[ntp:questions] Re: gps; why pps?
Richard B. Gilbert
rgilbert88 at comcast.net
Wed Mar 29 00:57:43 UTC 2006
Brian Garrett wrote:
> "Folkert van Heusden" <folkert at vanheusden.com> wrote in message
> news:20060328155232.GP26463 at vanheusden.com...
>>Could someone please explain me why you want to only use the PPS signal
>>of a GPS receiver and not the time indication?
>>Folkert van Heusden
> Presumably because of the GPS receiver's tendency to delay the time display
> anywhere from a half-second to two seconds behind the actual time. The
> processor's man task is calculating location and its firmware is dedicated
> to that; by the time it gets around to spitting out time-of-day info, the
> moment has passed. Even using the raw NMEA data can get you into trouble if
> you aren't clear about whether each sentence pertains to the second just
> passed, or the one coming up.
> Brian Garrett
I don't think that's quite correct. The processor solves a system of
four simultaneous equations in four unknowns: latitude, longitude,
height and time. All GPS receivers can do that. It does take a
substantial fraction of a second to solve those equations.
Receivers designed for timing have maybe three special features: PPS
output, the ability to go into "position hold mode", and priority given
to reporting the time rather than the position. Position hold mode
allows a timing receiver to solve one equation in one unknown since the
antenna is in a fixed, known, position. Not all receivers are designed
for timing and not all receivers have all the features I mentioned.
It is essential to understand the relationship of the reported time to
the PPS output if you are writing your own driver. The reported time
can mark either the last pulse or the next pulse depending on the design
of the receiver.
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