This is one of the most informative articles I've seen on the GPS problem: Monday, August 16, 1999

GPS owners advised they may fail soon

The Associated Press KENAI - A tiny receiver the size of a cigarette package can pinpoint your position, give or take a football field, anywhere on earth. But owners of Global Positioning System receivers need to take care later this week when a programming glitch could cause some to fail.

Most GPS receivers will continue to work, said Cmdr. Leonard Allen, chief of operations planning for the Coast Guard Navigation Center in Alexandria, Va. Older versions are particularly likely to fail sometime between Thursday and Sunday.

"We're advising people that they should be aware of the problem, just in case they are relying on it for safety or for navigation," he said. "It's more of a problem for older receivers, but that doesn't mean it's not a problem for newer receivers."

The Coast Guard advises boaters, pilots and hikers to check with manufacturers to be sure their GPS receivers will work after Thursday.

The GPS glitch is similar to the Y2K computer problem. The Y2K issue arises because computers that record the year with two digits may confuse the year 2000 with the year 1900.

The GPS glitch arises from the rollover of the 1,023-week .calendar on which the system is based. At midnight Aug. 21, the calendar rolls from week 1,023 to week 0000, an event that occurs roughly every 20 years. The satellites were designed to accommodate the so-called End of Week rollover, Allen said. But some manufacturers of GPS receivers did not account for it.

The Global Positioning System is a satellite-based navigation system originally established for the U.S. military. GPS receivers calculate position by triangulating from a system of 27 satellites. They calculate distances from the time it takes satellite radio signals to reach them, Allen said.

To time the radio signals, they rely on an almanac that tells the positions of the satellites at any given time. Each day, the Department of Defense transmits a new almanac to the satellites. When you turn on your receiver, it downloads the current almanac.

The government will start transmitting almanacs containing the Aug. 21 rollover beginning Thursday at 1 p.m. Alaska time, Allen said. A faulty GPS could fail any time between Thursday and Sunday, depending on which satellite it consults for the almanac.

While some receivers may fail to calculate a position, Allen said he knew of none that will give the wrong position.

As for backups, many mariners still carry LORAN, a system that triangulates from shore-based radio stations to calculate position. The Coast Guard had planned to close the LORAN system in 2000, officials said, but that plan drew a flood of complaints.

"We may end up continuing LORAN until 2008 or beyond," Allen said. "We're looking at LORAN as a backup for systems like GPS."

Lt. Cmdr. Eric Walters, of the Coast Guard Marine Safety Office in Anchorage, said the LORAN stations are Y2K compliant. However, navigators should still consult manufacturers to be sure their receivers will work after Jan. 1.

http://www.adn.com/stories/T99081636.html

-- Gayla (privacy@please.com), August 17, 1999

Answers

August 21 is Saturday. Otherwise, great posting! I wonder how many small boats will be "lost" this week-end...

-- Mad Monk (madmonk@hawaiian.net), August 17, 1999.

See previous thread on GPS and Map & Compass 101 in Alaska.

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=001FRS

-- Nelson Isada (isada@alaska.net), August 17, 1999.

also idiot hikers who rely on gps and their cellular phones to save them- no trail maps, no topo's- nada-

-- farmer (hillsidefarm@drbs.net), August 17, 1999.

y2k wackos are going to look stupid (again) when nothing happens. Can hardly wait to start making fun of you.

-- weare (laughing@you.com), August 17, 1999.

MM, the reference to "Thursday" does not seem to be a reference to August 21. I don't understand what it means for "the government" to be "transmitting almanacs", but apparently whatever almanacs are transmitted by the government will cover the rollover, starting Thursday. I had thought that the satellites transmit almanac information (AIUI, to help the receivers to locate the satellites), but I might not understand this correctly.

-- Lane Core Jr. (elcore@sgi.net), August 17, 1999.

A new and quite thorough article about the GPS rollover by Lane Core Jr....

http://www.y2ktimebomb.com/Computech/Issues/lcore9933.htm

"The August 21-22 GPS Rollover: What It Is, And What It Is Not"

-- Linkmeister (link@librarian.edu), August 18, 1999.