[time-nuts] LEA-M8T

[Gregory Maxwell]

On Fri, Apr 8, 2016 at 12:06 AM, Tom Van Baak <tvb at leapsecond.com> wrote:
> Meanwhile it would not surprise me if each GNSS system gives a slightly different position and a slightly different time than GPS does.

One could easily imagine a system that when signals from both
constellations were strong it fit a model matching a secondary system
to a primary.  Then when the primary was degraded/unavailable used the
secondary data with the model-- which would hopefully fix any bias
between the systems-- assuming that it's fairly stable. But that would
be a fairly specialized mode of operation.

If the solutions were pooled with weighing and knowledge of their
process errors adding more shouldn't make it worse... but might not
add anything: the ideal weight might be zero.

I wonder how much of these negative experience with glonass are due to
non-calibrated antenna: Glonass does frequency division multiplexing,
and so the antenna's angle/frequency dependent phase can harm
performance... many GPS antenna greatly attenuate the glonass signal
too.

For time-nuts operation where the GPS is conditioning one (or more)
nice OCXO or atomic clocks in a world where tens of gigaflops of CPU
are among the least expensive toys we can buy, I imagine that fairly
different signal processing approaches would be ideal: e.g. instead of
just computing second by second solutions and putting the error into a
PLL, one could collect hours of observation data and produce after the
fact correction data that uses prior assumptions about the stability
of the local oscillator.  Such a processing mode could be more robust
against disturbance from multipath and SV failure, as that data could
be excluded as hopelessly inconsistent compared to modes that didn't
assume a stable local clock and didn't use potentially hours of
observation that watched the SV move across the whole sky.

It's somewhat annoying that this kind of experimentation first would
require something like getting a software GPS implementation working
well; when you just want to try changing around secondary processing
after all the coorelators and such.