[time-nuts] LEA-M8T

Bob Camp kb8tq at n1k.org
Fri Apr 8 07:42:24 EDT 2016


Hi

In my case, the antennas were nice brand new NovaTel L/L2/L5 Glonass rated 
survey antennas. The were up far enough to have a 360 degree view of the sky
down to 10 degrees of the horizon. 

The runs I did used a number of modules, not just uBlox products. Some modules
did worse than others on position. I dropped them out of the group before I moved
on to timing. 

No attempt was made to compare the self survey location to any sort of “correct” survey 
data. The main goals were:

1) Can multiple systems give better self survey? 
2) Can multiple systems give better timing? 

There also was no real attempt to turn the project into a full blown “paper” level 
experiment. The only reason for doing the work was to be able to answer the very
basic questions of multi system timing as it applies right now. 

One important note - If you are in an antenna challenged location, the answers 
may be different. If you are in a < 4 sats situation 80% of the time, the added 
systems will likely improve things. 

In any case, a “perfect” solution would use an independent self survey location 
for each system. It also would use a separate sawtooth output for each system. 
So far, I have not seen a small module that does this. 

Bob

> On Apr 7, 2016, at 11:14 PM, Gregory Maxwell <gmaxwell at gmail.com> wrote:
> 
> 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.
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