Speed Through GPS Versus Old Fashioned Paddle Log

[thinwater]

So funny. The casual reader would assume very few of the posters have ever sailed a dingy and that they are also blind and deaf. We are giving the new generations of sailors the completely silly and pointlessly expensive idea that they need all this stuff, and worse, that it is more important than the senses. Just get your head out of the cockpit.

I have electronics, know very well how they work... and I don;t need any of them and don't turn them on many days.

Just silly.

As for the guy that thinks he's significantly breaking the harbor speed limit in a sailboat LOL.

[transmitterdan]

Originally Posted by lordgeoff

I'll try one more time......
GPS is designed to locate your vessel's Position (Lat and Long) on the Globe. That is why they call it Global Positioning System.
The GPS calculates the vessels position (Lat and Long) based on packets of data from the GPS Satellite Network. The vessels Position is established frequently and relentlessly. All calculations of Speed, SOG, VMG, ETA, TTG etc etc are calculated by comparing consecutive positions of the vessel continually and relentlessly. By comparing two consecutive positions very simple mathematical calculations can be made. Determining a heading between two points is Navigation 101. Calculating Speed is simply DISTANCE/TIME. The satellite time stamps position data.
Distance between two points is a simple calculation.
ETA, VMG, SOG are just very simple manipulation, by the receiver/plotter of the data on hand.
These calculations are all based on the very simple strings of data sent from the satellites in NMEA format.
The speed of transmission of the data makes GPS results extremely accurate. This is why you can get the accurate speed of a vessel travelling at 0.5 Kn or a car travelling at 100 kph or a Formula 1 travelling at 300kph.

Lord Geoff,

Technically, GPS is a PNT (positioning, navigation and timing) system. GPS is sort of a trademark like GLONAS and others. The US GPS system provides many features beyond positioning alone. It is used as a time and frequency standard by millions of devices. They will be distressed to learn that it only provides position capability.

Technically, speed is not distance divided by time. That would be the "average speed". Speed is defined as an instantaneous value. You do not have to wait until you reach another place to know speed. The simplest example of that is an old fashioned cable driven car speedometer. They do not measure the distance traveled and divide by time. Neither does a GPS receiver have to do that. Mathematically speed is the first derivative of displacement with respect to time and is an instantaneous value. Speed does not just exist at discrete points along the journey. Interestingly, speed is also the integral of acceleration over time. Just as distance traveled is the integral of speed over time. So it is possible to (and many inertial navigation systems can) measure the distance traveled without ever knowing a single position during the entire trip. Again we can look to the example of an old fashioned car odometer. It measures the distance traveled but it never knew where it was.

Doppler shift is caused when a transmitter and a receiver move in relation to one another. The amount of shift is a linear indication of that relative velocity. The GPS receiver measures enough of these doppler shifts from multiple satellites so that it is possible to compute speed very quickly and accurately. For low speed applications such as walking, running, boating and the like Doppler speed is much more accurate than distance/time and can be updated as fast as desired.

Much of what you wrote is not accurate (e.g. GPS satellites know nothing about NMEA) but it doesn't matter for this thread. If you are interested in learning more about this subject you can PM me for an offline discussion.

[goboatingnow]

Originally Posted by thinwater

So funny. The casual reader would assume very few of the posters have ever sailed a dingy and that they are also blind and deaf. We are giving the new generations of sailors the completely silly and pointlessly expensive idea that they need all this stuff, and worse, that it is more important than the senses. Just get your head out of the cockpit.

I have electronics, know very well how they work... and I don;t need any of them and don't turn them on many days.

Just silly.

As for the guy that thinks he's significantly breaking the harbor speed limit in a sailboat LOL.

