GPS World - February 2008 - (Page 32)

SYSTEM DESIGN & TEST | Galileo p FIGURE 2 Availability of GESS data during November 2007 p FIGURE 3 Comparison between orbit predicted using navigation message and orbit estimated a posteriori for Galileo, for which the necessary real-time parameters will be broadcast in the Galileo navigation message; analogous to Klobuchar ionospheric model for GPS). Orbits and Clocks. The following errors contribute to the orbit and clock performance: Non-modeled correlated errors in the GESS receivers’ raw measurements Inherent errors in orbit and clock estimation process Limitations in the underlying models (Keplerian orbits, parabolic polynomial model) Limitations in the dynamic range of the parameters used in the navigation message. FIGURE 3 shows the error a user would experience computing the position. The error is less than 1 meter after 5 hours in the radial direction, the most important from a navigation point of view. FIGURE 4 provides an independent means of checking the adequacy of the computed coefficient, by comparing the navigation message’s predicted orbit with the orbit determined using laser-ranging data. Note that in this figure the errors are expressed in WGS-84 rectangular coordinates and the fit interval is 4 hours. FIGURE 5 shows the navigation message clock errors. The error is below 5 nanoseconds (ns) peak-to-peak over one day. Experimental Time Offset. GGTO is the offset between the GPS and Galileo system times (GPSt and GST, respectively) and is generally a function of time. Since GPSt and GST both have stringent stability requirements and are both steered/monitored versus UTC, GGTO should be quite stable as well. GGTO is mainly needed by users of combined Galileo+GPS receivers. GPS satellites provide navigation information (clocks) referenced to GPSt. Galileo satellites provide navigation information (clocks) referenced to GST. GGTO is needed to enable users to compute p FIGURE 4 Navigation message versus European Space Operations Centre satellite laser-ranging orbit computed a posteriori (interval 4 hours) 32 GPS World | February 2008 p FIGURE 5 Comparison between clock predicted using naviga- tion message and clock estimated a posteriori 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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