Often wars have been remembered for advances in military science that shaped the face of battle, never more so than in the 20th century, that was marked by an exponential rate of technological development. As World War II introduced the jet engine and the atomic bomb, so the 1991 Gulf War saw the first widespread use of precision weapons, mainly utilising laser designation.

As with most technological breakthroughs, limitations are ignored until a later conflict reveals them and counter-measures erode military advantage. So it is with the global positioning system (GPS), a constellation of some 24 satellites that provides navigational accuracy for civilian as well as military applications.

In spite of the spectacular all-weather results of GPS weapons during the Kosovo campaign in 1999, the technology is old. The satellite constellation was designed in the late 1970s, primarily for military use. Following a Korean Airlines' crash in 1983 because of lack of accurate navigational information, the GPS network was authorised to be used for civilian purposes.

GPS now threatens the capabilities it was designed to support because of overlapping civilian and military demands. To appreciate the problem, the GPS signal characteristics must be understood. There are three pseudo-random noise (PRN) ranging codes offered by GPS, operating on two frequencies. The open-access service provides a course/ acquisition (C/A) code that is on only the link-1 (L-1) frequency at 1575.42 MHz. This is used by all civilians accessing the GPS system and also by armed forces to acquire the more accurate military code. The military service operates on L-1 and L-2 (1227.6 MHz) and is known as the precision (P)-code or, if the anti-spoofing (A-S) mode is in operation, the (Y) code (sometimes referred to as the P-Y Code).

The initial GPS cluster consisting of block I satellites was launched in 1978. Designed for a life of five years and constructed to validate the operating concept, many have exceeded their specification. All block I satellites are now out of service. Block II satellites, launched from 1989 onwards, were the first intended to provide full operational capability, with 24 functioning vehicles as well as a number of orbiting back-ups. This was achieved in April 1995, although a partial service had been available for some years. Block II has been deployed in three variants: block IIs, IIAs and IIRs. Since 1991 block II GPS satellites have provided non-authorised recipients (those without access to

Y-Code) with selective availability (SA), designed to degrade the C/A code sufficiently to deny accurate navigational information to potentially hostile users. The C/A code provides a predictable positioning accuracy of 100m (95 per cent) horizontally and 156m (95 per cent) vertically. The military codes (with or without A-S mode on) are accurate to at least 22m by 27.7m respectively.

Under pressure from the US and civilian organisations worldwide, in May 2000 President Clinton authorised the capability of the selective availability function to be switched off. C/A codes are now far more precise than was envisaged, giving a total spherical accuracy of 7m compared to 4m for the Y-code, creating further problems for military planners attempting to protect friendly access to GPS and hostile forces' exploitation of it.

The weakness of GPS

From a military perspective, the current situation has created two weaknesses. First, denying accurate information to hostile users because the same frequencies that would have to be jammed to degrade this capability are necessary for friendly forces to access the Y-Code. Secondly, the GPS signal is so weak it is very easy to jam. For airborne applications, it has been estimated that a 1kW jammer at 40,000ft over the Balkans could degrade the GPS signal across most of Europe. The system is line of sight, so small variations in location could determine whether a GPS receiver would work through a jamming environment if the disrupting power were ground based.

To tackle these problems, the US GPS modernisation and navigation warfare (NAVWAR) programme was established to improve communications security (COMSEC) and anti-spoofing techniques and reduce electronic warfare fratricide. The plan is to separate civilian and military bands while trying to introduce extra frequencies to cope with requirements from both users. It is intended to create a new Lc code, centred on the existing L-1 and L-2 codes, for use by civilians and the military in peacetime. Either side of these frequencies will lie dedicated military codes that will be directly accessible. This would allow the military to employ far higher levels of anti-spoofing measures to protect their friendly codes, but ignore civilian codes or jam them if hostile forces use them. To cope with enormous demand on the GPS network a new frequency will be established for civilian use at 1176.45MHz.

To speed up addition of the new civil code to L-2 and to frame L-1 and L-2 with the new military only code (designated M-code Earth), Lockheed- Martin will modify 12 of the remaining block IIR satellites in storage. Launches of these are expected between 2003 and 2006, with initial operational capability by 2008 and full capability by 2010. From 2005-2010, the more advanced block IIF will add the third civilian code, L-5, to provide more robust navigational information for civilian users.

For the next generation, GPS block III vehicles launched after 2009 will offer a military spot- beam capability (M-code spot) from the satellite to give increased signal strength on specific areas of Earth to counter jamming. Unfortunately, these changes can be introduced only as the satellite constellation is replaced. Because of longer-than-expected lifetimes of existing satellites this could take a decade. Even block III levels of M-code spot power will not remove the jamming threat because satellites are constrained by the power they produce in case they jam their own signals.

In parallel with enhancements to the satellite vehicles, efforts are underway to improve inertial navigation systems (INS) that work with GPS data. If INS systems can be upgraded to retain their GPS co-ordinates long enough for weapons to engage their targets or navigational systems to gracefully degrade until the next GPS fix, the jamming environment can be worked through. New, etched silicon boards offer this ability at reasonable cost.

Improved GPS receivers can create nulls in response to a jammer but it is difficult to avoid parasitic nulls being directed towards the satellite. So more flexible steerable receivers, possibly using software reprogrammable architecture, should be able to direct an enhanced gain towards a satellite, providing more protection against hostile jamming. The array-size problem still remains. These can be fitted to cruise missiles, but not to some smaller weapons.

Since its use during the Gulf crisis, GPS has been adopted by armed forces to assist in navigation, command and control and weapon targeting. Even hand- held receivers have had an effect on the tempo of operations, allowing commanders to manoeuvre their forces more accurately. With a new generation of vehicles, missiles and systems offering embedded GPS, reliance on the constellation is becoming absolute. Allied forces have already experienced attempts to jam GPS-guided munitions. Failure to protect the network could spell catastrophe for the conduct of operations, and so enormous efforts will be devoted towards expanding the military and civilian use of the network while improving anti-jam techniques.