There are five monitor stations, three up link stations, and one master control station. The U S Air Force has responsibility for this segment. The user segment tracks and receives the radio navigation signals and computes user position and time. Ll at 1.575 GHz and L2 at 1.227 GHz. Spread spectrum modulation is used.
P code is encrypted and for military use only.
The satellite receivers determine time and position by process which starts with measuring the time of arrival of the satellite signals. Each satellite broadcasts information which permits calculation of the position of the satellite. Hence, for most real world computer synchronization applications whether SA is on or off is not a relevant concern . ACTS is provided by the National Institute of Standards and Technology, NIST, in Boulder, Colorado. There is no charge for accessing the ACTS service other than standard long distance charges.