Sweden and some other countries are currently evaluating and verifying the potential impact of a newly developed, satellite-based GNSS transponder system on future CNS/ATM and ATN systems.

The concept provides the possibility to introduce a distributed, mobile, real-time data processing system, for all properly equipped users in the global civil aviation community being interacting members (air/air, air/ground and ground/ground).

Inmarsat commissioned the field study to be carried out by Signal Computing Ltd. in association with the Royal Greenwich Observatory. The study aimed to identify the commercial suitability of Wide Area Differential GPS (WADGPS) corrections. The corrections provided to users will be relayed via Inmarsat-3 geostationary satellites and are required to be valid over the footprint of an entire Inmarsat ocean region (approximately one third of the Earth's surface).

The study has been conducted within the Inmarsat Atlantic Ocean Region East. Trials took place over a five-month period to achieve a representative set of data. Six widely-distributed collection sites were used to ensure that the maximum duration of satellite data were received and that a representative model for atmospheric effects was obtained. Analysis is restricted to the use of GPS carrier and phase data obtained using the C/A code transmitted by Block II satellites only.

This paper presents the analysis of data collected during a four-day trial period. The ability to determine the GPS satellite ephemeris (independent of GPS broadcast ephemeris), Klobuchar model parameters tailored to the oceanic region, and satellite clock corrections using GPS data obtained from this limited deployment of collection sites is addressed. The primary degradation effects on stand-alone GPS positioning of Selective Availability (SA) are corrected using WADGPS, resulting in a significant improvement in position accuracy.

This paper and the following 5 papers were presented at the Institute's NAV93 Conference on The Application of Advanced Systems. The conference was held in London in November 1993. The full Proceedings, containing some 42 papers, are available on application to the Director, price £30 for Members and £40 for Non-Members.

The paper opens with a brief description of the formation and roles of the Civil GPS Service Interface Committee (CGSIC) and its various Sub-Committees, defining clearly the concept of civil GPS status and health information, and the information services which are necessary. The paper then concentrates on international implications, referring to the activities of the CGSIC's international Sub-Committee, the International Information Sub-Committee (IISC). The concepts of national points-of-contact and national information centres are introduced together with a description of their perceived functions, and this is followed by discussion of the products which they are expected to make available. The paper continues to describe the consequences that have been encountered by the IISC, and the action that has been or is being undertaken. The concept of so-called ‘active distribution’ is defined, and is followed by discussion of the dissemination not only of GPS status and health information, but of information about how that information can be obtained. The possibility of legal implications and the consequences is also discussed and developed towards the arguments for appropriate international agreements regarding the civil use of the Global Positioning System.

Recently, there has been wide-spread interest in the use of the Global Positioning System (GPS) for precision approaches in a civil aviation environment. However, most of the work to date in Europe and North America has treated a GPS/inertial reference system (IRS) combination as the source of guidance for this phase of flight. Since the US has a very large general aviation population, the FAA is very interested in determining the role of a stand-alone implementation of differential GPS (DGPS) as a navigation sensor for precision approaches. This paper has two parts: a summary of the analysis of this application of DGPS; and a discussion of the navigation satellite test bed that is under development at the FAA's Technical Center.

The first part contains proposed accuracy, integrity and availability operational requirements, and their air traffic implications. A stand-alone DGPS concept is developed, and analysed. The major results of the analysis indicate that stand-alone DGPS may not fully meet Category I (CAT I) accuracy requirements. The results of the analysis are then used to provide the rationale for the major recommendation: that FAA pursue an implementation of a wide-area DGPS (WADGPS) to achieve a Near CAT I precision approach capability. The second part of the paper discusses the three-phase navigation satellite experimental programme that is being developed to acquire the data to validate the enhancements to GPS, including a GPS Integrity Channel (GIC) and WADGPS. The objectives of each of the experimental phases are presented and discussed. Then the experiments of Phase I that address GIC/WADGPS are discussed.

A research and development programme performed in Sweden over a period of 9 years has created a substantial know-how on the design of satellite navigation user systems. Thus, a basic user system, called the GP & c Total System has been developed and tested over 3 years. This system can serve as a basis for the tailoring of an almost endless number of applications. The system is currently using data from the GPS satellites, but can also be operated with combined GPS/GLONASS receivers. The Swedish GP & c System is an example of how the communication, navigation and surveillance (CNS) concept and the various elements (ADS, ATM, etc.) for a new global Air Navigation System published by the ICAO FANS Committee can be implemented and integrated into a user-friendly operational system. The ICAO 10th Air Navigation Conference (5–20 September 1991) endorsed the FANS concept in which the main elements are based on the implementation of satellite navigation technology.

In 1991 Trimble Navigation introduced a Global Positioning System (Gps)-based attitude determination receiver capable of 3-axis solutions with accuracy to several milliradians for airborne, sea and land platforms. This paper discusses the physical, architectural, and operational features of this receiver system. Analysis of system performance will also be reviewed for various configurations and user applications. The Trimble Navigation attitude determination receiver uses differential carrier phase techniques to determine azimuth, pitch and roll angles of a 3-antenna array. This product is designed to operate in a variety of user applications and to withstand rugged operating environments. Trimble Navigation has expanded its proven GPS sensor architecture to incorporate three independent RF sections and additional processing channels to measure and process the differential phase between antennas on two orthogonal baselines. Direct measurement of differential phase utilizing a common local oscillator provides highly accurate relative phase data. The navigation and attitude processor computes azimuth, pitch, and roll angles as well as position, velocity and time. The solution accuracy and stability statistics are sensitive to various parameters. Antenna baseline length, signal multipath, platform dynamics and filtering are investigated. Test data from static and various dynamic platforms are also presented.

The principal characteristics of the GLONASS system, its current status and deployment outlook are considered. Comparison of the main characteristics of the GPS/GLONASS systems from the user's viewpoint is made. Proposals focused on the operations to be carried out to improve navigation determination accuracy for the users, provided with the combined equipment, are given. The data presented, received simultaneously on the user&s GLONASS equipment and a GP–500 GPS system, indicate that navigational determination accuracies of both systems are approximately equal. The key characteristics of 6–channel user facilities for civil aircraft and helicopters are presented. This equipment has been developed in the Institute of Space Device Engineering (ISDE) in accordance with a standard draft ARING on the combined system. The principal directions of the accuracy characteristic of the GLONASS system updating are considered. It is suggested to use the quantum–optical measurements on the Etalon satellite, launched into GLONASS satellite orbit, so that ephemeris information can be improved. The questions concerning supplying the system integrity and differential corrections information are discussed. It is proposed to combine the global system of integrity signal generation and transmission with differential corrections for regional systems. The version of the information frame developed in ISDE and intended for transmission of this information to users via geostationary satellites by means of signals similar to GPS/GLONASS, and without additional complication of user equipment, is presented here.

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.