Originally Posted by pshute
Yes, but you said it needed a fresh copy after 7 days. How do non-cell GPS units get a fresh copy?
Because the cell phone only keeps a 7 day subset due to memory limitations.
Non-cell devices have the complete version stored in the device. When you do a map update or connect the GPS to the Internet, the ephemeris is also updated if needed. It isn't something that is constantly changing. Time and location are the primary variables. The orbits are known and fairly constant as is the trigonometry to figure it all out.