GPS World - February 2008 - (Page 34)

SYSTEM DESIGN & TEST | Galileo Outlier due to GIEN station maintenance p FIGURE 8 RMS of the difference over 1 day between the GGTO prediction and the a-posteriori determination p FIGURE 7 Prediction performance of E-GGTO2 baselined for the GIOVE navigation message (3 different arcs) GPSt) with the GPS satellite clock estimation done within the GPC (since the GIOVE and GPS satellites are processed together by the on-ground E-OSPF software).This is designated as EGGTO2. Finally, two-way time-transfer techniques between INRiM and USNO can also be envisaged leading to E-GGTO3, although this requires dedicated hardware and implies more technical complexity. In FIGURE 6, the real performance of the first two methods can be found, in terms of E-GGTO1 (green) and E-GGTO2 (red) values (left-hand scale). Note, for E-GGTO1 an approximation has been used (E-GGTO1 approx) since the calibration of USN2 to GPSt was not yet finalized during the experimentation. In blue (and right-hand scale) the value of USNO(UTC)USN3 – GPSt is plotted, in yellow the standard deviation of E-GGTO2. It can be obtained that E-GGTO1approx and E-GGTO2 coincide at the few-nanosecond level. The E-GGTO2 standard deviation is the standard deviation when averaging the satellite clocks at each epoch (stdev of GPS time from satellites). It is not the uncertainty of E-GGTO estimation. With the current configuration (GPS Block IIR only, 2-sigma rejection), this value stays around 2 ns. E-GGTO estimations are predicted into the future for dissemination to users (GIOVE navigation message). Here, we use a linear model and E-GGTO2. This value/method is independent from USNO data or the USN3 station. In the current setup, all the estimation arcs are used to fit the prediction model. The 34 GPS World | February 2008 p FIGURE 9 C1C-C7Q interfrequency bias temporal evolution p FIGURE 10 Group delays values estimation comparison between GPC and IGS predictions are compared against estimations in FIGURE 7. The E-GGTO2 prediction error is fairly stable over time. We obtain an RMS error over 1-day predictions usually below 2.5 ns. The best case so far is 0.75 ns (see FIGURE 8). Delay and Bias. The navigation message includes parameters to enable the single-frequency user to correct the ionospheric delays at his location. Ionospheric delay is one of the biggest contributors in the single-frequency user error budget. www.gpsworld.com http://www.gpsworld.com

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GPS World - February 2008 Contents Out in Front Expert Advice Galileo's Second Coming Soon UPS First to Use ADS-B European Industry Group Galileo Validation 2008 Antenna Survey Tsumani Detection by GPS Classifieds Advertisers Index Seen + Heard

GPS World - February 2008

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