The US Department of Transportation has accepted that the global positioning system (GPS) is vulnerable to interference, and will ensure adequate back-up systems are maintained for critical applications. Which back-ups will be maintained for aviation has not been identified.

After a delay, the DoT has formally endorsed last year's report by its Volpe National Transportation Systems Center that the GPS signal is vulnerable to unintentional interference as well as deliberate disruption. Released in September, the Volpe report ended any hopes of GPS being approved for sole-source navigation.

The DoT says safety-critical transport applications that use GPS have adequate back-ups, but adds that future actions will be necessary to build redundancy into systems under development.

The department plans to complete an assessment across all modes of transport to identify the best mix of radio navigation systems, on capability and cost grounds, for the next decade and beyond. The DoT hopes this will result in a reduction in the number of different systems needed to support all users.

The department's action plan includes completion of an evaluation of the long-term need for Loran-C.

Last month, the US Federal Aviation Administration completed its evaluation of ground-based Loran's applicability to aviation. Proponents say the results achieved were accurate enough to allow Loran to be used as a second, dissimilar, source of positioning to back up GPS. Loran is also being proposed as an alternative to satellites for broadcasting data from the GPS-based wide-area augmentation system.

Source: Flight International