THE CASE OF THE DISAPPEARING SATELLITES No. 3

THE PROBLEM. I visited the ship SS Bubuk, an LNG carrier,  because the SMIDS "doppler" docking system was not working perfectly. SMIDS displays show the strength of each satellite signal being tracked and it was immediately obvious that that strengths were jumping up and down in a random way. Strengths could vary on a scale of 0 to 9 from 3 to 8 at any instant. Each satellite behaved differently, very very noisy.

INVESTIGATION.  Radio interference seemed likely to me so I made a quick test using another antenna temporarily outside the bridge door, and quite close to the bridge. All signals were immediately stable. The Captain's gast was flabbered.

Every piece of electronics on the bridge was shut down, looking for some source of interference. This included all communications, radars, computers, electronic compass, TV amplifier, but to no avail. These tests were made with the ship at sea at least 20 miles from the mainland, so cross-modulation from shore transmitters was eliminated. I would have liked to have shut down everything electronic on board but the cargo control system was in operation de-gassing the ship underway, and I was told "no chance."

The antenna was at the masthead, normally an ideal place. The downlead was longer than the specification allowed, so I shortened it. Results were the same; noisy signals.

I tried a replacement downlead, using RG213 as before but no change.

The temporary antenna was used as a search sensor on the end of the RG213 lead, in an attempt to find some place where interference was maximum and killed all signals. No good. At the same time I was trying to find a place where we had strong and stable reception, but there was nowhere on the monkey island or mast suitable.

The antenna was tried at the extreme bridge wing, on top of the navigation light box. Perfect on both port and starboard wing. Then, extraordinarily, it was raised by hand above head height and it was immediately noisy again. A difference in height of one metre changed results from perfect to bad.

CURE.  The antenna was fitted on a stub above the port navigation light. The Captain suggested that side since they normally berth starboard-side-to and it would be furthest from shore installations. Good thinking Captain Copeland! The bridge wing position is good as it reduces mast and funnel shadow compared to a more inboard position.

THE FORWARD ANTENNA. Another mystery. The forward SMIDS antenna at the mast head also suffered from noisy signals. Again the answer was to mount the antenna on the focsle head rail.

ADVICE. The cause of the problem remains a mystery. Whilst this note concerns SMIDS/GEOLOG satellite receivers I am quite certain that the ships other GPS receivers are affected, but they are not being used for such a precision application, and I did not investigate their performance. 

My advice is to expect the unexpected and make a temporary antenna connection before committing to a final installation of a GPS. 

Andrew Fairgrieve 23/12/99

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