SAR/GMTI "spot maps" for targeting weapons, and for producing recce imagery and BDA imagery of specific target areas. Such attack radars typically employ a conventional nose mounted antenna, and usually include also conventional real beam mapping, Doppler Beam Sharpening and air-air engagement modes. Essentially they are digital multi-mode attack radars which incorporate the Digital Signal Processing power and the necessary signal processing algorithms to perform SAR imaging and GMTI target detection and tracking. Their primary optimisation is that of targeting weapons and providing supporting intelligence, with effective range performance between 30 and 100 NMI. Some have the ability to generate SAR strip maps, but usually with limited swath width and range. Many such radars incorporate software specifically to support GPS guided bombs and glide weapons, and will actually overlay the bomb delivery footprint over the radar map image, making the delivery almost trivial to fly, ie steer the "shape" over the selected aimpoint and pickle the payload.
A SAR/GMTI reconnaissance and surveillance radar (Pave Mover, APY-3 (JSTARS), ASTOR, Ingara, Dornier ATLAS or AWARDS, Hughes ASARS or HiSAR, Norden APY-6) is optimised to produce large wide area "strip maps" to provide surveillance and reconnaissance over large areas of interest, at extended ranges. While these radars can be and are used to produce targeting intelligence for air strikes, their purpose is far more general. This class of radar typically employs a large sidelooking antenna in a ventral canoe shaped radome, or side mounted blister, and are carried on transports, UAVs, or dedicated recce assets. The radar is designed to typically produce maps to ranges of up to 200 NMI, including both SAR and GMTI imagery, usually with provisions for an operator to produce additional spot maps of areas of interest. Some radars in this category also include an additional vertically separated antenna and associated receiver package, which allow them to produce 3D maps, rather than simple 2D maps. Often the computer packages for such radars are built as ruggedised rather than Milspec systems, using commercial computer hardware and software.
The simplest analogy is that SAR/GMTI attack radars are to SAR/GMTI surveillance radars, what Pulse Doppler Air Intercept radars are to AEW&C radars.
A modern SAR/GMTI attack radar would also address to some degree the emissions issue, and a planar array antenna which is standard with such types would reduce the frontal radar signature of the aircraft.
An issue which ought not go unmentioned in the context of the attack radar is the issue of the Law Of Armed Conflict (LOAC), and the "CNN Factor". Australia is now a signatory of LOAC, and therefore by law dropping bombs onto or shooting standoff weapons into the wrong target is essentially illegal. A bomber crew or their commanding officer can now be held responsible legally for any casualties produced by collateral damage on a strike sortie. And if the law does not act, we can rest assured that the mass media will, and therefore any bomb or missile which hits the wrong aimpoint will become the leading sound-bite on the six o'clock news, internationally. Given the "shoot-first, ask-questions-later" behaviour of much of the international media, and their all pervasive coverage, even a single bomb landing in the wrong place can compromise a government's political position when doing battle.
Recent historical experience suggests that there are two leading causes of collateral damage: guidance failure in weapons and errors in targeting. The former is less serious, in the sense that a weapon which has lost its guidance impacts essentially randomly, and therefore is as likely to miss as it is to hit anything. Errors in targeting however will usually result in a perfectly executed attack against the wrong aimpoint, the destructive effect intended for a valid military target then being expended against hapless civilians.
There is only one means via which such errors in targeting can be avoided. The quality of reconnaissance and targeting sensors must be improved to ensure that there is no ambiguity in the selection of targets. In this respect, a precision SAR/GMTI attack radar is an excellent tool, since it can image the target of interest and its surroundings under arbitrary weather/visibility conditions, precisely in relation to the bomber's geographical coordinates, even at ranges required for shooting standoff weapons.
There are good military reasons for adopting a SAR/GMTI attack radar for the F-111, however, there are also just as good legal/political reasons for doing so. The consequence of not fitting such a capability to the F-111 will be diminished combat effectiveness when using standoff weapons, reduced survivability when using guided bombs, and increased political risks in the use of the aircraft, be it in regional combat operations, or international coalition operations.
