In message: <[EMAIL PROTECTED]>
"Tom Van Baak" <[EMAIL PROTECTED]> writes:
: > In addition, since GPS time is TAI - 19s, the GPS-UTC difference will
: > eventually overflow any fixed-sized transmission packet (if transmitted
: > as a delta or as a table, it makes no difference in the end).
:
: True, but the GPS signal format has a number of
: fixed length fields and they do not cause a roadblock
: for receiver firmware engineers. There are fixed-sized
: 256 and 1024 week number fields that "overflow",
: for example, and all modern GPS receivers get them
: right. It is trivial for a receiver to handle the equivalent
: 256 leap second rollover should one occur in the next
: hundred years.
Many of the week number roll over algorithms use the leap second count
as a good first guess which 1024 week epoch you are in. If that field
rolls over, then these algorithms will need adjustment. So it is
likely doable, but it might not be completely trivial due to the
irregularity of leap second insertions.
: On the question of UTC updates; it's true that a cold
: GPS receiver has to wait up to 12 minutes for the
: correct GPS/TAI/UTC delta. I am wondering, though,
: if anyone knows of an example of a GPS receiver
: that caches the delta value from the last power-up?
: It seems to me this would take care of the delay in
: all but the most extreme cases.
Most receivers cache some value. That is why I've been careful about
saying "cold" since there are different definitions of "cold" between
receivers. Having a cached almanac greatly speeds satellite
acquisition time. This is why many GPS receivers do well when off for
up to about a week, and then have a much longer acquisition time when
they are turned on after a longer haitus. They have to 'guess' at
what satellites are in the sky and rotate through their guesses until
they happen to hit on one that is in the sky and can download more
accurate almanac information. Fortunately, that almanac data is
transmitted more frequently so once you have one, it goes pretty
fast to acquire the rest.
Warner
