GRAHAM WARWICK / WASHINGTON DC
GPS may have become a victim of its own success as the system struggles to adapt to the needs of a rapidly broadening user community
The global positioning system (GPS) has become an everyday utility - almost as prevalent as laptop computers and mobile telephones - since the first satellite was launched in 1978.
Designed for the US military, GPS has found international application in civil aircraft, vehicle and vessel navigation, as well as recreation, surveying, tracking and timing. The explosive growth in worldwide sales of products and services since GPS became fully operational in 1995 has highlighted two aspects of the system: the limitations of the original satellite signals; and the fact that operation and evolution of the constellation continues to be funded entirely by the US military.
Signal limitations will begin to be addressed in 2003, with the launch of the first GPS Block IIR-M satellite capable of broadcasting new, more robust, civil and military codes. Funding is likely to become an issue as the US Department of Defense (DoD) moves ahead with development of the next-generation GPS III - the first to be designed from the ground up to cater for both military and civil requirements.
GPS has far exceeded expectations, but circumstances have changed dramatically since the original system was deployed. A system designed for the military en-route navigation mission is now being expected to provide precise guidance for everything from airliners to munitions. Today, the US Government has concluded, GPS is "minimally prepared" to be the dual-use civil/ military system on which the world is coming to depend, and which has an exponentially growing economic impact.
Recognising the growing importance of GPS to both sectors, a modernisation effort is underway to meet the military requirement for improved anti-jam performance and the need for a better, more reliable civilian service. The result promises to be an entirely new ground and space architecture that, for the first time, meets the safety-of-life requirements for aircraft navigation.
Free access
Since its inception, GPS has broadcast two basic signals - coarse acquisition (C/A) and precise encrypted (P(Y)) - on two frequencies: L1 at 1,575MHz and L2 at 1,227MHz. While civil users have had free access to the C/A signal on L1 from the outset, only the US military and its allies can use the P(Y) codes on both L1 and L2. Until May last year, the USA deliberately degraded the standard positioning service available to the civil community using selective availability (SA). With SA on, GPS was accurate to within 100m (330ft). Now, with SA off, horizontal accuracy is better than 22m. The precise positioning service available to military users is more accurate still, as receivers can compare both frequencies to eliminate ionospheric errors in the signals.
The first step in modernising GPS will be to provide civil users with access to both frequencies. This will come in the form of a new civil signal on the previously military-only L2 frequency. The original plan was to replicate the L1 C/A code on L2, but it was decided to develop a new signal, known as L2CS, which would improve the performance of civil GPS. L2CS comprises two codes, one with navigation data and one without. The structure is similar to that planned for a third civil frequency, L5 at 1,176MHz, which will be introduced in 2005 in the next phase of modernisation.
With SA off, signal delays caused by the atmosphere are the largest source of GPS errors. With access to two frequencies, civil users will be able to correct the ionospheric error, increasing accuracy to better than 9m. L2CS makes it easier to pick up and track a weak signal, and makes it possible to decode the navigation message when barely tracking the signal. L2CS also eliminates problems experienced with C/A code when a strong GPS signal interferes with a weak one.
It is not yet clear how much aviation users will benefit from the second civil frequency, as neither L1 nor L2 are in frequency bands set aside for aeronautical radio navigation service. For civil aviation, the biggest change will come with availability of the third civil frequency, L5 - the first safety-of-life GPS signal. The dual-code L5 signal will be more robust than C/A code and L2CS and will provide accuracy comparable to P(Y) code.
As L5 uses the same frequency band as DME ground navigation aids and the military Link 16 datalink, there is potential for interference, particularly over the USA and Europe. Any degradation will be limited to higher altitudes and is expected to decrease as DME frequencies are reassigned away from L5.
Anti-jamming
At the same time, the military signal structure will be improved with the introduction of a new M code on both the L1 and L2 frequencies. The aim is to improve performance in jamming, and particularly to allow the USA to deny an enemy access to GPS within a theatre of operations by jamming the civil signal while still allowing its military receivers to operate.
The civil signal is broadcast at the centre of each frequency band, overlapping the military P(Y) signal, but M code will be broadcast towards the edges, and with more power. Higher power will increase jamming resistance, while spectral separation of the signals will make it easier to protect GPS while denying it to others.
