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

The GPS phase measurements described were obtained using two similar multi-channel GPS ASHTECH Z12T receivers belonging to the BIPM and the BNM-LPTF. These receivers are based on the conventional geodetic ASHTECH Z12 unit which has been modified to meet the stability requirements of time and frequency comparisons. Comparison of the two receivers operated side by side in different antenna configurations shows typical short-term noise of 1.1 ps to 3.5 ps. Longer term variations indicate a temperature-sensitivity in the equipment which constitutes a limit to the performance of the GPS phase method. One of the receivers was successfully operated using a temperature-stabilized antenna, and the ASHTECH antenna which feeds the second receiver was placed in an home-built oven, maintained at a constant temperature. These precautions make it possible to reduce a number of systematic effects. A separate study of frequency comparison was carried out between two hydrogen-masers, located at the BNM-LPTF, and the PTB, Braunschweig, Germany, using `ASHTECH Z12T-like´ receivers. The relative frequency stability obtained is about 6×10^-15 for an averaging time of 30000 s, an interesting result facing the outstanding performance of new ultra-stable frequency standards