At over 20,000 kilometers above sea level(20,180 kilometers (12,540 miles)) is a constellation of satellites, each orbiting Earth every 11 hours and 58 minutes. These satellites are continuously beaming data down to us on earth, which in turn is received by devices such as your phone or navigational units in your cars, allowing you to see where you are on the planet. There are a lot of misconceptions about how GPS’s actually work, an example being that your phone and the GPS satellites are both talking to each other. So let’s get down to it: how do GPS’s work?
The Navstar system, which is simply referred to as GPS, is what we will be focusing more on – although most phones and devices tend to have the capabilities to use both GPS and GLONASS. GPS satellites are set up in such a way that from almost anywhere on the surface of Earth you should have a direct line of sight of at least four GPS satellites. This is quite important on the basis that GPS point positioning requires at least four satellites to calculate three position coordinates and the clock deviation.
With the satellites all sending exceptionally accurate time down to Earth, your phone or GPS receiver can compare the difference of time between the signal being sent and received to work out the distance between you and the satellite. By multiplying this time difference with the speed of light (as the signal is sent as the speed of light), you can get the distance you are from the satellite. As the satellites are also sending whereabouts they are, you can begin to draw spheres around the satellites, with you being somewhere on the outside border of the sphere.
There are a lot of factors that escalate the potential error, but the most significant is due to the ionosphere, a part of the upper atmosphere extending from 60 km to 2,000 km, where free elections occur frequently enough to have an appreciable influence on the proportion of electromagnetic waves passing through this layer. This error is substantially smaller when satellites are directly overhead, compared to being larger the closer satellites are to the horizon relative to you as the path between you and the GPS satellites goes through more of the atmosphere compared to being directly overhead. Even things such as small variations in the atomic clocks found onboard these satellites can cause major errors.
Combining the 7 microsecond a day delay due to the satellite’s velocity relative to Earth and the GPS’s being further away from Earth, this adds up to a 38-microsecond delay, which if left uncorrected would translate through to a 10 km/day pseudo-range error, rendering GPS’s invalid from the get-go if this was not to be taken into consideration. This is compensated by the GPS’s clock frequencies being slightly slowed down from 10.23MHz to 10.22999999543 MHz to cancel out the effects of relativity. So next time you turn on your GPS, you should realize just how much physics and maths is going into you finding your own location.
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