Europe’s Galileo one-ups DOD’s GPS

 - April 16, 2008, 5:36 AM

Speakers from Eurocontrol and the European Space Agency last month informed attendees at a meeting of the FAA’s Satellite Operational Implementation Team (SOIT) that their organizations would accept liability for system failures when the Galileo satnav system was used in critical applications requiring high-accuracy guidance, such as approach and landing operations. This policy is in contrast to that of the Department of Defense, which has steadfastly rejected the notion of liability and has clearly stated that GPS service is offered totally at the user’s risk, regardless of application.

Forecast to begin worldwide operations in 2008, Galileo will be interoperable with GPS–meaning both will use common receivers–and will bring up to 30 additional satellites to the present U.S. constellation. But while most user groups have acknowledged the availability and performance benefits of having two complementary satellite navigation systems in the future, there are still some underlying concerns within the Departments of State and Commerce about Europe’s satnav initiative, which will negate the GPS monopoly and is also seen as having potential adverse effects on U.S industry and space leadership. The acceptance of specified liability responsibility by the Galileo authorities serves only to increase these concerns. “They’ve stolen a march on us,” said one GPS industry attendee at the meeting.

Liability would be recognized only when certain chargeable high-accuracy services were being used, and would not be offered to users of the normal navigation service, which will be roughly equivalent to today’s basic unaugmented GPS. But U.S. government and industry officials are reported to fear that aircraft hull insurers, for example, might require operators who need high-accuracy guidance to use Galileo exclusively.

This concern is exacerbated by Galileo’s forecast transmission from the outset of both the basic L1 and the new L5 frequencies. These are both required to compensate for ionospheric disturbances, which are now the main source of satnav position errors. L1 and L5 are sufficiently removed from each other in the frequency spectrum that they are affected by different ionospheric events, and next-generation satnav receivers will measure and then automatically compensate for their different errors. This capability is a necessary prerequisite in the absence of a local-area augmentation system (LAAS) for precision approaches using the GNSS landing system (GLS).

But the new L5 frequency may not be available from GPS worldwide until around 2015, when its current satellites–which lack L5–are completely replaced by new GPS III satellites that will carry it. Launch of the first GPS III satellite is forecast for 2008, but the renewal of the full 24-satellite constellation will be a gradual process, due to the DOD’s “launch on demand” policy, which replaces satellites in orbit only when they are approaching the end of their lives. However, the current GPS satellites are showing surprising longevity. Additionally, GPS satellites are launched singly, typically atop Atlas rockets from Vandenberg AFB in California, while Galileo satellites will be much smaller and lighter– due to the absence of additional military payloads and nuclear-hardened construction–and will be launched in clusters of four or five satellites atop large commercial booster rockets from launch sites around the world.

For the time being, both the Europeans and the DOD are downplaying the notion that there is a growing “space race” developing between Galileo and GPS III, but as 2008 draws closer we can expect to hear much more from Europe about the benefits of liability coverage, and the elimination of ionospheric errors by the L1/L5 technique.