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VSAT technology and installation >> Dish pointing and alignment >> NOC Tech in need of cross-pol instructions

Message started by JamesVSAT-BP on Sep 22nd, 2009 at 3:12am

Title: NOC Tech in need of cross-pol instructions
Post by JamesVSAT-BP on Sep 22nd, 2009 at 3:12am
I work alone at night in a Network Operations Center.  I'm wiling to learn more about how to do NOC functions better in order to succeed in what I do.  

My first issue is performing a cross-pol with a remote site.  I do not know the first thing about doing a cross-pol (always called our satellite provider - SES).  I don't even know what "cross-pol" means.  

Could anyone recommend a great book regarding VSAT technology/cross-pol so I can mentally visualize what is going on to understand it?  Can someone explain what cross-pol is, or how to perform a cross -pol?  

This is what I know:
The remote technician puts up a clean wave (CW) carrier on the satellite at a certain frequency given by the NOC.  While the NOC Technician is looking at a spectrum analyzer (with the center frequency being that CW on that satellite and transponder), the remote technician adjusts the feedhorn/LNB based on how high the carrier comes up in the Spec An.  Once the peak is achieved, the remote technician then adjusts the azimuth and elevation to achieve a higher peak.  

From here, the NOC technician says "OK, your co-pol is "-xx" and a cross-pol of "-xx" with an isolation of "xx" db.

Don't get me started on 1db compression tests.  I don't know what that means either.  

But if my boss says that we need to learn how to do cross-pols in order to stop having to rely on SES or another satellite company for this, then I need to learn and have someone explain it to me.  

Thank you for your help.  I really want to learn to become better, but the only way to start is to ask for help.

Title: Re: NOC Tech in need of cross-pol instructions
Post by Eric Johnston on Sep 22nd, 2009 at 10:24am
The frequency chosen for the CW test must be one which is unused on both polarisations on the wanted satellite and also unused on adjacent satellites in the orbit.  This is to avoid the CW carrier causing interference with anyone else. Only the satellite operations centre can authorise CW carrier tests.

The large antenna at the teleport hub is where your VSAT transmit co-pol and cross-pol measurements are made.  The hub antenna has two PLL LNBs or LNAs, one for each polarisation.  The wanted co-pol signal is measured, e.g. -10 dBm.  The weak cross pol is also measured, e.g. -43 dBm.  Simultaneous measurements are possible if they have two spectrum analysers but it may be easier to switch between the two receive paths.  Either way, calibration is important.  The hub wants you to adjust your transmit polarisation to minimise the crosspol level so as to obtain a high isolation (43 - 10 = 33 dB isolation in the above example).

It is not easy measuring low levels so, when requested, turn your VSAT feed by a very small amount (e.g 1 deg) and leave it there till the hub is able to measure the new level. It may take them 30 seconds to get a good reading, so be very patient and don't rotate the feed again until the hub say to do so.

The objective it to have at least 30 dB isolation at the beam centre, when the antenna under test is pointed directly at the satellite.  This 30 dB isolation must be maintained when you are mispointed by -1 dB in any direction so the test may be repeated 8 more times, by mispointing the antenna up, up right, right, right down, down, down left, left and up left.  This more complex cross-pol test should be done on larger VSATs, for example 3.5m with subreflector or VSATs with high power BUCs or HPAs.  All the emphasis is on reducing interference to other people's services.

Regarding BUC P-1dB gain compression tests.  The drive level into your BUC must not exceed the P-1dB level otherwise your transmit signal will be distorted, causing high error rate, and, more important due to spectral regrowth, causing interference to adjacent carriers.  Also you BUC output transistors will be damaged.  While transmitting a CW carrier the teleport hub measures your level.  The output power from the modem is increased in 1 dB steps and the level at the hub measured also.  Plot the graph.  As you get near BUC saturation the BUC output level no longer goes up in 1 dB steps. Once the BUC output level is lagging the input by 1 dB you are at the P-1dB gain compression point.  Stop and record the modem output level.  This level must never be exceeded in service.  The level varies from site to site according to the cable length. If you are going to transmit 8-PSK or 16-QAM modulation it is best to keep 3 dB below the P-1dB compression point as even slight distortion of the transmitted signal will increase the error rate.

Title: Low snr
Post by ando on Oct 15th, 2009 at 1:40pm
Hello Forum, Please i need your help. I have been working in one site for more than two months now. i install 1.8m dish sat. is badr. the signal is very very low, infact the snr is below 7 which cannot do anything. I have change the complete the feed, jet no changes. please someone should help me out of this problem.
Aniekan from Nigeria.

Title: Re: NOC Tech in need of cross-pol instructions
Post by Oasis Networks on Oct 15th, 2009 at 2:06pm
Do you have some pictures from the site?

What SNR are you supposed to get?

Did you check the dish is installed properly (the feed is placed in the exact position in front of the reflector, at the correct angle?).

Did you check the cables maybe?

Did you try to check the signal with a different modem or with a spectrum analyzer?

Title: Re: NOC Tech in need of cross-pol instructions
Post by ando on Oct 16th, 2009 at 2:01pm
Hello, I think the snr should be 9-10

yes everything is in place, but the site is close to sure,
so i dont know if that could be the problem or could it be the size of the dish.

please just help me let me solve this problem.


Title: Re: NOC Tech in need of cross-pol instructions
Post by Eric Johnston on Oct 16th, 2009 at 3:33pm
What BADR satellite?.

If BADR-6, C band at 26deg east, are you using a circular polarisation feed ?  Have you assembled the feed for linear polarisation by mistake ?  The OMT should be at +/-45 deg to the two lines of pins/slots inside the polariser.  45 deg one way will work perfectly.  45 deg the other way not at all.  Half way (i.e. in line and linear pol) will give at least 3 dB degraded and totally degraded if there are cross-pol carriers.

Is the feed in the correct location relative to the dish ?  Are the feed support struts in the right holes ?  Is the front rim of the dish flat ?  Pictures might help.

What kind modem, bit rate, FEC ? Reducing the bit rate  and trying 1/2 rate FEC might get something working.

Best regards, Eric.

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