The the primary offensive sensor package on the F-111C AUP comprises the sixties technology APQ-169 real beam analogue attack radar, supplemented by the late seventies technology AVQ-26 Pave Tack thermal imager / laser designator. This sensor package is optimised for the blind radar/laser assisted delivery of dumb bombs, and the delivery of laser guided bombs with high accuracy. While the Pave Tack is now dated technologically compared to newer electro-optical/laser pods, it is still regarded to be a highly accurate sensor with excellent jitter performance, suitable for bombing from all altitudes. The APQ-169 is a variant of the APQ-113, with better ECCM and maintainability, adequate for the delivery of bombs at short release distances, but not accurate enough to target modern standoff weapons (82 WG RAAF).
SAR/GMTI Attack Radar Technology
There are several types which could be adapted to the F-111. The primary engineering issues are meeting the volume, power and cooling constraints of the existing APQ-169/171 installation, and adapting the antenna installation. The Forward Equipment Bay (LH) radar rack provides about 5 cubic feet of usable volume for the attack radar installation, a slightly lesser amount is available in the TFR rack (RH).
Essentially, there are two approaches to solving the problem of a new attack radar. The first is the "sixties" strategy, employed in the F-111 and B-1A (APQ-144/130 common to F-111F), retaining the roll stabilised antenna pedestal, fitting a new SAR/GMTI attack radar, and either retaining, modifying or replacing with new technology the dual redundant terrain following radar. The second is the "eighties" strategy, used on the B-1B (APQ-164) and B-2A (APQ-181), which replaces the attack radar and TFR with a dual redundant attack and terrain following radar, which uses a shared fixed electronically scanned phased array antenna. In this scheme, one radar channel is in hot standby, while the other interleaves terrain following and attack radar modes, through the shared antenna. The APQ-164 TFR mode also concurrently scans to either side, to facilitate terrain masking, and by virtue of a much bigger antenna and better receiver, can provide the same range as dedicated TFRs with significantly lower emitted power levels thus reducing detectability even without specific use of LPI techniques.
There are no technological or operational advantages to the "sixties" strategy, however it decouples the problems of terrain following radar capability from the attack radar capability, allowing either system to be modified or replaced separately. The "eighties" strategy, by virtue of current active electronically scanned array (AESA or "active phased array") technology, provides a smaller, lighter, very much more reliable and flexible installation, which by virtue of using a phased array antenna, can incorporate Low Probability of Intercept (LPI) modes both for the TFR and the attack radar. In the F-111, this approach would remove the troublesome roll stabilised antenna pedestal, and potentially provide greater roll angles limits in TF flight. The RCS of the antenna bay could be dramatically reduced, and space possibly freed in the FEBs.
Solving the problem using the "sixties" approach suggests two immediate candidates for the attack radar, these are the Northrop-Grumman (Norden) APG-76 Multi-Mode Radar System, used on the Israeli F-4E upgrade, and the Raytheon (Hughes) AN/APG-73 to be fitted to the RAAF Hornet.
In terms of existing SAR/GMTI modes, the more capable candidate at this time is without doubt the four channel APG-76, capable of producing genuine realtime SAR and GMTI imagery concurrently, with resolutions down to 1 ft, and tracking and identifying moving surface targets. A full package of air-air modes is included. The deployed variant of this radar is at 625 lb / 8.6 ft^3 too bulky for the F-111. It employs late eighties computer technology and a planar array antenna package designed for the F-4E.
Recent development effort by the manufacturer has seen the signal and data processing software ported to C language, and the original vector and data processors replaced by a high speed COTS (Commercial Off The Shelf architecture) VME processor, with further improvements in resolution performance, support for precision bombing, moving target imaging, 3D SAR imaging, sea surface search, and all up weight reduced down to about 450 lb. Adapting it for an F-111 installation would require primarily repackaging, using a airborne tactical Milspec rated VME/COTS processor, and adapting the antenna/mount to the existing F-111 roll stabilised pedestal. Redesignated the APY-6, this radar has been bid for JP129, and this would offer potential for commonality.
