Time transfer data, derived from single frequency observations of GPS satellites, have shown a unique bowing characteristic in the residuals with an amplitude which sometimes amounts to about 80 ns. Investigations attempting to explain this characteristic have so far been unsuccessful. The studies have been concerned with hardware and software effects. The hardware studies have:

(a) compared data obtained with single frequency receivers with data obtained simultaneously with dual frequency receivers which eliminate the effects of the ionosphere;

(b) compared data obtained with different kinds of single frequency receivers; and

(c) compared data obtained simultaneously with different kinds of antennas, including one designed to minimize the effects of multipath.