ALOS Data Users Handbook

ALOS Data Users HandbookSection 7 Overview of Processing Algorithms7.1 Preconditions7.1.1 Time Distribution SystemIn ALOS, spacecraft time is managed on the basis of the GPS time system. 1PPS (pulse per second) referencepulse which is a base and constitutes the spacecraft time is distributed in timing to synchronize with aninteger second of the GPS time. The time data is distributed in the format of the GPS time (GPS weeknumber, accumulated seconds of week). Time management in ALOS is unified as follows:1) The DMS (Data Management System) distributes 1PPS reference pulse / 1MPPS reference pulse tothe mission instruments. This reference pulse is set to the spacecraft time.2) 1PPS reference pulse and 1MPPS reference pulse are distributed to the DMS from the GPSR (GPSreceiver).3) The DMS distributes the reference pulse after calibrating its pulse by the reference pulse distributedfrom the GPSR.4) The DMS distributes the spacecraft time to the AOCS (Attitude and Orbit Control System) and othermission instruments.5) If an anomaly occurs in the GPSR, the DMS distributes the reference pulse based on the DMSinternal clock, not performing calibration by the reference pulse from the GPSR.6) It is judged by “time system status” in the TT&C system telemetry whether the DMS is calibratingthe spacecraft time by the reference pulse distributed from the GPSR or not.7) Even if the 1PPS reference pulse which is distributed from the GPSR shifts to the DMS internalclock, time data is distributed as a continuous sequence data before shifting the reference pulse.8) If the GPSR is recovered to a normal state while the DMS time system is in use, the time system ofthe reference pulse is switched automatically to the GPS time system from the DMS time system. Ifthe operation by the DMS time system continues for a long time, a time lag may occur, but returningto the GPS time system from the DMS time system is carried out by a command according to theoperation constraints. Since this is done manually, an automatic switch of the time system from theDMS to the GPS during observation does not exist.7-1