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GPS Part II - GPS and DGPS Munition Guidance

by Carlo Kopp

published in Australian Aviation, September, 1996

(c) 1996 Aerospace Publications, Pty Ltd, (c) 1996 Carlo Kopp

The availability of GPS and highly accurate Differential GPS navigational aids has created a revolution in aircraft navigation. What is less commonly known is that GPS and DGPS are about to transform what we understand to be the nature of precision bombing. Indeed, the introduction of GPS and DGPS guided munitions will have an impact not unlike the introduction of laser guided bombs, with the resulting force multiplication effects significantly improving the potency of Western air forces as a strategic power projection tool.

GPS Guided Munitions

The central idea behind the design of DGPS/GPS/inertial guided weapons is that of using a 3-axis gyro/accelerometer package as an inertial reference for the weapon's autopilot, and correcting the accumulated drift error in the inertial package by using GPS PPS/P-code. Such weapons are designated as "accurate" munitions as they will offer CEPs (Circular Error Probable) of the order of the accuracy of GPS P-code signals, typically about 40 ft. The next incremental step is then to update the weapon before launch with a DGPS derived position estimate which will allow it to correct its GPS error as it flies to the target, such weapons are designated "precise" and will offer accuracies similar to laser or TV guided weapons, potentially CEPs of several feet. Because the GPS package is highly accurate a cheap inertial package may be used, while the GPS package is inherently cheap to manufacture as it uses wholly electronic hardware which can be built by automated production equipment (robots) used commercially. Providing that the servo mechanisms and weapon airframe are designed for cheap mass production, the DGPS/GPS/inertial guided bomb can be built as cheaply as standard laser guided munitions. Only should an opponent capable of jamming GPS signals be encountered, will more expensive inertial packages and ECCM equipped receivers be required (NB: even so a good military receiver can cost as little as US$5k/unit in volume).

For an aircraft to support such munitions, it will require a DGPS receiver, a GPS receiver and interfaces on its multiple ejector racks or pylons to download target and launch point coordinates to the weapons.

The development of purely GPS/inertial guided munitions will produce substantial changes in how air warfare is conducted. A GPS/inertial guided weapon which is updated with DGPS corrected position will, if properly designed, offer accuracy only slightly lesser than a proportionally guided laser guided weapon or TV guided weapon. Unlike a laser guided weapon, a GPS/inertial weapon does not require that the launch aircraft remain in the vicinity of the target to illuminate it for guidance - GPS/inertial weapons are true fire-and-forget weapons which once released, are wholly autonomous and all weather capable with no degradation in accuracy. Existing precision weapons require an unobscured line of sight between the weapon and the target for the optical guidance to work. GPS/inertial weapons are oblivious to the effects of weather, allowing a target to be engaged at the time of the attacker's choosing.


The impending deployment of GPS guided bombs and glidebombs will revolutionise air warfare as we know it. Affordable, all weather attack on multiple targets by single aircraft will become the norm. This diagram depicts the relative accuracies of established laser and imaging optical weapons, against the published performance figures for the first generation of GPS and DGPS guided weapons (Author).


From a tactical perspective, this removes many of the traditional constraints which forced delivering aircraft to have to penetrate through defences to guide a weapon - standoff launches become virtually the standard for this family of weapons. What is even more significant, is that the traditional constraint of laser guidance, illumination for each target/bomb, no longer exists. An aircraft can program its whole load of weapons each for individual targets, release these from standoff range almost concurrently, and then immediately egress the target area. As well, weather over the target is no longer an operational constraint.

The ability to concurrently attack multiple targets with a single aircraft from standoff ranges is an unprecedented force multiplier for air power as an offensive tool. Providing that an aircraft can penetrate to launch range, it can then saturate target defences with its whole payload of weapons, which if not engaged by point air defences, will destroy their programmed targets autonomously.

