There are two ways this generally works:
- A low-current hot buss that is always connected to the system battery
- A small rechargeable battery that powers low-power time and memory circuits
The first is common in aircraft with hard-wired systems, while the second is the case for nearly every handheld-style GPS.
While GPS does not necessarily need to be powered all the time, it dramatically speeds up the satellite position lock if it is
- close to the correct time and
- can produce the GPS 'almanac' (coarse orbit) from memory
- can produce valid ephemeris (detailed orbit) data from memory
- has a valid last known position
The time, position, and almanac information give the GPS hints on which satellites to search for first. Once the first satellite is locked on to it can acquire a time signal and the time is quite accurately synchronized to the GPS system. The almanac is successively used to search for more satellites that should be in view, but will fail over to look for others if these are not found. Once a satellite is found, if the ephemeris data has expired, it will listen for the ephemeris information which takes around 30 seconds to receive. This is the main difference in the 'hot' 'warm' and 'cold' start time for a GPS receiver.
If no power is maintained in the GPS, at the very least the time will be lost, and more typically all of the almanac information is wiped out as well, resulting in a position acquisition time of a minute or more.