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

[Paul Fertser]

Hello,

On Sat, Oct 19, 2013 at 09:37:57AM -0400, Eric S. Raymond wrote:
> When a GPS asserts PPS, is it top of second for the in-stream 
> data *preceding* or *following*?  

Some information from official documents for Mediatek devices:

1. Here asserting means rising edge on PPS pin: "Pulse length (high
state) is 100 ms and it has 1us accuracy synchronized at rising edge
to full UTC second with nominal GPS signal levels". Section 3.9 of [1]

2. "Confirm that the UTC offset has been downloaded ... and that the
receiver is generating 3D fixes... Capture the time from the ZDA
packet. Once time is acquired, add 1 to the whole second on the next
PPS for the correct time". [2]

I'm not sure how you plan to deal with the fact that there's no
standard way to learn if the receiver already knows the current amount
of leap seconds or not; for MTK a proprietary $PMTK457 command is
available, but even that doesn't help much: on earlier models it just
outputs the current assumed offset, so you'd need some external data
to verify its correctness.

[1] 
http://www.u-blox.com/images/downloads/Product_Docs/UC530M_DataSheet_%28FTX-HW-12007%29.pdf
[2] 
http://www.naelcom.fr/app/download/5781977313/022542-034_Anapala-Silvana_TN_0112.pdf
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