The pace at which GPS can be upgraded is dictated by the rate at which new satellites are launched to replace those in orbit. The original plan was to introduce the new capabilities with Block IIF follow-on satellites to be built by Boeing but, with the existing Rockwell (now Boeing)-built Block II/IIA satellites exceeding their design lives, the DoD has decided to accelerate modernisation by modifying 12 of the Block IIR replacement satellites already built by Lockheed Martin and awaiting launch. At the same time, the DoD has cut back its planned GPS IIF purchase to 12 satellites and accelerated development of the next-generation GPS III.
The modified GPS IIR-M will provide the new civil L2 signal and military M code, while the L5 third civil frequency will be introduced with GPS IIF. GPS III will include all the new civil and military frequencies as well as a spot-beam capability to increase signal power and anti-jam performance in a theatre of operations.
Constellation upgrade
For the new capabilities to be useable, at least 18 suitably equipped satellites must be in orbit, so it will take time to upgrade the constellation, which consists of 29 operational satellites: 23 Block II/IIAs and six unmodified Block IIRs. The first modified GPS IIR-M is scheduled for launch in 2003, the first GPS IIF in 2005 and the first GPS III in 2009. Initial operational capability (IOC) of civil L2 and M code is due in 2008, with full operational capability (FOC) planned for 2010. IOC of the third civil frequency is set for 2012, leading to FOC in 2014.
Lockheed Martin received a $33 million contract to modify up to 12 of 20 already-built Block IIR satellites in September last year. Boeing was awarded the original $1.3 billion, 33-satellite Block IIF contract in April 1996, but this is being renegotiated because of the cut in numbers and the addition of the L5 frequency. The company has submitted its final proposal for production of an initial six modified satellites and hopes for a go-ahead before year-end. Lockheed Martin, meanwhile, is believed to be proposing that the DoD procure additional GPS IIR-Ms as an alternative to buying a small number of IIFs. This could delay the availability of the L5 civil signal.
Boeing and Lockheed Martin are completing GPS III concept exploration work under $16 million system architecture and requirements definition contracts awarded in November. Spectrum Astro, which did not win one of the two study contracts, has continued self-funded work and will bid for the next phase of the programme: system definition and risk reduction (SD/RR). The request for proposals (RFP) for the 24-month SD/RR phase is expected to be released shortly, and the award of two $100 million contracts is scheduled for March next year.
In 2004, the DoD plans to select one contractor as lead integrator for the entire GPS III system. It will be responsible for managing development, production and launch of the satellites, and for operation of the ground segment. In the system development and demonstration phase, the contractor team will build one ground qualification vehicle and one flying development vehicle to be launched in 2007. The DoD will then buy production satellites, probably in lots of three, for launch beginning in 2009.
The number of GPS III satellites, expected to be around 30, could be cut if the DoD proceeds with development of the Global Multi-mission Support Platform (GMSP), a spacecraft that wouldprovide all the same civil and military navigation services as GPS III but also wideband and narrowband communications services for military and intelligence users. GMSP is not yet funded and is not a replacement for GPS III, the DoD stresses, but the concept involves augmenting a constellation of 16-24 GPS IIIs with six to nine GMSPs.
As the DoD prepares to release the RFP for GPS III, it is asking how far the system should go in supporting civil needs. Programme officials point out that many of the performance shortcomings of GPS are specific to civil requirements. A draft system specification for GPSIII has been circulated to industry. It sets threshold and objective levels of capability, which incorporate some or all of the civil community's needs. One observer says the threshold level is "better than what we have today", while the objective level "is a whole lot better".
There is optimism within industry that GPSIII, with its new end-to-end architecture, will finally be the safety-of-life navigation service civil aviation is seeking. But with worldwide sales of GPS products and services forecast to reach $16 billion a year by 2003 - 70% outside the USA - and Europe proceeding with plans to develop a competing satellite navigation system which civil users will be charged to access, it remains to be seen whether the USA will continue to provide free civil access to a system that is funded and operated by an increasingly budget-conscious military.
Source: Flight International
