the 1st satellite gives the time reference.
2nd gives a circle, but not where on the circle.
3rd gives a second circle and they will intersect, twice. Past information,
manual or from memory, eliminates one point.
4th gives solution without past information, and height.
the relative geometry of the visible satellites have strong impact on the
accuracy; more satellites just refine the solution, as does WAAS.
Leslie.
used GPS to fix orbiting satellite position to a few centimeter, with other
tools (almost 20 years ago)
________________________________
From: Marek Dziedzic <[email protected]>
To: [email protected]
Sent: Monday, January 28, 2013 8:37 AM
Subject: Re: Stus-List Sextant
Obviously, this must be winter and we have nothing
better to talk about...
What Eric described applies to 2D position (assuming
(which not that far of) that the Earth is a sphere. Most GPS receivers require
4
satellite fixes to calculate the position (the 4th one gives you the error (the
accuracy)). You need more satellites to get a 3D fix.
Marek
C&C24 Fennel in Ottawa
Message: 5
Date: Mon, 28 Jan 2013 08:23:51 -0500
From: "Eric
Haberfellner" <[email protected]>
To: [email protected]
Subject: Re:
Stus-List Sextant
Message-ID:
<[email protected]>
Content-Type:
text/plain; charset="utf-8"
Just to clarify. The only GPS satellites that
are in geosynchronous orbit are
the ones that provide WAAS correction data.
The ones used to generate a
position fix are not in geosynchronous
orbit.
The constellation of about 24 GPS satellites orbits at about
12,600 miles
and these are not in equatorial orbit. If fact in order to
generate a fix, it
is critical that the satellites not be arranged in a
straight line as all
geosynchronous satellites are along the equator. This
would be a classic case
of bad satellite geometry. The fact that the
satellites are not in
geosynchronous orbit and are therefore moving relative
to the earth's surface
is critical in GPS calculations. This relative
movement allows the GPS
receiver to calculate the satellite's true position
by using the Doppler
shift. The receiver can now calculate its distance from
the satellite. Once
you know the distance you know that the receiver has to
be on a point on the
surface of a sphere with a radius of that distance with
the satellite at the
center of the sphere. By limiting the points on
the surface of the sphere to
points that intersect the surface of the
earth gives you a circle of
position on the earth's surface that the receiver
lies on. By then repeating
the calculation for at least two more satellites
and seeing where the circles
of position intersect you get a position fix.
Just like using a sextant and
three lines of position.
--
Eric
Haberfellner
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_______________________________________________
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http://www.cncphotoalbum.com
[email protected]