The AN/APG-73 multimode air-intercept radar to be installed in the Hornet upgrade (HUG) is another viable candidate, which in its RUG I/II variants has a respectable SAR capability, support for GAM/GATS (pseudo-differential) GPS guided bomb delivery, and optional provisions for reconnaissance strip mapping via an onboard recorder, which records raw video for ground processing. The radar includes an inertial sensor for very high resolution SAR imaging, but is not configured to process onboard such at this time.
At 350 lb / 4.5 ft^3 the APG-73 is a relatively easy fit, and may not even require repackaging of its four line replaceable modules (WRAs), at most the transmitter packaging may need to be altered for a better fit, without displacing any AUP boxes from the rack. Again the antenna mount would need to be adapted to the F-111 pedestal. The radar is evolved from the APG-65 which has been adapted to the AV-8B Harrier and the Luftwaffe F-4F.
While the APG-73 cannot at this time match the top end capabilities of the APG-76/APY-6, it could be modified to similar capability using software developed for the APG-70/AC-130U, and it would offer virtually 100% commonality with the HUG radar in hardware, identical software, and is almost a "drop in" fit. Since the radar is used in the USN F/A-18C/D/E/F, there would be no issues whatsoever with long term software and hardware upgrades and support.
Solving the problem using the "eighties" approach suggests two candidates for the combined attack/TF radar, these would be a dual redundant APG-73 variant using the planned RUG III active phased array, or a dual redundant APG-68 variant using the Agile Beam Radar (ABR) active phased array, expected to deliver in 2001. Both of these radars have design provisions for automatic terrain following, and incorporate SAR and GMTI modes. Since such radars are modular, the system could either be built up to be symmetric, with identical radar configurations for both paths, or asymmetric, with the backup radar minimally configured to support only TF and basic navigation functions, thus reducing redundant package cost.
The advent of active array antenna technology now allows such an installation in an aircraft the size of the F-111, without the weight and reliability penalties of dual TWT transmitters. Indeed, applying the Mil-Std-756B reliability model to either the APG-73 or APG-68 in dual redundant configuration yields an MTBF of 350-400 hrs, which is 2.5-3 times as reliable as the AUP APQ-171 TFR ! Both Raytheon and Northrop-Grumman produced their existing operational APQ-181 (B-2) and APQ-164 (B-1B) designs using hardware components from the APG-70 series and APG-68 respectively, and both thus have existing proven software and hardware to accommodate shared attack/TF radar operation. Both have extensive experience with LPI techniques through the B-2 and F-22 programs.
Clearly there is no shortage
of available technology in the marketplace. Other than cost considerations in integration
and testing, the best medium to long term approach would be to employ a dual redundant
attack/TF radar with an active array and LPI capability, based either upon the APG-68 ABR
or APG-73 RUG III. However from a pure SAR/GMTI performance/capability perspective, the
APG-76/APY-6/MMRS would appear to be a more attractive solution. From a cost/commonality
perspective, the APG-73 RUG II would be the easiest solution.
The GBU-31 Joint Direct Attack Munition
With the USAF, USN and USMC now adopting the JDAM GPS guided bomb tailkit (http://www.jdamus1.eglin.af.mil:82/map.html) across their fighter and bomber fleets, a Kerkanya clone JDAM variant a very likely proposition in the near future, a SAR/GMTI radar would make such weapons easy to use while providing them with all weather accuracy competitive with the current Paveway II.
The accuracy and flexibility of GPS guided
bombs such as the JDAM depends wholly upon the targeting method used. The least demanding
of aircraft sensor capability but least flexible is to bomb blind using nav-attack
preprogrammed GPS coordinates derived from satellite or aerial photo/SAR reconnaissance,
accuracy determined mostly by the quality of image registration. Much more flexible, but
potentially limited in accuracy is targeting the GPS guided bomb using a real beam mapping
(eg APQ-169) or Doppler Beam Sharpened radar. Equally flexible but much more accurate is
delivery using a SAR/GMTI radar. The most accurate and flexible delivery is that using a
SAR/GMTI radar and GAM/GATS GPS targeting (eg B-2 GAM/GATS or APG-70/73), or the same with
Wide Area Differential GPS support. The latter modes achieve accuracies competitive with
the best laser guided weapons, without any weather limitations. Other alternatives also
exist, such as using a thermal imager / laser rangefinder, such as the Pave Tack, to
accurately measure the target position and update the nav attack coordinates, and then use
GAM/GATS GPS targeting, to provide similar accuracy to a laser guided bomb, albeit weather
Since GPS guided bombs are preprogrammed before launch, it is feasible to program individual bombs in a payload for individual aimpoints in the target area. This is the approach used in the B-1B and B-2A, where multiple aimpoints are selected before release, and the bombs after launch each guide to their respective target. Therefore, a single aircraft in a single pass attacking a target such as an airfield, can allocate individual bombs to individual parked aircraft, fuel tanks, HAS', C3 buildings etc.
