Long term effects of antenna cables on GPS timing receivers

We report efforts to minimize the effect of reflected signals in GPS antenna cables on the delay stability of common-view time-transfer receivers. The day stability of interest is at long-term time intervals from five days to about one year. We measured cables for a commercial receiver with the signal transmitted from the antenna to the receiver at the L-band frequency. In a typical system the lock point of the receiver could change almost 60 ns with cable lengths and temperature variations. This would occur if the reflected signal was 25 dB below the direct signal, and the electrical length of the cable was 217 ns and changed 165 ps, or about 5 cm. By terminating the antenna cable with good 50 /spl Omega/ matching, the reflection loss was 50 dB below the direct signal. This reflection loss, along with the use of phase-stable cables with delays under 120 ns, should keep the lock point change due to coherent reflections in the cable below 200 ps. A common-clock common-view experiment using 90 days of data showed stabilities of the differential delays below 100 ps for averaging times from 1 to 10 days. Whereas before the matching, stabilities were worse than 300 ps.