gpsd-dev
[Top][All Lists]

Re: gpsprof: higher resolution

 From: Hal Murray Subject: Re: gpsprof: higher resolution Date: Mon, 17 Aug 2020 03:53:41 -0700

```gem@rellim.com said:
[decimation]
>> Do I just throw away 9 out of 10 samples?
> That is one way, then low pass filter the data, or not.

I think that's the reason I was confused enough to inquire.  Normally when I
see decimation, it's part of a scheme to reduce bandwidth so it makes sense to
filter before decimation.

But here, I think we are just averaging.  There is no signal processing.  The
term bandwidth doesn't apply.  So what does decimation mean?  Why do it in
this case?

> 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.

I think you are correct, but you didn't show your work.

>From John's graph, he is at 84W.  Call it 90, or 1/4 circle so we have a round
number to work with.  The Earth's circumference is 40,000 kilometers.  1/4 of
that is 10,000 km.  or 1E7 meters, 1E10 mm.  If we have 12 hours of data at 1
sample per second, that's 43000 samples.  That means the sum is 4.3E14.
That's not quite 1E15, but real close.  A bit more than 24 hours of data would
be too much.

You could do it with 64 bit integer arithmetic using micometers.  You need 10
more bits for mm to um but you get back 11 from the floating point exponent.

--
These are my opinions.  I hate spam.

```