Basics of Satellite Antenna Positioning
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.
The basic steps involved in calculating the pointing angles to a satellite are as follows:
• Determine the orbital location of the satellite. Satellite locations are referenced by their longitudinal position over the equator. Depending on the source providing the satellite information, the orbital locations may be referenced by Longitude east or west of the Prime Meridian (i.e. Zero Longitude) or may be referenced in absolute form from the Prime Meridian east. This is an important piece of information to have, because a satellite located over the United States, for instance, at a Longitude of 110 degrees West may also be referred to as being at 250 degrees (that’s 250 degrees EAST of the Prime Meridian). The antenna pointing calculator you chose will require the orbital position of the satellite to be entered in one of these specific ways in order to output an accurate result.
• Determine the ground station location on the Earth’s surface. A reading of the antenna’s Latitude and Longitude is needed in order to make the calculations. Again, depending on the antenna pointing calculator you choose, you may have to provide these coordinates in a particular manner. Latitude is almost always referenced as number of degrees North or South of the Equator. This value may be referenced by “N” and “S” or by “+” and “-“ with “+” being equivalent to North of the equator. Longitude may be referenced, as with the satellite locations, as being East or West (“+” or “-“) of the Prime Meridian or in absolute values from the Prime Meridian eastward.
• Conduct a site survey. With antenna azimuth and elevation known, the user should conduct a short site survey at the actual antenna location to determine if that location can “see” the satellite in question. Large hills or other terrain features as well as thick vegetation in the transmission path can have a detrimental effect on the link, even to the point of failure. Using a compass and a device to determine angle up from the ground (a clinometer, transit or similar instrument is best) site along the calculated path and make sure that clear sky is visible. Stevens offers the Empire Magnetic Polycast Protractor to help users position antennas correctly and ensure proper transmission with geostationary satellites.
Install the antenna and point it at the satellite. Using the calculated values, a compass and clinometer, adjust the mount for the antenna so that it matches the calculated values. Test and adjust position slightly as necessary to obtain the best signal.
Common mistakes that are made in this process are almost always related to how the azimuth is determined. Many of the available satellite look-angle calculators provide the azimuth reading result in degrees true, rather than magnetic. If a user is not aware of this, and uses a magnetic compass in the field to determine the azimuth, then the effects of magnetic declination are not accounted for. Make sure that the method of determining the azimuth in the field agrees with the values provided by or corrected from the calculator chosen.
Low Earth Orbiting (LEO) satellites present a different sort of challenge. While a geostationary satellite appears to “hang” in a single spot in the sky all the time, LEO satellites “rise” and “set” again and again relative to a single location on the ground. In most cases, it is impractical to install an antenna that tracks the movements of these spacecraft, so omnidirectional antennas are normally used. Power levels for these types of installations must be carefully calculated so that a sufficient level of energy reaches the satellite regardless of its position in the sky at any given moment.
Figure 2 illustrates the general concept of a LEO satellite. LEO orbiting altitudes are typically in the range of approximately 100 to 1,200 miles (160 to 2000 km). To maintain a certain level of coverage for users, LEO satellites are sometimes deployed as “constellations” of multiple spacecraft in slightly different orbits.
Figure 3 illustrates what the user’s station will “see” when using a LEO satellite. As the satellites orbit, they appear to “rise” and “set” relative to the user’s location as the portion of the earth’s surface in which the user is located comes in to view of the satellite’s ground track. The ground system must incorporate some type of system to know when the required satellites will rise and set since data can only be transmitted when the satellites are in view. Depending on the system in question, this function may be accomplished with beacons on the satellites or one of various types of orbital prediction tables or software.