On Mon 2005-09-26T10:27:11 -0400, John.Cowan hath writ:
> 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).
Yes, but (as already mentioned) that does not preclude getting the
difference between GPS and UTC right for the immediate future (by
which I mean the expected useful life of most GPS receivers) with the
addition of a little bit of heuristic algorithm that predicts the
magnitude of the 8-bit signed Delta-t_LS as compared with the 10-bit
unsigned
Today is a big day for GPS, for the launch of the first of the next
generation of SVs just happened.
http://www.cnn.com/2005/TECH/space/09/26/rocket.launch.ap/index.html
The new GPS L2 ICD that describes the Block IIF SVs
is available via this web page
http://www.navcen.uscg.gov/gps/modernization/default.htm
under the link that reads "ICD-GPS-200 Rev. C"
There are two significant additions in the Block IIF signals.
6.2.5 and 20.3.3.5.1.13 indicate that bits 7 through 22 of word ten in
page 25 of subframe 5 will be a 16-bit integer giving the calendar
year (curiously it does not specify whether this is signed or
unsigned, but the Control Segment has around 14000 years to clarify
that point).
30.3.3.1.1.1 indicate that bits 39 through 51 of L2 CNAV message type
1 will be a 13-bit unsigned integer which extends the range of the
existing "Transmission Week Number" from 1024 weeks to 8192 weeks.
This extends the ability of a GPS receiver to tell when it is from not
quite 20 years to over 150 years, which should be longer than any GPS
receiver is likely to last. Unfortunately, 30.3.2 indicates that
message types 1 and 2 are temporary and will be replaced by the as yet
undefined messages 7 through 9. This no doubt will reduce the
likelihood that a GPS receiver will bother to use them.
The inclusion of calendar year is an interesting addition to the
original week-based scheme. The week-based scheme was perhaps chosen
while noting that the week remained intact when Pope Gregory (and
then, eventually, all the protestants) switched calendars. Thus the
GPS scheme is probably robust against anything short of adoption of
the doomed-to-fail "World Calendar" schemes which proposed the
intercalation of weekday-free days, but which had little hope of ever
being adopted. As such the inclusion of calendar year does not
prohibit the adoption of new calendars with new year schemes so long
as the change is adopted with sufficient lead time to permit the
firmware in GPS receivers to be updated.
With the calendar year available, the fact is that the signed 8-bit
quantity Delta-t_LS is no longer a limitation. It will be something
like 30000 years before the combination of calendar year and
Delta-t_LS values is incapable of producing an unambiguous result.
On Mon 2005-09-26T11:26:00 -0700, Tom Van Baak hath writ:
> The other point I was trying to make is that there are
> web pages that still claim that "GPS WILL FAIL" on
> such and such a date because of the 8-bit leap second
> field. This sort of hyperbole is uncalled for.
Mea culpa. I have more homework to do on my web pages to incorporate
these very details. It remains the fact that most existing GPS
receivers will fail by around 2070 or so, but that really should not
be much of a surprise or inconvenience to their owners.
--
Steve Allen <sla_at_ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858
University of California Voice: +1 831 459 3046 Lng -122.06014
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
Received on Mon Sep 26 2005 - 23:57:42 PDT