Satellite Visibility and Availability A Ganesh and R Narayanakumar The satellite visibility and availability is an essential parameter for precise positioning and surveying. Users of GPS must know where, when and what satellites should be tracked to attain the best results. The following are the essential measures a GPS operator is expected to know, during positioning. The elevation angle: – is the angle from the antenna between the horizontal and the line of sight to the satellite.
The azimuth:- is the clockwise angle from north to the location of the satellite in the sky. Mask angle:- . The mask angle refers to the elevation angle below which GPS signals will not be recorded. A satellite is said to be visible if it is above the specified mask angle provided there is no obstructions are present. GPS receivers, processing software, or both may have an option to set a specific mask angle or cutoff angle. Obstructions:- Obstructions are objects which block the path between a satellite and receiver.
For example, if a desired satellite is at an elevation of 20° and azimuth of 70°, and a building is located at the same elevation and azimuth, the satellite signal will be obstructed. The avoidance of obstructions is very important to the successful application of GPS positioning. Almanac files: – for any given location on the earth and time, it is possible to predict which satellites will be available and their location in the sky. This is accomplished by using almanac files which contain satellite orbit parameters, in conjunction with software designed to use almanac files to compute satellite visibility. pic] Elevation and Mask Angle Azimuth (Courtesy: – Geomatics Canada, Information Services, 615 Booth Street, Ottawa, OntarioK1A 0E9) Software packages to compute satellite visibility are available commercially and often accompany commercial GPS receiver software. When computing satellite availability, one should be careful to use only recent almanacs, no more than one month old. Satellite Geometry: – Sky plots are generally used to represent satellite visibility. Each concentric ring represents an elevation angle, while each radiating line represents an azimuth.
In the figure, the shaded area, below 15° elevation represents the mask angle. The path of all visible satellites over a two hour period is plotted. The numbers indicated on each plotted line are the satellite numbers. For example, satellite 13 is shown in the plot as tracing a path from an elevation angle of 40° and azimuth of 270° to an elevation angle of about 60° and azimuth of 10°. [pic] A Sample Sky plot Satellite geometry has a direct effect on positioning accuracies.
The best single point positioning accuracies are achieved when satellites have good spatial distribution in the sky (e. g. one satellite overhead and the others equally spread horizontally and at about 20° elevation). Sub-optimal geometry exists when satellites are clumped together in one quadrant of the sky. The geometry of satellites, as it contributes to positioning accuracy, is quantified by the geometrical dilution of precision (GDOP). Satellite configurations exemplifying poor and good GDOP are illustrated in Figure. [pic]