The Delay Lock Loop

Code Correlation

Source: GPS for Land Surveyors

Once correlation of the two codes is achieved with a delay lock loop (DLL), it is maintained by a correlation channel within the GPS receiver, and the receiver is sometimes said to have achieved lock or to be locked on to the satellites. The receiver can continue to log the signal from the satellite and stay correlated unless it is somehow interrupted by a cycle slip or an obstruction.  If that happens, the receiver is said to have lost lock. However, as long as the lock is present, the NAV message is available to the receiver. Remember that one of its elements is the broadcast clock correction that relates the satellite's on board clock to GPS time, and a limitation of the pseudorange process comes up.