Ian Sheppard/LONDON
A worldwide observation campaign to define more precisely the characteristics of the Russian Glonass satellite navigation system will run from September to December this year. It will be conducted by the geodetic community, which is relying on combining the Glonass with the US global positioning system (GPS) to give millimetre accuracy 100% of the time anywhere on Earth.
Achieving full GPS/Glonass interoperability will not be easy, but the International Glonass Experiment (IGEX 98) team has the advantage that a similar programme was run in 1992 to improve understanding of the GPS.
The co-ordinator of IGEX 98 is Pascal Willis, from the French National Geographic Society in Champs-sur-Marne, who says that "-there are a lot of implications for navigation", despite the project being primarily aimed at the geodetic community and the demand for real time, millimetre-accuracy applications. With interoperability between the two systems, users will have six or seven satellites in view at all times where now they regularly have only three or four GPS craft, says Willis, not enough to filter out multipath and other effects.
The main objective of IGEX is to determine satellite orbits more accurately in the international standard reference frame (ITRF), rather than the PZ-90 system used by Glonass, and to determine transformation parameters to map these on to the GPS WGS84 reference frame, while also connecting the two system times.
The offset between system times is around 20s and, while GPS time is maintained to within 20 nanoseconds, Glonass time varies by up to 7s. Russia, therefore, is considering broadcasting the difference as part of the navigation message, and has been prompted to steer its system time closer to GPS.
With the advent of dual-frequency Glonass receivers, there is a requirement to demonstrate that the system can operate in differential mode, so only receivers capable of collecting pseudorange and carrier-phase observations are to be used, while the test sites where receivers are positioned, if not co-located with GPS receivers, must have well-defined ITRF co-ordinates .
Transformation parameters are already programmed into combined GPS/Glonass receivers now being offered, says Willis, although these are often limited to specific regions. The basic seven-parameter transformations used are to be improved to give nominal values which will work everywhere, and which will be available to all manufacturers.
The first results of the experiment will be available by the end of this year, says Willis.
MONEY MATTERS
Charles Trimble, president of US GPS receiver manufacturer Trimble Navigation, says that the additional ranging signals which Glonass provides are essential because of the demand to augment the GPS to allow positioning "100% of the time". He is disappointed, however, that Glonass is down to around 13 useable satellites, with a failure rate of four a year.
Another problem is that "-the Europeans got a bad taste with the [International] Space Station", believes Trimble, putting them off linking with the Russians, even though Eurocontrol admits to being serious about the possibility of coupling the Glonass with the GPS and the proposed European Geostationary Navigation Overlay System (EGNOS) as a step towards a second-generation global navigation satellite system, called the GNSS-2.
Although Glonass funding uncertainties have cast a shadow over the system's long-term usefulness, Europe and the USA have also run into trouble with plans for new satellites to augment the GPS. Both the EGNOS and its US equivalent, the Wide Area Augmentation System (WAAS), have met opposition, the former from European airlines concerned that they will have to pay for the system and the latter from the US Congress, anxious over spiralling costs.
The European Commission sees the EGNOS as a way of gaining independent expertise and is determined to push on with the augmentation system, for which the first contract is due to be placed this month.
Another area of controversy is the frequency spectrum allocation for positioning services, which the European Commission (EC) wants to use for data communications. Trimble says: "We are fighting hard as part of the GPS community to preserve the frequency spectrum, including [that for] Glonass."
The EC proposal would remove 8MHz at the lower end of the GPS spectrum, which some have warned would undermine the "safety-of-life" aspect - the guaranteed availability of the signal - critical to its use in civil aviation.
Source: Flight International
