In search of a new primary GPS receiver for NIST

A previous publication [1] showed problems with the current NIST Time and Frequency Division primary GPS receiver when used for Precise Point Positioning (PPP)-based carrier phase time transfer. We confirm that, for this receiver, boundary discontinuities during overlapping data runs tend to be biased away from zero on average and that this bias increases as the a-priori pseudo-range sigma increases. We show that this problem does not occur for other receivers at NIST, even receivers of the same model or make. Next we review results for selecting a new primary receiver from others now at NIST, focusing on two desired properties: an average overlap bias close to zero using PPP, and a low code instability. We want at least one year of good data on a receiver before considering it as a replacement.