[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [gpsd-dev] Very basic PPS question:

From: Gary E. Miller
Subject: Re: [gpsd-dev] Very basic PPS question:
Date: Mon, 21 Oct 2013 15:03:20 -0700

Yo Hal!

On Mon, 21 Oct 2013 14:17:03 -0700
Hal Murray <address@hidden> wrote:

> > 1) devices grabs raw fix data at PPS edge
> > 2) device computes fix from raw fix data
> > 3) device emites a fix report
> I think you are reading between the lines way too much when you try
> to explain how/why it works.

Yes, most is now done in DSPs and CPUs, but originally this was all done
in TTL.  I have seen the schematics to GPS in TTL and at least some
GPS 15 years ago still did it that way.

> The right answer is because the documentation says so.  The firmware
> can easily do it either way.

This is too fast and critical to do in firmware.

Check this out:

The critical part of a good GPS is keeping the VCO's in sync with the
satellite frequency.  PPS is then counted out from there.

> Some NMEA devices are off by a second.  I assume that's because their 
> software geeks didn't read the documentation and/or didn't understand
> it, or maybe just got it wrong and nobody noticed.

We see GPS off by 1 second, 2 seconds, 15 seconds, even GPS epochs.  Many
ways to do it wwrong!

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

Attachment: signature.asc
Description: PGP signature

reply via email to

[Prev in Thread] Current Thread [Next in Thread]