posted 03-30-2011 11:38 AM ET (US)
I am not an expert of GPS receivers, but I believe that happens when a GPS receiver is first activated is along these lines:--the receiver wants to know the date and time
--the receiver wants to know is approximate position
--based on the date, time, and position the receiver predicts what satellites may be in view to it
--the receiver tries to find a signal from a satellite using a particular psuedo-random noise coding.
The signals from GPS satellites are all sent on the same frequency and are modulated with different psuedo-random noise (PRN) encodings. This is called a spread spectrum technique or carrier division multiple access (CDMA). The signal to noise ratio of satellite signals received here on earth are not very great, and the PRN encoding actually helps them be received with better immunity to interference.
The receiver makes an educated guess about what PRN to use to listen for a signal. If the PRN is not the correct PRN for one of the satellites that is actually in view, the receiver will not find any signal. After a while it will give up and try another PRN. It repeats this process until it finds the first satellite.
Since there are 32 PRN sequences, it can take a while for the receiver to stumble onto a satellite if there is a serious error in the data, time, or position.
To predict what satellites will be in view, the receiver needs to have an ephemeris of the orbits. If the ephemeris is out of date or cannot accurately predict the current satellite position, the acquisition of the first satellite signal may take a long time.
I have a very old GARMIN model 300 hand held receiver. It is so far out of date, about 20-years, that it is very slow to acquire satellite signals. The last time I got it to work I left it outside and running for about 48-hours before it finally stumbled onto some satellites.
I recommend you let your old unit sit outside with a clear view of the sky and some fresh batteries for a day or two. You may find that it will eventually find satellites.
Older receivers could only track one satellite at a time and had to time share among available satellites. Newer receivers can track as many as 20 satellites in parallel. Receiver sensitivity has also been improved.