Consider the scenario of an F-111 attacking an air defence radar site with laser guided bombs. To approach undetected it will have to penetrate at low altitude, and then toss the one or two bombs delivered at the target and illuminate to bomb impact. This profile exposes the aircraft to "trash fire", ie AAA, small arms and shoulder launched SAMs while at low level in the vicinity of the target, as well as being fuel inefficient and imposing fatigue load upon the airframe. While a soft target like an air defence radar could be easily destroyed by a 500 lb weapon, maximising the probability of kill would dictate the use of a pair of 2,000 lb weapons to ensure that either bomb guidance errors or operator tracking errors do not compromise the kill - reattacking a target is much more expensive than using a pair of bigger bombs.


This diagram depicts the envelopes of some commonly used air defence weapons, against the envelopes of established guided munitions and the new GPS guided munitions. Significantly, the combination of GPS and glidebomb technology defeats most air defence weapons completely, while offering this capability in weapons which may cost $100k or less. This diagram includes recently published figures for subtypes of the SA-10 Grumble missile (Author).


Consider now the same target being attacked with DGPS/GPS/inertial guided bombs. The F-111 could fly an indirect approach to the target at medium altitude and much higher TAS, and when approaching the limits of area defence SAM coverage, rapidly accelerate to supersonic speed, quickly change heading toward the target, and toss its payload of eight 500 lb GPS/inertial guided bombs for maximum standoff range. Several of these bombs could be programmed to hit the primary target, but others could be programmed to hit the command vans, point defence SAM systems and AAA batteries surrounding the radar van. In this tactical scenario, the single aircraft has inflicted the same damage as a multiple aircraft strike, whilst also having minimised exposure to all threats other than fighters.

For an unpowered weapon, standoff range is maximised by launch altitude and airspeed, and weapon glide performance. An example profile is the 48th TFW's GBU-15 attack on the heavily defended Al Ahmadi oil pumping stations during the Gulf War. The F-111F aircraft approached the coastal target at supersonic speed at 20,000 ft and launched the weapons from about 20 NMI distance, they then immediately turned away and guided the GBU-15s to impact through datalink commands. Were the aircraft delivering GPS/inertial guided weapons, they could have immediately left the target area at supersonic speed to frustrate any potential interceptor threat. Whereas the Al Ahmadi strike required two aircraft to take turns at hitting two targets, requiring loiter in the vicinity of the target, and this inviting a fighter attack with any competent opponent, the use of GPS/inertial guided glidebombs would have simplified the sortie quite significantly as a single aircraft could engage both targets with a single pass.

The ideal weapon for this style of air attack is a highly aerodynamically efficient glide-bomb which allows a supersonic toss delivery - this allows the launch aircraft to impart the maximum amount of energy to the weapon during launch and thus maximise standoff range. Ranges of 40-70 NMI become quite feasible, and this will defeat most area defence SAM systems. As the aircraft is supersonic at high altitude, it will be a difficult target for an interceptor, moreso since it will not need to loiter. Because the weapon is unpowered, it is substantially cheaper to buy and to maintain than a powered weapon, and expensive datalink pods or laser targeting equipment are no longer required. Where the aircraft is stealthy, the defence's warning time may be non existent. In any event the task of engaging a 900 kt target at 30,000 ft will be extremely difficult for a Combat Air Patrol, and even more difficult for a ground launched interceptor.


The GPS guided glidebomb allows the single bomber to reclaim the upper portion of the penetration envelope. As these weapons can be released from above 30,000 ft at transonic speeds, and glide for up to 75 NMI, they allow a bomber to engage its target from ranges where SAMs are wholly ineffective, and fighter CAPs are hard pressed to perform without AEW and tanker support. This provides a significant advantage to the attacker, who can saturate defences with multiple weapons (Author).


The model postulated here assumes the air defence system is functional, however should it become subjected to intense radar and communications jamming and direct attack, and should fighters be available to threaten the defending interceptors, this profile becomes both highly survivable and very dollar efficient, particularly in a low air defence density environment such as the Asia-Pacific. In any event, this approach defeats all AAA and point defence SAM systems, which are a plague during low level operations.

