Stevens Water Monitoring Systems, Inc.

Basics of satellite antenna positioning for environmental monitoring

Users of geostationary satellite based telemetry systems have one unique challenge that other types of telemetry users do not – that is, calculating the angles needed to point their transmitting antennas to orbiting satellite. The process is actually fairly straightforward, although there are some common pitfalls that can be avoided with a little pre-planning. First, we need to look at how the satellite and ground stations are related to one another. We’ll start with geostationary satellites. A geostationary satellite is one that is orbiting at an altitude and location where its orbital period (i.e. the time it takes to go around the Earth once) matches the rotation of the Earth on its axis. The altitude is approximately 22,300 miles (35,900 kilometers) and the location is directly over the equator. By orbiting a satellite with these parameters, the satellite appears to “hang” in one spot in the sky, allowing fixed antennas to be pointed at them full time.

Pointing a satellite antenna is a matter of calculating the elevation and azimuth for the ground station antenna so that it points directly at the orbital location of the satellite.

In Figure 1, we see the Earth in an equatorial view and a polar view. The user station is illustrated in each view. In the top view, we see the needed elevation, relative to the local horizontal, to which the antenna needs to be raised. In the bottom view, we see the needed azimuth to which the antenna needs to be rotated. Azimuth may be measured relative to true or magnetic north, but the reference used must remain constant in all calculations and field checks.

Since we’re pointing the antenna from a spherical surface, the mathematics involved in the calculations is not as simple as if we were calculating the angles between objects in reference to a flat plane. However, the user does not have to be burdened with the details of the math, since there are automated look angle calculators available in the Internet, such as the one available at NASA's AERONET website.

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