SOG 0.3kts but I am not moving!

[LifePart2]

Here I am, sitting at anchor on a calm night. We are static. Not even swinging more that a few degrees. But OCN is consistently showing my SOG as between 0.2 and 0.4kts in a northward direction.

The direction would make sense, since the little wind is from the south. But, honestly, we are not moving.

My NMEA data is:

20:28:39 $GPGSV,3,3,12,19,40,110,36,20,72,278,48,25,17,309, 46,35,,,00*4A<0x0D><0x0A>
20:28:39 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:39 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:39 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:39 $GPRMC,012837,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*6E<0x0D><0x0A>
20:28:39 $STALK,90,00,03*67<0x0D><0x0A>
20:28:39 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:39 $GPGGA,012838,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*76<0x0D><0x0A>
20:28:40 $IIMWV,7.5,R,7.7,N,A*3F<0x0D><0x0A>
20:28:40 $IIHDM,166.0,M*23<0x0D><0x0A>
20:28:40 $IIDBT,2.3,f,,M,,F*10<0x0D><0x0A>
20:28:40 $GPGSV,3,3,12,19,40,110,38,20,72,278,48,25,17,309, 46,35,,,00*44<0x0D><0x0A>
20:28:40 $IIVHW,,T,,M,0.0,N,,K*7B<0x0D><0x0A>
20:28:40 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:40 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:40 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:40 $GPRMC,012838,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*61<0x0D><0x0A>
20:28:40 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:40 $GPGGA,012839,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*77<0x0D><0x0A>
20:28:41 $GPGSA,A,3,02,05,06,09,12,17,19,20,25,,,,02.0,01.0 ,01.7*07<0x0D><0x0A>
20:28:41 $IIMWV,11.0,R,8.1,N,A*04<0x0D><0x0A>
20:28:41 $GPGSV,3,1,12,02,48,342,46,05,49,234,47,06,33,045, 43,09,16,055,34*76<0x0D><0x0A>
20:28:41 $IIHDM,166.0,M*23<0x0D><0x0A>
20:28:41 $GPGSV,3,2,12,11,50,347,00,12,45,276,50,13,11,176, 00,17,24,121,41*7F<0x0D><0x0A>
20:28:41 $IIDBT,2.5,f,,M,,F*16<0x0D><0x0A>
20:28:41 $GPGSV,3,3,12,19,40,110,38,20,72,278,48,25,17,309, 46,35,,,00*44<0x0D><0x0A>
20:28:41 $IIVHW,,T,,M,0.0,N,,K*7B<0x0D><0x0A>
20:28:41 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:41 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:41 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:41 $GPRMC,012839,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*60<0x0D><0x0A>
20:28:41 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:41 $STALK,90,00,03*67<0x0D><0x0A>
20:28:41 $GPGGA,012840,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*79<0x0D><0x0A>
20:28:42 $IIMWV,13.5,R,7.7,N,A*0A<0x0D><0x0A>

The RMC says I am doing 0 kts. Is there something else in there saying I am moving? How do I fix that?


Thanks

Noel

[LifePart2]

Now the dashboard plugin (but not the status bar at the bottom - that now shows zero) says that I am doing 4.3kts! Fortunately we still have not crashed into the rocks just 100m behind us!

The NMEA data stream is still live.

I closed and reopened the dashboard, but no change. I then closed and reopened OpenCPN and now the dashboard shows SOG --- as it should.

So clearly something is getting stuck on the dashboard.

Also, now my boat heading is wrong. It is pointing north, when actually we are pointing south.

On my laptop, using the same NMEA stream, I am pointing south.

At other times (not infrequently) on the RPI I lose the heading, the boat then points in some random static direction (often at about 90 deg to our real heading) for many minutes, and then suddenly will revert to a live heading for a while, and then it will again lose that live heading and flip over to an incorrect static heading again (similar but not identical to the previous static heading).

Any suggestions?

Noel

[verkerkbr]

Originally Posted by LifePart2

Here I am, sitting at anchor on a calm night. We are static. Not even swinging more that a few degrees. But OCN is consistently showing my SOG as between 0.2 and 0.4kts in a northward direction.

The direction would make sense, since the little wind is from the south. But, honestly, we are not moving.

