Poul-Henning Kamp skrev:
In message <e660d55b75514e29a5735af7dcb48...@athlon>, "Ulrich Bangert" writes:
The receiver can use this redundant information in two ways:
a) to improve the solutions for the position
or
b) to improve the solutions for the time
but not both at the same time.
That's just bogus.
First of all, you need four sats for a complete solution: X+Y+Z+T, second
the more sats you add after that, provided they do contribute gainfully,
will improve both the position and time solutions, for the very simple
reasons that they are one and the same solution.
Once you go to position-hold mode, all the sats contribute to is the
time solution, and in principle one sat is enough to get a solution,
because, as the name implies, you stop treating X+Y+Z as variables.
I totally agree. This is well covered in the books that go into the deep
details of GPS navigation.
3D positioning requires at least 4 sats for resolving X, Y, Z and T
coordinates, which translates to Lat, Long, heigth and T.
2D positioning requires at least 3 sats for resolving Lat, Long, T
(really X, Y, Z and T which a fixed relationship between X, Y and Z so
given two the third will be given, as the heigth is assumed).
T positioning requires at least 1 sat for resolving T.
Also, you can use the redundant information to identify false-tickers
and remove them before final position is calculated, this is done by
making a preliminary calculation and then compare the calculated time
with the pseudo-range value for each and let those being significantly
off be removed.
The pseudo-range system make the time of the receiver a critical
variable to establish. The stability of the receives time will therefore
also be a critical parameter in order to establish good quality
positional values. High short-term stability oscillators is being
deployed even in simple L1 receivers to reduce LO phase noise and its
effect on code and carrier pseudo-range measures. All pseudo-ranges will
depend on the actual distatance, but also on the time of the sat and the
receiver. The sat time is being corrected into propper GPS time by
additional correction values, such that remaining timing errors is to be
found in the receiver. Phase offsets of the signal from the sats center
of mass is also given, since it is the center of mass which the
positional values of the sat indicate.
The receiver uses the previous time estimates to correct its own clock
and advanced receivers use Kalman filtering for optimum clock
estimation. Each positional solution also feeds the clock algorithm so
that the clock is steered towards a zero offest. The pseudo-ranges is
samples with a sample clock, which has known deviation from the local clock.
In the end, I can't see how this type of receiver would fit the claim
that one has to optimize for position or time. It does not make sense to
me, as I know the system. What is true is that not all receivers has the
algorithms to provide optimum time solutions in the fixed geografical
position (it's not fixed in time position). The same receivers where one
has the time option performs the same on normal positioning. In fixed
position the solution part of the receiver must know that the position
is fixed in order to resolve all pseudo-ranges into time-offset only.
So, 3D positioning does not give the same time-stability as a fixed
position does, that is true, but it is not the same as being claimed.
Cheers,
Magnus
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