How GPS Can Go Wrong
How your dot ended up on the wrong side of the street.
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Sometimes the accuracy or availability of GPS signals becomes an important issue, either in immediate situations, like figuring out what side of the street you are on, or in a legal case, where important facts can hang on the reliability of GPS logs. Understanding how GPS works and how GPS errors happen can remove some of the mystery, and also reveal how not all GPS receivers are created equal.
I will preface this by saying that I have built and programmed high-performance GPS receivers, from the raw radio signal up to position and velocity estimates, and what I present here is based on my personal experience. And everything here probably applies to other GNSS systems, like GLONASS, Galileo, etc., although I have no firsthand experience with those systems.
Basic Principles
We will start with a simplified version of the story, and then add details.
The GPS system works from a “constellation” of satellites in precisely known orbits. Each satellite transmits a specific sequence of bits that is designed to make it possible calculate the propagation delay from the satellite to the receiver, to within a few billionths of a second. That’s the time it takes light to travel a few meters. This propagation delay defines a sphere of constant distance around the satellite, and the receiver must be somewhere on that sphere. If you have three such spheres in three-dimensional space, the point where all three intersect is the location of the receiver. So the basic operation of this simplified GPS receiver is:
- Receive the signals from at least three satellites.
- Estimate the propagation delay to each satellite, which is directly proportional to the distance, related by the speed of light.
- Calculate the location of each satellite and the constant-distance sphere around it. (Each satellite also sends information about its orbit, so that its location can be calculated to within a few centimeters.)
- Calculate the place where the three spheres intersect. That is your location.
That is also a huge over-simplification, but a starting point.
