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 instability of GPS common-view time transfer receivers. The delay instability of interest is at long term time intervals out to about one year. Our measurements of various cables indicate a range of power for multi-path signals from the cables arriving at the receiver. For NBS-type receivers with traditional cables we find significant variations of the reflected power with frequency changes around the 75 MHz IF. The levels range from -16 dBc to -27 dBc. For a commercial receiver with the signal transmitted from the antenna to the receiver at the L-band frequency, the best result is for a special cable system we have built with a reflection loss of -30 dBc. With new cabling systems, any effects due to reflected signals in the cables for either the NBS-type or the commercial receivers should be below 1 ns. We include several weeks of data from receivers using these cables. A full evaluation of the results will require many months, and perhaps years, of data.