Use of GPS ASHTECH Z12T receivers for accurate time and frequency comparisons

The GPS phase measurements described in this paper were obtained using two similar multichannel GPS ASHTECH Z12T receivers belonging to the Bureau Int ernational des Folds et Mesures, BIPM, and the Laboratoire Primaire du Temp s et des Frequences, BNM-LPTF. These receivers are based on the conventiona l geodetic ASHTECH Z12 unit, which has been modified to meet the stability requirements of time and frequency comparisons. Comparison of the two recei vers operated side by side in different antenna configurations shows typica l short-term. noise of 1.1 to 3.5 ps. Longer term variations indicate a tem perature sensitivity in the equipment, which limits the performance of the GPS phase method. One of the receivers was successfully operated using a te mperature-stabilized antenna TSA from 3S Navigation, and the ASHTECH antenn a, which feeds the second receiver, was placed in a home-built oven maintai ned at a constant temperature. These precautions made it possible to reduce a number of systematic effects. A separate study of frequency comparison w as carried out between two hydrogen-masers located at the BNM-LPTF (Paris, France) and the PTB (Braunschweig, Germany) using receivers similar to ASHT ECH Z12T receivers. The relative frequency stability obtained was about 3.3 X 10(-15) for an average time of 15 000 s, an interesting result comparabl e with the outstanding performance of new ultrastable frequency standards.