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Seventies F-111D was the first digital F -111 variant, as well as being the first ever digital tactical fighter, fitted with a Rockwell Mk.II avionic suite and Norden cockpit display set. The F -111D cockpit included a "glass" ADI, combined moving map/TFR display, dual HUDs and a digital radar display which included frame freeze capability. Unique among USAF variants, the F -111D was designed with an intended air-air role, and was to have initially carried both AIM-7G and AIM-9J MMs. In service, the aircraft was tasked with strike duties, as was its sibling variants. (USAF)
Technology Explained – F-111 AUP Update Pt 2
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New Tusks For The Pig
The F-111 Avionics Upgrade Programme Pt I
The General Dynamics F-111, affectionately termed the “Pig”, is one of the RAAF’s most important assets. These aircraft are Australia’s strategic deterrent, and are tasked with long range land and maritime strike roles. In the event of hostilities, these aircraft would carry the fight to the enemy, destroying targets deep in hostile airspace and threatening hostile shipping to a substantial radii. While the F-111’s superlative combat radius and high aerodynamic performance are key aspects of its ability to perform its demanding roles, its avionics suite is no less important. The aircraft’s offensive avionics, comprised of sensors and navigation equipment, are critical to the aircraft’s ability to locate, identify and successfully attack its targets. The defensive avionics, on the other hand, determine whether the aircraft will survive its incursion into contested airspace, to fly and fight another day. When it was first deployed in the late sixties, the F-111 carried the most complex and technically sophisticated avionics systems ever fitted to a tactical aircraft. This complexity translated into substantial support costs, moreso since the system was largely built with analogue electronics which as a technology are somewhat temperamental. As time progressed, the maintenance of this equipment became increasingly expensive, and by the late 1980s the pressure to upgrade the RAAF’s aircraft became substantial, particularly as the USAF was expected to convert all of its remaining analogue models to contemporary digital equipment. Cost and operational readiness pressures forced the USAF to upgrade, and some indication of the scale of the support problem can be gained from published US figures (1981) which indicated that the SAC FB-111A had an aggregate aircraft MTBF of 2.2 hrs (still somewhat better than the USN A-6 with an MTBF below one hour). The digital refit of the USAF fleet would have made many analogue spares effectively unobtainable. In seeking approval for an upgrade to the F-111 avionics, the RAAF was largely constrained to retaining existing capabilities, the opportunity to substantially enhance the aircraft’s capabilities was unfortunately not to be exploited. The upgrade programme is thus focused on replacing sixties avionics technology with contemporary equipment of broadly similar capabilities, with the objective of greatly increasing reliability and reducing maintenance costs in the latter period of the aircraft’s life cycle. Incidental gains have been made, in that contemporary navigation equipment is substantially more accurate, and a digital weapon system can be easily adapted to accommodate new munition types. Aircrew workload is also reduced, as 1990s avionics provide for a greater degree of automation. Given the technological complexity required to support the various sensors, munitions, flight profiles and delivery modes inherent in the platform, an avionics upgrade of this scope is not a technically trivial exercise, as some would have us believe. Every major weapon system integration programme in the last several decades has been a protracted affair, this is simply the nature of the process. Where dozens of avionic black boxes need to be functionally melded by hundreds of thousands of lines of computer software, in order to create the illusion of a single weapon system, there are simply no easy shortcuts. To believe otherwise is to defy engineering intuition as well as historical experience. In 1990, the RAAF contracted Rockwell Electronics (Australasia) to upgrade the F-111C and RF-111C avionic systems, replacing the Mk.II analogue weapon system with contemporary digital equipment (NB: the RAAF’s four F-111A airframes have already been retrofitted with the “big” wing and FB-111A undercarriage, thus effectively converting them to F-111C airframes). Choice of prime contractor was substantially influenced by Rockwell’s then very recent experience with the F-111D/F Pacer Strike programme. Rockwell’s first involvement with the F-111 was in the late 1960s, when they developed the offensive avionic systems for TAC’s F-111D and SAC’s FB-111A aircraft, followed by work on TAC’s F-111F. The F-111D’s Mk.II avionics system was a particularly important technological milestone, as it was the first ever wholly digital airborne weapon system fitted to a tactical aircraft. The nucleus of the Mk.II avionics system was a digital computer, with most weapon system functional integration performed by software. The F-111D’s primary sensor, the largely digital AN/APQ-130 attack radar, was also designed by Rockwell. Rockwell as associate contractors thus created the Mk.II weapon system for the F-111D, and later the Mk.IIB weapon systems for the FB-111A and the F-111F. The latter system was blooded during the 1986 El Dorado Canyon mission to Tripoli, and most of the aircraft used in the Gulf would have still carried this equipment. It was therefore no surprise to the technical community that Rockwell subsequently won the 1989 USAF Pacer Strike contract to upgrade the USAF’s F-111D/F fleet with a new build offensive avionic suite, replacing the Mk.IIAIB systems. The Pacer Strike upgrade replaced the AJN-16 digital bomb nav, the AYK-6 computer and a miscellany of cockpit displays with a pair of 64 kWord Singer-Kearfott AN/AYK-18 Mil-Std-1750A digital computers, the AN/ASN-41(LN-39/H423) Standard Inertial Navigation Unit (SINU), cockpit MultiFunction Displays (MFD) and an Integrated Communication Navigation Identification System (ICNIS), all tied together by Mil-Std-1553B databusses. Initially Pacer Strike covered 79 D models and 84 F models, but this was subsequently cut to 58 F models, after the collapse of the USSR and the onset of subsequent force reductions.