Another factor which falls out of this paradigm of air attack is that electronic combat operations can defeat the air defence system by taking out only the strategic early warning, strategic SAM acquisition and Ground Control Intercept radars, the economically costly process of lobbing Anti-Radiation Missiles (ARM) against every SAM and AAA system fire control and acquisition radar becomes largely redundant. Once the long range early warning, strategic and interceptor control radars are down, the air defence system is in dire straits. The need to saturate the lower tiers of the system with suppressive ARM fire, cluster bombs and jamming is no longer required in order to close to weapon release distance.

The availability of a GPS/inertial guided cluster munition, and a highly accurate rangefinding radar warning receiver (ie 0.1 degrees DF accuracy/0.2% range accuracy) would allow the suppression of most area defence SAM radars without having to expend expensive Anti-Radiation Missiles, only the most capable and expensive systems such as the S-300 (SA-10/12) would require ARMs for suppression. With a powered munition providing 50 NMI of range, even systems such as the S-300 would become ineffective. Because such SAM systems are expensive, large numbers will not be deployed, and not every operator will be competent to use the weapon to its fullest.

The GPS/inertial guided weapon is thus a potent force multiplier in strategic air warfare, as it allows single attacking aircraft to engage multiple targets simultaneously, day or night, under all weather conditions, from standoff ranges. This automatically defeats all target point defence systems, and most area defence weapons. This means that an attacking air force only has to deal with strategic air defence weapons and fighters, which can then be dealt with more easily as a larger proportion of resources will be available to defeat them. Because a single aircraft may engage multiple targets on a single sortie, the GPS/inertial guided weapon is a force multiplier on the scale of the laser guided bomb, when first introduced. Whereas before the LGB, it was a case of many aircraft/bombs for one target, the GPS/inertial weapon swings this equation around, with one aircraft for many bombs/targets.

In terms of performance parameters for strike capable aircraft, high payload radius and aerodynamic performance becomes a major asset as it allows the best possible exploitation of the capabilities inherent in the GPS/inertial guided munition. As these weapons make high and medium altitude attack more attractive even in the opening phases of an air campaign, the importance of electronic combat capabilities oriented against strategic early warning, GCI and SAM acquisition radar, communications, command and control is increased relative to the importance of defending against tactical and point defence SAMs and AAA. Defensive ECM will need to be reoriented against fighter radars, air-air missile seekers, strategic SAMs and early warning radars first and foremost. Opponents unable to field the top tier of air defence weapons will be highly vulnerable to air attack by GPS/inertial guided standoff weapons.

In the Australian context, the deployment of cheap GPS/inertial guided weapons will increase the value of the F-111 significantly, as it is the aircraft which can best exploit the capabilities of this family of weapons. The use of such weapons would strongly reinforce the case for upgrading the aircraft with a current generation powerplant, as this would allow supercruise operations which fit this paradigm so nicely. It would also reinforce the case for boom equipped tankers, as this would allow the aircraft to carry a substantial payload of such weapons at radii of thousands of nautical miles (NMI). Importantly, the focus of the upcoming EW upgrade for the F-111 should take this shift in operational paradigm into account.

Targeting GPS Guided Weapons

The deployment of DGPS/GPS/inertial weapons will create some interesting problems in the area of targeting. Whereas existing laser and TV guided weapons have an operator in the loop to refine the aimpoint and minimise collateral damage, generic GPS guided weapons are wholly autonomous and their accuracy is determined primarily by the accuracy of the target coordinates loaded before launch. Once released, they are committed and no corrections are possible. Only should the weapons be equipped with a datalink receiver, capable of feeding target position updates into the autopilot during flight, are aimpoint corrections or attacks on moving targets feasible. It is worth noting that a one way datalink of this variety is a technically much simpler proposition than the wideband video datalinks used by TV guided weapons, and hence such a datalink receiver will be much cheaper to build.

