Wildly inaccurate GPS position and velocity

Question

I'm writing a toy car-navigation app for Android, mostly to learn the Android SDK.

While testing my application, it worked fine for a few hours. Then, suddenly, it reported my position as being in a town 100km south of my real position, travelling at 300km/h. I was in fact travelling at approximately 120km/h.

I changed application to Google Maps, and it also showed the same erroneous position and approximate velocity. Since I was driving a car in dense traffic at high speed, for safety, I decided not to start fiddling with the phone any more, and just put it away. The next time I took it up (maybe 15 minutes later), the position and velocity was correct again.

I will add diagnostics to my program to show the reported accuracy of the GPS-fix.

I have a few questions:

• Is this a hardware/firmware problem in android phones? (I have a HTC Hero)
• Is this a known failure mode of GPS-receivers?
• What could make the GPS-receiver report such inaccurate positions?
• I had another GPS receiver in the car (a Garmin car navigator). Could this receiver have interferred with the HTC Hero unit?

Answer 1

Handheld GPS devices are much less accurate than is typically claimed, but being off by 100 km (or by whatever the distance is between Bolton and Madrid) is pretty spectacularly unusual. It sounds like something weird may have briefly happened to the GPS system.

The other GPS receiver in your car would not have caused this - the device antenna just receives signals, it doesn't broadcast anything.

Update: might be solar flares. The article laughably claims they'll produce errors in the range of 30 meters; it's laughable because GPS devices are less accurate than that under normal conditions.

Answer 2

Due to the nature of GPS and satellite locking, it just sounds to me like you dropped below the required number, and fell back to some other method, cell-tower-based method of guessing location...

In DC I see this sometimes when traveling on the train due to the fact that the GPS is loosing signal when I go into the tunnels and the "tower" based location kicks in.

The only thing I can think of is to do some sort of geo-distance reasoning about your position... but what you would do once you knew you had bad geo data I don't know. :-)