Speed Through GPS Versus Old Fashioned Paddle Log

[Robin3]

Originally Posted by MarkSF

You forgot one thing that's essential to do DR. An accurate timepiece. So I guess redundancy means at least 2 watches.

Well a reasonably accurate timepiece will do, except of course for celestial navigation when an accurate timepiece Is essential. So to illustrate that we can all have blind spots, I used to do celestial navigation calculations using the time taken from..... my GPS.

Magnetic Compass deviation and swinging it. Now that could be a thread to last forever too!! Hands up who hasn't swung their compass and got a current deviation card for it ? I'll start: I had swung and made proper deviation cards for each of my last two sail boats in the UK but not for the motoryacht we lived on here in the USA nor our current sailboat as we never got round to it on the liveaboard motoryacht and it is still on the 'to do' list for the current sailboat, before we head out farther than we do right now although the electronic compass on the autopilot was electronically swung as per it's instruction manual . We do have a hand bearing compass on board too and another in our binoculars so can do a rudimentary compass bearing check on the course being steered by using the HB compass as a comparison from a position on board that is well away from lumps of keel iron etc or any electrickery stuff.

I was once on a passage across the English Channel in company with some friends on another boat and wondered why their heading was much different for the same destination at the same speed. On investigation I then discovered that my shower stuff bag was hanging on a hook in the shower instead of put wellaway in a locker and it was hanging on the other side of a bulkhead from the fluxgate compass sensor. My electric razor in the shower bag was the villain of that error and me the clown wot forgot it. Had we continued we would have missed our destination about 70 miles away by ten miles or more but worse still would have been dragged by the tide current down into a tide race and some serious rocky bits. This was pre the niceties of GPS navigation although we did have a rudimentary RDF set to use for a confirmation of EP 'fix' from some very distant aero beacons.

[Wotname]

Originally Posted by Seaworthy Lass

This is a subject I have a keen interest in .
I add my voice to opinions expressed by OceanSeaSpray, Dockhead, Wotname and Stu Jackson. Dockhead's lengthy explanations are spot on.

CTS was all discussed at length a couple of years ago and made a very interesting discussion. It was condensed in this thread (links to the original threads are provided in the first post):
http://www.cruisersforum.com/forums/...ad-115361.html

This is not just a "personal opinion" any more than saying 2+2=4 is a "personal opinion".......

If you sail/motor along the rhumb line with variable tide during any journey then you are
taking the slow route. The current does not have to be symmetrical, nor does it have to be reversing for it to be inefficient to follow the rhumb line. It just needs to be VARIABLE for the journey. If the current is not variable, then then calculated CTS will simply take you along rhumb line ...

SWL

I was hoping that the vector maths post might pique your interest and get you aboard this thread

Your description of why and when CTS is important is both simple and easy to understand and I do like your tablecloth analogy for understanding a straight line through water; I must have missed it earlier.

However in keeping with the ethos of the thread, I should take you to task about whether your arithmetic is fact or opinion

[Sailor Doug]

Wotname do you still have a compass?? What vector are you trying to take us on. I have been part off many safety audits and understand system redundancy do you??

Still having fun don't know why.


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[Wotname]

Originally Posted by Sailor Doug

Wotname do you still have a compass?? What vector are you trying to take us on. I have been part off many safety audits and understand system redundancy do you??

Still having fun don't know why.


Sent from my iPad using Cruisers Sailing Forum

Dunno about what vector , maybe more like drift - thread drift that is

And is the compass question a trick question??? Or have I missed a joke .....

Anyway the straight answer is: I have two magnetic mechanical compasses, one in the cockpit and one down below. I also have two electronic magnetic compasses, one in the tiller pilot and the other is a hand compass.

All are swung, deviation card for cockpit compass only, all better than 3 degees of error except the tiller pilot which has up to 5 degrees on some headings.

For redunancy, I tend to rely on the Mk 1 eyeball with mechnical optical aids

[Seaworthy Lass]

Originally Posted by Wotname

I was hoping that the vector maths post might pique your interest and get you aboard this thread

Some kinds of thread drift are always perfectly acceptable in my book .

Originally Posted by Wotname

For redunancy, I tend to rely on the Mk 1 eyeball ....

Built in redundancy right there (except for up close) for most of us (just my personal opinion of course ).

SWL x

PS I must behave better.
I must behave better.
I must behave better.
I must behave .......

[lordgeoff]

Originally Posted by goboatingnow

Lordgeoff. I cannot understand why you continue peddling this nonsense. The fact is people that understand this technology at an engineering level have explained to you how this works. Equally the internet is full of technical explanations that explain this technology. The trouble is GPS is a conceptually simple system that's actually very very complex in reality , hence the incorrect explanations that abound,

GPS systems derive instantaneous speed from the Doppler effect which appears in the carrier wave from the sat. This speed for example is fed into the kalman filter to achieve better position accuracy.

All modern chipsets compute Doppler speed

Indeed as you say heading ( on single antenna ) systems are derived from position on RTK systems with two antennas heading is derived using inferometry and not position computation

Note . What appears on the NMEA output is often different. Garmin for example had traditionally not stored instantaneous speed in its track log for example in some products

Also. GPS satellites do not repeatedly transmit position. The pseudo code that is continuously broadcast merely serves to indentify the particular satellite , its primary purpose is to allow clock recovery / sync in the receiver.