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The Raaf F/Rf-111c Avionic Upgrade Programme
The RAAF’s F-111 AUP has much commonality with the Rockwell F-111D/F Pacer Strike upgrade, and the earlier USAF/Grumman F-111A/E Avionics Modernization Programme (AMP) upgrade, particularly in the choice of avionic subsystems. Much of the mission computer software is also derived from the Pacer Strike system software. 34 Australian Aviation August 1995 there are however a number of substantial differences, particularly at a system level, and these have been largely dictated by the RAAF’s need for a high level of system reliability on long range sorties and remote deployments. Other important factors which influenced the RAAF’s design specification were the need to introduce more commonality between the F-111C and RF-111C, and experience with the USAF upgrades which suggested the need for a better GPS receiver, computers, displays and stores management. The RAAF’s support for weapons such as the Harpoon was another influence upon the project, as well as the need to have commonality in stores interface with the RAAF’s F/A-18 fleet. The RAAF AUP system has been described as two to three times as complex as the Pacer Strike system, particularly in software. To fully understand the scope of the upgrade it is useful to briefly review the functional aspects of the aircraft’s offensive avionics. The aircraft’s primary sensor is its attack radar (ARS), which is used both for navigation and blind bombing of surface targets. Penetrating at very low altitudes, the aircraft also employs a terrain following radar (TFR) which allows it to hug the ground. The TFR must be closely coupled to the flight control system (FCS), as it generates automatic pitch control inputs for this purpose. Precise navigational reference is provided by an inertial nav system (and GPS post AUP). If laser guided munitions are to be used, a Pave Tack thermal imager/laser designator pod is carried. Because of the need to provide selective arming and release of stores, a stores management system must be used. These systems must be seamlessly integrated, so that the weapon system provides the capability to navigate and attack targets from low level in precision first pass attacks. The high level of integration is driven by this mission profile, which is also common to the larger B-1 Lancer. Indeed, the B-1A offensive avionic suite shared many components with the F-111F.
An/Apq-169 Attack Radar And An/Avq-26 Pave Tack
The RAAF’s 1980s Pave Tack upgrade to the F-111C saw the elderly GE AN/APQ-113 J-band attack radar incrementally upgraded to AN/APQ-165 configuration, which incorporated a number of module changes common to the F-111F AN/APQ-161. The AN/APQ-161 was derived from the earlier F-111F AN/APQ-144, and was fitted as part of the USAF Pave Tack upgrade. Interestingly, the USAF had earlier tested an upgrade to the APQ-144 which included MTI for blind attack against surface moving targets such as vehicles, and a higher frequency K-band transmitter, but neither were introduced into service. The subsequent APQ-161/165 upgrade saw the addition of a digital scan converter, which takes analogue radar video signals, digitises it, and remaps it into an EIARS-170 875 line/60 fps analogue raster scan format suitable for the Pave Tack display, and the Airborne VCR. This allows a single raster scan CRT (tube) display to accommodate radar, Pave Tack and EO/IR weapon imagery, as well as alphanumeric symbology generated by the Pave Tack’s onboard computer. The AUP upgrade will see further incremental changes bringing the radar up to the AN/APQ-169 standard. Notable gains in the AN/APQ-169 upgrade are improved signal processing, improved ECCM beyond the APQ-113’s frequency agility and sidelobe cancellation, and automatic beacon tuning. The APQ-113 antenna assembly and pedestal are retained. The RAAF F-111G also uses the AN/APQ-169, but is fitted with a different cockpit display, which has only a single but much larger CRT display. Fitted to the F-111C during the mid 1980s, the AN/AVQ-26 Pave Tack is a podded thermal imaging turret which is boresighted with a 1.066 um Nd:YAG laser designator and rangefinder. The Pave Tack pod is mounted on a fuselage pallet, which fits in the fuselage weapon bay. While the Pave Tack equipment will be retained unchanged, the analogue and serial digital interfaces in the fuselage pallet will be tied into the new digital avionic system via the ACU unit (refer illustration). The Pave Tack contains its own internal computer, and supports a very wide range of search and tracking modes. This versatility was put to very good use by the USAF during the Gulf War, where the Pave Tack equipped 48th TFW flew both strategic and tactical interdiction sorties at all altitudes, using the 500lb GBU-12, 2000lb GBU-10 and GBU-24 laser guided weapons, as well as the GBU-15 glidebomb and preproduction AGM-130 standoff weapons. The 48th TFW is credited with kills against 52 bridges, 920 armoured vehicles and a large fraction of the 350 Hardened Aircraft Shelters destroyed. The Compact Airborne VCR (CAVR) is primarily a Bomb Damage Assessment tool, which records radar, Pave Tack or EO/IR weapon imagery during the attack on the target. The equipment used is a TEAC V-250AB-R, which is equipped with an RS-170/343A video interface to accept radar, thermal imager and cockpit MFD video signals. The CAVR is controlled from the cockpit strike camera control panel (SCACP). The V-250AB-R is a compact, ruggedised two head helical scan 1/4″ VCR, built to withstand a Mil-Std-810C environment, and capable of operating at sustained load factors of up to 10G. The CAVR is powered off the aircraft’s 28V DC rail.
