Pave Tack And The Gbu-15 Greatly Expand Raaf Strike Capabilities

The Flir head itself contains several systems, most significantly the Texas Instruments AAO-9 Flir assembly (see TE March ’84). The AAO-9 operates in the 10-micron IR band, with dual fields of view of 7. 5 and 2 degrees, with further 2X magnification if selected. The Flir window then mounts a yaw-stabilised yoke, that in turn mounts a pitch-stabilised mirror. The mirror is used to stabilise both the Flir picture and the laser beams, that with an accuracy of around 1 /1000 degree, reference provided by gyros. The AVO-25 laser designator/ rangefinder mounts on either side of the fair, the transmitter with its 1. 06-micron solid state Nd:YAG optically pumped pulse mode laser and beam expanding/pulse modulating optics on the left, the receiver with its beam focusing optics and solid state avalanche diode optical detector on the right.

Laser range is classified; the range error in rangefinding has been described as 10 ft rms. The laser range is though, far less than the 10 nm+ detection range of the Flir, against large targets. Both the Flir and laser equipment see the outside world through a large IR window, with two glass viewing ports for the laser systems. The base section of the pod drives the turret with a motor Oust as the turret drives the head) and ring gear, both motors driven by power amplifiers mounted in the aft of the base section. Divided into three areas, the aftmost of which also contains the Flir electronics, power supply and auto tracker, the base section is quite densely packed. The central area houses the laser power supply, system power supply, pod control electronics (both operating mode control and stabilisation) and last but not least the video recorder which turns Pave Tack into both a damage assessment camera and all-weather photo recce tool.

The forward area of the pod then contains the environmental control systems and the pod’s digital computer. A computer is a 16-bit machine with a modest 32 kbyte of nonvolatile magnetic core memory, which is adequate for built-in-test tasks and the pod’s internal housekeeping functions. In RAAF F-111Cs, which are devoid of digital computers, this machine will carry launch software for both the GBU-15 and particularly the AGM-84A Harpoon (most likely block ID) and will carry out the necessary launch sequencing tasks.

The pod’s systems are very modular, line replaceable units may thus be easily replaced at an intermediate-level maintenance shop, such as the unit currently being equipped at Amberley.

As is apparent, the pod is driven with digital signals-controlling pod aimpoint, operating mode, laser mode and laser coding, together with the analogue signals controlling the Flir. This has created problems for the RAAF, as the F-111C’s analogue nav/attack doesn’t have available digital reference signals. This has forced custom design a system of interface circuits, Pave Tack attack profile. Entering the target area, the Pave Tack pod is extended and cued onto a known geographical feature. The laser rangefinder is fired and the computer uses the precise position data to update the INS. The pod is again cued, now onto the target as estimated by the nav-attack system. The navigator will then take a close look at the target, to select the best impact point for his laser-guided munitions. This could be a bridge support, a powerplant generator housing, an airbase runway or the command bunker of a SAM site, The Pave Tack crosshairs are placed on the impact point and the navigator initiates tracking, manually or with auto track providing the necessary analogue/digital, digital/analogue conversion and clocking functions to tie the pod into the AJQ-20.

The Pave Tack pod is mounted in a roll cradle, which in turn fits into the weapons bay, virtually filling. The cradle rotates by 180 degrees to either stow the pod inside or extend it for operation, which takes about 5 seconds, and can be carried out at speeds up to Mach 1. 4. Extended, the pod’s drag cuts off 2 nm of range per minute of extension; retracted its drag is negligible.

The RAAF is acquiring 10 Pave Tack pods, a sensible number in view of the 12 aircraft’s strong nominal strike force and high reliability of the pods. One would anticipate all aircraft but the RF-111Cs being fitted with the cradles and interface systems, allowing the selective fitting of pods to specific aircraft flying specific missions.

Pave Tack has been in operational use with the USAF’s 3rd Air Force since mid-1982, carried by the F-111Fs of the 48th Tactical Fighter Wing based at Lakenheath in the UK. This significantly increased the lethality of the aircraft in the high-threat central European theatre; it is interesting to note that the USAF equipped all of its European F-models with cradles, maintaining though, a 2. 5 to 1 aircraft-to-pod ratio, suggesting the use of the pods only on particular missions.

