WAAS Up?
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The Government Accountability Office, a Federal watchdog agency, reported on May 7th that the Global Positioning System of satellites used for navigation and many other business and scientific purposes as well as for proving that your teenage son was actually driving down the Interstate at 100 miles-per-hour last Thursday night when he claimed to be bowling, well that satellite system is in danger of becoming unusable because satellites are not being replaced quickly enough by the U.S. Air Force.
Only it isn’t true.
Right now on Google News you can find more than 400 stories all saying the same thing with varying degrees of alarm. The Air Force is three years late in launching a new generation of GPS satellites. The replacement program is over-budget by more than $700 million. The whole mess has been incompetently run and ought to be fixed. All this is true. What isn’t true is that it matters very much to the real world operation of the GPS system or its users.
The GPS system has 31 satellites in orbit right now, the oldest of which has been operating since 1990. For the system to work perfectly it must have 24 or more satellites functioning. The GAO says it is only 80 percent certain that the Air Force can maintain full coverage before replacement satellites can be launched. This lack of confidence is not based strictly on the idea that eight or more satellites will go dark over the next couple years, but that some undetermined number of satellites will go dark, the Air Force will make no progress in replacing them, and that the remaining satellites will be unable, for some reason, to be moved into new positions, filling gaps in coverage. That’s quite a combination of improbable events and makes me very suspicious of the 80 percent number.
For the GPS system to work requires that the receiver in your car, airplane or iPhone be able to simultaneously track at least three satellites (four if you require altitude information). If your receiver can show the satellites it is tracking (many can) you’ll see the number in sight is usually five to seven satellites with the rest being over the horizon and out of view.
If your GPS equipment was purchased in the last couple years it probably makes use of the Wide Area Augmentation System (WAAS), which is a system of ground stations and two geosynchronous satellites that help your receiver correct for ionospheric variations that can slightly degrade GPS performance. Without WAAS your GPS is only accurate to 7.6 meters. With WAAS accuracy is about one meter. The reason we care about this is because GPS is used now to land airplanes and the difference between 3.8 meters above the runway and 3.8 meters below the runway could ruin your whole day.
In addition to improving GPS accuracy over North America (and just North America — there are different systems for Europe and Japan), WAAS also effectively adds two virtual satellites to the GPS constellation. These are the two geosynchronous reporting satellites, which for ease of use in the system are treated by receivers like regular GPS satellites except they for some reason don’t seem to move in the sky. For WAAS-enabled GPS receivers, then, it is possible to maintain acceptable accuracy with only ONE (not three) of the regular GPS satellites in view.
The chances of the GPS system going down are very remote — FAR lower than the 20 percent suggested by the GAO. That’s because the GAO ignored completely in its analysis the implications of WAAS.
So what’s going on here? Why is this even a story?
The Air Force is late and over budget and the GAO wants to make a point of that. The best way to make that point is by putting the technical story in the worst possible light, which the GAO has done to an extreme that I think is excessive. This is just political infighting.
What’s worse, though, are those 400+ news stories that miss the point entirely. Where is a professional and questioning press? It looks to me like 400+ media outlets rewrote the GAO press release and left it at that, giving-in to the fear-mongering that has become the way government policy is promoted these days.
Some stories quoted experts saying a failure isn’t likely. Some stories said the GAO likely has a non-technical agenda. But I couldn’t find any stories that put the whole thing together and questioned whether there was any news value at all.
We need smarter, better-informed, and less gullible reporters. THAT’s the story.
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