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By Adam Baddeley

In both the US and Europe, modernisation of space based navigation is an ongoing process. As accuracy, availability and reliability grow militaries are seeking even greater numbers of applications. This is true not least for land forces who are quickly realising the benefits to operations of Global Positioning Systems (GPS). These systems are of benefit not only in Armoured Fighting Vehicles (AFVs) but also in the man platform, initially through GPS integration with Combat Net Radios (CNR) today and later this decade through integration in future soldier systems.

GPS modernisation

Speaking at the SMI’s Military navigation conference in October, John Lundquist Vice president Lockheed Martin Navigation Systems explained how GPS III would significantly reduce current shortfalls.

The GPS modernisation plan will take users from the current Block IIA/IIR basic GPS capability, through the imminent Block IIR-M and IIF improvement until its is replaced by GPS Block III, due to enter service in 2012.
The current basic GPS provides a standard service with a 16-24m SEP (Spherical Error Probability), to civil users, via a single L1-frequency and unencrypted C/A code navigation. The precise service at 16m SEP or less requires the addition of a second L-2 frequency and P-code navigation. This second frequency band is provided to military and other authorised users giving them greater accuracy when added with encrypted code This also provides users with a service less prone to spurious signals and with a higher resistance to jamming.
Each GPS Block builds on these capabilities. The IIR-M satellites have an additional signal on L2 (the L2C) and provide earth coverage Military code (M-code). on L1 and L2. The Block II F satellites have a third civil signal (L5), which provides a further ‘safety of life’ signal for aviation users. A factor in both the Block IIR-M and Block IIF satellites is a flexible power upgrade, which adds a modest ability to counteract enemy jamming. It has been noted that legacy receivers perform poorly with increased power from the spot beam. The first of the eight Block IIR-M satellites is due to be launched in the late summer of 2004. The first of 16 planned IIF satellites, produced by Boeing, will begin entering service from 2006.

Block III will provide a comprehensive improvement of GPS across the board with improvements to Guaranteed service, accuracy, availability, controlled integrity and survivability. The M-code spot beam provides an anti-jam spot beam, which trades earth coverage for concerted power to defeat jamming. The spot beam of M-code will have at least 20db greater signal power. With a nod to the requirement for Transformational communications it will also provide Nav-related messaging and will be an adjunct to Blue Force tracking. Another gain will be a move away from the current UHF crosslinks, which will increase security. Lundquist stated that there is no current requirement for multiple spot beams, but there was space for them if the budget was increased.

When jammed the current GPS has a delay, affecting operational tempo until the jammer can be identified and eliminated. The delay is not helped when the back-up systems are rarely ready. GPS III will make a considerable improvement in these areas with its goal of full spectrum dominance. Lundquist said that although GPS III would provide benefits in penetrating heavy foliage and in urban environments, the gains would be modest.

The advantages that GPS III undeniably brings are tempered by some drawbacks. The addition of a second open access signal on the L2 frequency means that adversaries need to be barred from both signals. Furthermore with the addition of the L5 signal there is the potential for interference with the JTIDS/MIDS Link 16 signal.

The two teams competing for the GPS III contract are currently in the midst of 24-month Systems Requirement Review Ph

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