Monitoring selective availability dither frequencies and their effect on GPS data

The signals ttansmitted by Block II satellites of the Global Positioning System (GPS) can be degraded to limit the highest accuracy of the system (10m or better point positioning) to authorized users. This mode of degraded operation is called "Selective Availability" (S/A). SlA involves the degradation in the quality of broadcast orbits and satellite clock dithering. We monit IO, kIn can be examined with short test baselines if the receiver clocks are deliberately mis-synchronized by a known and varying amount. Our results show that the maximum effect of satellite clock dithering on GPS double difference phase residuals grows as a function of the clock synchronization error according to: S/~Ita=O.04 cm/msec. and it increases as a function of baseline length like: S/~"" =0.014 ctn/IOO kID. These are equations for maximum observed values of post-fit residuals due to S/A. The effect on GPS baselines is likely to be smaller d1an the 0.14 mm for a baseline separation of 100 kID. We therefore conclude. for our limited data set, and ffX'the level of SlA during our tests, that SI A clock dithering has ~gligible effect on all terrestrial GPS baselires if double difference processing techniques are employed and if the GPS receivers remain synchronized to better d1an 10 msec. S/A may constitUte a problem. however. if accurate point processing is required. or if GPS receivers are not synchronized. We suggest aIxi test tWo different medtods to monitor satellite frequency offsets due to S/A. SlA m<xiulates GPS carrier frequencies in the range of -2 Hz to +2 Hz over time periods of several minutes. The meth<xis used in this paper to measure dte satellite clock dither could be applied by the civilian GPS community to continuously monitor 51 A clock dithering. The monitored frequencies may d high accuracy point p~itioning applications in a postprocessing mode (Malys and Ortiz 1989), and differential GPS with poorly synchronized receivers (Feigl et al. 1991). Introduction