Equally when you go home , you can of course get there by using your horse, and then as it's dark light your house using oil lamps. This method works just like using your car and electricity and will of course more or less achieve the same thing. Funnily we don't find them in common use for those purposes any more

As for harbour limits. Mine in places is 5 knots. I can easily get the boat to 9 at a push

Dave




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[Sailor Doug]

Doppler log vs gps vs paddle wheel! For us plain old sailors for fun do our gps units use Doppler effect for speed? Please DONOT include devices that measure shifts between near by objects like sea bottom or trees. Please provide brand names and models.


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[goboatingnow]

Originally Posted by Sailor Doug

Doppler log vs gps vs paddle wheel! For us plain old sailors for fun do our gps units use Doppler effect for speed? Please DONOT include devices that measure shifts between near by objects like sea bottom or trees. Please provide brand names and models.


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Yes your plain old consumer devices use Doppler effect. Making it quite accurate as a speedo.

Certainly it's more accurate then the paddle wheel , even if of course , it's measuring a different thing


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[goboatingnow]

Originally Posted by goboatingnow

Yes your plain old consumer devices use Doppler effect. Making it quite accurate as a speedo.

Certainly it's more accurate then the paddle wheel , even if of course , it's measuring a different thing


In reality SOG and STW can , outside of a few fairly extreme circumstances be used interchangeably in real life

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[Sandero]

GPS derived speed accuracy is between .5mph (auto applications) and .1 mph marine. My B&G speed measures/displaces to .01 knots... I don't know the accuracy of that system. It would depend on many factors, cleanliness of the hull and transducer, position in the hull, calibration and so forth. But it does show very small changes in speed and the trend. It is useful for sail trim.

[transmitterdan]

Originally Posted by Sailor Doug

Doppler log vs gps vs paddle wheel! For us plain old sailors for fun do our gps units use Doppler effect for speed? Please DONOT include devices that measure shifts between near by objects like sea bottom or trees. Please provide brand names and models.


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Doug, I think all GPS receivers you can buy today measure speed using Doppler effect. I have no way to verify that but I think it's a safe assumption. All GPS receivers measure Dopper shift so it's only a matter of software to calculate speed. And Doppler speed calculation requires less battery power. I can't imagine why a GPS engineer would use the slower, less accurate and power hungry distance/time method.

I have done a bit of Google searching on this and I find there is a lot of "wrong" explanation about how GPS works on the internet. Much of what I found is just people writing what they imagine rather than what they know.

[goboatingnow]

Originally Posted by transmitterdan

Doug, I think all GPS receivers you can buy today measure speed using Doppler effect. I have no way to verify that but I think it's a safe assumption. All GPS receivers measure Dopper shift so it's only a matter of software to calculate speed. And Doppler speed calculation requires less battery power. I can't imagine why a GPS engineer would use the slower, less accurate and power hungry distance/time method.



I have done a bit of Google searching on this and I find there is a lot of "wrong" explanation about how GPS works on the internet. Much of what I found is just people writing what they imagine rather than what they know.

Most GPS systems today have the horsepower to to N satellite averaging. They implement a Kalman filter , which is a form of quadratic estimation system. They feed into that carrier derived velocity , n sat range determination , noise , and usually past data points. , out of that they get an improved estimate of position

TransmitterDan. Here's one for you, the unit computed by carrier phrase correlation using a
L 4 sats is actuality a velocity , ie speed and direction

Does the instantaneous heading display use that info or is it position derived ?


Reading about it. It seems that both attitude and heading can be determined by interferometric methods. But do consumer GPS units use this for such displays

Dave


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[goboatingnow]

Belay by last question, I already know the answer. It requires dual independant phase measurement systems like my sat compass. --- doh. I should have realised that


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[Robin3]

I keep reading on here about 'speed' but that is only half of what the paddlewheel tells. DISTANCE gone through the water is IMO more important navigationally and is why I would miss having a proper 'log'. Water Speed to me is useful but not essential. Our Garmin paddlewheel feeds boat speed and distance data to a Garmin GMS10 multi function display head ( which also shows windspeed/direction, true or apparent, VMG to windward, and depth). It is also displayed on our 4012 multi plotter come radar come AIS display Within the system which is all linked via NMEA2000, we can select whether boatspeed data is GPS or paddlewheel derived. I choose to have paddlewheel data speed and the wind/depth stuff reading on the separate GMS10 display head at the wheel and to useGPs data derived stuff on the big plotter display on the coachroof visible from everywhere in the cockpit.

