Speed Through GPS Versus Old Fashioned Paddle Log

[transmitterdan]

Originally Posted by Sailor Doug

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

GPS calculated velocity is extremely accurate at pretty much any speed. Much more accurate in terms of velocity relative to the ground than a paddle wheel knot log.

You can try a simple experiment with a hand held GPS. Wave it around in the air for a few seconds as fast as you can (preferably when no one is watching ) and see if speed is displayed even though it went nowhere.

The rate with which speed updates come from Garmin units has more to do with the 4800 baud serial port interface and the communication software than anything else. The speed display on the screen probably updates faster than the position update.

[The Smokester]

Originally Posted by Sailor Doug

...At 5kt it would require some pretty slick electronics...

The pulse in your wrist can be measured using the Doppler shift from an impinging laser.

The breathing motion from persons buried in rubble can be measured from the Doppler shift of RF waves flooding the debris.

[goboatingnow]

TransmitterDan, I can see we are battling uphill here !!!


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[The Smokester]

Originally Posted by lordgeoff

I'll try one more time......
...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...Calculating Speed is simply DISTANCE/TIME...

If your explanation is true, can you please explain how a handheld GPS can determine my speed within a few tenths of a mph as I walk less than ten feet across the house.

Please allow in your explanation for the inconvenient fact that the GPS unit doesn't know where I am to within ten feet.

[fryewe]

Originally Posted by Robin3

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.

DR position is simply the advancing of position from the last known accurate fix using the steered course and ship's speed for the interval since the last fix.

When the DR position is adjusted for leeway, current, and other effects, it becomes an EP (estimated position). While an EP is useful for route decision making, between fixes, for safety, it is useful to evaluate an expanding circle of position uncertainty based on set and drift, instrument inaccuracy, and steering errors, and keep the entire circle clear of danger until a new fix is gotten.

With a working GPS system, the fix interval makes these procedures old hat...but it's a hat worth keeping the dust off of, if you sail offshore.

[Sailor Doug]

Goboatingnow/transmitterdan this is a down hill battle, you decline to identify brand of marine gps units that use Doppler effect.


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[Wotname]

Originally Posted by Sailor Doug

Goboatingnow/transmitterdan this is a down hill battle, you decline to identify brand of marine gps units that use Doppler effect.


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All of them.

Please be assured that GBN & TD are quite correct in their explanations of how GPS works.

[Wotname]

FWIW, Garmin have been makeing GPS units since they became availabe to the general public, sometime back in the very early 80s IIRC. At that time, they were at the forefront of GPS developement for general aviation and the leisure market. IMO, they have always been at or near the top w.r.t. of GPS technology for consumer applications.

[Sailor Doug]

Originally Posted by Wotname

FWIW, Garmin have been makeing GPS units since they became availabe to the general public, sometime back in the very early 80s IIRC. At that time, they were at the forefront of GPS developement for general aviation and the leisure market. IMO, they have always been at or near the top w.r.t. of GPS technology for consumer applications.

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[Robin3]

Originally Posted by fryewe

DR position is simply the advancing of position from the last known accurate fix using the steered course and ship's speed for the interval since the last fix.

When the DR position is adjusted for leeway, current, and other effects, it becomes an EP (estimated position). While an EP is useful for route decision making, between fixes, for safety, it is useful to evaluate an expanding circle of position uncertainty based on set and drift, instrument inaccuracy, and steering errors, and keep the entire circle clear of danger until a new fix is gotten.

With a working GPS system, the fix interval makes these procedures old hat...but it's a hat worth keeping the dust off of, if you sail offshore.

Agreed, and yours is a much better explanation of what I was trying to say, but it still makes distance moved through the water an important input and one not available unless some kind of distance log is fitted. In years of old we used to tow a 'Walker Log' a spinner on a line attached to a mechanically recording mileometer on the taff rail. later we went more modern with speedometer type cable driven thru hull rotator spinnner to a 'Sumlog' or a cheaper one called a 'wasp' log, these morphed into paddlewheel logs of varying sophistication although we had one boat with a Brookes & Gatehouse through hull propeller type spinner that fed a digital mileometer and an analog speed dial Subsequent boats had Raymarine paddlewheel kit in various form from ST40/50/60 and currently we have all new Garmin stuff with an Airmar paddlewheel transducer ( actually a triducer as it gives speed, depth and water temp data) So in the unlikely event ALL 5 of our on board GPS receivers fail simultaneously we can do manual chartwork like wot we had to do a lifetime ago.

Maybe when I get more used to sailing only in areas of little or no current flows I will join the 'GPS is all you need' camp, but in the meantime will remain a boring old fart ! Maybe I should just switch to sailing a rocking chair and leave the new generations to their high tech only toys.

[Sailor Doug]

Originally Posted by Sailor Doug

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Wotnot I see you are a professional, have you read all the posts. The question is do marine gps determine position and speed by time differential or by Doppler shift. I understand that Doppler shift helps identify satellites in a constant orbit relative.


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[mrm]

Originally Posted by transmitterdan

That is more accurate the the paddelwheel on our boat for sure.

Naaaah, not after a third cork...

Personally I like to have both, but if log is not working, I won't dive to clean/free it - water is too cold here..

[Wotname]

Originally Posted by Sailor Doug

Wotnot I see you are a professional, have you read all the posts. The question is do marine gps determine position and speed by time differential or by Doppler shift. I understand that Doppler shift helps identify satellites in a constant orbit relative.


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First, thank you for reading my posts, I not sure that many others do

Anyway enough about me, back to your question; marine GPS units use Doppler shift techniques to determine speed. They don't determine speed simply by postion and time differential. In fact most (probably all?) GPS chipsets do it this way.

To those who think the GPS engine is both cheap and simple, let me assure you it is not simple. Cheap, yes, by way of mass production and turn over. Simple no, unless very very very advanced maths is simple.

For instance, the RF level of the satellite signal is way below the level of the background RF noise level by the time it reaches your antenna. Yet this "simple" receiver can determine what is signal and what is noise, slowy amplify the signal and not the noise until it raises the the desired signal to a level above the background noise; then it can process that signal and do stuff with it. This is sophisticated electronics. It is like someone whispering to you from one side of a extermley noisy bar or rock concert to a half deaf person on the other side.

Now I don't make this equipment, I just install it, fix it and sometimes fix up other people's stuff ups. I am sure that TD & GBN know more about the design techniques than me but I do know they are correct on this one.

[fryewe]

...The question is do marine gps determine position and speed by time differential or by Doppler shift.

At the link below is a good (a bit dated) description of the parameters used in calculations by your gps...with both pseudo-range (phase difference...which is time difference...key for position determination) and pseudo-range-rate (Doppler) used for speed calculations...from multiple satellites.

So, the answer is "both."

GPS Determination of Course and Speed

[goboatingnow]

Originally Posted by Sailor Doug

Goboatingnow/transmitterdan this is a down hill battle, you decline to identify brand of marine gps units that use Doppler effect.


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All of the current models do so and have actually done so for some time. For example all SiRF IIi and IV do so.

It's a side effect of digital methods of phase correlation so it's easy to implement.

Why do you think it's done any other way

Dave


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