GPS proves jam resistant: type the phrase "GPS jammer" into any Internet search engine, and you'll find dozens of sites assuring you that anyone with a modest degree of hobbyist electronic construction skills can build a jammer able to disrupt the Global Positioning System, an essential navigation aid in modern combat. But as the Iraqi armed forces learned in March 2003, GPS isn't so easy to knock out. (Navigation).

As the prospect of war against Iraq increased in late 2002 and early 2003, concern was voiced that in the event of hostilities, Iraq would jam the Global Positioning System (GPS), blunting the effectiveness of the huge US investment in GPS receivers and GPS-guided bombs and missiles.

Before the war began, the Iraqis were warned by the US Department of Defense of the futility of attempting to use GPS jamming. Anyone attempting the jam GPS could find himself singled out for attack. "Anyone who transmits on the battlefield can be found, and anyone who can be found can be targeted," said Lt Col John Carter, chief of space requirements at the US Department of Defense. "When the bad guys are picking jobs, [they] don't want to pick 'GPS jammer'." It was good advice, but Iraq chose to ignore it.

"We have noticed some attempts by the Iraqis to use a GPS-jamming system that they have procured from another nation", said Major General Gene Renuart, director of operations at US Central Command, during a press briefing on March 25. "We've been able to identify the location of each of those jammers, and I'm happy to report that we have destroyed all six of those jammers in the last two nights' air strikes ... they had no effect on us. In fact, we destroyed one of the GPS jammers with a GPS weapon."

The designers of GPS had always assumed that an adversary might one day attempt to jam the system. In 1996 the GPS Joint Program Office established the Navigation Warfare (Navwar) programme. This was intended to protect Department of Defense and allied use of GPS during times of conflict, while preventing its use by adversaries, and maintaining normal availability to civil users outside the area of conflict.

"From the day we built GPS, we've been working on ways to overcome jamming," said Lt. Col. John Carter during a pre-war interview. "We're very confident we can do that."

The US Air Force has begun a programme to make future GPS satellites more jam-resistant. "We recognise the fact that GPS can be jammed," says Peter B Teets, undersecretary of the Air Force and chief of the US National Reconnaissance Office. "We're taking steps to make it much more jam-resistant on the satellite side, on the control-element side and on the user-equipment side."

This effort started with eight unlaunched GPS IIR spacecraft that Lockheed Martin is modifying to the improved IIR-M standard. "One of the features of the modification is something called flexible power, which increases the power level radiated from the GPS", says Teets. "While this power increase will provide some anti-jam capability, a major improvement will come when the GPS III is introduced in about ten years time."

Anti-jamming measures are also being incorporated into GPS receivers. The main near-term solution involved fielding anti-jam antenna systems on weapons platforms. More than 1000 examples of the GPS Antenna System-1 (Gas-1) have been produced by the UK-based Raytheon Systems Limited. This is an analogue system whose anti-jam capability is provided by the formation of spatial nulls in the direction of interference.

As a longer-term solution, the Department of Defense plans to field a fully digital GPS receiver with nulling/multi-beam steering. Until this can be fielded, a significant near-term improvement is promised by the Digital Antenna Electronics (DAE) programme, which will incorporate digital signal processing to enhance jammer suppression by providing a limited beam-steering capability to assist in filtering out jammer noise, while preserving the current Gas-1 AE in form, fit and its interfaces.

In October 2002, Raytheon's Precision Guidance Systems organisation was awarded a $1.9 million contract option by the US Navy's Space & Naval Warfare Systems Center in order to fund the development, production and testing of prototype DAE hardware that would be compatible with standard aircraft-mounted anti-jam antenna systems. A second award made to Raytheon Systems Limited covered continued development of an alternative design.

The GPS Selective Availability Anti-Spoofing Module (Saasm) is a mandatory feature of all next-generation US GPS receivers that use the Precise Positioning Service, which is also to be used by US allies. The Saasm will be operated in stand-alone GPS receivers and in GPS sub-systems embedded into other products and weapons.

