GPS SOG vs. Log

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I am trying to calibrate a new Raymarine log. The manuals no longer explain the old way of doing it (by sailing a known distance and comparing to the trip reading), they only give the method of setting the GPS speed to be the speed of the log.

BUT when I tried this, and checked the result by sailing a known distance, the log now gave too high a distance, indicating that the calibration factor was wrong. I re-calibrated the old way, to get distance correct, and now I find a 0,3 - 0,5 kts lower speed reading on the log than on the GPS.

I recall, on my previous boat, that the GPS often showed a higher reading, and that log was calibrated spot on, which had been tested by sailing thousands of miles.

Has anyone else noticed this discrepancy ? Is there a "theoretical" explanation for it ?

Regards,
Andreas


Andreas
 
I used the traditional method to calibrate my raytheon log and it agrees very well with the GPS SOG. Of course they rarely give the same reading due to the tidal stream, which is rarely slack.
 
Yes, have noticed the same thing, calibrated log, (walker) on measured mile in the Haringvliet, on a windless day having established there was no current etc, and found the same just under 1/2 Kn difference. It was more noticeable then due to Sa still being on.

Dont know the theory for it but on asking walkers was told it is due to averaging in the software of the gps on speed calcs, and pointed out that this is why most of the builders were quoting gps speeds knowing it gave them another 1/2 Knot!!

David ps Take a look at my site www.yachtman.co.uk
 
There is quite an appreciable current on the Haringvliet, especially around Tiengemeten and Slijkgaat. This is particularly noticeable when swimming, something we do most weekends. My GPS and log will agree at about 5 knots when there is definitely no current but there is a significant difference according to which direction I am travelling.

At six knots the GPS is a little enthusiastic, at four knots it's the other way around. I tend to believe the GPS, rather than the log, because I have no reason to believe otherwise. Since SA was turned off the accuracy of positioning has been superb, with no exceptions that I know of.

My new GPS has no SOG averaging capability, this is a feature that seems to have been removed. My previous one was able to average over various time periods, giving a steady reading of COG and SOG, whereas in choppy conditions the repeater readout is now almost useless.
 
I get up to 3 knots of tidal stream thpough my mooring. I calibrated my log by drifting in my tender with my GPS past the mooring while someone noted the log readings. I then corrected the log for the difference between the log reading and the GPS. A purist would probably shudder but I seem to have got the answer roughly right.
 
Log \'deviation curve\'.

I calibrated my log against the kilometer posts on the Middleberg canal, in Holland, which is almost straight and there is virtually no current. The distances agreed precisely with the GPS.

My ST50 log is now accurate AT 5.5 KNOTS. Above this it over-reads, below, it underreads. Next time I visit I intend to construct a 'deviation' curve for the log.
 
An idea for calibrating

I´m quite sure that a log is not linear, it can only be calibrated correctly for one specific speed. I recognize your problem and have overcome it this way:
During my holiday cruise last summer I wrote down a table of the various GPS SOG values and the corresponding log values in two different columns. I did this about 5 - 10 times a day during several days. I tried to do this in various conditions, when sailing at different tacks, angles and speeds and when motoring in different speeds.
Finally I calculated the differences between log and GPS in every observation and made an average out of all these calculations and re-calibrated my log according to this.
I now have a log showing accurate speed and distance within a few %.
 
GPS - SOG discrepancies

I have a standard Garmin fixed GPS. I have tried calabrating the log to it when I know there is no tide, however the speed is so inconsistant with it showing 4 knots one second 6 knots next momemnt, and sometimes settles down to a steady reading. Do I have a problem with my GPS?? perhaps the siting of the arial.
 
Re: GPS - SOG discrepancies

Perhaps you are in a time warp - this is just how GPS used to behave, before Selective Availability was turned off. Or are you sailing in Iraq?

Otherwise, I suspect some sort of fault. My GPS signal is absolutely rock steady, alongside a berth position does not move at all over a period of hours. Speed measurement never varies. My antenna is on a short mast on the pushpit. Previously I had a Garmin 75, without an external antenna, below at the chart table. It ran most of the time but satellite coverage was not great and it sometimes lost signal altogether. Suggest you monitor your DOP and EPE for half an hour or so. These should be very low, DOP less than 1 and EPE somewhere around 10 - 20 feet. If they are not, investigate moving the antenna.
 
Re: GPS - SOG discrepancies

I had expected SA to improve this, so yes - I'm in a time warp

The EPE reading is usually about 35, anf the altitude around 45 feet. What is the DOP, as I dont remember this on the satelite asquistion screen. I have never seen any real inaccuracy when plotting - probably because to plotto that accuracy is well nigh impossible.

I have emailed Garmin for further advice, as obviously its not doing what it should. The instrument is a 4 year old 120XL, perhaps the newer ones are better.

Thanks for you comments though
 
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