[C320-list] GPS Antenna Mount Location?-list Digest, Vol 710, Issue 1

[Sean Kaldor]

[WARNING: UNSCIENTIFIC GOBBLEDEGOOK CONTAINED HEREIN]

For anyone else considering the possible mounting of a Raystar 125 GPS (the sensor used on the Raymarine C-70, C-80, etc.) under fiberglass, here are my personal findings on a 1998 C320.
I placed the GPS sensor in three locations. 
(1) On top of the aft rail, facing the sky, half-way between the port rail seat and the swim ladder, with a clear view of the sky ("best case"); 
(2) On the aft berth bunk, laying on its side, immediately under the the Edson helm ("worst case"); and 
(3) In the aft locker, port side, immediately under the step adjacent to the swim ladder, facing skyward, duct taped into temporary position ("test case").

I configured the sensor in each of the three locations and gave it 3 minutes to complete its acqusition. There was nobody in the cockpit during these tests. On a partly cloudy day with intermittent sprinkles, I checked the GPS status on the cabin-mounted C-80 for each of the locations while at dock.

Location 1 (Aft rail, "best case"): 12 satellites detected, one of which was minimal (approx 5%), full satellite differential fix achieved (SD-FIX) with overall HDOP accuracy of 1.0 ("ideal" per the manual). The estimated average percentage of each satellite signal strength was 56%. Excluding the one satellite with minimal detection, the average was 61%.

Location 2 (Aft cabin, "worse case"): 12 satellites detected, none of which were minimal, basic satellite GPS fix achieved (FIX OK) with overall HDOP accuracy of 2.0. The estimated average percentage of each satellite signal strength was 31%.

Location 3 (Under port aft step, "test case"): 12 satellites detected, three of which were minimal (approx 5% each), full satellite differential fix achieved (SD-FIX) with overall HDOP accuracy of 1.0 ("idea" per the manual). The estimated average percentage of each satellite signal strength was 48%. Excluding the three satellites with minimal detection, the average was 63%

INSIGHTS:
Placing the GPS directly under fiberglass does not seem to seriously degrade the signal. However, placing it in a position with increased obstructions to the sky (e.g. the helm, the refrigerator compressor, the bulk of the boat, etc.) does have an effect. My desired "test case" placement actually achieved higher signal strength from the satellites it picked up, but did miss a couple more satellites than the "ideal" location. The result, with more than 9 satellites detected and reception of over 70% on seven of them, was excellent accuracy. In heavy sea conditions, the low positioning may suffer from further obstructions by following waves.
The benefits of the "test case" location include no obstruction from people/objects in the cockpit, and no chance of damage from lines (fishing or sailing) or poorly placed hands.
As another note, I recently came 400 miles up the California coast with the GPS receiver placed on the shelf over the galley sink, with excellent reception throughout the trip. And on a final note Garmin specifically manufactures an under-fiberglass mount for their GPS receiver with specific instructions that it can successfully be placed there.

CONCLUSION:
I will mount my receiver under the aft port step and see how things go for the next year or so... I've left plenty of cable in case it needs to be moved up to the aft rail after all... :)

Sean
Liberte #499
San Francisco
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