Timing accuracy of the Ulysses GRB experiment

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Abstract
In the triangulation, or arrival time analysis method of burst localization, the most widely separated spacecraft pair generally provides the strongest constrains on the final error box. Thus a knowledge of the systematic timing uncertainties in these two spacecraft is quite important. In the 3rd Interplanetary Network, spacecraft pairs involving Ulysses have usually been the most widely separated, since the Earth‐Ulysses distances reached 6 AU. It is therefore essential to be able to demonstrate that no large systematic errors are present in the Ulysses timing. The design goal was to maintain such errors in the range of several ms at most. Here we describe an in‐flight, end‐to‐end calibration procedure which is carried out about twice every year, to verify the timing accuracy. It involves sending precisely timed commands to the GRB experiment through the Deep Space Network, and timing the arrival of the signal at the Ulysses spacecraft. We show that it is possible using this method to verify that the design goal has been met, and therefore that, in practically all cases of interest, systematic timing errors do not dominate the location uncertainties of IPN bursts.

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