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## Re: gpsprof: higher resolution

 From: Gary E. Miller Subject: Re: gpsprof: higher resolution Date: Sun, 16 Aug 2020 13:58:53 -0700

```Yo Hal!

On Sun, 16 Aug 2020 12:59:17 -0700
Hal Murray <hmurray@megapathdsl.net> wrote:

> gerry.creager@noaa.gov said:
> > Also, a caveat I'm sure you're aware of, is before you simply
> > average your NMEA data, decimate it to reduce the impact of
> > autocorrelation in the standard position output.
>
> Could you please say more.  That sounds like something I should
> understand but I don't get it.

I keep looking for authoritative definition as well, but the few theat
claim to understand it all use different methods.

> What does "decimate" mean in that context?

In this context, merely hypothetical.  I've not seen an implementation.

> Do I just throw away 9 out of 10 samples?

That is one way, then low pass filter the data, or not.  But the
literature I've read on that said the reason was to reduce the data set
to something your CPU could manage, not to improve the results.

> Or average blocks of 10 then take their average?

I have not seen that.  I have seen just low passing the data so the
effective samplefrequency was 1/10th.

> The latter sounds like a dodge for numerical precision

Personally, I also hate throuwing away data points.

> My back of the envelope says it should fit in a double.

gpsd uses gps_fix_t that uses double for lat/lon.  A double is 53
significant bits.  Wikipedia says that is 15 to 17 decimal digits
of precision, so we are good for a while longer.

https://en.wikipedia.org/wiki/Double-precision_floating-point_format

> How does autocorrelation get involved and/or how does it screw things
> up?

Autocorrelation is how the receiver knows part of the solution to the
psuedorange from the receiver to the sat.

The topic is quite involved, but discussed well in the lecture series:

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com  Tel:+1 541 382 8588

Veritas liberabit vos. -- Quid est veritas?
"If you can't measure it, you can't improve it." - Lord Kelvin
```

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