Enabling Accurate Differential Calibration of Modern GPS Receivers

摘要

The difference between the local time reference and a GPS receiver's internal time base is an essential contributor to the calibration of that receiver when used as a timing device. In the past, in the absence of a PPS (Pulse-Per-Second) output signal provided by the receiver, a one-time "tick-to-phase" measurement together with the reference PPS cable delay were used to represent the difference between the local time reference and the receiver's internal time base in the calibration process. The availability of a 1 PPS signal actually derived from the receiver's internal time base renders the "tick-to-phase" measurement obsolete, allowing for the measurement of the "tick-to-tick" time difference, which is a better representation of the relation between the receiver's time base and the local time reference. In the context of the synchronization between the MINOS (Main Injector Neutrino Oscillation Search) remote sites, NIST has developed an auxiliary electronic system to be associated with each receiver. Its core feature is the on-board time-interval counter (TIC) that continuously measures the receiver's "tick-to-tick" time difference, allowing for a dynamic and appropriate calibration of the receiver. The on-board TIC also measures the "tick-to-phase" time difference, albeit only for diagnostic purposes. Additional features of the auxiliary electronics, described in this paper, include RF power detection and conditioning, and pulse buffering and conditioning. Multiple calibration results for the MINOS synchronization setup will be presented and discussed.1