Unfortunately, the accuracy of some aspects of the information included in the NAV message deteriorates with time. Translated into the rate of change in the three-dimensional position of a GPS receiver, it is about 4 cm per minute. Therefore, mechanisms are in place to prevent the message from getting too old. For example, every two hours, the data in subframes 1, 2 and 3, the ephemeris and clock's parameters, are updated. The data in subframes 4 and 5, the almanacs, are renewed every six days. These updates are provided by the government uploading facilities around the world which are known, along with their tracking and computing counterparts, as the Control Segment. The information sent to each satellite from the Control Segment makes its way through the satellites and back to the users in the NAV message.
The question might arise in your mind, where does this Navigation Message and the information embedded there come from? It comes from the Control Segment. There are three segments in the GPS system: the User Segment, the Control Segment, and the Space Segment. The Space Segment is obviously the satellites themselves, the User Segment are all of us with our GPS receivers, and the Control Segment, well, here's an illustration that shows you the expanded Control Segment that is currently operational.
It's a good thing that we have the Control Segment a little bit modernized from what it once was. We'll talk more about that in the future, but here we have the stations, some of them monitor stations and then some of them actually have upload capability as well. In general, what happens is this. The GPS satellites are tracked by at least, now, two of these stations simultaneously all the time. Now, by tracking them, they are able to do several things. For example, since the stations already know their positions on the surface of the Earth, they are able to collect the signals from the satellites and know how much they have been delayed or attenuated by the atmosphere. They're also able to track the satellites and determine their path, their orbit. From this, their ephemerides can be calculated. The control station are also able to compare the signal coming from the satellite with the station's atomic clock, see the difference between the two, and then upload a clock correction to the satellite.
This is basically how the information is derived. It then goes from the tracking stations to the Master Control Station in Colorado Springs at Schriever Air Force Base where calculations are done and the components that we've just seen in the Navigation Message are generated.
Those corrections go back to the upload stations and are uploaded back to the satellites. That's where the information that is in the Navigation Message comes from. It's the control stations in the Control Segment on the surface of the Earth comparing the received signal from the satellite with their satellite clock, with their atomic clock and with their actual known position. This way, they can derive the corrections that go into the satellite's Navigation Message.