Satellite glitch rekindles GPS concerns

Could ubiquitous GPS coverage soon get spotty?

News today that the Air Force is investigating signal problems with its latest Global Positioning System satellite are likely to rekindle the flames of a congressional report last month that said the current GPS coverage may not be so ubiquitous in the future. 

The Air Force stated that routine early orbit checkout procedures determined that the signals from the Lockheed-built GPS IIR-2 (M), which was launched in March, were inconsistent with the performance of other GPS IIR-M satellites.

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The Air Force said it has identified several parameters in the GPS IIR-20 (M)'s navigation message that can be corrected to bring the satellite into compliance with current GPS Performance Standards. Over the next several months the team will be testing impacts the corrective actions may have on military and civil GPS user equipment. The test strategy implements engineering analysis, modeling and simulation, and testing of real-life GPS receiver equipment to the greatest extent possible to ensure that there will be no inadvertent impacts to GPS users, the Air Force stated.

A Wall Street Journal article on the problem said: The Lockheed satellite is the first to include a new civilian frequency - known as L5 -- designed for, among other things, use by future nationwide air-traffic control systems. But that signal, part of test package, apparently is interfering with other signals from the satellite and reducing their accuracy, according to industry and Air Force officials. The degraded signals are accurate only to about 20 feet, versus about two feet for typical GPS signals, the article stated.

The article went on to say the glitch could complicate deployment of a new family of 12 Boeing GPS satellites currently being built that also feature the L5 signal.

According to a GAO report just last month, the Air Force has struggled to successfully build GPS satellites within cost and schedule goals; it encountered significant technical problems that still threaten its delivery schedule; and it struggled with a different contractor. As a result, the current satellite program has overrun its original cost estimate by about $870 million and the launch of its first satellite has been delayed to November 2009-almost 3 years late, the GAO said.

The GAO said the GPS system has had its share of technical difficulties as well.  For example, last year, during the first phase of a critical test to determine space-worthiness that subjects the satellite to space-like operating conditions, one transmitter used to send the navigation message to the users failed, the GAO stated.  The program suspended testing in August 2008 to allow time for the contractor to identify the causes of the problem and take corrective actions.

Delays in the launch of the new satellites will increase the risk that the GPS constellation will decrease in size to a level where it will not meet some users' needs, the GAO said. If the GPS constellation falls below the number of satellites required to provide the level of GPS service that the US has committed to providing, some military and civilian operations could be affected. The defense department is evaluating alternative approaches that could mitigate the gap. However, procurement of additional GPS satellites does not appear to be feasible, the GAO said.

Because there are currently 31 operational GPS satellites of various blocks, the near-term probability of maintaining a constellation of at least 24 operational satellites remains well above 95%. However, DOD predicts that over the next several years many of the older satellites in the constellation will reach the end of their operational life faster than they will be replaced, and that the constellation will, in all likelihood, decrease in size, the GAO stated.

In response to that report, the Air Force proclaimed the system healthy and stable. "No, the GPS will not go down," stated Col. Dave Buckman of the Air Force's Space Command. "GAO points out, there is potential risk associated with degradation in GPS performance."

"The issue is under control. We are working hard to get out the word. The issue is not whether GPS will stop working. There's only a small risk we will not continue to exceed our performance standard," he said.

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