So, which one of these is more accurate?

[Jackdaw]

As different devices will have different strategies and formulas to calculate 'max speed', it is quite understandable and normal for these to value to be different. Heck, my Velocitek ProStart reports two different values for MAX at the same time!

But different steady-state readings are different. Assuming a good antenna position for both and and un-occluded view of the sky, both units should read with a few 10ths over a 10 second average. If that is not the case, something strange is going on.

 

[Michael ray]

Looking at a more simplified version.... Your knot matter will tell your speed in the water, this means if you're going with or against the current or there is no current, they will be different from the GPS which gives you the actual speed ( in relation to your actual location from point a to b). Knot meters can not determine your location, true point a to b speed. Just how fast water is turning that small impeller/paddle wheel.

 

[ifmdalvey]

Bill,

There is is nothing 'random' about the the approach angle or amount of Doppler shift at all... It has to be known to the reciever find the pseudo-code signal sent from the bird in the first place. Otherwise, the reciever would have no idea where to find the signal... The so-called 'cold start' which can take 30 minutes. Once tracking, the VERY SMALL amount of shift from the expected frequency (the so-called apparent-Doppler) is used to calculate speed. Run this stream through the Kalmann filter and the speed estimate is better than the delta model you describe. This capability started with the SiRF STAR 3 chipset introduced in (about) 2005.

I tend to agree with Bill, for a couple of reasons.
Firstly , i was a professional navigator flying and teaching on maritime patrol aircraft - admittedly 10 years ago now.
The faster you travel the more accurate both the SOG and COG, that is because the distance between individual fixes is further apart and therefore makes the result more accurate.
On my chart plotter, i display both the heading line and the track line (COG). When i make a turn, the EV1 follows that fairly accurately but the track line lags way behind because it is using the fixing method, if it were using doppler, the answer would be instantaneous.
As an aside, the doppler effect was how i tracked submarines when directional sonobuoys were not available.

 

[Tom]

GPS = speed across the bottom.
LOG = speed through the water.

 

[capta]

Ar

Bill,

There is is nothing 'random' about the the approach angle or amount of Doppler shift at all... It has to be known to the reciever find the pseudo-code signal sent from the bird in the first place. Otherwise, the reciever would have no idea where to find the signal... The so-called 'cold start' which can take 30 minutes. Once tracking, the VERY SMALL amount of shift from the expected frequency (the so-called apparent-Doppler) is used to calculate speed. Run this stream through the Kalmann filter and the speed estimate is better than the delta model you describe. This capability started with the SiRF STAR 3 chipset introduced in (about) 2005.

Aren't these geostationary satellites? Wouldn't that mean that there was almost no approach in the approach angle? It's not like we're zooming across the sky at 600 mph like a plane? Wouldn't our measly 5-9 knots be minuscule to the satellites?
Not arguing, just trying to understand what you are saying.

 

[capta]

as for GPS and compass agreement, I've always found that my GPS course and compass agree quite well once you account for variation, deviation and current.

And for those whose GPS and compass do not agree, it is a very simple fix. Either learn to swing your own compass or call the guy who does it locally.
Every prudent sailor should have a fairly recent deviation card for their compass aboard.

 

[Jackdaw]

Ar

Aren't these geostationary satellites? Wouldn't that mean that there was almost no approach in the approach angle? It's not like we're zooming across the sky at 600 mph like a plane? Wouldn't our measly 5-9 knots be minuscule to the satellites?
Not arguing, just trying to understand what you are saying.

No they are not Geo-stationary, and that's the reason they Doppler-shift in and out all the time.

 

[Jackdaw]

I tend to agree with Bill, for a couple of reasons.
Firstly , i was a professional navigator flying and teaching on maritime patrol aircraft - admittedly 10 years ago now.
The faster you travel the more accurate both the SOG and COG, that is because the distance between individual fixes is further apart and therefore makes the result more accurate.
On my chart plotter, i display both the heading line and the track line (COG). When i make a turn, the EV1 follows that fairly accurately but the track line lags way behind because it is using the fixing method, if it were using doppler, the answer would be instantaneous.
As an aside, the doppler effect was how i tracked submarines when directional sonobuoys were not available.

Sorry, but your view is too simplic. Go re-read what I posted. Modern GPS units use Doppler to calculate speed. Full stop.

 

[cdarwincole]

Lots of great information and amusement here! I for one really appreciate the efforts of those of you who have taken time to share your insights or humorous thoughts on the subject.

