Philip Pemberton skrev:

The 1 us figure is a historic figure relating to a worst case degrades
GPS constellation situation when the 24 sat constellation has degraded
significantly etc. This number comes out of ICD-200. A more commonly
referred figure is 340 ns which is what the GPS constellation with SA
enabled. In the SA-disabled world seeing lower numbers as 60-40 ns is
not unreasnoble. Old 6 or 8 channel receivers was adequate for the older
constellation situation, but seing 10-12 sats in todays world is not
unreasnoble and naturally will the bias effects and other noise
processes be lower. In addition has receiver technology advanced to
better suppress various imperfections such as multi-path, weak signals
and quick locking. In the other end, awareness of how the PPS is being
used have improved how the PPS signal is generated and producing
"sawtooth corrections" enables lower time quantization noise.

Old receivers can perform better in todays world, so we could modernize
the specs by comparing them with newer boards in todays environment.

I am not sure if you really got a real answer, but hopefully may some of
the difference become explainable to some degree.

Cheers,
Magnus

Phil,

as soon as a receiver sees more than 3 sats there is redundant information
available. 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 implies that there are good position receivers available and good
timing receivers but not any that is suited well for both purposes (which
matches the market siuation pretty well). On a not-timing receiver the PPS
is more or less a useless gimick which may have accuracies as bad as 1 us or
even more. Dont't worry: These receivers are not made with a precise PPS in
mind. On the other hand: If a good timing receiver has ended his "site
survey" it's position messages stay constant, even if you move it around.
Obviously that is not what you expect from a navigation system. Keep
navigation and timing receivers clear apart from each other. They are as
different as horse and zebra.

Best regards
Ulrich Bangert

In message E660D55B75514E29A5735AF7DCB484B2@athlon, "Ulrich Bangert" writes:

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.

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Never attribute to malice what can adequately be explained by incompetence.

Poul-Henning Kamp skrev:

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

Hi Ulrich,

We had this discussion a few years ago here!

You are still wrong. The best timing receivers available are the geodetic
quality receivers that have an external frequency input. Sometimes they
are slightly modified versions of the standard geodetic receivers, but
they remain top class navigation receivers never-the-less.

Tell me a modern receiver used by a national time-lab for time-transfer to
other labs that is a bad navigation receiver!

--

Björn