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Message started by satnew on Nov 21st, 2010 at 8:47am

Title: Tracking level
Post by satnew on Nov 21st, 2010 at 8:47am
Does any one have idea why the tracking level of a 13m on step track mode decreases over time?
 Considering that a regular station keeping is done.


Title: Re: Tracking level
Post by Eric Johnston on Nov 21st, 2010 at 12:08pm
Once the step track system has peaked up and the antenna stopped moving, the beacon level will then gradually decrease as the satellite moves away from the antenna beam centre.  Once the level drops below some threshold the tracking system will reactivate and it will peak up again.  Such step tracking peak-ups may occur several times per hour. Some may occur as a result of the level dropping, some may occur because the antenna controller has learned the orbit and knows it is about time to peakup again.

For a 13m antenna, the level will drop 0.5 dB with 0.02 deg of satellite movement. If the satellite moves +/-0.15 deg (0.3 deg total movement) during the day at least 15 movements will be required.  More frequent movements will occur during learning and in normal operation.  

Satellite north-south stationkeeping may occur several times per month, but possibly every 12 or 24 hours. Following a stationkeeping maneuver the tracking controller must relearn the new orbit.

It is a skilled job to configure a tracking controller - there are many parameters relating to thresholds, motor start and stop times, backlash, smoothing, antenna size, frequency etc.  Get help and training from the specific antenna tracking controller manufacturer if you have problems.

It is important that your tracking controller has a linear input signal from the beacon receiver.  You may need to put an attenuator between the LNA/LNB output and the beacon receiver input to get the levels right. Alter the attenuator by exactly 1 dB and the output should also vary by exactly 1 dB.  Checking the linearity of the LNA/LNB is more difficult.  You need a broadband power meter, the LNA/LNB specification, a high power satellite and spectrum analyser. The differential between main beam gain and first sidelobe gain should be the same regardless if you are pointed at a high or low power satellite.

Be alert to the possibility of the tracking system locking on the first sidelobe ring.

It is recommended that you keep a paper record of the normal pointing angles next to the tracking controller. Go and have a look at the actual physical axis scales and record those readings also.

The image below shows a typical satellite movement during one day. The satellite is on same longitude as you, so the top of the geostationary orbit appears horizontal.

The wide beamwidth of a small VSAT dish covers all of the daily satellite movement and so the VSAT may be fixed pointing.

Best regards, Eric.

Title: Re: Tracking level
Post by satnew on Nov 21st, 2010 at 7:29pm
Hi Eric,

What are the consequences if i manually track the satellite and revert back to step track as soon as i peak up the the antenna?

thanks and regards

Title: Re: Tracking level
Post by Eric Johnston on Nov 21st, 2010 at 10:03pm
Before doing that study the manual and also record the current pointing, both on the controller and also on the physical scales. Do you have manual handles ready in case you break the motor drive system?.

When you put it back to automatic make sure you are within 5 dB of the beam peak (i.e. on the main beam) and not on a sidelobe.  Watch it for at least 30 minutes to make sure it really is peaking up properly and is starting its learning process again.

To track manually it helps to have a spectrum analyser tuned to the beacon frequency in zero span with 30 min sweep time so you have a graph of the level. Use a resolution bandwidth that is much wider then the frequency drift.  You can do this to monitor the step track operation and watch the zig-zag and exploratory steps.  See if you can get the controller to display/print out a log of its activities.

Be very careful. Tracking systems on a large dish like yours involves high voltage motors, powerful drive mechanisms, soft and hard end stop limits etc. Mistakes can be fatal, especially at heights, in the dark, rain, snow, high winds etc. It is quite possible for the controller and drives to do very strange things, like suddenly mispointing the antenna a very long way from nominal for no obvious reason. Wear a hard hat.

I hope the above does not sound too alarmist, but I speak from experience.   Tracking is a good example of a case where the maxim "If it works, don't fix it" applies.

Best regards, Eric.

Title: Re: Tracking level
Post by satnew on Nov 22nd, 2010 at 3:34am
Hi Eric,

does not sound too alarmist at all...thanks for all the info...


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