Inertial Navigation And Gps
The primary navigation reference of the F/RF-111C AUP bombing and navigation system are a pair of Honeywell H423 Ring Laser Gyro inertial units, termed SINUs (AN/ASN-41) as per USAF terminology. These are highly stable, accurate and mature systems, widely used in USAF and export equipment. The USAF SINU programme saw the Standard Inertial Navigation Unit equipment standardised across a wide range of tactical and strategic aircraft, with Honeywell supplying the H423 and Litton the LN-39, as interchangeable Line Replaceable Units (LRU). These are therefore used by all of the USAF’s upgraded F-111 variants. The SINUs are supplemented by a Rockwell 3M (MAGR) 5 channel GPS receiver, which provides additional navigational reference. The 3M GPS receiver is a higher performance receiver, in comparison with the 3A model standard to the Pacer Strike upgrade. Kalman filtering techniques are used in the navigation software to provide a best possible position estimate, with GPS position and velocity employed to calibrate the SINU outputs. Accurate position fixes may also be inputted from the Pave Tack (using its laser rangefinder) and the attack radar. This scheme provides very high accuracy as the system software can selectively choose the most accurate navigational reference from any of the available sources, but also reflects in very high mission availability as failure of multiple sensors or nav references will not disable the system, but merely degrade accuracy. This strategy is very similar to that employed in the F-111G nav system, which uses Doppler velocities to calibrate SINU velocity readings.
Mission Computers And Software
The nucleus of the F/RF-111C AUP offensive avionic suite are a pair of IBM AP-102 Mil-Std-1750A instruction set 16-bit Mission Computers (MC). The AP-102 is the successor to the AP-101F which is the core of the B-1B weapon system. The two MCs are tied into a pair of dual redundant Mil-Std-1553B databusses, which provide access to the Stores Management System, Avionics Converter Unit (fudge box), SINUs, GPS, Air Data Computer, ICNIS, SC-DDS in the RF-111C and a Programmable Display Generator (EXPDG), which drives the two cockpit displays. The mission computers are a major difference from the AMP and Pacer Strike upgrades, which employed a pair of Singer-Kearfott AN/AYK-18 processors, each dedicated to running a part of the mission software. Failure of either would cause the loss of significant system functionality. The RAAF’s AP-102s deliver higher computer performance (~1 USAF MIPS) and have larger memory capacity than the AN/AYK-18, and each runs the full mission software package, thereby providing true dual redundancy. Analogue instrumentation outputs and sensor outputs, and discrete I/O channels, are interfaced to the Mil-Std-1553B bus via the GEC-Marconi ACU, which contains discrete I/O interfaces, analogue to digital and digital to analogue converters, and an embedded processor to manage them. The use of “fudge boxes” such as the ACU is very common in bussed avionic systems, as many smaller items of instrumentation or equipment do not warrant the use of dedicated Mil-Std-1553B bus terminals. The ACU contains a significant amount of board level firmware in EPROMS, as well as non-volatile memory containing configuration data. The configuration code, cca 200 lines of C language and Assembly Code source, allows further integration of any unused channels in the device, or alterations in configuration should this be required in the future. The controller in the ACU has 32kBytes of EPROM and 32kBytes of RAM. This system topology is designed for maximum reliability, and the provision of redundant mission computers and redundant busses ensures that the loss of any single component would not preclude mission completion. Multiple redundancy is a strategy first employed in the F/A-18 and has proven to be particularly robust for mission critical applications. The mission software handles navigation, weapon delivery and control of bus interfaced mission avionics. The software has been written in Assembly Code and Jovial, with some older Fortran modules recoded into Jovial.
