7Chapter GPS Mission Planning
Fact: In a given area, the GPS data collection process works better at some times than at others. Determining a good (or just satisfactory) time to take data is called “mission planning.”
Several years ago, this discussion of mission planning would have had to appear at the front of this book, because there were so few GPS satellites in orbit that you had to go into the field at specified times to collect data. Now with the NAVSTAR system at full operational capability (31 satellites up and healthy in 2008), you can almost always “see” enough satellites to get a pretty good fix. But is it a fix that is accurate enough to meet your needs? (Our needs in this book were simply to show you how the system worked. So we left the mission planning details until now.)
It might have been simpler if the GPS satellites were simply parked over various areas of the world so that, in a given location, you always dealt with the same set of satellites in the same positions. Unfortunately, this would mean that they were always parked directly over the equator, because, due to the laws of physics, that’s where all geostationary satellites are. (If they were orbiting the Earth anywhere else, as they would have to be in order to provide worldwide coverage, they would be moving with respect to the ground underneath.)
So the rule for GPS satellites is “constant change.” The number of SVs your receiver can track changes. The geometric pattern they make in the sky changes. The local environment in which you want to take data changes-perhaps there are objects that are blocking signals from reaching your antenna. If you want to collect data at the optimum times-for examples, when the DOP is very low, when there are enough satellites in the right part of the sky, or when there are a large number of satellites available for overdetermined position finding-you might want to use mission planning software.