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Finding Satellite Beacon

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VSATEngineer
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Dec 1st, 2015 at 1:37pm  
Could you please explain what is the nominal beacon value and what is the worst? And do we need to track it automatically?

thanks in advance.
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« Last Edit: Dec 10th, 2015 at 12:01pm by Admin1 »  
 
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Reply #1 - Dec 1st, 2015 at 6:10pm  
Satellite beacons are provided in accordance with the specification by the customer.  Typically the satellite radiated power (EIRP) might be in the order of +8 dBW, equivalent to 6 watts into an omni-directional satellite antenna. It could be different, as some beacons are transmitted via a directional antenna, such as a global beam horn or toroidal bicone. Some examples: satellite beacons

Whatever, the NOC will have details. Ask them for the beacon EIRP radiation pattern. You need to know the exact EIRP towards your location.

Many beacons are un-modulated, CW signals. Some beacons are modulated with telemetry and ranging data, in which case you will be interested in the power of the central carrier.

Note the polarisation of the beacon. Circular polarisation may be used to enable successful reception by linear polarisation earth stations, regardless of their operating polarisation adjustment angle. In such cases take off 3 dB.

Beacon receivers may have a frequency tracking capability to deal with doppler frequency changes. Such receivers may also have the ability to rapidly reacquire the signal following a severe rain outage.

A CW beacon has a very high power flux density per Hz, so it is possible, if the frequency stability of your LNB is good (PLL suggested), to measure beacon levels using very narrow receive filters (<100 Hz) and thus have C/N values in the order of 25-35 dB. This is good for antenna pattern measurements.

It would be interesting to hear from anyone measuring satellite beacons using FFT spectrum analyser software which can do waterfall displays with resolution bandwidths measured in milli-Hz!   The leading CW preambles of TDMA bursts might similarly be detected.
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VSATEngineer
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Reply #2 - Dec 9th, 2015 at 8:57am  
Thanks Eric for your explanation.

if you don't mind Eric, for antenna under test, I would like to know how to use the spectrum analyzer properly in order to find/latch to the beacon of the targeted spacecraft?

Thanks in Advance
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Reply #3 - Dec 10th, 2015 at 12:00pm  
I'm assuming you have a large antenna which needs motor driven tracking to follow the slight movement of geostationary satellites.

You need a beacon receiver with an output like 0 - 10 volts to go to the antenna tracking controller.

To see the beacon on a spectrum analyser you need a high stability type LNB, such as PLL type, preferably with external reference using a GPS or atomic clock 10 MHz frequency standard. This will stop the beacon frequency drifting about when you are using very narrow resolution bandwidth filters in the spectrum analyser. If your analyser accepts a 10 MHz input reference, use it.

If you already pointed at the wanted satellite, tune the analyser to the beacon frequency and set a span like 1 to 4 MHz.

If you have not yet found the satellite and it has lots of known carriers (e.g. TV) on it then use the spectrum analyser is wide band mode (span=600 or 1000 MHz)and find the satellite using manual antenna movements. Set the elevation accurately first using an inclinometer.

If the satellite is new and has no traffic, find out all about the beacons and set the analyser to an appropriate frequency and span.  If there are 2 beacons close together make the span embrace both and think about what the picture will look like.

Move the antenna till the beacon(s) appear. This may take some time as the beacon(s) will be visible far off from the antenna beam centre. Try resolution bandwidths from 100 kHz downwards.  When you think you have found the satellite in the main beam move the dish in azimuth and elevation till you are sure that you have just one peak in the centre and two first sidelobe peaks approx equal level either side, approx -10 to -16 dB lower. Plot the antenna sidelobe patterns in azimuth and elevation for reference.

If you are trying to find some weak beacon such as deep space satellite, use a very narrow resolution bandwidth filter plus averaging and waterfall display if you have it. It may take a long time. Once found make a proper record of frequency and its drift rate, the spectrum analyser configuration and az/el drive encoder readouts.

Useful links:

http://www.satsig.net/satellite/inclined-orbit-operation.htm

http://www.satsig.net/pointing/how-to-make-inclinometer.htm

http://www.satsig.net/sub-reflector-alignment.htm

Calculate the feasibility of detection of SETI signals / very weak signals / very long range signals.
http://www.satsig.net/seticalc.htm
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« Last Edit: Dec 10th, 2015 at 1:56pm by Admin1 »  
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