GPS World - February 2008 - (Page 31)

Galileo | SYSTEM DESIGN & TEST satellites being tracked. Today GIOVE-A routinely transmits navigation messages that are uploaded automatically at a given epoch by the GIOVE-A Ground Control Centre (GSC-A) in Guilford, United Kingdom. In principle, the navigation message transmission proceeds through the following steps. See the OPENING GRAPHIC, GIOVE Overall Architecture, for data flows inside the system: 1. Raw measurements are collected from the 13 GIOVE Experimental Sensor Stations (GESS) in the mission segment, distributed worldwide. 2. GIOVE-A orbit and clock evolution are determined by the Orbitography and Synchronisation Processing Facility (OSPF) every two hours using a set of measurements collected over a two-day period. www.gpsworld.com 3. Keplerian parameters plus corrections are estimated that locally fit the determined orbit. The implemented fit interval is eight hours. 4. Second-order polynomial parameters are estimated that fit locally to the clock evolution. 5. The parameters are assembled in the navigation message as required, and files are generated, one for each carrier frequency, and stored in the archive (the Data Storage Facility, DSF) of the GPC. 6. These files are retrieved from the DSF and transmitted from the GPC to the GSC-A. 7. The GSC-A periodically (each day at 12:00 GMT) collects the most recent navigation messages, checks and assembles them in order to uplink them to the spacecraft during the subsequent contact. To provide some autonomy, the most recent navigation message is copied across 3-hour buffers that enable transmission, without contact with ground, for 14 days (see FIGURE 1). 8. The GIOVE-A satellite applies the measures to reduce data corruption and loss (Forward Convolutional Encoding and Block Interleaving) and transmits the navigation message on top of the modulated carriers. 9. The 13 GESS collect raw measurements from GIOVE-A, including the navigation message bits, and deliver them to the GPC. 10. Data generated in steps 2 and 3 is compared to data collected in step 9. Important operational aspects such as timeliness, robustness, and actual quality/ performance, were analyzed and validated in a complete campaign carried out over the past few months. This article reports the main results of the campaign. Operational Aspects It has been veriﬁed that enough data is available from the GESS to estimate precisely the GIOVE-A orbit and clock. As an example, the availability of the observables can be seen in FIGURE 2. Factors influencing the availability of data collected locally in the stations are related mainly to the lack of observables due to degraded quality of the RF environment at the sites and the reliability of the GESS themselves. Furthermore, since the GIOVE Mission Processing Centre is not operated during the weekend, some of the navigation message transmission gaps cannot be recovered in a timely fashion when operators are not present. Performance Aspects Different criteria can be considered when discussing the performance of the navigation message: The predicted computed position of the spacecraft at a given epoch based on Keplerian parameters transmitted in the navigation message and the corresponding a posteriori estimation. Similarly, it is interesting to determine the difference between the predicted computed clock timing based on the polynomial model transmitted in the navigation message and the a posteriori estimation. Related to the clock performance, for GPS+Galileo users the time offset between both systems is transmitted and validated in various ways. For dual-frequency users, an accurate estimation of the interfrequency bias of the satellite is needed from the navigation message. For single-frequency users, an accurate estimation of the ionosperic delay is needed in terms of model coefficients (NeQuick, the ionospheric model p FIGURE 1 Navigation message closed-loop rough timeline February 2008 | GPS World 31 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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