Inertial navigation, some aircraft have three separate units, some only have one, you feed in the Lat/Long to the unit and after it spins up it can detect what direction you are moving without any outside help from radio aids, in the old days a platform took about 30 minutes to spool up, a modern laser ring gyro can accurately sense a 1˚ change in angular displacement directly, and hence 7 minutes at mid-latitudes is sufficient to find the direction of true North.
An aircraft has to be stopped during this spool up process, an aircraft having an ir problem may attempt a quick alignment at the end of the runway to fix the problem.
180MPH
