If not, please delete this message now.
I had previously heard about this early date problem from a friend, Chris
Peachment, who works in the "year 2000 problem business". When the note came
around this morning, I thought I would ask him. Below is his response.
Scott Martin
Actuary
Manufacturers P&C Limited
phone (246) 228-4910 fax (246) 228-4929
scott_martin@manulife.com
---------------------- Forwarded by Scott Martin/Reinsurance/Manulife on
07/12/99 11:29 AM ---------------------------
ctp@format2000.com on 07/12/99 11:54:03 AM
To: Scott Martin/Reinsurance/Manulife
cc:
Subject: Re: Fwd: Year 2000 - A Strange Date
Hi Scott:
The "problem" with some GPS navigation receivers originates
with the American navy software designers' decision to have
the satellites report the date in terms of a week number
plus the day within the month. Week number 1 was back in
1979-80 when the system first started, and each week since
then has been assigned the succeeding number. Computers
store integer numbers in a limited number of bits. Ten bits
will hold decimal numbers in the range 0 to 1023. Five bits
will hold decimal numbers in the range 0 to 31 - just enough
for the day of month. The total is 15 bits which together
will fit within the standard 16 bits for a standard integer
number. The extra bit is used for the sign, allowing
negative numbers which are not meaningful in this situation.
At 52 weeks to each year ( 1999 - 1979 ) x 52 = 1040 so
sometime in 1999 the upper limit of 1023 will be reached and
the week number will cycle back to zero and start again.
This will happen in the satellites and the receiver must be
able to recognise the implications when it reports the date
in one of the usual formats of day, month, and year.
I have seen a report listing various manufacturers and the
state of their receivers. Most of the recent ones have been
fixed to report the date correctly. Some of the older ones
will definitely fail and the situation with some others is
unknown.
Since a date in August is imminent, people using the faulty
receivers will know soon enough. The cost of a replacement
receiver is from US $ 100 to US 1000 so it will not be a
critical purchase.
Since the date is an informational field and is not related
to the geographic location report, it is only those
receivers which are attached to recording devices which may
cause some confusion. I can not think of any reason why the
reported location would be dependent upon the date.
The technology depends upon detecting the time it takes to
receive a signal from each of a minimum 3 satellites. The
time difference is measured in microseconds (millionths of
a second), and the date has no relevance. Each signal
contains the time the transmission started and the receiver
must first synchronise its own clock with the actual time
by subtracting the signal delay time for the satellite with
the shortest delay. Each delayed signal represents a distance
between the satellite and the receiver. Since the altitude
of each satellite is known, there is a circle of position on
the earth's surface corresponding to the length of the signal
path. Two such circles (from two satellites) will intersect
at one or two positions and three such circles will intersect
at just one position. Four satellites permit determination of
altitude as well.
There are inherent errors in the system and the American navy
has introduces an artificial error which limits current
accuracy to +- 100 metres, 95% of the time. There are navy
receivers which can correct for the artificial error, other-
wise one must buy a differential GPS receiver and be within
range of one of the fixed position earth stations which
broadcasts the difference between its reported and actual
positions. Apparently, this difference can be used to correct
a local computed position to an accuracy of +- 10 centimetres.
The rapid development of new models of receivers means that
the feature set, reliability, accuracy, etc., has increased
rapidly and I would expect only private individuals are
still using the older obsolete devices. I do not know of any
reason why they could not continue to be used after the week
rollover - just ignore the reported date.
Hope this helps.
Chris
Visit the CAS Web Site at http://www.casact.org
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