For example, future tactical radio systems will use embedded GPS to allow automatic position reporting, so Saasm technology will be needed to maximise jam resistance. This trend is illustrated by the awarding of a contract to Rockwell Collins in October 2002 for the supply of between 6000 and 34,086 security devices to General Dynamics for use in the British Bowman tactical radio programme.

In the field of ground-based standalone GPS receivers, one of the most significant (and hotly contested) programmes is that to develop the Defense Advanced Global Positioning System Receiver (Dagr), a unit intended to replace the widely used Precision Lightweight GPS Receiver (PLGR). The Dagr will provide authorised Department of Defense, US federal civilian and Foreign Military Sales users with a hand-held dual-frequency Y-code receiver that employs next-generation tamper-resistant security module technology.

Rockwell Collins and Raytheon have recently been awarded contracts worth around $ five million to continue their work on the Dagr. Final selection of the Dagr production contractor is expected during the fourth quarter of 2003, after completion of first-article testing.

Technology continues to shrink both the size and the weight of GPS hardware, but for the infantryman, that shrinkage can't come fast enough. Some US Army soldiers openly carry privately purchased, commercial-grade GPS receivers when on operations.

"When we build our military GPS receivers, we build them to counter threats, and in the process of doing that, the size increases slightly in order to accommodate some of the issues associated with threats", says Col Steven F Fox, director, US Army Space Support. A soldier trying to minimise the weight of equipment he must carry may therefore opt to take his personal GPS receiver rather than the heavier military-standard unit issued to the squad or platoon.

"It's their money; they can buy what they want", says Fox. "But ultimately, if a [EW] threat does occur, they can always rely upon the military-procured system, which is designed to counter the threats."

The small size and decreasing cost of GPS receivers allow them to be used in throwaway form to guide missiles and other munitions. They can even be incorporated into existing weapons in which little free internal space was available. Jam-resistant GPS subsystems are now used on a range of American missiles including the Tomahawk, the Joint Stand-Off Weapon (Jsow), the Paveway, the Standard Missile and the EGBU-15.

Existing weapons likely to acquire GPS guidance include the Lockheed Martin INS-guided Wind Corrected Munitions Dispenser (WCMD) and the IR-guided Raytheon AGM-65F Maverick. The WCMD was always seen as a potential GPS/INS weapon, but the addition of GPS to the AGM-65F is a move never foreseen when that variant entered service in 1989. It would allow the weapon to conduct through-cloud attacks, as it would rely on target acquisition after launch.

In our 2002 feature on artillery fuzes, we reported on the use of GPS to provide course correction for artillery shells by deploying a drag-inducing mechanism to correct trajectory errors in range. Similar techniques can be used to correct the flight of artillery rockets. The Guided MLRS (GMLRS) rocket being developed by an international consortium for service later in the decade will carry a combined GPS/INS guidance system and course-correcting canards, while a similar scheme is being studied by Diehl Munitionssysteme for an extended-range MLRS round able to carry a payload of 'smart' submunitions.

GPS can also be used as a mid-course guidance system for anti-ship missiles. In the mid-1990s, China tested missiles that used GPS receivers in place of traditional INS systems, while MBDA is adding GPS navigation to its long-established Exocet. The MM40 Block 2 Mod 2 version will have a new digital guidance computer and a GPS receiver, so will be able to fly via GPS-specified waypoints. In one of its final reports to the UN before the invasion of Iraq, Unmovic revealed that Iraq had been working on GPS-guided versions of the HY-2 and AM 39 Exocet.

Starved of funding, Russian's Glonass satnav system continues to deteriorate. By the end of last year only seven spacecraft were operational--four in plane I, none in plane II, and three in plane III. The oldest satellites were two launched on 30 December 1998, while the youngest were two launched on 1 December 2001.

