COG bounce/ erratic

[TomHalfmoon]

Our recently purchased trawler came equipped with RM E120 and E80, AutoPilot and Radar plus Brookhouse MUX and AIS.

The RM GPS antenna failed and I have replaced it with a suitable non-RM antenna. All appears to function normally except that the COG is erratic.''I have verified that the GPS sentence for SOG & COG is correct and consistent.

I switched everything ON except the Autopilot, so no head displayed. While underway the COG was rock solid and accurate.

Any thoughts on why the AP / heading would disrupt COG??

Thank

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum Tom,

It is recommended that you verify that only one source of GPS data has been interfaced to the system. If so, then please note that some GPS sensors, including early Raymarine GPS sensors, will report COG when the vessel is stationary. By definition, a stationary vessel has no COG. Raymarine's latest generations of GPS sensors (i.e. Raystar 150 and Raystar 130) offered significant improvements in filtering to not produce COG for stationary vessels.
Should the movement of the Chart application's screen in response to COG not be desirable, when stationary or maneuvering, then it would be recommended that the Chart application page/pane being viewed be configured to North Up chart orientation and to have the vessel icon centered on the screen rather than be offset. Once underway and making way, the Chart application may be reconfigured to Head Up and any offset then applied.

[TomHalfmoon]

Hi

There is only 1 GPS connected via the MUX as "talker". The E120 display is connected as "listener".

Next time out I'll try the N up mode. It seems that the Motion option (RM<TM<AR) is more the issue. TM is the most stable.

Thanks

Tom

[Chuck - Raymarine - Moderator]

You're welcome.

[TomHalfmoon]

Ref: COG erratic
We were out on the water this weekend and I was able to try a few different options and take some notes.

While underway 5-8 kts. with consistent heading the COG arrow generally moves from an accurate reading to a consistent in-accurate one. Say i"m heading 093, for a short period of time the COG will approximately match 093 and then swing around to say 352 deg. Estimating 20 % of the time correct (093) and 80% incorrect (353) 352. If I change heading the new "incorrect" COG reading will change to a new value but again remain consistent.

With the Autopilot engaged and steering to a WPT the boat will track perfectly to the WPT with a consistent Heading but a bouncing COG.

Question: Doesn't the AP use the COG data to steer to?

Thanks for any suggestions

Tom

[Chuck - Raymarine - Moderator]

Tom,

It sounds like there may be a problem with your system's GPS sensor. Autopilots do not steer to COG, autopilots steer to heading from a magnetic north seeking compass which is typically part of the autopilot system.