Trip Counter Accuracy

Taniwani

Furuno Super Fan
Just had the first longer trip with all my new Furuno equipment and had some time getting used to it. All rather well, especially the radar is incredibly good.
But one thing I don't seem to get working for me is the trip counters and odometer. I had a route set up that was in total 272 miles and we were following it quite exactly, sailing at 6 to 9 knots all the time. But upon arrival both trip counters that were reset when we started showed 255.6 miles. The odometer increased by the same amount (255.6).
Both TZT2s show the same, but I think they are synchronised. Both recorded a track at 0.1 miles distance, which seemingly worked as expected.
GPS positions come from a Thrane LT1000 NRU which seems to work rather precise and sends position and heading updates at 10Hz.
Now looking back a few days, we traveled a route of 29.2 miles and counted 27.5 miles on the odometer.
Any ideas?
 
Just had another shorter trip (36nm) and again the odometers miss about 7% of the distance sailed.

Thought it might have to do with the damping I had on SOG/COG and took that off today, but still short by the same percentage. Checked a couple of times along the trip and the sum of distance sailed and distance to go kept decreasing pretty proportional to moving on ending up as 34.12 versus a route length of 36.6.

Tracking seems to work fine, even if based on a distance interval. Using 0.1 miles as tracking interval I get exactly one mile between 10 track points and summing up all track legs in excel I get the true distance traveled. This would indicate the MFD knows when it traveled 0.1 miles exactly and places the track point correctly. This seems to suggest that the GPS data cannot really be the problem.
 
Japan said that the trip is based on distance of movement in 1 meter increments based on the LAT/LOG position provided by the GPS.
 
Thanks for the info Johnny. Still very mysterious and wondering why nobody else has seen the problem. I‘ll be in harbour for a week, but plan to try running with the built in GPS on the next leg and see if that works.
 
Have been doing a trip using the built-in GPS in the cockpit mounted TZT2 which has a fairly free sight of the sky just under a canvas dodger. The result was essentially the same as with the pole mounted GPS. We went exactly along a preset route of 139.4 nm and the trip counters accumulated only 131.1 nm. I had tracking on in the TZT2 with a distance interval of 0.1 nm.
Exporting the track and summing up the distances between the track points along the route, also comes out almost exactly at the route length with 139.6.

I believe that the algorithm used to run the odometer and trip counter is flawed using a too short sample interval, truncating the tiny legs to 1m accuracy. The error is hence depending on the speed of the boat, getting worse, the slower you go. We have been going between 5kn and 9kn averaging 7.6kn. That's about 2.6 - 4.6 m/s. Assume you are sampling at 3sec intervals then we would have travelled 7.8-13.8 meters. If you cut off the 1/10s of meter, you would loose 0.5 meters on average per sample. At about 10m per interval, that would be about the 5-7% error I'm seeing.
 
I still haven't found a way to get a working odometer or trip counter.

Am I the only one having this problem?
Can anybody confirm that your TFT2 odometer is actually working correctly?

I think the error gets bigger at lower speeds, so maybe fast boats wouldn't see a huge discrepancy.

I currently have no other way than to process the exported track file to determine the actual distance covered. Just had a trip where the shortest route would be 617 miles and we had to deviate from the rhumb line quite a bit due to unfavourable wind. When we arrived the trip counter showed 592, summing up the track legs gives 657.
 
I asked Japan to look at your posting because I am not an expert on this or how it is programmed. They verified the damping or smoothing setting has nothing to do with the distance calculation. Distance for the trip and odometer values are counted every second. If you continue to have difficulties or questions, I recommend you contact your local Furuno distributor and get them involved with Japan on this subject.
 
Thanks Johnny for trying to help. Problem still persists and per your suggestion I have now written to Furuno Germany and asked for help. Have little hope now that this gets fixed. I think a 1 second interval is way too short, given GPS accuracy and rounding errors. We just move 3 meters in that time.
 
I'm bringing this up again, as the problem is still unresolved (didn't get any response from my distributor on two inquiries) and I have some more observations/information:

I have systematically recorded what the trip counter shows, versus the actual distance travelled using 3 different GPS sources top supply positions.

Using the built-in GPS or the main GPS (a Thrane LT-1000) gives the same result, with the trip counter showing about 94% of the actual distance.

Using the AIS-transponder (EmTrack 350b) has the opposite effect and Trip counter then shows 103% of the actual distance traveled.

Looking at what these GPS's send out on PGN 129025, I can see that the AIS truncates the resolution only providing 3 digits after the decimal, thus setting resolution to about 11 meters. I cannot tell if it rounds or truncates.

Both the TZT2's GPS and the LT-1000 provide 6 digit resolution where the last 2 digits on the LT-1000 output change fast as the boat rolls and swings the antenna. The built-in GPS seems more smoothed. But they agree on accuracy and both have the same effect on the trip counter coming out at 94% of actual distance.

The actual distances were determined using track recording at 0.05 miles (93meters). If anything, this could only come out short if the legs wouldn't be straight and we were mostly travelling along a route or on a fixed course.

Track processing takes some time and effort and is currently the only way I can determine my distance travelled and I can only do it after the fact and not while I'm travelling.So I'm not too happy with that situation. I think somebody should look into how this is coded.
 
Please PM me your distributor details and who you have been attempting to speak with. Your distributor should be assisting you.
 
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