Analysis of Precise Time Data Out Messages for the GEM-S GPS Receiver onboard BIRD

Kurzfassung

In the framework of the BIRD mission, it is intended to steer the onboard clock using the onboard GPS receiver GEM-S. When the GEM-S is in the so-called NAV mode, and has decoded the UTC-GPS time offset from the GPS navigation message, a 1 Pulse-Per-Second (1PPS) is output with a corresponding Precise Time Data Out message indentifies the UTC rollover of the 1 PPS, and will be output within 100 ms of the corresponding pulse. This note analyses the Precise Time Data Out messages, that have recently been collected in the course of a hardware- in-the-loop simulation. To this end the messages have been decoded and the time difference of GPS and UTC time as well as the Time Figure of Merit (TFOM) has been computed and analyzed. To relate these numbers with the number of tracked satellites and the receiver acquisition performance, the Time Mark Out Message (ID 3) has also been decoded. The following results were obtained: 1. The Precise Time Data Out message and the 1 Pulse- Per-Second (1PPS) are transmitted by the GEM-S only after the receiver has undergone a certain startup phase. 2. The Precise Time Data Out message contains valid date and time data only when the Time Figure of Merit (TFOM) is 8 or less. 3. The time difference of GPS and UTC time can be derived from the Precise Time Data Out message or the Time Mark Out message. 4. Valid UTC times are expected at about 2 minutes after the initalization of the receiver. 5. The current year is not given in the Precise Time Data Out message. It has thus to be provided either by the Time Mark Out message or by external sources. Althougt the great majority of tests for a steering of the BIRD onboard clock still have to be done, this note provides a first step towards a time synchronization concept.