Blanket GPS Coverage in the Cross Hair

The results of spottier GPS coverage could include the following:

Flights: Intercontinental commercial flights use predicted satellite geometry over their planned navigation route, and may have to delay, cancel or reroute flights.

Emergency calls: Enhanced-911 services, which rely upon GPS to precisely locate callers, could lose accuracy, particularly when operating in "urban canyons" or mountainous terrain.

Defense: The accuracy of precision-guided munitions that rely upon GPS to strike their targets could decrease. To accomplish their mission, military forces would either need to use larger munitions or use more munitions on the same target to achieve the same level of mission success. The risks of

collateral damage could also increase.

Directions: Another important consideration is that both the standard positioning service and precise positioning service performance standards assume that users have unobstructed visibility to nearly the entire sky, an

assumption that does not hold for the large number of users operating in

moderately mountainous terrain, in the "urban canyons" of large cities, or

under forest foliage.

So what's to be done? Well, the Air Force is looking at lower cost alternatives to current GPS satellites. In 2007 the Air Force Scientific Advisory Board examined whether small satellites -- which can be developed more quickly and at relatively low cost-might help. The board concluded that small satellites may eventually have operational utility in augmenting GPS capabilities. However the need for an extensive control segment infrastructure to monitor and control these small satellite augmentations, combined with the need to develop, produce, and install user equipment, would make it very challenging to field a near-term small satellite augmentation, the GAO said.

Powering down some components of current GPS satellites is also an option. The operational life of a GPS satellite tends to be limited by the amount of power that its solar arrays can produce. This power level declines over time as the solar arrays degrade in the space environment until eventually they cannot produce enough power to maintain all of the satellite's subsystems. However, according to Air Force representatives, the effects of this power loss can be mitigated somewhat by actively managing satellite subsystems-shutting them down when not needed- thereby reducing the satellite's overall consumption of power. It would also be possible to significantly reduce the satellite's consumption of power by shutting off a secondary GPS payload. This would buy additional time for the navigation mission of the satellite at the expense of the mission supported by the secondary payload, the GAO stated.

In the end of its report the GAO called for the Secretary of Defense to appoint a single authority to oversee development of GPS space, ground control, and user equipment, to ensure they are synchronized, well executed, and potential disruptions are minimized. For its pat the DOD agreed with the GAO. We'll se where it goes from here.

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