Raymarine C95 MFD Giving Incorrect SOG

[Esmerelda]

Our Raymarine C95 MFD continually gives a slightly incorrect SOG, to the extent that it shows 0.1-0.2 knots SOG when the boat is out of the water on the hard

Does anyone out there have any thoughts on why this might be happening?

Cheers

 

[Lucky Duck]

Isn't this just that the calculated position is slightly different from time to time giving the impression that the receiver is moving?

 

[Esmerelda]

But the other chartplotter on the boat shows zero when the boat is stationary, as did the one on the previous boat...

 

[johnalison]

My e7 was showing anything up to a knot SOG at rest and similar errors when on the move, making it next to useless. I whinged to the agents and also to Raymarine, which resulted in a patronising letter from them about the variability within the system and so on. It didn't matter how I set the filter, it stayed the same. Fortunately, there was a recall because of a display problem and lo and behold, the replacement machine worked OK and gave correct SOGs.

 

[pvb]

Our Raymarine C95 MFD continually gives a slightly incorrect SOG, to the extent that it shows 0.1-0.2 knots SOG when the boat is out of the water on the hard

Does anyone out there have any thoughts on why this might be happening?

What's your HDOP reading?

 

[Esmerelda]

What's your HDOP reading?

Might need some help with that - HDOP reading?

 

[pvb]

Might need some help with that - HDOP reading?

It's a measure of the horizontal precision, you should find it on your satellite page.

 

[Esmerelda]

I'll check at the weekend. What would I be hoping to see in terms of a "good" reading?

 

[pvb]

I'll check at the weekend. What would I be hoping to see in terms of a "good" reading?

Close to 1 would be good. Is the display at the helm, with clear sky view? Or inside?

 

[haydude]

GPS readings have always position errors from 5 meters minimum to an average of 15 metres depending on signal strength and the random error introduced by the military to prevent precision use by non military organizations.

This results in erratic position calculations whilst standing still. One second you get a position (with error) a minute later another position (with a different error) that moved up to 30 metres because of the random error that your plotter detects as movement.

You can configure a Raymarine plotter to ignore movements below a certain threshold. Look into the GPS or SOG configuration parameters. However even with this filter it is likely that you will continue to notice a movement whilst in a marina because of the larger error due to background radio signal interference caused by all sort of devices around you (as an example I have a boat just behind me that splatters RF up to S5 from his battery charger on all frequencies, as reference normal background RF noise is S3 in a populated area, out at sea is S1-S2).

- Why do I not see this jumping on my TomTom? Because TomTom's software actively amend the position to put your car always on the nearest road, otherwise you would see cars running over buildings all the time. Classic is the jump effect you see due to the adjustment when you are in an uncharted off-road area or car park.
- Why Raymarine's plotters do not amend the position? Because on sea charts there aren't vector representations of roads that can be used as reference to put a boat onto, but mostly because boats can be anywhere and require a more accurate indication of their true position.

 

[pvb]

You can configure a Raymarine plotter to ignore movements below a certain threshold. Look into the GPS or SOG configuration parameters. However even with this filter it is likely that you will continue to notice a movement whilst in a marina because of the larger error due to background radio signal interference caused by all sort of devices around you (as an example I have a boat just behind me that splatters RF up to S5 from his battery charger on all frequencies, as reference normal background RF noise is S3 in a populated area, out at sea is S1-S2).

Can you do this with a c95? There doesn't seem to be any mention of it in the manual.

As for radio interference, the OP says his other plotter correctly shows zero SOG when stationary, so there can't be that much interference.

 

[Ric]

But the other chartplotter on the boat shows zero when the boat is stationary, as did the one on the previous boat...

Are your two chartplotters networked together? If so you need to disable the GPS on one of them.

 

[pvb]

Are your two chartplotters networked together? If so you need to disable the GPS on one of them.

