Adam Davis wrote:
>
> Correct me if I'm wrong, but I was under the impression that the gps receivers
> were set up to take their position at 1 second intervals.  Since their time is
> set by one of the gps receivers in the sky, then they should have taken the
> readings at the beginning of each second, which will be within msec of each
> other.
>
> Aside from that, the gps receivers select 4 satellites, one to get the time, a
nd
> three the triangulate the position.  Both receivers would have to select the
> same satelite for time, and the same three for triangulation, in the same area
> for very good correlation.  There is a great chance that they would select the
> same four satellites, but not necessarily assign the same roles to the same
> sats.

The reciever doesn't use one for time and three for positioning.
There are four variables, X,Y,Z and the time difference between
GPS and local time. In order to resolve the navigation solution,
the receiver needs to have four equations to solve for four unknowns.

Another misconception, is that SA consists of jitter in
the timing from the space vehicles. SA is made up by
artificially added jitter, as well as artificial errors
in the ephemeri (space vehicle position).

The reason that two receivers *have* to use the same sats
for the navigation solution with DGPS, is that the correction
in position at the base, is reversed into corrections in the
pseudoranges to the four space vehicles. These 'corrections'
to the pseudoranges only approximate the corrections that
should be made to the ephemeri and clock. If one or more
of the space vehicles that the rover uses differ from the
set used by the base, you can end up (theoretically) with
an even worse position. This doesn't happen often, of
course. Neither does the fact that some navigation solutions
also have a second solution *outside* the constellation
come up often.

One thing about GPS that gets to me, is when people refer
to the receivers as "GPS's". It is like calling your car
an "internal combustion", or your cell phone a "GSM".

There, I said it.

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