GPS units

[Eric Zurcher]

Hello all,

Since there has been some discussion about GPS units recently, this may be
a good time to point out a potential "time-bomb" which may be lurking in
many of these units. The situation is somewhat analogous to the infamous
"year 2000" problem. The information broadcast by the GPS satellites
includes a "GPS week number", specifying the number of weeks since 6
January 1980. This number is modulo 1024, and on 22 August 1999, the GPS
week number count will "roll over" from 1023 to 0.

Depending upon the manufacturer and model of your GPS receiver, this may
cause serious problems. Some receivers may display inaccurate date
information, some may also calculate "incorrect navigation solutions" -
that is, give totally bogus location information.

Don't but a new unit without first being sure that its software will deal
with the "rollover" correctly. If you already own a unit, check with the
manufacturer to be sure that the unit won't become worse than useless after
mid 1999.

Although this problem is apparently well understood within the military, it
has not yet received a lot of attention among civilians. Spread the word!

The US Naval Observatory does have some web pages about the problem. Their
GPS home page can be found at http://tycho.usno.navy.mil/gps.html

Cheers,


Eric Zurcher
CSIRO Division of Entomology
Canberra, Australia
E-mail: ericz at ento.csiro.au