Yes, and if they're not networked, the OP could disable the GPS on his c95 and feed the GPS data from the other plotter in to the c95 by NMEA.

 

[Esmerelda]

I checked the HDOP at the weekend and it was 1.5 - looks like this could be the problem. The MFD is in the cockpit but enclosed in a hard wood pod type case (hideous, to be replaced at some point!). I guess the best thing I could do for the moment is connect it to the external GPS receiver that the other chartplotter uses?

Thanks for all the advice

 

[NormanS]

GPS readings have always position errors from 5 meters minimum to an average of 15 metres depending on signal strength and the random error introduced by the military to prevent precision use by non military organizations.

This results in erratic position calculations whilst standing still. One second you get a position (with error) a minute later another position (with a different error) that moved up to 30 metres because of the random error that your plotter detects as movement.

You can configure a Raymarine plotter to ignore movements below a certain threshold. Look into the GPS or SOG configuration parameters. However even with this filter it is likely that you will continue to notice a movement whilst in a marina because of the larger error due to background radio signal interference caused by all sort of devices around you (as an example I have a boat just behind me that splatters RF up to S5 from his battery charger on all frequencies, as reference normal background RF noise is S3 in a populated area, out at sea is S1-S2).

- Why do I not see this jumping on my TomTom? Because TomTom's software actively amend the position to put your car always on the nearest road, otherwise you would see cars running over buildings all the time. Classic is the jump effect you see due to the adjustment when you are in an uncharted off-road area or car park.
- Why Raymarine's plotters do not amend the position? Because on sea charts there aren't vector representations of roads that can be used as reference to put a boat onto, but mostly because boats can be anywhere and require a more accurate indication of their true position.

Surely the "random error, introduced by the military", called Selective Availability, was done away with years ago.

 

[Lucky Duck]

Surely the "random error, introduced by the military", called Selective Availability, was done away with years ago.

I think we had Bill Clinton to thank for the removal of SA. Even so I get the odd 0.1 or 0.2 knot reading for SOG when the boat is alongside.

The maps on my phone appear to have a similar feature where the position is adjusted to the nearest road which is mildly interesting when travelling on the train.

 

[pvb]

I checked the HDOP at the weekend and it was 1.5 - looks like this could be the problem. The MFD is in the cockpit but enclosed in a hard wood pod type case (hideous, to be replaced at some point!). I guess the best thing I could do for the moment is connect it to the external GPS receiver that the other chartplotter uses?

Yes, input GPS data via NMEA. You'll need to configure the c95 to disable its internal GPS.

 

[Buck Turgidson]

HDOP depends on the number of satellites in view and their geometry. It will vary with time as the constellation moves. Check your antenna is pointing up and has a clear view of the sky.

With the boat on the hard it's quite possible you are receiving path errors due to signal bouncing off various nearby structures and poor geometry due to some of the sky being obscured. These errors are common and it's why phones use assisted GPS which despite what some people preach actually helps reduce these errors by correcting from known cell and wifi positions.
if you plot your track you should see it moving around within an area of around 15 meters. If you have a new gps it should be SBAS enabled which will remove atmospheric errors but it still can't help with path or geometry. On the open sea with SBAS you will have an error of less than a couple of meters most of the time.

 

[pvb]

HDOP depends on the number of satellites in view and their geometry. It will vary with time as the constellation moves. Check your antenna is pointing up and has a clear view of the sky.

With the boat on the hard it's quite possible you are receiving path errors due to signal bouncing off various nearby structures and poor geometry due to some of the sky being obscured. These errors are common and it's why phones use assisted GPS which despite what some people preach actually helps reduce these errors by correcting from known cell and wifi positions.
if you plot your track you should see it moving around within an area of around 15 meters. If you have a new gps it should be SBAS enabled which will remove atmospheric errors but it still can't help with path or geometry. On the open sea with SBAS you will have an error of less than a couple of meters most of the time.

See post #3.