US Department of Defense (DoD) officials were due to meet at the end of February to decide the future of global positioning system (GPS) modernisation. Development of GPS's next generation, Block III, was put on hold earlier this year - to address security and vulnerability issues, industry sources suspect.

The US Air Force had planned to release a request for proposals for the pre-acquisition phase late last year, with Boeing, Lockheed Martin and Spectrum Astro expected to compete for two $100 million, 24-month system demonstration and risk-reduction contracts leading to selection of a winning contractor for the $2-5 billion GPS III programme in 2004. Now the USAF is expected to award all three competitors contracts worth $2-3 million to continue studying GPS III requirements and costs for a further year.

Block III is the final leg in the USA's strategy to upgrade GPS to improve protection of military signals and enhance civilian use of the system. The first step, to stop deliberately degrading the standard positioning service available to civil users, took place in May 2000 when selective availability was turned off. The next step is to modernise the space segment by launching satellites providing new military and civil signals.

The pace at which GPS can be modernised depends on the rate at which the satellite constellation needs to be replenished. This is slower than expected because the present Block II/IIA spacecraft are exceeding their design lives. To accelerate availability of the new services, the DoD decided to upgrade replacement satellites already built, but not launched, as well as those on order, to incorporate the new military and civil signals.

Last year, Lockheed Martin was awarded a contract to modify up to 12 already built Block IIR replacement satellites to provide a new civil signal on L2, the second GPS frequency, and a new military signal, M-code, on both the L1 and L2 frequencies. The ITT-produced payload will also increase available power on all signals, with the ability to adjust power in orbit to enhance anti-jamming capability. The first modified GPS IIR-M is scheduled for launch next year.

Boeing, meanwhile, has been awarded a contract to produce up to 12 follow-on Block IIF satellites equipped to provide the civil code and new military code on the L1 and L2 frequencies, and modified to introduce a third civil signal at a frequency designated L5. For civil users, the new signals will improve the accuracy and redundancy of the system, and L5 is the first GPS signal to meet the safety-of-life requirements for aircraft navigation. The first GPS IIF-M is scheduled for launch in 2005.

The new capabilities require at least 18 suitably equipped satellites to be in orbit, so it will take time to upgrade the services provided. Initial operational capability (IOC) of civil L2 and M-code is due in 2008, with full operational capability (FOC) planned for 2010. Under the current schedule, GPS III launches should begin in 2009, and IOC of the third civil signal in 2012, leading to FOC in 2014.

If the DoD changes the schedule, it could affect availability of the L5 signal. The USAF says GPS III has been put on hold to address concurrency between development and production, and to prepare implementation of the latest high-level decisions on GPS. Industry sources expect the programme to be modified as a result of two events last year: a report by the US Department of Transportation's Volpe Center confirming the vulnerability of GPS signals to unintentional and intentional interference; and the 11 September terrorist attacks.

The US Federal Aviation Administration is re-evaluating its satellite- based navigation plans in the light of GPS's documented vulnerability, and is looking at providing a ground-based back-up, probably Loran-C. The Volpe report is also expected to affect future decisions on two systems intended to increase availability and integrity of GPS: the wide-area augmentation system (WAAS) and local-area augmentation system (LAAS).

The delayed WAAS is now due to become operational in December next year, but with reduced capability. A decision on whether to pursue further development of WAAS to achieve the promised Category I precision-approach capability is due this month or next. A contract for full development of Cat I LAAS is due in July, with the first of 46 systems to be operational by the fourth quarter of next year. Acquisition of 114 Cat II/III systems is planned to begin in 2003, with the system to become operational in 2006.

Source: Flight International