Remember the position of the satellite is a far smaller contribution to positional error then clock error. The complexity in GPS is to regenerate the accuracy of the onboard atomic clock in a cheap receiver powered by a simple crystal oscillator. In fact if you just used a crystal oscillator uncorrected , after a week the position error would put you on the far side of the moon.

Hence the complexity in clock recovery. The position of the satellites is much less of a concern and can be transmitted much slower them anything else.


Overlaid on the c/A pseudo code is a data transmission, that contains the satellites almanac and more importantly ephemeris. This data is transmitted at a rate of 60 bits per second , very slow.

This is because the satellites orbit is tightly constrained and known in advance. The online data ( navigation message ) is primarily ephemeris corrections.

The navigation. Message information can actually come by lots of comms methods like in the case of assisted GPS.

PS. No one hear has said the receiver transmits to the sat

Note that VMG have nothing to do with GPS per se


It's all here http://www.gps.gov/technical/ps/1995...cification.pdf


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....I'll try one last time. Go to the link below. I think we agree Garmin knows a little about GPS.
The whole of the link is a good summary for understanding by yachties so read the lot.
The pertinent paragraph reproduced below is what I have been saying.

"A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."

Garmin | What is GPS?

[hoppy]

If you don't think you need speed through water, then you are not a sailor, just a boater with a sailboat.

[Wotname]

I'mm sorry but nothing in the quite basic GPS tutorial by Garmin supports your claim except by inferance.

[goboatingnow]

Originally Posted by lordgeoff

....I'll try one last time. Go to the link below. I think we agree Garmin knows a little about GPS.

The whole of the link is a good summary for understanding by yachties so read the lot.

The pertinent paragraph reproduced below is what I have been saying.



"A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."



Garmin | What is GPS?

Its a sales description for the Luddites. It doesn't actually explain what's going on. Of you follow some of the GPS tech forums. You will see Garmin engineers discussing Doppler etc

[goboatingnow]

Originally Posted by hoppy

If you don't think you need speed through water, then you are not a sailor, just a boater with a sailboat.

I wish people would stop parroting nonsense based on rote learning or something a book told them.

So few people understand navigation from first principles

Ships measured distance through the water cause that was all historically they had. They then used that as in effect as ground distance

They used the information

[lordgeoff]

Originally Posted by goboatingnow

Its a sales description for the Luddites. It doesn't actually explain what's going on. Of you follow some of the GPS tech forums. You will see Garmin engineers discussing Doppler etc

Last time then I am off to the football....
GPS= Global Positioning System.
Satellite SIGNAL to GPS RECEIVER= POSITION ON THE GLOBE.
Clever GPS RECEIVER = FROM CONSECUTIVE POSITIONS CALCULATES SPEED, VMG, TTG, HEADING, XTRAK ERROR, SOG etc etc etc etc.
GPS = BEST PIECE OF NAVIGATION EQUIPMENT ON THE BOAT.....INCLUDING PAPER CHARTS
Cheers I'm off to the footy.

[hoppy]

Originally Posted by goboatingnow

I wish people would stop parroting nonsense based on rote learning or something a book told them.

So few people understand navigation from first principles

Ships measured distance through the water cause that was all historically they had. They then used that as in effect as ground distance

They used the information

To understand how your boat is performing under sail you need to know your speed through water. Without it you don't have the full picture.

[monte]

That's interesting as I never try to trim sails to STW. I'm going somewhere, it's a fixed point, I sail the fastest I can to that point which may involve tacking, altering course etc, but why not trim to SOG. If a small adjustment adds some speed it's all good, why bother to see the same information on a less reliable meter. The times I do check the log are when the boat doesn't seem to be performing as well as it should to compare SOG to STW to analyse currents and try better the situation if possible. So why is it that it's better to trim to STW? Any increase would also increase the SOG wouldn't it?

[hoppy]

Originally Posted by monte

That's interesting as I never try to trim sails to STW. I'm going somewhere, it's a fixed point, I sail the fastest I can to that point which may involve tacking, altering course etc, but why not trim to SOG. If a small adjustment adds some speed it's all good, why bother to see the same information on a less reliable meter. The times I do check the log are when the boat doesn't seem to be performing as well as it should to compare SOG to STW to analyse currents and try better the situation if possible. So why is it that it's better to trim to STW? Any increase would also increase the SOG wouldn't it?

I'm not saying it's better to trim to STW, but having it in addition to SOG helps you understand the boats performance.

If you trimmed your sails to get the best performance on a port tack, then after a tack you find you are getting 2 knots less. Having both STW & SOG can help you troubleshoot. Also, if you know your boat well enough, then you know what to expect for given conditions and again STW & SOG together can help.

[Stu Jackson]

Originally Posted by monte

So why is it that it's better to trim to STW? Any increase would also increase the SOG wouldn't it?

Nope. For example: What would happen if you are sailing well, trim is great, but into a building current? Your SOG would drop, right? But your STW would remain the same.