Satellite Beacon Primer
Kindly written for UHF-Satcom.com by Mr. N. Syder from the USA.
Anyone with more than a passing interest in satellite communications or monitoring has come across what appears to be continuous carriers on a satellite and have perhaps wondered why they are there. Why would anyone waste valuable downlink power broadcasting them? The truth is, those CW carriers are an important function of satellite operation. They have many purposes, including those which are listed below:
Transmit information about the satellite back to the respective control centre. (TT&C) Aid in ranging of the satellite. Enable Earth stations to track them. Serve as a basic identification mark. Help earth stations adjust the polarity of their antennas.
For the ground station TT&C facility (tracking, telemetry, and control) to know the health of a satellite, it needs data back from the bird. This data contains such information as battery voltage and current, voltage and current the solar cells are providing, fuel left onboard, internal and external temperatures, how many transmitters and receivers are operating, and how much current each function of the spacecraft is using. This is just the short list of parameters that are monitored to make sure that the satellite is functioning correctly. Not all satellites transmit this information continuously. Many ham radio and some scientific satellites do, but military, navigation, and commercial birds do not until commanded to do so.
The control facility sends a command on a specific uplink frequency to the satellite telling it to transmit its information. When the satellite responds, the C.W. beacon becomes modulated with data. What you will see is the centre carrier and two or more sidebands (see the picture below). Most satellites transmit in the F.M. mode, ham radio satellites being the exception as they use a mixture of F.M. or S.S.B., depending on the satellite. The beacon is also used to acknowledge a command that has been received by the satellite and executed. For example, when a transmitter, receiver, instrument, etc. goes bad, a command is sent to switch to a backup unit. Once this command is received and acted upon (or not), the response is transmitted back to the control facility. There are numerous commands that are sent to a satellite. Commands are sent immediately following launch to instruct the satellite to separate from the launch vehicle, to put it into a transfer orbit, and then to send it into final position. After it is in its final position there are more commands to keep it in normal operation, such as firing station-keeping thrusters, adjustments to the solar array angles, and power regulation. Finally, when the satellite is at the end of its life, the last command issued is to send it into a graveyard orbit.
Control facilities perform what is called ranging to know if an orbital manoeuvre or mid-flight correction is necessary to keep a satellite on course. What happens is the control facility sends a command that will make the satellite respond back to the control facility over a period of time. The control facility measures how long it took to issue the command until it received back at the control facility. Knowing the constant for RF, and taking into account how long it would take to get through any of today's networks, the control facility can figure out how far away the satellite is. Then any north/south, east/west, or mid-flight course correction manoeuvres can be applied. When the beacons aren't being used to send data back to the TT&C facilities, they are unmodulated carriers.
Geo-synchronous satellites in an inclined orbit (satellites that have had the north and south station keeping discontinued, but still are being adjusted on the east and west) or satellites in mid flight need to be tracked. An Earth station uses what is called a beacon receiver. It is tuned to the beacon coming from the satellite, and measures its signal strength. Normally the beacon receiver outputs a D.C. voltage that is a representation of the beacon signal strength. This D.C. voltage is sent to the antenna's control unit. When the signal strength falls below a preset level, it causes the antenna control unit to move the antenna in azimuth and elevation in small increments. This will continue until the maximum preset signal strength is achieved. Here is where beacons can help you. One purpose of a beacon is to serve as a basic identification mark to let you know you are on the correct satellite. Once you have moved your antenna in azimuth and elevation to find the satellite, you can verify that you are on the right satellite by using the beacon and a spectrum analyzer.
Most satellites have a distinct frequency or frequencies that are used. This is the case for commercial, military, navigation, amateur and scientific satellites. When you have found the beacon, is it on the correct downlink pole on your antenna? If you are using a circularly poled antenna and it isn't you know to change the LNA / LNB cable connections or the antenna phasing connections. If you are using a linearly poled antenna and it isn't you can rotate the feed/antenna/phasing 90 degrees. Once you find the beacon and have verified that you see it on the correct polarity, you can check to see if you have your polarity set correctly on a linearly poled antenna.
If you have a dual polarity antenna system swap your spectrum analyzer input connection over to the opposite polarity to see if any of the beacon is showing through. If it is, rotate the feed/antenna to null out the beacon to where it isn't showing thru. Once you have this done (or if you have a single downlink antenna) you can try to get maximum received signal out of your antenna by making small adjustments in the azimuth, elevation. In the case of a dish antenna, move the feed in and out to set the correct focal depth. When you see that you have the maximum amount of signal on the spectrum analyzer display, you are 100% on the satellite. As you can see, the beacons serve a multitude of functions. By knowing how the owner/operators use them, you can use them to help identify the satellite you are looking for and align your antenna for maximum receive levels.