The question is not clearly stated since "speed" has not been defined as either "speed through the water" or "speed over ground," though I suppose we are to assume the latter.

I have never sailed on the Great Lakes but am curious as to what effect a strong and steady wind might have on surface water, thereby perhaps creating enough quasi-current to affect SOG and log readings. That the thermocline in any given lake will tilt down at the leeward side certainly implies that water at the surface (epilimnion) is moving. Is that something Great Lake sailors need to consider?

I sail in Long Island Sound and thereabouts. Currents are a major factor. The log gives me a good sense of my speed through the water -- with decent dead-reckoning results after factoring in current -- while my two GPS units (Garmin and iPad) give me a pretty reliable SOG. The iPad and Garmin are generally in agreement, BTW.

 

[cdarwincole]

Ian, love that "Experience is something you don't get until just after you need it." LOL.

 

[Tally Ho]

Was out Wednesday night in 22knots of wind and 8-9 foot waves. Reefed jib only.

Quite a wild ride.

"Surfed" down a wave for the first time, and recorded 10.8 knots !

Not sure how accurate that is, but it sure was an exhilarating sail!

Greg

 

[dlochner]

I have never sailed on the Great Lakes but am curious as to what effect a strong and steady wind might have on surface water, thereby perhaps creating enough quasi-current to affect SOG and log readings. That the thermocline in any given lake will tilt down at the leeward side certainly implies that water at the surface (epilimnion) is moving. Is that something Great Lake sailors need to consider?

There is a general downstream current on the Great Lakes as they drain out the St. Lawrence. Normally this is not a significant factor, less than half a knot. There are also currents from the various rivers and streams that flow into the lake and these can have significant local effects depending on flow and wind direction.

The downstream current on the lakes (at least on Lake Ontario) is augmented by the prevailing westerlies. Depending on wind speed and duration the effect can be negligible to quite pronounced, I've experienced currents near 2 knots at times.

The most interesting phenomena is a seiche (Seiche - Wikipedia, the free encyclopedia). Often caused by a strong cold front the lake sloshes back and forth, often by several feet. These also produce currents that are less predictable. Once watched as the water level in the harbor dropped by about 3 feet in a matter of a few minutes in front of a strong cold front.

 

[cdarwincole]

Thanks, dlochner. Intersting!

Greg, how old is your GPS?

 

[walmsleyc]

See: http://www.glerl.noaa.gov/res/glcfs/glcfs.php?lake=o&ext=sfcur&type=N&hr=00

 

[cdarwincole]

Very cool, walmsleyc. Thanks! You and dlochner make it clear that there are indeed currents in the Lakes. That should answer Greg's question about which is more accurate: GPS for speed over ground, the log for speed in mobili (for Verne fans).

I'd bet that Greg's top speed number was derived in a more reliable way in the Garmin than in the iPad. Are there any Apple techs here who can tell us how an iPad does its GPS thing? It might depend on how the boat was pitching (as SFS pointed out) at the moments the iPad took its bearings.

 

[DaveJ]

I'm backing the 'current in Lake Ontario' statement. Last week I was heading southwest, motoring across the lake, and both my GPSs read 8.2 knots! My max is about 6.2 at that RPM....so I'm assuming there was a 2 kt current. My speed log is not accurate so I cannot compare to it. There had been 3 days of big west winds, pushing water to the eastern end....guess it was going back home.

Cheers

 

[Bill Roosa]

another comment on compass and GPS. I like the format of capta's deviation card but find that I usually know the magnetic course I want and need what compass course I should steer to get it. Having all the magnetic courses "jumbled up" (598, 12, 14, 26,.... etc) and the compass courses all nice an neat (0, 10, 20,...etc) makes that a lot harder. I prefer having all the magnetic coursed nice and neat so I can easily interpolate the course I need into a compass course to steer. capta's format is handy to shoot bearings with however so I'm thinking would want both formats.
on a related topic, how many different "courses" can you think of?
I'll start with the easy ones.
True course (TC) - the direction the boat is traveling over the ground referenced to true north
Magnetic course (MC) - the direction the boat is traveling over the ground referenced to magnetic north TC+variation=MC

 

[Tally Ho]

Thanks, dlochner. Intersting!

Greg, how old is your GPS?

I don't really know. It was on the boat when I purchased it 2 years ago. Why? Newer ones more accurate?

Greg