SatSig topic:

Symbol Offset errors

(Read 8046 times)

Mar 19^th, 2010 at 1:37pm

What would be the cause?

We are using 7.1.1 version iDirect modems. They are 3100 series.

Mar 19^th, 2010 at 4:49pm

No idea if this is the cause here, but we recently saw these errors with a mobile remote (on a ship), which was wrongly set to a fix location.

Symbol offset increased continuously while the mobile station moved away from the entered fix position.

Regards,
Gregor

Mar 19^th, 2010 at 6:50pm

The symbol offset (SO) in iDirect is simply a measurement of phase differential between the ideal and the actual. The SO reported via iMonitor (or monitored from a telnet session) is really not a number that corresponds with a specific degree measurement, but it does happen to fluctuate up and down depending on the movement of the spacecraft. As the spacecraft moves further from the remote site, the time it takes for the signals to travel between the site and spacecraft takes more time - which moves the phase....which yields the values seen in iMonitor or from command line. It is normal for most remote sites' symbol offset to fluctuate +/- 200 symbols every day (that fluctuation is station box movement…read: those figure 8’s we satellite people all speak of).

Normally, if the SO gets higher than 500 the remote site probably needs repointing…esp in instances where a spacecraft tracking ability (beacon tracker, SNR tracker, etc) is not available. There is no problem with seeing SO numbers as high as 300, but if it gets too far out of phase the Hub may not be able to decode the signal and the remote will drop out of the network. The limit of the SO in the iDirect system is about 800 symbols, but it varies from network to network because the pointing of the Hub Antenna adds a little variance.

Mar 26^th, 2010 at 9:25am

You need a right position (latitude and longitude ) for the both remote places (hub, remote) and the corect longitude for the satellite.

If you have a mobile remote, you need to set up the remote like mobile GPS to transmit his coordinates to the hub.

Regards,
Adrian

Mar 27^th, 2010 at 2:50pm

Agree with the above (Adrian). When seeing SO errors, the Geo location of the remote and hub should be checked. Furthermore, it is also a good idea for the HNO to hunt for ACQ errors on the Rx line cards. This will help assess the problem and determine a course of action. If there are ACQ errors present, then the remotes onboard GPS (if mobile) or its fixed geo location (hardcoded) are likely the source of the problem.

Mar 31^st, 2010 at 9:18pm

Thanks all.

If the geolocation is correct, then what could be the issue?

Mar 31^st, 2010 at 11:03pm

Is the location of the hub correct ?
Is the location of the remote correct ?
Is the satellite longitude correct ?

Is the satellite in inclined orbit ?

Where are the sites and which ones are affected ?
Note the offsets of all sites every 2 hours and look for patterns and so identify any abnormal site, but make sure the hub site location is exactly correct first ?

Best regards, Eric.