At 10:28 AM 2/12/01 +0000, Walter Blanchard wrote:
At 21:17 11/02/01 Sunday, VE2IQ wrote:
...............I propose to use GPS ..........:
The only real requirement is a GPS receiver with a 1-PPS output good to
within a few microseconds of UTC.
Be careful - don't confuse relative with absolute time. The 1 pps output
from cheapo GPS sets is only relative time, not absolute. If you like, it
gives frequency not phase and phase is essential to your
application. Recovering absolute time from GPS to microsec accuracy is not
easy and you won't do it with an ordinary positioning receiver.
I checked with Garmin on this and they're standing by their 1 microsecond
spec. They suggest using the GPS25-LVS (which apparently has a mode where
you can specify that the location is fixed and let it concentrate on
calculating the time accurately) - and say that under good signal conditions
the 1PPS output will track UTC "well within" the 1 microsecond limit.
They say typical accuracy is 100 nsec RMS with 50 nsec jitter.
That is more than good enough for our purposes.
I still haven't sorted out what the difference is between absolute and relative
time, but as long as everyone agrees on the same system it should
work fine.
Bill VE2IQ
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