My NMEA data is:

20:28:39 $GPGSV,3,3,12,19,40,110,36,20,72,278,48,25,17,309, 46,35,,,00*4A<0x0D><0x0A>
20:28:39 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:39 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:39 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:39 $GPRMC,012837,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*6E<0x0D><0x0A>
20:28:39 $STALK,90,00,03*67<0x0D><0x0A>
20:28:39 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:39 $GPGGA,012838,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*76<0x0D><0x0A>
20:28:40 $IIMWV,7.5,R,7.7,N,A*3F<0x0D><0x0A>
20:28:40 $IIHDM,166.0,M*23<0x0D><0x0A>
20:28:40 $IIDBT,2.3,f,,M,,F*10<0x0D><0x0A>
20:28:40 $GPGSV,3,3,12,19,40,110,38,20,72,278,48,25,17,309, 46,35,,,00*44<0x0D><0x0A>
20:28:40 $IIVHW,,T,,M,0.0,N,,K*7B<0x0D><0x0A>
20:28:40 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:40 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:40 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:40 $GPRMC,012838,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*61<0x0D><0x0A>
20:28:40 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:40 $GPGGA,012839,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*77<0x0D><0x0A>
20:28:41 $GPGSA,A,3,02,05,06,09,12,17,19,20,25,,,,02.0,01.0 ,01.7*07<0x0D><0x0A>
20:28:41 $IIMWV,11.0,R,8.1,N,A*04<0x0D><0x0A>
20:28:41 $GPGSV,3,1,12,02,48,342,46,05,49,234,47,06,33,045, 43,09,16,055,34*76<0x0D><0x0A>
20:28:41 $IIHDM,166.0,M*23<0x0D><0x0A>
20:28:41 $GPGSV,3,2,12,11,50,347,00,12,45,276,50,13,11,176, 00,17,24,121,41*7F<0x0D><0x0A>
20:28:41 $IIDBT,2.5,f,,M,,F*16<0x0D><0x0A>
20:28:41 $GPGSV,3,3,12,19,40,110,38,20,72,278,48,25,17,309, 46,35,,,00*44<0x0D><0x0A>
20:28:41 $IIVHW,,T,,M,0.0,N,,K*7B<0x0D><0x0A>
20:28:41 $IIMTW,28.0,C*19<0x0D><0x0A>
20:28:41 $IIVHW,,T,,M,0.00,N,,K*4B<0x0D><0x0A>
20:28:41 $IIMTW,28.2,C*1B<0x0D><0x0A>
20:28:41 $GPRMC,012839,A,2329.0884,N,07544.1609,W,000.0,000 .0,130322,,,A*60<0x0D><0x0A>
20:28:41 $STALK,9C,D1,25,FA*4E<0x0D><0x0A>
20:28:41 $STALK,90,00,03*67<0x0D><0x0A>
20:28:41 $GPGGA,012840,2329.0884,N,07544.1609,W,1,09,01.0,0 0011.6,M,-034.0,M,,*79<0x0D><0x0A>
20:28:42 $IIMWV,13.5,R,7.7,N,A*0A<0x0D><0x0A>

The RMC says I am doing 0 kts. Is there something else in there saying I am moving? How do I fix that?


Thanks

Noel

It is not OpenCPN. It is the wandering of your GPS fix. What GPS are you using ?


Here at home I have a USB GPS mouse, doing GPS, Glonass and Gallileo combined.


But still wanders somewhat and it is not because my house is moving.



Bram

[LifePart2]

Hi Bram,
I am using a Standard Horizon smart GPS.

I wondered if it was just the innate GPS inacuracy, but then one would expect the SOG and COG to fluctuate rather than be consistently at 0.3kt going northward.

Which part of the NMEA stream shows the GPS wandering? As I read the RMC sentence it says the SOG is zero.

Also, any idea why the boat's heading on the RPI is different to that on the laptop, both using the same datastream?

Noel

[sailormed]

GPS signal are subject to many outside errors. Most common is a reflected satellite signal, so the same packet is arriving twice at slightly different times. The software can then arrive at the conclusion that the antenna (read boot) is moving. I suspect that your installation is ok, your environment is playing games.

[chris95040]

Originally Posted by LifePart2

Also, now my boat heading is wrong. It is pointing north, when actually we are pointing south.

Your heading, if you don't have an actual heading sensor, is simply computed from your gps positions. There's averaging and stuff but basically its just the angle between your current position and your last position. It has nothing directly to do with which way the boat is pointing, just which way it's moving.

Of course this only works if you are actually underway. If you aren't really moving, the heading computed will be a pointless value representing the angle between a cluster of points as you slowly wiggle a few feet here and there.

[bdbcat]

Noel...
Please verify that Settings->Ships->"Calculate SOG and COG from position changes" is disabled (unchecked).


