Tlog on Standard Horizon CP300i over recording by 5ish

[RIBW]

Has anyone any idea why Tlog on the chartplotter is racking up the miles at roughly 5 times the actual distance travelled?
I only noticed it at the end of this week's trip and haven't had time to investigate or reset the Tlog.
The only external connection is from a Raymarine AIS250 receiver
All clues/inklings gratefully received
Bob

 

[lustyd]

Has anyone any idea why Tlog on the chartplotter is racking up the miles at roughly 5 times the actual distance travelled?
I only noticed it at the end of this week's trip and haven't had time to investigate or reset the Tlog.
The only external connection is from a Raymarine AIS250 receiver
All clues/inklings gratefully received
Bob

Are you sure it's measuring in miles? I assume you're travelling a known distance and the plotter is recording 5 times as far rather than comparing to a paddlewheel log which may or may not be calibrated correctly? You're also resetting the trip distance each time?

 

[LadyInBed]

Strange, their's nothing on the distance conversion chart that has anything near a factor of five.

 

[Simondjuk]

Long shot, but how is your satellite reception and how does your track look on the plotter? A jumpy fix might have the track zig-zagging hither and yon, thus adding miles.

That said, a factor of 5 is massive, so the fix would have to be doing more jumping than fixing.

 

[RIBW]

Are you sure it's measuring in miles?
Yes, Nautical miles

I assume you're travelling a known distance and the plotter is recording 5 times as far
Exactly that!

rather than comparing to a paddlewheel log which may or may not be calibrated correctly?
There is only the AIS250 connected - the CP300i is otherwise autonomous

You're also resetting the trip distance each time?
As I mentioned, I haven't reset - I was expecting Tlog to be cumulative. However I plan to reset next trip just in case I have hit a firmware glitch.

Thanks lustyd - I have interposed some replies in your text above

 

[RIBW]

Strange, their's nothing on the distance conversion chart that has anything near a factor of five.

I'd be interested to know what/where is a 'distance conversion chart'.
Thanks
Bob

 

[RIBW]

Long shot, but how is your satellite reception
Reception is really good - no physical impediments on simple straight passages


and how does your track look on the plotter?
Perfect - very regular track plots

A jumpy fix might have the track zig-zagging hither and yon, thus adding miles.
The recorded track looks straight

That said, a factor of 5 is massive, so the fix would have to be doing more jumping than fixing.

Thanks Simon - I have interposed some replies in your text above.

In the end it will be something very simple - but what?

Cheers
Bob

 

[lustyd]

My thinking on the reset was that you hadn't accounted for a load of miles already present - if that were the case then your problem isn't really a problem but I get the impression you're thorough enough to have thought of this one.

Are you on the latest firmware? It could very well be a bug but it would be an odd one.

Have you tried disconnecting the AIS to rule it out? There's an outside chance it's rebroadcasting the GPS signal back to the unit so your smooth track could actually be a one step forward, two steps back type affair - a sort of straight line version of a zigzag.

Other than a bug I can't think of a reason why a satellite only system would misread if you're getting a good plot. I assume you've not taken it home and powered it on or something like that - my handheld sometimes includes a plot at home, adding 80 miles to the end of the journey. I can't see why you'd do that with the SH unit though as there is very little benefit to powering it on at home.

 

[RIBW]

My thinking on the reset was that you hadn't accounted for a load of miles already present - if that were the case then your problem isn't really a problem but I get the impression you're thorough enough to have thought of this one.
I see what you mean, but it was regularly clocking faster i.e. each mile was recorded as five.

Are you on the latest firmware? It could very well be a bug but it would be an odd one.
Probably, but will check next visit.

Have you tried disconnecting the AIS to rule it out? There's an outside chance it's rebroadcasting the GPS signal back to the unit so your smooth track could actually be a one step forward, two steps back type affair - a sort of straight line version of a zigzag.
I haven't tried disconnection yet. I know what you mean 'tho - I had considered that as a possibility. Disconnection will determine it one way or another. The AIS250 does also act as a NMEA mux and will take the CP's input on the AIS250 4.8k port and resend it on its 38.4k (AIS) port back to the CP. The CP will therefore receive its own GLL message, possibly repeatedly! [If this hypothesis works out, the answer will be just remove the CP to AIS connection, but I am still intrigued as to why it would produce the over reading].

