8/22/99 should be a non-event in the GPS world unless you own one of the following GPS units.
From joe.mehaffey.com
The following GPS receivers reportedly will have EOW problems around
August 21, 1999. These problem will range from longer startup times
to failure to lock during week zero. The firmware should be upgraded
before August 21 1999. As far as is known, these units will report
the correct POSITION as soon as the units acquire lock.
Autohelm GPS - All units manufactured before January 1997
Autohelm ST50 - All units manufactured before January 1997
Furuno - Units earlier than GPS 500 MK2
Leica models MX 100/200/300 - older units
Magnavox (Leica) MX4200 - all models
Magnavox (Leica) MX9000 series - some models
Trimble - original placer 400 series
Trimble 10X, Transpak and Transpak II - no fix available; must replace
Trimble NT Series - firmware below Rev 2.46
Trimble NTCG - firmware below Rev 30.02
Trimble NavTrac
Trimble NavTrac XL - firmware below Rev 2.14
Trimble NavTrak - firmware version 2.09 and below
TrueTime GPS clocks - all models -- check with TrueTime.
There may be others of which I'm not aware.
Most consumer-grade units, including all Garmin units, will sail
through 1999 without a hitch.
--
Marc Brett +44 181 560 3160 Western Geophysical
Marc.Brett@waii.com 455 London Road, Isleworth
FAX: +44 181 847 5711 Middlesex TW7 5AB UK
More details from joe.mehaffey.com
November 1997 LETS MAKE A MOLEHILL OUT OF AT LEAST one MOUNTAIN!
Is the Year 2000 GPS "problem" REALLY a Problem at all?
===============================================================
I asked one of our GPS engineering consultants who is well versed
in GPS technology theory and practice to tell us what spectacular
event would happen to a GPS that was NOT "Y2000 compliant". Not
much says he. Read on for further comments. Since he does not
have time to respond to questions from the newsgroup, he has
asked that his name not be published. Here are his comments.
================================================================
Joe, you wanted to post my comments about the Y2k and EOW
"controversy" (let's see now, that means End of World in Year
2000, doesn't it, like the television psychics want to predict).
Well, here is The Short Version (consider it an abstract).
Consider this oversimplified. But I note that at least one of
your readers won't be satisfied with anything less than an
official manufacturer's certifying stamp on each unit, accompa-
nied by an international press conference and appearances on
Oprah, Nightline, and Larry King Live and attested to by Bill
Clinton (;->).
Hey, ya know what? As far as position and navigation goes, Y2K
(and any other artificial calendar counting time) doesn't matter
one bit to a GPS receiver. The position of the SV is found from
the orbital elements and the difference between the epoch of the
element set as transmitted in the navigation message and the time
of transmission of the message. It's only a _difference_ in time,
not the absolute time. And both the orbital element time and the
message time are contained in the message. The computer doesn't
care whether you are measuring on a Gregorian calendar, a Moslem
calendar, a Chinese calendar, a Jewish calendar or one you just
made up. The algorithm just takes the times given in the message
and calculates the current Mean Anomaly, from which you get the
True Anomaly, from which you ultimately get the position of the
SV in GPS coordinates (NOT lat/lon, UTM, or other geography
units). The only place the date appears in some sort of everyday
calendar format is on your display screen.
And as has been pointed out many times before, the receiver's
clock is, at most, only used to get the initial search configura-
tion. It is _not_ used for the PVT solution iteration (Position-
Velocity-Time). When you put your receiver in free search mode
("autolocate"), it doesn't even use its own clock. The "message
received" time is determined as part of the solution in deriving
the pseudoranges for each satellite used. Sort of like the
lat/lon vs UTM in 100 different datums. The number shown isn't
what the computer in the GPS uses anyway - it's only there for
display purposes. The system coordinate system is an Earth-
centered Earth fixed Euclidian system, no latitudes, no
Eastings. Hey, guess what? It's like GPS time vs UTC vs your
local standard or daylight time. The GPS uses GPS time for its
computations, then displays whatever you want. In the vast
majority of units (all the consumer toys and virtually all the
marine and air navigation units) you can't display the GPS time,
even if you want to. The time units are 1/403200 of a week, which
is about 1.5 seconds, not seconds or nanoseconds (it's 1/806400
of a fortnight, though (8>D).
The one problem is the week rollover. And, ya know what? That
really is only a problem for a unit that doesn't have a current
ephemeris for a given SV for a short time around the rollover
date. If you are more than a few hours past the rollover, all the
SVs will have a new ephemeris, your internal clock will be
counting up mod 1024 weeks, and you are fine. The problem comes
when you have an ephemeris which is epoch 1023.xxx weeks and your
clock has rolled over to 1024.yyy, which means it reads 0.yyy. It
will compute a negative time, unless your unit has a way of
catching that (as do all units from the major manufacturers in
the past 5 or 10 years). But, since the ephemeris is uploaded a
couple times a day, at worst your position computation will be
wrong for a day or so in August 1999. My understanding from the
manufacturers I have talked to (4 of the major ones), or
indirectly from ones Joe, Sam, and Jack have had contact with,
is that current units catch even that small problem. Some older
units will have a hard time locking on because they are calculat-
ing visibilities from the canonical orbital elements stored in
ROM, but once they have locked on and updated their ephemeris
set, they will give the right location and time of day (but not
the right calendar day, just off by about 230 days).
Sigh! What it comes down to is that there are a bunch of folks
who have absolutely no concept of how orbital calculations are
done. Sorry folks, the Earth isn't flat, it isn't the center of
the universe, and even the Sun isn't the center of the universe.
I groan every time I see another of these anthropocentric,
ethnocentric, or worse yet, egocentric postings that claims that
the poster's way of viewing things is the only way, and the rest
of the universe can --- whoops, the universe can't jump in the
lake can it? (My calendar is the only one, my footruler is the
only way to measure, my view of the world is the only correct one -so there!) |
