A dose of reality does the global positioning system no harm in the long term

Graham Warwick/WASHINGTON DC

Anyone observing events surrounding the global positioning system (GPS) over the past few months would be forgiven for believing that European scepticism about the system has been justified. But Europe's reluctance to embrace the GPS has more to do with the political unacceptability of the system's US military origins than the technical uncertainties which have surfaced in the USA in recent weeks.

Ironically, technical solutions developed for the issues now surrounding civil use of the GPS could go a long way to overcoming European concerns. And the political accommodations involved in ensuring civil access to the system should allay fears over its control.

Having spent billions of dollars developing the GPS to give its forces an edge in combat, the US military now sees its Government promoting the system as a "global utility", available free of charge to all users. Moreover, safeguards designed to prevent hostile exploitation of the GPS are being circumvented or removed with the complicity of the US Government.

Hardly surprising, therefore, that negotiations over improvements to the system specifically for civil users have been long and tough. In particular, talks between the Departments of Defense and Transportation (DoD and DoT) over a second civil frequency have been the first test of the effectiveness of the Interagency GPS Executive Board (IGEB), established by President Clinton in 1996 to manage the system and co-chaired by the DoD and DoT.

A second civil frequency is required to improve the availability and accuracy of the GPS. Access to two coded signals would allow civil receivers to correct for atmospheric distortion and two separate frequencies would be more difficult to jam - unintentionally or intentionally. There are two military GPS frequencies, but currently only one civil signal.

Now the IGEB has agreed to make not one, but two new civil signals available. One will be in the frequency of the current military L2 signal, and the other will be at a different frequency which has still to be determined.

That the DoD has agreed to share its L2 frequency with civil users has more political, than technical significance, as the military already shares the L1 frequency. Sharing the L2 frequency as well will make it difficult to deny GPS signals to adversaries while still making them available to allies, the DoD has argued.

One of the two new civil signals, to be useable for air navigation, must be protected internationally from interference from other spectrum users. The current shared L1 frequency already has this so-called "safety of life" protection. The military L2 frequency does not, and the DoD is not keen to see that change.

Its reasoning is easy to understand - safety of life protection for the L2 frequency would tie its hands in any conflict. The military would prefer that such protection be given the third civil signal - the one it will not share with civil users - but there is a hitch.

Despite long-running efforts, the IGEB has yet to identify a suitable slot in the crowded L-band spectrum for the third frequency. Nevertheless, its has undertaken to determine the frequency - and decide which should receive safety of life protection - by August.

In the high stakes arena of GPS techno-politics, meeting deadlines matters. The DoD and DoT pledged last March to decide on a second civil frequency within a year. They did, leaving the crucial issue of safety of life protection still undecided, in a bid to establish the credibility of the multi-agency IGEB as the controlling force behind the GPS.

Now they have a new deadline. Additionally, they must negotiate the thorny issue of satellite availability, as the new civil signals require new payloads on the GPS spacecraft. The frequencies must be decided by August if the DoD is to meet its deadline for awarding Boeing a contract for the next batch of GPS Block IIF satellites.

In theory, the first GPS satellite that can provide the new civil signals is the seventh Block IIF, scheduled to be launched in 2004. But the US Government has committed to making the new signals available in 2005 - which is cutting it fine. In reality, the DoT is hoping that the DoD will agree to modify some of the GPS satellites that are on order, but yet to be launched.

There are 19 Block IIR replacement satellites to be launched, plus six Block IIF follow-on spacecraft on order and scheduled for launch beginning in 2002. If the IIFs, as well as later IIRs, can be modified, then the US Government has a chance of meeting its 2005 deadline. The DoD, however, is concerned that modifying satellites on order could interrupt its constellation replenishment schedule and jeopardise GPS coverage.

There is also the question of who will pay. The DoT has already agreed to pay for one of the two new civil signals, but the other, which is intended mainly for survey and scientific use, may be paid for by the Department of Commerce.

Then there is the issue of which agency will pay to modify satellites, or to accelerate the launch schedule, if necessary, to meet the Federal Aviation Administration's requirement that a second civil frequency be fully operational in time for the peak in signal-degrading sunspot activity in 2009-2010.

There are plenty of challenges still facing the IGEB in establishing non-partisan control of the GPS - and therefore to the USA in promoting the system as the global standard for navigation.

Source: Flight International