It is worth noting that tossing a JDAM or winged JDAM from low level is somewhat less exposing an experience than tossing a Paveway, since the JDAM has more range than a Paveway due to better autopilot algorithms (in excess of 5 NMI for low level toss), and the JDAM is launch-and-leave, not requiring aircraft exposure to paint the target with a laser. Judicious choice of toss speed, angle and terrain would allow in many circumstances a delivery of the JDAM from below the radar horizon of the target thus wholly defeating the terminal defences of the target.
The GBU-31 JDAM is now to become the USAF's standard guided bomb, carried by the B-2A, B-52G/H, B-1B, F-15E and F-16C. The baseline weapon provides precision or near precision accuracy in all weather conditions, and is a fully autonomous launch and leave weapon, with a delivery and carriage envelope virtually identical to the Mk.83/Mk.84 Slicks. The tailkit is available for the 1,000 lb Mk.83 and BLU-110, and the 2,000 lb Mk.84 and BLU-109. It appears at this stage that the USAF is also interested in a winged standoff JDAM variant, in effect a clone of the DSTO devised Kerkanya glidebomb. Depicted is a USAF B-1B rotary launcher carrying the bunker busting BLU-109/B variant of the JDAM(USAF Photo).
Tactically, a typical profile would see the F-111 approach at low level, pop up at about 20 NMI for several seconds to get a SAR/GMTI spot map of the target, the navigator would then designate the chosen multiple aimpoints, and if necessary pop up again briefly at 10-8 NMI to refine the aimpoints. From that point on the aircraft remains at low level until the toss manoeuvre, which can be designed to minimise time above the radar horizon of the target defences. If conditions are right, the aircraft may never be exposed. Should a glidebomb variant of the JDAM be used, then the bomb can be tossed at 15-25 NMI. Moreover, the JDAM can be programmed with parameters such as target impact angle, enabling the bomb to hit the most vulnerable point on the specific target, thus no flexibility is lost in targeting.
An important point not to miss here is that the navigator finalises the targeting of the JDAM at the 10-8 NMI point prior to release, and no further intervention is required, unlike during laser guided weapon delivery. The pilot completes the delivery by following the ADI/HSI steering cues, flying to the appropriate point and tossing the weapons. This means that the navigator can focus on electronic combat and defending the aircraft, during the critical delivery phase, rather than guiding his weapons. The survivability advantages in delivery profile against the Paveway are therefore further enhanced.
One argument which seems to surface from time to time in relation to GPS guided bombs is "what if the US denies us access to the PPS crypto codes ?". This is now utterly immaterial, since the GPS dither on the civil C/A code is soon to be removed, the latest generation of anti-jam antennas provides enough margin to beat all but very clever jamming even of the C/A code, and finally the use of pseudo-differential (GAM/GATS) techniques nulls any residual C/A errors. If jamming succeeds, the bomb will revert to pure inertial guidance and at most lose some accuracy. If these arguments are still deemed inadequate, we can always point out that most of the weapons being bid under AIR 5398 rely almost wholly upon GPS midcourse guidance.
Integration of the JDAM is very simple, primarily involving addition of existing software modules into the AUP Mission Computer and Stores Management System Operational Flight Programs (common to the the USN F/A-18C/E), and the necessary flight testing. The bomb is aerodynamically a straked Mk.83/84, and employs a standard Mil-Std-1760 smart interface, directly compatible with the AUP interfaces. Because of the GBU-31 JDAM's similarity in weight, shape and aerodynamics to the basic Mk.84, any clearance testing effort will be simplified and thus much cheaper to perform, as the USAF cleared the Mk.84 on internal and external stations early in the development of the F-111.
