Satellite coverage

Map projections

The region illuminated by a satellite is called the coverage area or footprint. Within this coverage, service may only be offered to a selected region called the service area of the satellite. The shape and size of the coverage area depends on the satellite's antenna radiation pattern . The most elementary antenna pattern consists of a bell shaped circular pattern. The limit of useful coverage depends on the elevation of the satellite from the ground; the higher the elevation angle, the more reliable is a mobile satellite radio link but the lower is the coverage area. Minimum elevation angles of 5, 10 and 40' have been proposed. To make better use of satellite resources and reduce the size of the mobile terminal, it is usual to divide the coverage into smaller zones called spot beams. The number of spot beams range from a few to several hundred. To determine the coverage area of a satellite, its antenna pattern is projected to a map of the Earth as isocontours of antenna gain, or elevation angle, transmitted power or received power as necessary. A number of map projections are used, each suited to the application at hand.

Mercator projection is used commonly and is named after the Flemish cartographer Geraradus Mercator, who published a world map on such a projection for ship navigation in 1569. In this type of projection, the longitudes are represented by vertical lines spaced equally and latitudes by horizontal lines which are spaced closer near the equator than towards the pole. In such a map, coverage patterns get distorted.

Satelli-centric or hodocentric projection is used when it is necessary to preserve the shape of the antenna pattern. This projection is a view of the Earth from a satellite.

Polar projection is used by radio amateurs as it is quite simple for plotting ground tracks. In rectangular projection the X axis is represented as longitude and the Y axis as latitude.

The coverage contour of a satellite in GEO is stationary and hence visualization and design of the service area is simple as the relative position of any location within the service area is invariant. In non stationary orbit systems, the path geometry varies with time which requires a statistical description of visibility statistics from any given point. Section 2.3.8 summarizes visibility statistics of some non geostationary satellite systems.

Consider the salient coverage features of a few satellite orbits used in an MSS.

Polar orbits provide unbiased coverage to all geographic regions a single satellite can view the entire Earth over a period of time. Satellites in polar orbits have a large number of eclipses and require regular station keeping to maintain their orbital position. These orbits are well suited for applications, such as world wide MSSs and global Earth resource surveys. Satellite constellations in polar orbit tend to favour polar regions because all the satellites in the constellation converge towards the pole.