Gang, the geometry of the visible satellites at any given time can significantly affect the achievable GPS accuracy at a given location. This is calculated as the Dilution of Precision (DOP).
Most or the time, because there are 24 satellites, there is usually a decent configuration visible so we get decent accuract (within 10-15 feet). However, from time to time the DOP can get bad, and you'll have poor accuracy.
DOP can be forecast. There are several online sites that will calculate DOP for a location and time -- you can use this to plan when to execute a mission that requires the best accuracy possible.
I use a PC app called Trimble Planning that shows all sort of great GPS-satellite data this purpose.
Regardless of what forecasting (or not) you try to do, there are GPS apps for both iOS and Android that will give you real-time information about the satellite constellation, including DOP and it's specific components (VDOP, HDOP, PDOP, TDOP), so you can decide if the the satellite geometry is good enough in real time to run your mission.
The more precision you need, the close you want DOP to be to 1.