The use of any GPS/inertial guided weapons will place a premium on the quality of targeting information. Whereas contemporary satellite, aerial and radar reconnaissance can tolerate some inaccuracy as the delivering aircraft can visually acquire the target and correct the aimpoint if required, generic GPS guided weapons must be targeted accurately from the outset. If the reconnaissance picture used for target selection is poorly registered against the maps used, or the maps are inaccurate, this error could not only compromise the attack on the target, but also produce politically problematic collateral damage. A commander who unloads 8,000 lb of GPS guided bombs on infrastructure targets, only to find that a 0.5 mile error in his maps has placed the payload on a baby milk factory or religious or cultural artifact, is likely to be politically crucified if not by his own chain of command, then certainly by the lay media whose appetite for controversial death and destruction footage is insatiable.

The technological means of solving this problem exists, but is yet to be widely deployed. It is the high resolution imaging synthetic aperture radar. Such radars have resolutions of about 1 metre, and if tightly calibrated should be capable of locating a target with an accuracy of feet at standoff ranges of tens of nautical miles. As a result, such radar could be used by inbound bombers to confirm the aimpoints perviously programmed into the nav-attack system, before weapon release. Furthermore such radar, if supported by Ground Moving Target Indicator (G-MTI) modes, can locate surface targets of opportunity such as vehicle and armour convoys for subsequent attack. The GATS (GPS Aided Targeting System) on the USAF B-2A is a good example of such a system.


The Texas Instruments AGM-154 JSOW is a USN/USMC/USAF program to provide a 1,000 lb class GPS guided glidebomb. This USN F/A-18 is carrying four such weapons during trials. Upon release these glidebombs deploy their wings and glide to impact over ranges in excess of 40 nautical miles. The JSOW is reviewed in detail in Part 3 of this feature (Texas Instruments).


Extending the Paradigm

The availability of cheap DGPS/GPS autopilots raises other interesting possibilities. One of these is the "Robot Kamikaze", where retired fighters can be fitted with such autopilots and used as heavyweight cruise missiles or decoys for air defences. As the aircraft are flying a one way trip, their useful range is effectively doubled. A retired 350 NMI mile radius tactical fighter becomes, with one or two 2,000 lb bombs attached, a 700 NMI cruise missile. As the aircraft will have been paid off, the starting cost is zero. Stripping all non-essential equipment items will reduce takeoff weight and improve effective range. The only cost incurred are the GPS autopilot, its interfaces to the flight control system, and installation and testing costs. Ongoing maintenance costs are minimal as the airframe is not flown until needed, and by retaining manual flight controls and minimal instrumentation, the weapon can be ferried to its deployment base.

In use, the autopilot could be programmed to route the aircraft around known defences, and then expend remaining fuel in a supersonic afterburning dive against the target. Large targets such as industrial sites and petro-chemical plants would be ideal targets for such weapons, which will add significant incendiary effect to the explosive effect of the payload. Used as decoys in the opening phase of an air campaign, they will draw the fire of the air defence system thus forcing the expenditure of ready rounds on launchers, as well as forcing air defence radars to light up and thus expose themselves to SEAD aircraft, positioned in anticipation of this.

Vulnerabilities of GPS/DGPS Guided Weapons - ECM and ECCM

Whilst the deployment of DGPS/GPS/inertial guided weapons promises an almost order of magnitude increase in the destructive potential of a suitably equipped air force, it also creates problems to be dealt with. The first is the potential for an opponent with suitable technological skills to jam the satellite signal, thereby degrading weapon accuracy and removing much of what is gained by using the technology. The second problem is that competent opponents may use the same GPS signal to guide their own weapons, thereby acquiring a capability they may not otherwise have.

