Transcript:

So, GPS satellites are 20,200 km above the surface of the Earth, which is more than twice the diameter of the Earth. And when they are so far away up, the rate at which time passes for those GPS satellites is significantly different than the rate at which time passes for us on the surface of the Earth. And this time difference has to be accounted for, and there are some smart people who account for this time difference. 

Except the model that is used to determine the surface of the Earth is a sphere, and it is close enough. We all know that the Earth is not a perfect sphere, but it's close enough. How much error does this model impart? About a few meters. So, if you use a spherical model for the surface of the Earth, you'll be a few meters off from where the GPS thinks you are. And a few meters is not a big deal because you're using a GPS to locate an address. And when you see the address, you stop using a GPS. But a few meters makes a huge difference when a GPS is used to fire, let's say, a missile. In that scenario, the model that is used for the surface of the earth is much more accurate.