XTE error

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Kantara

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I have a Raymarine RL70RC chartplotter and Smartpilot S1G autopilot fed from a Garmin120 GPS. During some of the longer legs of our cruise this year when the autopilot was set to track, the boat would suddenly sheer 20 degrees or so to port or starboard, then revert to course having shifted 40 or so yards. Close examination of the XTE readings showed that the XTE would suddenly jump from 0 to .03 or 0.04NM, then back to 0. This would only happen when the waypoint was more than about 6 miles away - less than that, the XTE would change from 0 to 0.01NM and the autopilot would gently alter by a few degrees.

Anyone know of any solution to this?
 
I don't think the .03 or .04 off track will cause this but check and adjust the response rate, if set too sensitive the autopilot will turn sharply to correct any deviation from the track, if you reduce the sensitivity the autopilot will respond slower making it less twitchy.

Does it only happen in certain weathey conditions?

Jim
 
Well, it was usually raining!
I tried adjusting the response rate but it didn't help. I think it is a chart plotter problem rather than autopilot - all the AP was doing was responding to what it was told. My gut feel was that it was to do with the distance to the waypoint where the minimum angle change resulted in a XTE of greater than .01NM. However, I would have expected the initial XTE to drop gradually as the WP got closer. Watching the XTE closely as we got closer it never jumped to 0.02NM it was always either 0.03 or 0.04NM - and that even applied when the WP was over 20 NM away.
 
I take it you've tried all the connections to make sure they are all making good contacts?

I am not sure then if you've adjusted the response rate, that was my first thoughts.

If no one else can come up with an answer I'll see what else I can find out.

Jim
 
Sounds as though it must be a problem with the GPS receiver or the Plotter, depending which has the route set up on it. Did you happen to notice if the XTE 'jumps' were preceeded by big changes in COG? To be self-consistent the GPS can't suddenly be 70-odd yards away from the rhumbline without there being a big change in the COG beforehand!

If the GPS is sending its stuff every second or two then it's hard to see how it can suddenly jump .03/.04nm off track - unless you're travelling very fast indeed at about 90 degrees to your intended course!
 
It's the plotter that has the route set up on it. And, no, there wasn't a big change to the COG, just a gentle drift. I think the connections are OK - are you thinking that the GPS is only talking to the plotter intermittently? I don't believe that can be the case as on the odd occasion that the GPS signal goes the plotter bleeps like a BBC broadcast of a Billy Connolly sketch.
How is XTE calculated? Is it done using distance and angle from a waypoint or does it draw a line in its little brain and measure how far off it is? If it is the former, could the fact that I was either going just about North or South when it happened have anything to do with it?
 
Chris
[ QUOTE ]
How is XTE calculated?

[/ QUOTE ]It could be done (for shortish distances) by either of the methods you describe. It could be calculated as the perpendicular distance from the rhumbline to the current position or as the product of the distance-to-target and the sine of the difference between the current bearing and the original bearing. The methods would give similar results.

[ QUOTE ]
could the fact that I was either going just about North or South when it happened have anything to do with it?

[/ QUOTE ]Wouldn't have thought this would make any difference. There's a slight complication if the two bearings are either side of North but simple enough to resolve.

Hmmm...
How about checking the track points recorded by the chart plotter and the GPS? Maybe you could check whether they agree for the period when one of these anomalies happens? Do either/both of them show the sideways movement? You may need to adjust the track point intervals to get meaningful results.

Out of interest: do you know how long the plotter allows between GPS fixes before it gets upset and bleeps?
 
I take your point - but the issue here is the XTE from the plotter. If you manually steered following a rolling road, you would still find the XTE jumping from zero to 0.03NM, making it look as though a quite substantial course change was required
 
I think I follow you. The only reason why I thought North might come into it was because calculating a TAN (or is it a COTAN) requires an angle to be subtended with something and I thought it might be North.
I haven't tried crosschecking between the plotter and the GPS tracks. Whenever I have crosschecked positions, they are identical and updated very quickly (sub-second).
I think the plotter takes about 5 secs before it realises it's not getting position info, based on turning everything off in the wrong sequence.
 
If you're sure it only happens on routes greater than 6 miles then it looks like the GPS is ruled out as you're only using it as a position transducer.

To try and pin down the problem I think I'd try setting the track point interval to, say, 10 seconds and then wait till the problem occurs. Then zoom in on the track and see if you can see the sudden deviation.

Can't see how you can 'suddenly' be 50+ yards away from the rhumbline. Even with a significant change in heading or a sudden change in current flow it would surely take a minute or so and you shouldn't see the XTE suddenly step from 0 to 0.03.

I suppose it's not possible to set the route up on the GPS 120 and feed its NMEA directly to your a/p - and see if you still get the problem? This would rule out or rule in the plotter!
 
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