[goboatingnow]

Originally Posted by Robin3

I keep reading on here about 'speed' but that is only half of what the paddlewheel tells. DISTANCE gone through the water is IMO more important navigationally and is why I would miss having a proper 'log'. Water Speed to me is useful but not essential. Our Garmin paddlewheel feeds boat speed and distance data to a Garmin GMS10 multi function display head ( which also shows windspeed/direction, true or apparent, VMG to windward, and depth). It is also displayed on our 4012 multi plotter come radar come AIS display Within the system which is all linked via NMEA2000, we can select whether boatspeed data is GPS or paddlewheel derived. I choose to have paddlewheel data speed and the wind/depth stuff reading on the separate GMS10 display head at the wheel and to useGPs data derived stuff on the big plotter display on the coachroof visible from everywhere in the cockpit.

In reality distance over the water is in fact very little use. In most cases we plot DRs or even EPs by in effect correcting water distance to actual distance.
Out side of that distance through the water has little direct use.

Dave


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[Sailor Doug]

Can't find any reference that my simple Garmin gps uses Doppler effect. Units with high price do use Doppler for improved position accuracy and there are systems that use near objects doppler effect for very accurate speed.

Back to the first question of paddle wheel. It has a much less speed latency than standard gps and can help detect currents when used with fixed position navigation.

I and some others like a fast speed change response when sailing. Gps is only very accurate for constant speed.


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[Robin3]

Originally Posted by goboatingnow

In reality distance over the water is in fact very little use. In most cases we plot DRs or even EPs by in effect correcting water distance to actual distance.
Out side of that distance through the water has little direct use.

Dave


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Until you need to navigate properly as in without GPS. Plotting a DR position is not just plotting the lat/long readout off the GPS. It is calculating where you are as best you can by knowing a start position and then plotting how far you travelled in what direction and then how much other factors like leeway and tide/current drift affected that too, then you end up with an approximation of your current position that can hopefully be confirmed or refined by other means like a fix derived from bearings of known charted objects, even from sextant data if available.

I'm not trying to be pedantic or in any way superior but although I have all the electronic gizmos and multiple backups I just like to understand how it all fits in with the real world of wind, water and waves I carry paper charts on board but rarely look at them preferring our GPS Plotter or using OPEnCPN on a laptop or Navionics on my Tablet and smartphone I think we have 5 GPS receivers on board currently so not exactly electronically deprived

Now it Is time to go out to the boat for a fix. feel good fix not positional that is!

[goboatingnow]

Originally Posted by Robin3

Until you need to navigate properly as in without GPS. Plotting a DR position is not just plotting the lat/long readout off the GPS. It is calculating where you are as best you can by knowing a start position and then plotting how far you travelled in what direction and then how much other factors like leeway and tide/current drift affected that too, then you end up with an approximation of your current position that can hopefully be confirmed or refined by other means like a fix derived from bearings of known charted objects, even from sextant data if available.

I'm not trying to be pedantic or in any way superior but although I have all the electronic gizmos and multiple backups I just like to understand how it all fits in with the real world of wind, water and waves I carry paper charts on board but rarely look at them preferring our GPS Plotter or using OPEnCPN on a laptop or Navionics on my Tablet and smartphone I think we have 5 GPS receivers on board currently so not exactly electronically deprived

Now it Is time to go out to the boat for a fix. feel good fix not positional that is!

Indeed and as a previous Chartwork instructor I couldn't agree more. What I was commenting that whether you use a paddle wheel log or a GPS log to determine DR ( yes it's somewhat incongruous to use GPS in a DR plot !!!! ) it matter not in real terms


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