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

[Hal Murray]

address@hidden said:
> It makes no sense to say that the PPS signal belongs to the previous or
> latter serial message.  Basically, they are independent of the actual
> data-stream telling about location, time etc. 

One of us is missing something important.

When you get a message on the serial port, it says something like "The time 
xxx 12:34.56"
I think the question being discussed is whether xxx is "will be" or "was"?  
Does it refer to the previous or next pulse?


> (If memory serves me, some GPS devices also take special care to even emit
> the PPS pulse at the correct time inbetween the local digital clock cycles
> its logic is running on.  Or at least tell how off it is in that respect.
> In this case, it again of course matters if this (serial) information
> relates to the previous or next PPS signal.) 

There is an additional concept.  Suppose the clock on your GPS device is 50 
MHz or 20 nanoseconds.  That means you can get a clock edge within 10 ns of 
any target.  If you want accuracy better than that you either have to fix 
your clock to tick at the right time, or tell the other end how far off it 
is.  The first requires hardware and adds to the cost of each unit.  The 
latter is just some more software.


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