Mapping Hacks

Improve the Accuracy of Your GPS with Differential GPS #60

Chapter 5, Mapping with Gadgets

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HACK

HACK

#60

Improve the Accuracy of Your GPS with

Differential GPS Hack #60

The GPS system is amazing, but here is a way to make it even better.

Why does your GPS receiver give you an error reading, and why are the val-

ues it gives you often in tens of meters? The accuracy of your signal depends

not only on how many satellites you can see, but also the atmospheric inter-

ference between them in low earth orbit and you with your feet on the

ground.

Satellite-based global positioning systems rely on the basic property of our

universe that radio signals travel at a constant speed through a vacuum. If

we can estimate how long a GPS signal has taken to travel to our receiver

from a satellite in orbit, we can multiply our time estimate by that constant

speed to arrive at an estimate of how far away the satellite is. If we combine

enough of these estimates, we can arrive at a pretty good idea of where we

are on the planet’s surface. So far, so accurate!

But this simple physics solution doesn’t work so well in the real universe.

GPS signals don’t travel through a vacuum to reach us; they travel through

Earth’s atmosphere, and, in much the same way that sound waves travel at

different speeds through air or water or other mediums, the radio signals

from GPS satellites end up traveling at different speeds to reach ...