It is well worth pointing out that the JDAM is much simpler to manufacture than a Laser Guided Bomb, since it is wholly devoid of any optical hardware. The licence manufacture of the JDAM in Australia is entirely feasible, with the exception of the HG1700 RLG package and the Silicon used in the internal hardware, every other part of the bomb could be manufactured locally. For GPS sceptics, this would enable the incorporation of locally developed antennas, receivers and software modifications to ensure that the weapon cannot be compromised.
A final and no less important point on the use of the JDAM is that it's suitability for internal carriage further enhances survivability. With a two round internal loadout the aircraft can penetrate clean for best performance and best combat radius. With a four round loadout of two rounds internal, and two rounds on the outboard (3/6) swivel pylons, the drag is still less than half that of a four round external load of GBU-10s, with the additional benefit of more flexibility in sweep angles permitted.
AN/AVQ-26 Pave Tack
The final sensor related issue worthy of comment is the future of the Ford Aeronutronics AN/AVQ-26 Pave Tack, which has now been obsoleted by the USAF with the retirement of the F-111F. The Pave Tack is unusual in its class of thermal imager / laser designator pods, in that it is designed to be retracted when not being used, and to deliver weapons with high accuracy from low, medium and high altitudes. Retracting the pod removes a major drag penalty, while isolating the pod from the harsh environment of external carriage. The requirement to bomb accurately from high altitudes means that the Pave Tack has arguably the largest optical window and best mirror stabilisation / jitter performance of any thermal imaging pod in service today, since newer pods have been optimised for low level deliveries. The field of view of the Pave Tack is much better than that of most current pod designs.
The problem the RAAF will have is maintaining the Pave Tack in the longer term. Its central computer is now hopelessly obsolete, the refrigerator is a high maintenance item, and the very bulky thermal imaging module, the AN/AAQ-9, is no less obsolete than the computer. The AAQ-9 is sixties rotating mirror / linear detector array FLIR technology which is not very reliable, and with a very low picture resolution (280x370 pixels) it does not do justice to the pod's optical system (see imagery). Moreover it operates in the 8-12 micron band which is not optimal for the tropics, since it suffers greater attenuation due to atmospheric water vapour at shallow slant angles typical of low level toss deliveries.
Replacing the Pave Tack is problematic, since new thermal imager / laser designator pods can be up to several million dollars apiece, and would need to be carried externally thereby increasing drag, RCS, reducing pod lifetime, and tieing up stations. Moreover, making a major long term investment into the purchase and integration of a new type of pod makes little sense given the current trend to use GPS guided bombs instead of Laser Guided Bombs. The LGB is now becoming a niche weapon for fair weather attacks, primarily on moving battlefield targets and high value targets difficult to identify on radar.
A good case can be made for a technology upgrade of the Pave Tack, since a modest investment into the existing design can produce an end product with performance equal or better to a new buy pod at a fraction of the cost (something to also consider in the context of putting a FLIR/designator on the G-models). The computer could be replaced with a current design Mil-Std-1750A unit, an AP102 variant, already used on the F-111, would be a good example. Rehosting the software to a new like architecture processor would verge on the trivial. The AAQ-9 thermal imager module could be replaced with a new technology design, to exploit the much greater reliability and far superior picture quality of current Focal Plane Array (FPA) technology, which can deliver 512x512 up to 1024x1024 pixel resolution.
Texas Instruments abandoned a multiple IR band "image fusion" technology based upgrade for the AAQ-9, which would have been a swap-out box level upgrade for the Pave Tack, when the F-111F retired. Given the wide availability of high performance FLIR modules, this is by no means a difficult, risky or large task, more so given DSTO's proven expertise in this area. It is essentially repackaging.
In summary, given the purchase and integration cost of new pods, a good argument can be made for acquiring additional boneyard Pave Tack pods and cradles, and upgrading these to current computer and thermal imager technology, for use across the whole F-111C/G fleet as standard equipment.
Part 4 completes the discussion of
possible future upgrades for the F-111.
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