Thanks again lustyd - I have interposed more replies in your text above

 

[lustyd]

Thanks again lustyd - I have interposed more replies in your text above

If that does end up the answer then the reason would be processing lag in the AIS so it sends old positions which intermingle with the current position on the plotter. It would only need to disagree by a small amount to have this effect since it's sending that argument thousands of times a second which would make the plotter jump back and forth.

I did have some advice for rewiring but it was useless when I realised it's an internal GPS unit. Does the AIS also have internal GPS and does it need a GPS feed? You may find that a fix becomes more complex if that's the cause

 

[LadyInBed]

I'd be interested to know what/where is a 'distance conversion chart'.
Thanks
Bob

Not a chart as such, I used Realcalc scientific calc android app.

 

[RIBW]

If that does end up the answer then the reason would be processing lag in the AIS so it sends old positions which intermingle with the current position on the plotter. It would only need to disagree by a small amount to have this effect since it's sending that argument thousands of times a second which would make the plotter jump back and forth.

Does the AIS also have internal GPS and does it need a GPS feed?

Thankfully, the AIS250 doesn't need anything other than its RF input, so disconnecting the NMEA shouldn't be troublesome.
I suppose that even if there is only one repeat of the GLL message that would mean each fix could be followed by its predecessor (-delta) followed by a real update which would then be +2xdelta. Its already looking like 3x over reading! I think I'll try this 'disconnect' fix asap and will let you know how it goes.
Cheers
Bob

 

[RIBW]

Not a chart as such, I used Realcalc scientific calc android app.

Understood
Thanks
Bob

 

[lustyd]

Thankfully, the AIS250 doesn't need anything other than its RF input, so disconnecting the NMEA shouldn't be troublesome.
I suppose that even if there is only one repeat of the GLL message that would mean each fix could be followed by its predecessor (-delta) followed by a real update which would then be +2xdelta. Its already looking like 3x over reading! I think I'll try this 'disconnect' fix asap and will let you know how it goes.
Cheers
Bob

That's good news then, and hopefully is the problem. If the AIS is slow enough at processing and resending you might find it even worse than a 3x reading as it all depends how far back the repeated message is. If it were a mile behind for instance, every time the plotter updates it would add a mile plus the distance from last plot. Clearly you're not seeing anything that extreme, but if the AIS is 5 plots behind which could only be of the order of 1/100th of a second at 4800 baud that would account for your difference in log. Assuming that the AIS gives priority to its own traffic I'd say that delay is feasible although I'd hope that SH would have accounted for this sort of thing in the software. Good luck and we look forward to the answer whatever it may be

 

[Caer Urfa]

Has anyone any idea why Tlog on the chartplotter is racking up the miles at roughly 5 times the actual distance travelled?
I only noticed it at the end of this week's trip and haven't had time to investigate or reset the Tlog.
The only external connection is from a Raymarine AIS250 receiver
All clues/inklings gratefully received
Bob

Hate to mention it but 'I assume' you do have it set in miles not Km, for example if you have done 10 miles then it will show 16.09 km!

Mike

 

[RIBW]

That's good news then, and hopefully is the problem. If the AIS is slow enough at processing and resending you might find it even worse than a 3x reading as it all depends how far back the repeated message is. If it were a mile behind for instance, every time the plotter updates it would add a mile plus the distance from last plot. Clearly you're not seeing anything that extreme, but if the AIS is 5 plots behind which could only be of the order of 1/100th of a second at 4800 baud that would account for your difference in log. Assuming that the AIS gives priority to its own traffic I'd say that delay is feasible although I'd hope that SH would have accounted for this sort of thing in the software. Good luck and we look forward to the answer whatever it may be

I must check if I can delete GLL from the incoming data set to the SH CP, but I have a feeling it isn't that sophisticated.
Will certainly report back in hopefully a couple of weeks.

Cheers
Bob

 

[RIBW]

Hate to mention it but 'I assume' you do have it set in miles not Km, for example if you have done 10 miles then it will show 16.09 km!

Mike

Mike,
I am certain it is set for Nm (the over read is approx 5x anyway)
Cheers
Bob

 

[nigelmercier]

... I have interposed some replies in your text above

That really doesn't work very well.

 

[RIBW]

I'm pleased to report that it was the feed of the Standard Horizon 300i GPS data to the Raymarine AIS 250 and its repetition back to the SHCP300i that was causing the problem.
The SH Tlog is now performing accurately. Thanks to all advisors.
Regards
Bob