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.

On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

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.

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.



On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

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.



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?!!

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

Sorry....(blush)

On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

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.



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?!!

Larry W4CSC wrote:

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?


--
Email sent to is never read.

Larry W4CSC wrote:
Oops....I also forgot TIME. GPS does provide very accurate TIME to
make the calculations in my other post....

Sorry....(blush)

On Mon, 26 Jan 2004 21:12:28 -0500, "wg992000"
wrote:

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.



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.
--
Remove "nospam" from return address.

"Larry W4CSC" wrote in message
...
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.

I have heard otherwise: A GPS derives it's speed from the doppler shift on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert

On Tue, 27 Jan 2004 12:34:35 +0100, "Meindert Sprang"
wrote:

"Larry W4CSC" wrote in message
...
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.

I have heard otherwise: A GPS derives it's speed from the doppler shift on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert


This might be true on some $70K sophisticated survey instrument or
other, but, here at least, our discussion is about GPS operation on a
cheaply-made piece of crap sold to the boating consumer at amazing
markups, not sophisticated electronics. What you're talking about
costs serious money. You won't find that at Waste Marine where price
isn't related to quality.....(c;




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?!!

"Larry W4CSC" wrote in message
...
On Tue, 27 Jan 2004 12:34:35 +0100, "Meindert Sprang"
wrote:

I have heard otherwise: A GPS derives it's speed from the doppler shift
on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert


This might be true on some $70K sophisticated survey instrument or
other, but, here at least, our discussion is about GPS operation on a
cheaply-made piece of crap sold to the boating consumer at amazing
markups, not sophisticated electronics. What you're talking about
costs serious money. You won't find that at Waste Marine where price
isn't related to quality.....(c;

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

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