How to debug a VSAT system.

Written by Tom on Monday 11/06/07

The other day the Downstream SNR of our VSAT broadband internet system dropped from 9.9 (good) to 4.5 (bad) the Tx indicator on the iDirect 3100 router was flicking on and off intermittently. So what’s the problem? Here is a step by step account of the debugging process.

1 Test the coaxial cable between the ODU dish and the IDU iDirect router. To do this you need a known working cable and a long power extension chord. Remove the IDU router and take it to the ODU. Disconnect the original cables and reconnect the known good cables. Power up the system and wait the three minutes while the router completes POST. If the SNR is still no good go to step two, it’s not the cable.

2 SNR has to do with the receiver (LNB) so try realigning the dish.

3 Ensure that the disk is correctly aligned by replacing the BUC/LNB with a C Band receiver, connect this to a Free to air TV decoder and TV. You can then measure the quality signal of a TV channel from the satellite to ensure that you are pointed in the right direction.

(We did get 2 db more it appears that our base had moved a bit while settling.)

4 Replace the LNB is the SNR better?

5 Replace the BUC does this help?

6 Replace the iDirect router still no go?

This was the position that we found ourselves in. Having realigned the dish ensuring that it was correctly aligned via C band free to air decoder we then replaced BUC, LNB and IDirect router, still our SNR was low.

It then occurred to us that the original site survey was in error. A tree about fifty meters from the dish on some else’s (the military) land had sprouted extra foliage with the on set of summer, the extra sap and leaves appears to be absorbing some or our signal. This would account for the Tx indicator being intermittent.

In most countries you could just ask your next door neighbour to top their tree but here in Pakistan where it is illegal to cut a green tree and a jail sentence if you are caught this is a job that will take tact and stealth (a sharp axe, strong rope, moonless night and good eyesight).