GPS is the modern answer to the ancient question about one’s place in the world yet it has its limitations. It depends on the time of flight of radio signals emitted by satellites twenty thousand kilometers above you. Like any system involving large distances and high velocities, this is bound to offer some challenges to precise measurements which result in a limit to achievable accuracy. In other words: The fact that GPS locations tend to be off by a few meters is rooted in the underlying principle of operation.

Today’s level of precision was virtually unattainable just decades ago, and we’re getting that precision with a handheld device in mere seconds. Incredible! Yet the goal posts continue to move and people are working to get rid of the remaining error. The solution is called Differential GPS or ‘DGPS’ and its concept looks surprisingly simple.

What’s fascinating is that you can use one GPS to precisely measure the error of another GPS. This is because the inherent error of a GPS fix is known to be locally constant. Two receivers next to each other pick up signals that have been affected in the same way and thus can be expected to calculate identical wrong positions. This holds true for distances up to several kilometers between individual receivers. So in order to remove the error, all you need is a GPS receiver in a known location to measure the current deviation and a way to transmit correction information to other units. DGPS does just that, using either terrestrial radio in some regions and satellites in others. Mobile solutions exist as well.

So a raspi with a USB GPS dongle in a known location should be able to act as a DGPS over IP base station, right? In theory, yes. In practice… fail.

Continue reading “Fail Of The Week: How Not To Build Your Own DGPS Base Station” →