GPS Accuracy

[wg992000]

Does anyone know if there is a set relationship between the accuracy / DOP/
Differenctial indicated on the GPS page to the accuracy of the compass
direction? ( of the GPS)

ie. Trying to determine how far off the compass rose would be off if WAAS
sat's were being received and there was high accuracy.

Kind of new at this. Any thoughts appreciated.

 

[Rodney Myrvaagnes]

Does anyone know if there is a set relationship between the accuracy / DOP/
Differenctial indicated on the GPS page to the accuracy of the compass
direction? ( of the GPS)

ie. Trying to determine how far off the compass rose would be off if WAAS
sat's were being received and there was high accuracy.

Kind of new at this. Any thoughts appreciated.

The compass course given by the gps depends more on how fast you are
going than whether WAAS is on. The magnetic course depends then on
whatever the software has for variation. Variation is a slowly
changing function of location, so won't depend on WAAS at all.

But, since you don't really know what the software does, you would be
better reading true if you are trying to swing your compass with the
GPS. Then use the variation from your chart.




Rodney Myrvaagnes J36 Gjo/a

Entering your freshman dorm for the first time, and seeing
an axe head come through the door on your right.

 

[Harry Krause]

GPS has no compass heading information, actually. This is why your
GPS doesn't know which way to point the "heading up" display on the
chart plotter when the boat is stopped, unless you have a compass
sensor input.

The heading information on a GPS-only system depends on measuring the
change in the lat/long over several measuring periods before it
displays heading information. If the boat merely rotates around its
anchor, this information will not be available or will be totally
inaccurate.

GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.

You forgot time.

And how long is the "so long" you refer to in indicating a change in
heading or speed? It doesn't seem "so long" on my GPS instruments.
Perhaps you wired them up wrong, eh?

 

[Dennis Pogson]

Oops....I also forgot TIME. GPS does provide very accurate TIME to
make the calculations in my other post....

Sorry....(blush)



Larry W4CSC

Is it just me or did the US and UK just capture 1/3
of the world's sweetest oil supply? What idiot wants to
GIVE IT BACK?!!

The "compass" in a GPS is surely historical, it tells you where you've been,
not where you are going? A very rough comparison with the magnetic compass
might indicate if the latter had major errors, but compass swinging using
GPS, someone must be kidding! Much better to use transits, or a hand bearing
compass.

 

[Dennis Pogson]

I'm going to stick my neck out and ask a very newbie type question.
Rodney mentioned reading "True" rather than "Magnetic." I've puzzled
for a while now over why we would use "True" at all. I know... I
should know...but I don't.
The compass is obviously "Magnetic" and it would be used for taking
bearings while under way. That data is then transferred to a paper
chart that has a compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program
or any other occasion?

Blushing Bob

Since the position of magnetic north is changing perpetually, it is standard
practice to give all bearings etc, in true notation, and apply the
correction factor appropriate to the time and location of the navigator.

The compass rose on charts, where magnetic north has moved almost as soon as
the chart is published and sold, is for guidance only.

GPS bearings and calculated courses are corrected for magnetic north as at
the time and place of the fix shown on the screen, but the user has the
option to display in true notation if he/she so pleases. I personally set my
GPS to true readout, but others may prefer to set theirs to magnetic.


Remove "nospam" from return address.

 

[Rodney Myrvaagnes]

I'm going to stick my neck out and ask a very newbie type question. Rodney
mentioned reading "True" rather than "Magnetic." I've puzzled for a while
now over why we would use "True" at all. I know... I should know...but I
don't.
The compass is obviously "Magnetic" and it would be used for taking bearings
while under way. That data is then transferred to a paper chart that has a
compass rose with a magnetic measurement in it.
In short, when would one use the "True" measurement on a chart, when
reporting ones position to another, in a electronic charting program or any
other occasion?

Sorry, I tried to make that clear. The 'true' measurement is derived
first from successive Lat-Lon measurements, so it is historical as
others have said. The 'magnetic' depends on a lookup table in memory
if it is done by the gps.

As you will see if you read the compass rose legend on a chart, the
variation changes slowly over the years. Unless you know if your gps
updates its table, and/or when its data were entered, you would be
taking a chance.

Even though the 'true' is historic, it is possible to use it for
swinging. You must motor along an ad hoc range line at constant speed,
the faster the better. A motorboat will be easier., but if the
direction reading doesn't change at all for some time. you should be
ok.

If you can hold the range and the speed over ground, your true
heading will also hold steady and be correct. If conditions prevent
you from holding, it's the wrong time to swing your compass.

HTH




Rodney Myrvaagnes J36 Gjo/a

Entering your freshman dorm for the first time, and seeing
an axe head come through the door on your right.

 

[wg992000]

Thanks for all the input -
It was the swinging the compass input I was after.

Thought process was that maybe with decent sat reception - swinging the
compass (adjustments and compass card) may be easier and more accurate with
the GPS then using traditional methods i.e.. lining up bearing points,
shadows etc with my 13 year old at the wheel and me trying to do this. Have
a 21 coastal boat with compass in 5 degree increments. Sounds like the best
idea is stick with traditional methods or farm it out. In any case thanks
for all the help and thoughts.

 

[BrianR]

Well, the datasheet of my "cheaply-made piece of crap" GPS module, costing a
whopping $70, clearly states the presence of doppler shift data in the raw
datastream I can extract from that module. The used chipset is a very common
one in low end GPS receivers. Do a google search on "gps speed doppler" and
you'll find this info.

Meindert

Tried a Google search for "gps speed doppler" and got the following message:

Your search - "gps speed doppler" - did not match any documents.

Suggestions:
- Make sure all words are spelled correctly.
- Try different keywords.
- Try more general keywords.
Also, you can try Google Answers for expert help with your search.

 

[BrianR]

Thanks, I left the quotes in originally as I was looking for the exact
phrase.
Now get the 13,300 hits, much appreciated.

 

[Ante Topic Mimara]

Thanks, I left the quotes in originally as I was looking for
the exact phrase.
Now get the 13,300 hits, much appreciated.

If you go to http://www.alltheweb.com and enter GPS Speed Doppler
without quotes, and use the "all the words" function, you will get
34,344 hits.

I don't know if the extra 21,000+ hits matter to you, but there is
a wealth of information on some of these pages.

I hope this helps!

 

[Bruce in Alaska]

Wouldn't it be nice if there were some kind of exact crossing ranges
setup for just compass swinging in some out of the commercial shipping
channel place. Two ranges pointing into an area unobstructed,
crossing in the middle. One N-S range and one E-W range with markers
on both ends so you could go either direction and KNOW you were 0 or
90 or 180 or 270.

Alot of Coastal Towns have such Ranges setup out here in the west.
In Seattle, all the compass rebuilders use the same range that is
setup in Lake Union. In the spring the whole Seattle Based fishing
fleet can be seen out there doing compass adjusting.

Bruce in alaska

 

[Bruce in Alaska]

Bruce,
Do you have any Seattle wet compass rebuilders you can recommend? We don't
have anyone left here in Portland who can add fluid, etc., since Captains
Nautical went out of business a year ago thanks to stores like West Marine.
Doug K7ABX

Doug,

Yea, there is a fellow that I used a lot in the past.
Eknes Instruments, in Ballard 826 NW 49th St 98107 (206) 783-3303
this fellow is a third generation Compass Adjuster/rebuilder.
Well respected in the North Pacific Fishing Fleet.

Bruce in alaska