This option is only to be used if you have a very old, or defective, GPS receiver that cannot calculate SOG/COG on its own, or does not transmit RMC.


Thanks
Dave

[Lepke]

My track left on for a couple weeks while tied to the home dock.

[Cwens]

Originally Posted by chris95040

Your heading, if you don't have an actual heading sensor, is simply computed from your gps positions. There's averaging and stuff but basically its just the angle between your current position and your last position. It has nothing directly to do with which way the boat is pointing, just which way it's moving.

What you describe is "course." "heading" is the direction in which your boat is pointing. Plain GPS provides course data as you describe, as computed from successive positions, and cannot provide heading data. A compass provides heading data but cannot provide course. Some "GPS" units also contain a magnetic compass and can provide heading data. Some fancy installations have two GPS sensors far apart on the boat and can compute heading data from the difference between them.

[BMD21]

You are aware that the primary purpose of GPS is military? They added a random noise, which has been scaled down due to public demand. I don't know but what is happening could be noise that the military puts in. That would also explain why we have 3 constellations giving the same information. Those things aren't cheap.


Boris

[bcn]

All this is completely normal with GNSS only sensors. An IMU with coupled GNSS will improve that a lot - regarding motion and heading, position some improvement.


Some GNSS chip set will filter noise, giving a smoother but not more correct solution. Or not. Then you see the spikes and jumps.



15m jumps in position are normal, not frequent. Over a longer observation period you will see those happen. As this is stochastic the spikes can happen now or within some hours.


No noise added by nobody, just atmosphere, solar activity, GNSS orbits and clocks and stuff like that.
In the case of clear not obstructed sky - what we have usually on a boat - that's it.
Moving in the country or in a town or between docks adds multiple returns/multi-path and outages resulting in a much noisier solution.



Staying quite - being moored has some residual movement - over a 24h or 48h reading will give some 2m circle (2d not 3D) with your probable real position.
SBAS corrections might improve this to 80cm.


If you have a receiver or software that does process differential corrections either in realtime (that's then called RTK precision) or in post processing 2 - 10cm can be expected.
Being lucky having a L1/L2/L5 receiver and differential corrections, then you can get <1cm (mm range) in stationary or <2cm in dynamic solutions.


GNSS compass with dual antenna do like L1/L2 very much as well.


A GNSS/IMU sensor will give you Heading and COG(real). Attitude and position. Precision range directly derived from the size of your wallet..

[El Pinguino]

This was all discussed at considerable length here on CF maybe 4 years ago so it is not something new.

I've just checked a two ports. Puerto Montt, about 40ºS, lots of nearby masts, lets call that 'mast density', every single yacht with AIS/GPS running is moving around in a quite remarkable fashion, always on about a 350º/170º axis.

Salina/ La Libertad very close to the equator - about 2ºS. A smaller sample mainly motor boats, lower mast density. Not a single one is showing movement greater than a boat's length.
Pic 1 typical movement in Montt
Pic 2 typical movement in La Libertad

[rgleason]

Please see Dave's Post
https://www.cruisersforum.com/forums...ml#post3591306

Please verify that Settings->Ships->"Calculate SOG and COG from position changes" is disabled (unchecked).

This option is only to be used if you have a very old, or defective, GPS receiver that cannot calculate SOG/COG on its own, or does not transmit RMC.

Maybe this should be in the manual's FAQ?

[CarCode]

To find out if the reason of this problem is caused by OpenCPN or by your GPS equipment the best solution will be to test with another similar software. Because I don't have the same GPS equipment as you have you should test e.g. with qtVlm which is a similar software as OpenCPN however not based with wxWidgets but with the Qt frameworks. It is available for free from https://www.meltemus.com/index.php/en/ and offered for all operating systems (even for Apples iPhone and iPad which OpenCPN cannot handle).

[goboatingnow]

Originally Posted by BMD21

You are aware that the primary purpose of GPS is military? They added a random noise, which has been scaled down due to public demand. I don't know but what is happening could be noise that the military puts in. That would also explain why we have 3 constellations giving the same information. Those things aren't cheap.


Boris

There is currently “no additional noise “ added by the “ military “SA was turned off some time ago .

Note that there are 4 operational GNSS systems we need to stop referring to one ( ie GPS) Galileo for example is not a military system and is not controlled by one. Most modern GNSS receivers now use all four constellations.

Positional inaccuracies in GNSS are a function of “ dilution of precision “ issues , multi path issues and transmit path latency , ie the basic physics