摘要：Is it true that the GPS satellite geometry repeats every day shifted by 4 minutes?
A: It is true that the GPS satellite orbits were selected to have a period of approximately one half a sidereal day to give them repeatable visibility. (One sidereal day is 23 hours, 56 minutes, and 4 seconds long or 236 seconds shorter than a solar day.) However, because of forces that perturb the orbits, the repeat period actually turns out to be 244 to 245 seconds (not 236 seconds) shorter than 24 hours, on average, and changes for each satellite.
The selection of a half sidereal day orbit causes the satellite ground track and the satellite visibility from any point on earth to be essentially the same from day to day, with the satellites appearing in their positions approximately 4 minutes (236 seconds) earlier each day due to the difference between sidereal and solar days. This was a particularly useful property in the early days of GPS when session planning was important to ensure adequate satellite coverage. With this easily predictable coverage, GPS users could schedule repeatable campaign sessions well in advance just by shifting their experiments forward each day by 4 minutes.
