
Geosynchronous Satellite Positioning

Author: Cass Lewart (rlewart@monmouth.com)

Program Description

Communications satellites provide "mirrors" in the sky, which reflect and 
amplify electronic signals. The effect of relaying these signals via a 
satellite is like having a very tall antenna, which provides extended 
coverage. The microwave signals in the Giga Hertz region of the frequency 
spectrum used for satellite communication travel in basically straight 
lines. Thus as long as the satellite is visible from a given location it 
can be used as a communications relay.

The idea of using artificial satellites as communications relays was first 
proposed by Arthur C. Clarke, best known for his film 2001, in a 1945 
article published in Wireless World. The article had to be approved by 
wartime censors, who passed it immediately considering it a piece of far 
out science fiction.

The movement of satellites in the earth's orbit is governed by Newton's and 
Kepler's Laws. The time of one revolution around the earth is proportional 
to 1.5 power of the height of the orbit measured from the center of the 
earth. Early communication satellites, such as Telstar, launched in 1962, 
circled the earth in circular orbits every 90 minutes at the height of a 
few hundred kilometers. These satellites were visible for only 10-20 
minutes at a time from a given location. Other satellites, such as those in 
the Russian Molniya series, were launched in highly elliptical orbits so 
that they would be visible for several hours at a time over certain areas 
of the earth. The operation of satellites in low circular or elliptical 
orbits requires large steered antennas which follow the satellite as it 
moves across the horizon.

Progress in rocket propulsion technology made possible launch and precise 
positioning control of satellites in much higher orbits. Geosynchronous 
satellites in circular orbits at an altitude of 35,800 km above the equator 
have an orbital period of exactly 24 hours and thus remain stationary 
relative to the surface of the earth. Since inexpensive fixed antennas can 
receive their signals, these satellites are used for communication 
purposes, including relaying of cable television programs. 

As the cost of receiving equipment decreases, many private individuals are 
installing antenna dishes in their backyards to expand their television 
reception. The satellite antennas are particularly popular in remote areas 
of the country, where only a few TV outlets and no cable are available. 

This program will help you to decide on the optimum placement for the 
"dish" antenna to avoid trees and buildings in the path between the antenna 
and the satellite. The elevation angle, which determines signal attenuation 
between the satellite and the antenna, will help in deciding on the best 
installation, selecting the antenna dish size, and the Low Noise Amplifier 
(LNA) with an adequate noise temperature.

The program calculates the azimuth, elevation, and compass heading of a 
geosynchronous satellite, given the observer's latitude, longitude, compass 
magnetic deviation, and the satellite's longitude. Magnetic deviation is 
the angle between magnetic and true north, azimuth is the angle between 
true north and the satellite's projection on the horizon measured 
clockwise. Elevation is the angle between the horizon and the satellite 
(see Figure 22.1). Compass heading is the azimuth corrected by the magnetic 
deviation. Latitude is positive north of the equator and negative for the 
southern hemisphere. Longitudes are positive west of the Greenwich meridian 
and negative east of it. Magnetic deviation is positive if the magnetic 
north is west of true north, otherwise it is negative. If the satellite is 
below the horizon, then the elevation is negative and there is no clear 
line of sight.


Program Instructions

All prompts are self-explanatory. All latitude and longitude inputs are in 
deg.mmss format. For example, type 123.0405 for 123 deg 4'5". After 
entering your location parameters and the longitude of the satellite (the 
latitude of a geosynchronous satellite is always 0 degrees), the program 
will display antenna aiming angles for the satellite. The outputs are in 
degrees, minutes, and seconds.

Example

From Holmdel, NJ (latitude = 40 deg 23'45" North, longitude = 74 deg 11'35" 
West, magnetic deviation = 12 deg), what are the azimuths, elevations, and 
compass headings of SATCOM 4 (83 deg West)?

Results:

Azimuth: 193 degrees, 26 minutes, 43 seconds
Heading: 205 degrees, 26 minutes, 43 seconds                   Elevation: 
42 degrees, 24 minutes, 48 seconds