Pave Tack will prove to be a valuable tool, extending the effective lifetime of the F-111C’s analogue nav/attack until a suitable digital upgrade is carried out, aside from its inherent use as a reconnaissance system.

Rockwell GBU-15 Cruciform Wing Weapon

The GBU-15 modular glidebomb came into being as the result of the USAF’s pressing need for a heavy-weight standoff weapon. Though the Paveway laser-guided bomb was effective it did require line of sight to the target, similarly, the HOBOS TV guided bomb had range limitations. Neither weapon was suitable for low-level standoff release. The GBU-15 was the solution to the problem, conceived as a modular weapon allowing the user the choice of seeker or warhead (rather bomb body).

The weapon operates in two distinct modes, direct and indirect attack. In a direct attack the operator on board the launch aircraft views the scene observed by the camera in the bomb’s nose on a head-down display, he then steers a set of crosshairs onto the desired target and engages a contrast lock. The bomb is then released and glides into the target.

The weapon’s primary mode of operation is however, indirect attack. To allow this, the bomb is fitted with a data link receiver and transmitter, likewise, the launch aircraft carries a receiver and transmitter in a pod. The transmission links use highly jam-resistant transmission techniques, allowing the bomb to transmit a TV picture to the aircraft and the aircraft a series of commands to the bomb. In executing an indirect attack, the aircraft lofts the bomb, preferably from behind a natural obstacle. GBU-15 will then climb and glide along a programmed flight path, its autopilot receiving midcourse updates via the datalink, until it reaches the target area. The operator may then steer the crosshairs onto a target or part of a target, guiding the bomb in that direction. The weapon may be locked on and use its contrast lock for the terminal phase, but the operator may also steer the bomb manually throughout the attack. Alternatively he may choose to change the exact impact point once the contrast lock is engaged.

As is very apparent the weapon is exceptionally flexible in both delivery and configuration. The latter aspect is more evident upon closer examination. Physically the GBU-15 is divided into six modular assemblies. Starting from the nose, we find the TV guidance module, containing optics, a stabilised platform mounting a vidicon and all the electronics for processing the video, together with the contrast lock. This unit is interchangeable with an infrared (IR) unit which allows day/night/adverse weather attacks. The day only TV unit is in production, the IR unit has been successfully demonstrated.

Either unit is then followed by the guidance adapter, in principle a cowl providing a smooth transition to the bomb body, fitted with a set of four strakes. The bomb body then follows, that being either a Mk-84 2000 lb demolition bomb or an S UU-54 submunition dispenser, the latter version of the weapon being in late development. The rear section of the bomb then carries the control unit, that itself mounting the wings, control surfaces and the final assembly, the datalink unit. The control unit houses the autopilot, the gyros, batteries for power and the control actuators. The datalink unit then contains the receiver and transmitter electronics, facing aft.

The GBU-15 thus allows the user the choice of both seeker and payload, tailored for a specific mission. Consider counter-air, night attack. The IR seeker combined with the Mk-84 would be used to attack the airfield, alternately the SUU-54 could be used with either cluster bomblets against parked aircraft or mines and penetrators against runways and taxiways.

The basic TV weapon with the Mk-84, as acquired by the AMF, Is usually employed against hard point targets, be it bridges, command posts, bunkers, industrial complexes, or even shipping. The SUU-54 version is even more useful, ideally suited not only to counter air but also to anti-armour, anti-vehicular, anti-personnel and defence suppression missions. In terms of performance, the GBU-15 is cleared for supersonic launch and has been extensively tested in a hostile ECM environment, either aspect emphasising its inherent compatibility with the F-111C and its operating environment.

The GBU-15 appears at this stage destined for a colourful future, particularly in view of the current development of an extended range version, with a ventrally mounted rocket sustainer which is jettisoned after burnout. The increase in range would thus be subject only to the burn time of the sustainer and the limits of the datalink range. Whether this version eventually enters production, or is abandoned along the way, the basic concept of the weapon will remain – modularity. That will ultimately ensure its survival.