[IGSMAIL-2248] GPS SV Testing, more re

[forward from US SPACECOM]

IGS Electronic Mail      Mon Apr 26 14:30:13 PDT 1999      Message Number 2248
******************************************************************************

Author: R. Neilan (forward from US SPACECOM)
Subject: GPS SV Testing, more re: IGSMail #2242

Note from #2242 (thanks to Willy Bertiger) that EOW roll-over testing is
scheduled for tommorrow, NANU's attached below fyi.

================================================

UNITED STATES SPACE COMMAND

News Release

DIRECTORATE OF PUBLIC AFFAIRS,
HEADQUARTERS, U.S. SPACE COMMAND, 250 S. PETERSON BLVD, STE 116,
PETERSON AFB, CO 80914-3190
PHONE: (719)  554-6889
DSN:  692-6889
UNITED STATES SPACE COMMAND WEB SITE ADDRESS:
http://www.spacecom.af.mil/usspace


April 20, 1999

Release No. 99-09

U.S. Space Command tests GPS

	PETERSON AIR FORCE BASE, Colorado - U.S. Space Command will conduct
Y2K compliance testing of the Global Positioning System on four separate
days between April 23 and mid May.  The test involves a single satellite
and will not interfere with day-to-day use by GPS users.
	Y2K testing of GPS is critical in order to ensure this national
asset is available to civilian, commercial and military users.
	During the tests, the clock on one GPS satellite will be moved
forward to specific dates to be tested in a Y2K environment.  Before moving
the clock forward, the satellite will be temporarily set "unhealthy" so
that its signal cannot be used by GPS receivers made in accordance with
manufacturing specifications*.
	There are presently three more GPS satellites on-orbit than
required for the full constellation of 24 satellites.  Even with one set
"unhealthy," GPS users will continue to receive navigation information that
exceeds accuracy the US Air Force has committed to provide.
	While this test will be totally transparent to the GPS user, as
with any maintenance on GPS satellites, users will still be notified of the
test via a Notice to Navstar Users (NANU).  This NANU can be found at the
Coast Guard Navigation Center web site at HTTP://WWW.NAVCEN.USCG.MIL.  This
site routinely provides information on the status of the GPS constellation.
-- more -
 	In addition to checking Y2K compliance, the test will also verify
GPS readiness for a phenomenon known as the End of Week rollover (EOW).
The GPS end of week rollover happens approximately every 20 years (1023
weeks)  because GPS system time, which is counted in weeks, started
counting on Jan. 6, 1980.  At midnight between Aug. 21 and 22, 1999 the GPS
week will rollover from week 1023 to 0000. This event is significant
because it is the first EOW rollover since the GPS constellation was
established and could be interpreted as an invalid date in GPS receivers
that were not designed to accommodate this phenomenon in accordance with
manufacturing specifications*.
	The Y2K issue stems from the fact that many computer programs use a
two-digit date field and assume that the year is 19XX. When the year 2000
occurs within the program, the two-digit date becomes "00" and could be
interpreted as an invalid date.
	For more information about the GPS constellation and Y2K and EOW
rollover, go to the US Space Command Web Site at
www.spacecom.af.mil/usspace/news8-99.htm.
-30-

*Interface Control Document GPS 200 Rev C specifications



NANU's
----------------------

23-APR-99 - GPS SATELLITE SVN27 (PRN27) WILL BE UNUSABLE ON JDAY 115
  (045)     (25 APR 1999) BEGINNING 0700 ZULU UNTIL JDAY 115 (25 APR 1999)
            ENDING 2100 ZULU.

23-APR-99 - GPS SATELLITE SVN27 (PRN27) WILL BE UNUSABLE ON JDAY 117
  (046)     (27 APR 1999) BEGINNING 0700 ZULU UNTIL JDAY 117 (27 APR 1999)
            ENDING 2100 ZULU.

23-APR-99 - GPS SATELLITE SVN27 (PRN27) WILL BE UNUSABLE ON JDAY 118
  (047)     (28 APR 1999) BEGINNING 0700 ZULU UNTIL JDAY 118 (28 APR 1999)
            ENDING 2100 ZULU.

23-APR-99 - GPS SATELLITE SVN27 (PRN27) WAS UNUSABLE ON JDAY 113
  (048)     (23 APR 1999) BEGINNING 0730 ZULU UNTIL JDAY 113 (23 APR 1999)
            ENDING 1940 ZULU.

25-APR-99 - GPS SATELLITE SVN27 (PRN27) WAS UNUSABLE ON JDAY 115
  (049)     (25 APR 1999) BEGINNING 0722 ZULU UNTIL JDAY 115 (25 APR 1999)
            ENDING 2058 ZULU.













[Mailed From: "Ruth E Neilan" <Ruth.E.Neilan at jpl.nasa.gov>]