As discussed earlier, there are some reasonably potent electronic counter-counter measures (ECCM/EPM) techniques which can be used to defend against jamming, however it is important that this be accounted for when designing GPS based weapon systems. Failure to do so could create a significant vulnerability. Jamming bombing navaids has a long history and the fate of the Luftwaffe during the Battle of the Beams (Blitz) should be a good reminder of the operational consequences of taking a simpleminded approach to the issue.

Hostile Exploitation of GPS Weapon Guidance

The more worrisome problem is that of GPS exploitation. Even during the Gulf War it was reported that the Iraqis used commercial GPS equipment to assist in calibrating Scud launch sites. The real problem will come about when Third World countries start dusting off their fifties and sixties technology cruise missiles and fitting them with commercial DGPS/GPS autopilots.

Most of these weapons used combinations of inertial autopilot, radio command link and anti-ship radar homing guidance to attack either shipping or area land targets. In the latter instance, they were never taken seriously due their poor accuracy. With DGPS accuracies they become very effective standoff weapons.

There are some very good examples. The Russians exported large numbers of AS-5 Kelt missiles, as well as ship launched P-21/SS-N-2 Styx missiles. The Chinese reverse engineered the Styx into the air and surface launched HY-2 Silkworm, and its derivatives, the larger HY-4 and C-601. These weapons typically carry 1,000 to 2,000 lb warheads, to ranges between 50 and 100 nautical miles. What is important is that the PRC is still manufacturing the Silkworm family of missiles and these have been very widely exported throughout the Third World.

To compound the problem, the CIS (formerly USSR) still has significant stocks of former AV-MF and DA anti-ship cruise missiles which were intended for use against Western shipping convoys in the event of war. The most potent of these is the liquid rocket propelled supersonic 13,000lb 200 NMI range AS-4 Kitchen, which carries a 2,000 lb warhead and was deployed both on the Backfire and Bear G cruise missile carrier. Late Eighties estimates placed the stockpile at about 700 rounds. The AS-4 is supplemented by the AS-6 Kingfish, which is slightly smaller, uses solid rocket propulsion, and has very similar performance and warhead type. Estimates placed the Kingfish stockpile at 300 rounds. With a stockpile of 1,000 rounds and a hyper-inflationary economy, we can have no doubt that the Russians would be most accommodating should a government offer to exchange their collection of boneyard ASMs, or a portion thereof, for hard cash. Equipped with DGPS autopilots these become quite serious weapons which can be very hard to stop either by fighter or SAM.

Another concern which has arisen, perhaps overly publicised by Dale Brown's Technothriller "Storming Heaven", is the possibility of terrorists fitting GPS autopilots to GA airframes and using these to attack targets from within the airspace of Western countries. Whilst perhaps somewhat fanciful, this idea should also not be ignored. All is fair in love and war, and the possibility of a Third World government despatching an engineer with a covert penetration group to implement such a scheme is not beyond the realm of possibility.

As it appears, the only real defence the Western Alliance will have against hostile GPS exploitation will be that of jamming the GPS SPS C/A code, and equipping Western military aircraft with suitably jam resistant receivers. Civilian aircraft will have to get by with VOR and DME, whilst the L1 carrier is being jammed. The only other alternative would be to encrypt the civilian SPS signal, and distribute keys only to authorised users in wartime situations.

Summary

The proliferation of GPS and DGPS guidance is a double edged sword. On the one hand, this technology promises a revolution in air warfare not seen since the laser guided bomb, with single bombers being capable of doing the task of multiple aircraft packages. On the other hand, GPS and DGPS may be exploited by relatively unsophisticated industrial nations to provide them with a capability which until now has been the almost exclusive domain of the Western Alliance. The ease with which basic GPS signals can be jammed will result in another major cycle of ECM and ECCM development, as defenders and attackers build jammers and jamproof GPS receivers to counter jammers. To complete the analysis of this paradigm shift in air warfare, Parts 3 and 4 of this feature will review current US GPS and DGPS weapons development programs.


Acknowledgments:

Special thanks to Dr Don Kelly then of the USAF EDGE Program for his review of the draft of this article.

 


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