If the Glonass constellation remains viable, the system could be used to guide 'smart' bombs. Russia's Bazalt State Research and Production Enterprise is developing an add-on guidance kit based on an inertial navigational system plus a GPS/Glonass receiver. The MPK (Modul Planirovaniya i Korrekciyi) modular add-on kit installs guidance and navigational electronics, foldout wings and a single movable control surface to existing 'dumb' bombs such as the widely used Fab-500 M62. Flight tests have begun, Bazalt announced, in the summer of 2002.

Just as with ground-based hardware, the Saasm is seen as a way of improving jam-resistance. In the autumn of 2002, flights tests began of Lockheed Martin AGM-158 Joint Air-to-Surface Standoff Missiles (Jassm) fitted with Saasm technology. The missile carried a new anti-jam receiver, which uses beam steering to direct its antenna reception pattern towards the GPS satellites as well as adaptive nulling techniques designed to ignore unwanted signals arriving from multiple directions.

Boeing is working on an upgrade to the Jdam (Joint Direct Attack Munition) GPS/INS guided bomb which would install an improved GPS directional antenna that is also compliant with the Saasm. If adopted for service, it could be added to new-production rounds, but a retrofit to existing weapons is another possibility.

Guided artillery shells can also benefit from Saasm. In October 2002, Interstate Electronics Corporation (IEC) announced that its Saasm-based TruTrak GPS receiver had successfully acquired and tracked nine satellites that were in view during a test of the US Navy's Extended Range Guided Munition (ERGM). The receiver provided positional data to the projectile's onboard guidance computer, guiding the round to impact at 39 nm downrange after launch from a five-inch/62-calibre gun.

Europe's plan to launch its own Galileo navigation satellite constellation is viewed with puzzlement in the US, where observers cannot see a logical reason why Europe should duplicate the massive US investment in GPS. Europe argues that satellite navigation has become so important that its provision cannot be entrusted to a single system or a single nation.

Galileo launches are expected to begin in late 2004 with the orbiting of the first experimental satellite, which will form part of the Galileo System Test Bed (GSTB). Intended to prove the critical technologies needed by the final system, it will be followed around 2005/6 by up to four operational satellites which will be used to validate the basic Galileo space and related ground segment. Once this In-Orbit Validation phase has been completed, the remaining satellites will be installed to reach full operational capability in 2008.

The full constellation will consist of 27 operational satellites and three spares, positioned in three circular medium earth orbit planes at 23,616 km altitude above the Earth, and at an orbital inclination of 56 degrees. Galileo navigation signals will provide a good coverage even at latitudes up to 75 degrees north and beyond.

Galileo will be inter-operable with GPS and Glonass, so that a user will be able to use current models of receiver to take a navigation fix from any combination of American, Russian and European satellites.

In urban areas, buildings obscure signals from satellites low on the horizon, reducing the number of spacecraft that receivers can use when determining position. When the Galileo constellation is fully deployed, the number of satellites visible under such circumstances will more than double.

Some of the end results of the improvements being introduced into the world of satellite navigation will be, firstly, greater reliability, and in the case of most guided munitions, greater accuracy. During developmental tests, the Jdam demonstrated a circular error probable (Cep) of 4.9 metres, but early tests following the introduction of a twelve-channel GPS receiver resulted in a Cep of only three metres.

Such accuracy figures could revolutionise air-to-ground targeting. Briatin originally considered a 454 kg (1000 lb) weapon to meet its Precision Guided Bomb (PGB) requirement, but is now convinced that a 227 kg (500 lb) weapon will have the required lethality. In the past, Britain has always used its own patterns of bomb, but will adopt the US 227 kg Mk 82 as the basis of its new PGB. It is not expected to buy any unguided Mk 82 bombs or other members of the US 'Mk' series, but will rapidly become an 'all-PGM' force. If other air forces follow Britain's lead, the days of the dumb bomb may be numbered.