RAMON LOPEZ / WASHINGTON DC
With the reliability and security of GPS becoming an increasingly hot issue, the US Government is deciding what track it should take
The Bush Administration is at a crossroads over the vulnerability of global positioning system (GPS) satellite navigation to being jammed.
The importance of GPS to the military was outlined on 7 October when US and UK forces launched airstrikes on terrorist training camps and military targets in Afghanistan using the latest GPS-guided weapons, such as the Boeing Joint Direct Attack Munition and Raytheon Tomahawk Land Attack Missile.
In response, the US Department of Defense (DoD) is fielding systems to improve the jamming resistance of GPS-guided weapons - and in a joint venture with Australia, it has demonstrated that the susceptibility of GPS receivers to interference can be greatly reduced. Meanwhile, the US Defense Advanced Research Projects Agency (DARPA) is working towards a full-scale weapons demonstration of a device designed to combat attempts to jam the GPS signal over a battlefield.
The general availability of GPS is causing concern in the civil aviation community. The US Department of Transportation (DoT) recently published a study on the effect of GPS jamming on civil aviation and other modes of transport.
The landmark report mandated by a presidential decision directive and prepared by the DoT's Volpe National Transportation Systems Center concludes: "GPS is susceptible to disruption from such causes as atmospheric effects, signal blockage from buildings, and interference from communications equipment."
Furthermore, "the GPS signal is subject to degradation and loss through attacks by hostile interests. Potential attacks cover the range from jamming and spoofing of GPS signals to disruption of GPS ground stations and satellites", it says. The report contains recommendations to deal with possible disruption and to ensure the safety of transport systems.
DoT secretary Norman Mineta says the research provides "a roadmap for addressing possible vulnerabilities in GPS. The DoT takes this report's findings very seriously, and we will be working to ensure that GPS will fulfil its potential as a key element of the nation's transportation infrastructure."
He has directed the heads of the various DoT agencies, including the Federal Aviation Administration's Jane Garvey, to review the study and consider the adequacy for backup systems to GPS. They are due to report to Mineta early next year.
Next generation
The Navstar GPS 28-satellite constellation provides precise positioning, velocity and timing information to both civil and military users. The GPS spacecraft has evolved from the Block I satellite fielded in 1978 to the current generation Block IIR replenishment satellite. The next generation of satellites, Block IIF, is under development.
GPS's utility first became evident during the Gulf War in 1991, when the first satellite-guided combat missions were conducted. Military historians say GPS was the key to manoeuvring in the featureless desert terrain. Operation Allied Force against Yugoslavia in 1999 would later spotlight the capabilities of GPS-guided weapons in adverse weather conditions.
Civil aviation had envisioned GPS-based navigation and landing systems becoming the sole guidance system installed in aircraft and the only navigation service provided by the FAA. But concerns over sole-means use of GPS began to surface in 1998, and the FAA said it was likely the GPS-based Wide Area Augmentation System (WAAS) would need a ground-based backup system, such as the long-range Loran-C, inertial navigation systems, VHF omni-range/distance measuring equipment (VOR/DME) systems, or an anti-jam antenna developed by the US military.
The FAA acknowledged that low-power GPS and WAAS signals were susceptible to interference. It said: "The potential exists for an individual or organisation to jam GPS or WAAS signals and cause a loss of service over a large area." That same year, the DoT's Office of Inspector General said a backup for WAAS would be needed for "the foreseeable future" because of uncertainties related to GPS signal interference.
But a year later, an independent risk assessment by the Johns Hopkins University Applied Physics Laboratory concluded that, with some improvements and augmentations, GPS could serve as the sole basis for radio navigation by 2010. It said threats to GPS signal reception could be managed and that unintentional interference was not a major risk factor.
It deemed intentional interference the largest risk area, but noted that planned avionics were designed to rapidly detect the onset of such a threat. As aircraft can be vectored away from jammed regions, the threat poses no safety risk, though traffic flow disruptions might follow. The study said methods to detect and stop GPS jammers must be put in place.
GPS jammer technology is readily available. The military has expressed concern over such products, adding that the civilian world is increasingly vulnerable to its misuse. A Russian company, Aviaconversia, has developed and is marketing a low-cost, 4W hand-held jammer intended to disrupt GPS for up to 200km (110nm). Meanwhile, a DoD report says China may be developing GPS jamming technology, and that Iraq may have used Russian jammers.
The DoT study concludes that affordable backups to GPS need to be developed, but adds that even improvements to GPS reliability can be disrupted. It also recommends implementing systems to monitor, report and locate sources of unintentional interference. In the face of intentional disruption, the report recommends military GPS anti-jam and false signal detection technologies be considered for civilian use.
GPS modernisation programmes, including the fielding of higher-powerspacecraft and the eventual availability of three civil frequencies, will reduce vulnerability, the Volpe Center says, "but like any radio navigation system, GPS is vulnerable to interference that can be reduced, but not eliminated." It also advocates development of affordable backups, such as GPS/INS, GPS/Loran-C systems, or VOR/DME.
In particular, the Volpe study says Loran-C modernisation should continue "until it is determined whether Loran-C has a role as a GPS backup system. If it is determined [it] has a role in the future navigation mix, DoT should promptly announce this to encourage the electronics manufacturing community to develop new Loran-C technologies."
Work in progress
Former FAA administrator Langhorne Bond says the FAA "is doing solid work in proving that Loran is an excellent navigation system for aircraft. It works well with GPS. Loran also provides a powerful jam- resistant timing source, so solving the GPS timing dilemma.
"DoT secretary Mineta should state that Loran is a permanent element of the navigation and timing infrastructure and will be continued until at least 2015. Loran is here to stay. We need to mate it with GPS to yield an invulnerable system."
Meanwhile, the DoD is continuing work on various programmes to protect GPS-guided weapons systems. For example, the Office of Naval Research recently awarded Northrop Grumman a $2.5 million research and development contract to develop a prototype of anti-jam, autonomous-integrity software designed to improve the resistance of GPS to jamming.
Meanwhile, Lockheed Martin and Rockwell Collins have combined forces to develop G-STAR, a spatial temporalanti-jam receiver. G-STAR is being incorporated into the Lockheed Martin AGM-154 JASSM Stand-off Weapon, and could be destined for other US GPS-guided weapons.
Last year, the USA and Australia demonstrated how the vulnerability of GPS receivers to interference can be greatly reduced. The GPS jammer locator trials showed how GPS jammers can be found and eliminated from a combination of ground and airborne platforms.
DARPA is taking a different tack incountering GPS jamming. The surrogate satellite, known as GPX Pseudolite, would be mounted on a high-altitude, long-endurance unmanned air vehicle, such as the Northrop Grumman Global Hawk or a large fixed-wing manned platform.
It would use deep antenna-nulling to receive the low-power GPS signals, despite jamming, and emit a higher-power GPS-like code frequency to receivers modified to work with the surrogate spacecraft.
No hardware changes would be required, the software co-existing in the receiver with an automatic switchover when the GPS signal is degraded or lost.
A full-scale weapons demonstration of the GPX Pseudolite is planned for 2003, possibly leading to operational use by 2005. The flight test will involve a full GPS Pseudolite constellation, involving two manned and two unmanned aircraft. It will determine if a GPS-guided weapon can perform in a heavy jammed environment.
A prototype, built by Rockwell Collins, is said to work better than expected, providing 10m (33ft) navigation accuracy despite the jamming.
Source: Flight International
