Both Galileo In-Orbit Validation Element (GIOVE) satellites can transmit prototype Galileo signals in all three bands, E1, E6, and E5a/E5b, with signal sufficiently representative to the envisaged final Galileo signals to enable the securing of the frequency filing, and to enable demonstration of preliminary navigation services. The provision of the navigation messages can be accommodated through the uploading of data files over the S-band communications link, which are then provided to the navigation signal generator and incorporated into the navigation transmissions.

The GIOVE-A navigation signal-in-space interface control document (SIS ICD) is available from ESA's GIOVE web site: http://www.giove.esa.int/images/userpage/GIOVEA_SISICD.pdf. Following in-orbit test review of GIOVE-B, its navigation SIS ICD will also be made available in summer 2008.

Generating Nav Messages

Transmission of the navigation message is a key element of any GNSS system. Without the information contained in the message, the user would not be able to fruitfully take benefit of the 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.

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.

Figure 1 Navigation message closed-loop rough timeline

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.