Digital data is further encoded on the S/Vís signal at a 50Hz frequency.
? Time, and known satellite clock error
? Approximate ephemeris for other satellites
? The entire message takes 12.5 minutes so a cold-started GPS can take a while to ďlock onĒ.
? Another complication is that the satellites are typically moving at several miles/sec relative to receiver. This Doppler-shifts the GPS carrier frequency. The receiver thus also has to search in frequency to lock on to them. This happens quicker if the receiver already has a good ephemeris, because then it can calculate the required shift, approximately, in advance.
? The receiver measured Doppler shift is used to help measure the receiverís velocity.
? Your GPS will have a page where you can enter your approximate position to help them lock on.