The use of substantial amounts of Assembly Code is common in embedded systems as this provides both speed and importantly, compactness of executable code. The choice of Assembler and Jovial, an older USAF standard, was almost certainly dictated by the availability of existing code modules from earlier Rockwell F-111 upgrade work, and by performance and compactness in comparison with the newer ADA, not renowned for speed or efficiency of memory usage. The development system is hosted on a DEC VAX/VMS system, using Mil-Std-1750A compilers to generate the machine loadable binaries. Binary executable code is loaded into the mission computers’ internal non-volatile memory (NVRAM) by Memory Loader/Verifier equipment, which is a conventional approach for this style of embedded system. The Mission Computer software (Operational Flight Programme or OFP in RAAF terminology) is a major difference from the Pacer Strike system. As the RAAF required full (hot standby) dual redundancy, both computers concurrently execute the same functions and a hardware failure in either machine must be transparently handled without interruption to function, with the failing unit placed offline. This is a major increase in functional complexity against a single CPU system, or a dual non-redundant CPU system (ie Pacer Strike), and significant effort is needed in developing the software to ensure a clean cutover between the processors. As either computer has the performance to carry out all system functions without degradations in performance or system accuracy, this approach provides very high mission reliability. The MC OFP is about 50-60,000 lines of source code, and is substantially more complex than its lineal predecessor in the Pacer Strike system. Mission parameters are uploaded into the computers via a Smiths 3200 series Data Transfer System (DTS). This allows mission profiles to be prepared on a ground based Horizon Technologies Mission Planning System (running on a PC) and then uploaded prior to takeoff using a 256kByte capacity NVRAM cartridge. The DTS is a very useful facility, as it saves the Navigator much time otherwise spent punching in waypoints on a cockpit keypad. The RAAF’s experience with the test aircraft indicates much shorter pretakeoff times on the taxiways as a result.
Stores Management System
The Stores Management System (SMS) is unique to the F/RF-111C upgrade and employs hardware used on the F/A-18 SMS Update programme. The SMS is a major technological improvement over the Pacer Strike system, and the RAAF AUP has seen the first ever software programmable digital SMS fitted to the F-111. Built around a Smiths Stores Management Processor, the scheme uses Smiths digital encoder/decoders to allow the stores management system to individually control and release munitions from specified wing stations. The decoders are tied to a third dual redundant Mil-Std-1553B bus, which allows the mission computer software to control, via the Stores Management System, each pylon. The AUP SMS internal processor runs internal system software specific to the F/RF-111C AUP. This is written in C language and Assembly Code, and at about 40,000 lines of source code is almost as complex as the mission software. The RAAF’s decision to use a unique stores control subsystem must be seen in the context of it providing the means to support weapons which are not standard to the USAF F-111 fleet, such as the Harpoon. This coupled with the ability to produce custom mission software and SMS software to support the weapons, will provide the RAAF with substantial flexibility in the integration of new weapon types over the life cycle of the aircraft. The potent radar guided sea-skimming AGM-84A-1C Harpoon anti-shipping missile, which in most airborne installations employs a dedicated HACLC command and launch computer communicating via the US Navy Mk.82 interface, will also be integrated into the new weapon system via the Stores Management System and Mission Computer software, considerably simplifying maintenance. The Harpoon requires unique serial and discrete signals for prelaunch seeker initialisation, as well as seeker and heater power feeds which are not standard to USAF aircraft pylons. Only a handful of USAF B-52Gs were ever fitted for Harpoon. The Harpoon has three range known launch modes, as well as a bearing only launch mode, and is a very flexible and effective weapon in the maritime role. An important aspect of the AUP programme is the Weapon System Support Facility (WSSF), which will be based at Amberley. This will be a comprehensive systems engineering, integration, software development and testbed environment which will allow the RAAF to maintain the software used in the new avionic suite. Development systems for the MC OFP (above), SMS OFP, ACU OFP, ICNIS OFP and PDG OFP will enable programmers to modify all software items which may require changes. This will allow the RAAF to introduce additional new capabilities with changes in mission requirements, as well as integrate new munitions throughout the life cycle of the system. The core of the WSSF is a mockup cockpit and forward equipment bay, which is driven by a one million line of software simulation system. This allows the RAAF to carry out comprehensive testing and debugging of new OFP code, before it is loaded into a “live” computer on a real aircraft. Part II will complete AA’s technical review of the F/RF-111C AUP Programme and will appear in our September issue. Readers interested in more information about the operational aspects of the F-111 offensive avionic suite and mission profiles flown are directed to the June 1984 issue AA (pp 29-387), which details the Mk.I analogue suite and Pave Tack in considerably greater detail. While back issues are unfortunately no longer available, AA can provide photocopies for a nominal charge.
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