This paper and the following five papers were presented at the NAV91 Conference on Satellite Navigation held at Church House, London, 6–8 November 1991. Copies of the conference proceedings are available from the Director, price £85 (non-members) and £65 (members).

In the last two years, the idea of a civil geostationary overlay (augmentation) to GPS and GLONASS has moved from the discussion/concept stage to something approaching reality. Many details of implementation have been or are in the process of being determined. Major developments that have taken place in this period of time include: (i) INMARSAT'S adoption of a four ocean region satellite deployment (three had been employed previously), now including one region (AOR-West) that covers the conterminous United States, (ii) Inclusion in the specifications, and contracting for, navigation repeater payloads in the four INMARSAT-3 spacecraft, now under construction, (iii) Major improvements in the Test Bed, bringing the test signals closer in function and structure to those to be provided through the INMARSAT-3 navigation payloads. (iv) Plans in the US and Europe to employ the Test Bed, or a facsimile thereof, for field trials of the GPS/GLONASS integrity channel (GIC) concept, (v) Endorsement of the ‘Wideband GIC’ concept (transmission at GPS L, frequency of a spread spectrum signal) by many members of the aviation community, (vi) Introduction and study of the possibility that the Gic might be able to carry some form of differential corrections in addition to the basic integrity and constellation augmentation functions. In other words, whereas two years ago we were still considering the possibility of a civil element of global satellite radionavigation, this year we are deeply in the process of putting the necessary elements into place.