Consideration of Practical Visibility of Geostationary Augmentation Satellites from Aircraft

George Kinal

Abstract:	The augmentation of the GPS system by means of geostationary satellites is now, after a long period of discussion and development, approaching realization, with the FAA’s Wide Area Augmentation System (WAAS) under procurement, and the first appropriate geostationary satellites, Inmarsat’s third generation, in the final stages of construction preparatory to launch. The practical utility which a mobile user gains from such an overlay will depend both on the data provided over the channel and on the availability of the signals under realistic operating conditions. In particular, some analysts have claimed that geostationary satelliies are not useful for aviation interests at latitudes greater than perhaps 45 or 50 degrees, since typical aircraft banking (rolling) maneuvers would restrict the visibility of the geostationary orbit [1,2]. Satellite signal ‘visibility” (more accurately, receivability) is relevant not only for geostationary satellites but for the GPS satellites as well. Aircraft maneuvers can, in principle, result in a loss of signal continuity through blockage by portions of the airframe. This paper reports on analytic work intended to estimate the visibility of satellites, particularly geostationary, in more detail. Optical sight horizons for several typical airframes are modelied. ft is shown that for a representative deployment of geostationary augmentation satelliies, typical aircraft maneuvers (for example, a 25 degree roll) do not result in simultaneous blockage of all the augmentation satellites available. This is because, even at high latitudes where all the augmentation satellites are at moderately low elevation angles relative to the earth’s surface, they are nevertheless at significantly different azimuths and so incur different changes relative to the airframe local horizon at any given time. Actual flight test data is presented and analyzed regarding the reception of actual GPS signals. These data indicate that the optical horizon is a conservative bound on satellite signal visibility. As one might expect from GPS antenna patterns, there is a reduction in gain, therefore signal strength, below 10 degrees. However, the gain does not drop off suddenly below the optical horizon, even to 5 or 10 or more degrees bebw. In one instance, a GPS satellite was tracked, albeit with further reductions in signal strength, to nearly 40 degrees below the optical horizon. A brief discussion of actual flight procedures is included, basically indicating that large changes in attitude do not occur in the final approach phase of flight, when the WAAS data are most critical, so that concerns about blockage from sustained 25 or 30 degree banking angles are in any event misplaced and misleading. Conclusions are drawn about installation guidelines for WAAS antennas, WAAS receiver specifications, and antenna designs. lt is concluded that a side effect of the FAA’s requirement for a large number of WAAS satelliies is that satellites will in fact be available over a range of azimuths, and that Europe, Africa, and the Middle East are, fortuitously, also well served by overlaps in Inmarsat-3 coverages. Satellite selection algorithms in WAAS receivers should employ both elevation angle and azimuthal separation in selection of which WAAS satellites to track, if more than two are available.
Published in:	Proceedings of the 1995 National Technical Meeting of The Institute of Navigation January 18 - 20, 1995 Disneyland Hotel Anaheim, CA
Pages:	79 - 87
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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