Question About Garmin "Hot Fix Data"

wrote:


=46our satellites aren't 'artificially set'. The maths of the position
solution require a minimum of three satellites for a 2D position and a
minimum of four satellites for a 3D position.

I agree that solving for two consecutive positions will provide enough
information to determine direction and speed but the use of Doppler
shift to determine speed requires only one solution.


I agree that the P solution relies on the time signals. However, Doppler
shift will vary according to the cosine of the satellite elevation and
the cosine of the angle between the direction of the satellite and the
direction of travel of the receiver. This means that the Doppler shift
from any satellite at right angles to the movement of the receiver will
be zero and from any satellite in line with that movement, for example
in line and on the horizon, will be a maximum.

I think we must agree to disagree but it's an interesting discussion
(and it's starting to make my brain hurt).
=20
--=20
Alan White
Mozilla Firefox and Forte Agent.
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in =
Argyll, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather

On Thu, 05 Aug 2010 09:01:38 +0100, Alan


What I meant was that 3 satellites are required for a 3D position, but
the probability of error would be significant so 4 (or more) are used,
with the 4th used as a sort of error correction against the first 3.

Wiki says it better than I can:

http://en.wikipedia.org/wiki/Gps#Basic_concept_of_GPS
Three satellites might seem enough to solve for position, since space
has three dimensions and a position near the Earth's surface can be
assumed. However, even a very small clock error multiplied by the very
large speed of light—the speed at which satellite signals
propagate—results in a large positional error. Therefore receivers use
four or more satellites to solve for the receiver's location and time.


We're in full agreement that there *is* Doppler shift and that the
receiver must deal with it in order to successfully receive and decode
the time signals. Where I think we diverge is whether that Doppler
shift plays a role in computing position, and by extension, velocity
and time. I say it doesn't, but I'm ok if you believe otherwise.

Thanks for the discussion.

On 05.08.2010 13:01, Sunshine wrote:



Wit 3 satellites it is 2D position and it is good as long you are on the
earth and you know your height. To get the height you need a 4th satellite.
Clock time propagation of a single satellite is a bowl,
area of equal clock time of 2 satellites is a circle, of 3 satellites it
is a line. With 4 satellites you get 2 points, one below and one above
the surface of the earth.

Jo

Sunshine wrote:



Go back to the old sci.geo.satellite.navigation newsgroup and you will
find many discussions and descriptions of the use of Doppler in GPS. I
used to think like you many years ago, but was convinced that Doppler
plays a significant role in calculating your velocity and direction of
travel. (and is used extensively when receiving a satellite's signal at
a particular shifted frequency).

On Thu, 05 Aug 2010 13:30:27 -0400, JF Mezei


Thanks for the invitation, but I have no desire to go clear up the
misconceptions in another group when there's plenty to go around in
this one. (Me included!) :-)