Richard B. Gilbert wrote:
Brian Garrett wrote:
"Folkert van Heusden" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
Hi,
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
Brian,
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.
As I recall, if SA is on, the accurate position is not known for some 30
days; position hold involves a very slow filter to try to remove the
affect of sthe clock variation imposed by Selective Availability. The
early units I played with solved the 4 equations with the postion and
time to be valid at the next 1 second tick, but those were rack mount
Collins units about 20 years ago. We also had some PCI based units (I
think Ashtech) that ran at 10 Hz solution rate, something like 15 years
ago. Have the manufacturers abandoned the "at the mark, the time and
postion will be" approach for "at some time in the past, we were at"
kind of solutions? Sounds dumb to me.
Tim Keck
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