QUICK AND EASY GPS TIME SETTING FOR COMPUTERS

I started an on-line search for a solution, hoping to find a way to interface one of my Radio Shack WWVB-controlled table clocks to the computer. I did not find that, but I did come across a wonderful little piece of downloadable freeware that allows virtually any GPS receiver to update computer time automatically. The GPS does have to have an NMEA-183 data output connection, but all but perhaps the lowest priced GPSs have that now. The applet is "GPSTime," only a couple of hundred k in size, downloadable from http://www.coaa.co.uk/gpstime.htm. I happened to have a nearly obsolete, 13-year old Garmin GPS-50 with data plug and external antenna. Hardly believing it could be that simple, I got a female DB-9 serial connector from Radio Shack, soldered on three wires, and plugged it in to the computer serial port. Easy wiring instructions with diagrams can be found on: http://pfranc.com/projects/g45contr/assemb.htm. Having put my GPS antenna on the window sill, acquired a "fix," and set my GPS menu to "data output" -- bingo, the computer clock jumped to the correct time.

GPSTime works on Windows 95, 98, 2k, and XP. The source for a Macintosh version is also available on the "pfranc" website.

The needed wiring consists basically of just connecting the data output ground wire (black on Garmin GPSs) to pin 5 of the DB-9, the "data out" line (brown) to pin 2, and the "data in" line (white) to pin 3. The "data in" connection might not be necessary, but I hooked it up in case some kind of handshaking might be needed. GPS output cables also generally include a power input line (red) that makes it possible to use an external power supply. I use a 6-volt supply buffered by a 6-volt gel cell to get around short power outages. Be sure to check the range of power supply voltage safely acceptable for your particular GPS (some must be less than 8 volts; some can go up to 40). The wiring instructions on the "pfranc" website above are for Garmin GPS models, but the same NMEA standard data lines are used by other manufacturers. They also give a source for the plugs for GPS units, and even complete cables if desired.

GPSTime can be set to update your computer only on manual request or (as I chose) automatically every minute. I did find that at first my time updates were running between one and two seconds slow, probably a result of the fact that my old GPS puts out its NMEA-183 time string, $GPRMC, only once every two seconds. The GPSTime applet, however, has an offset feature that can be set to the nearest whole second. Adding a positive two-second offset put my updating consistently to within a fraction of a second of WWV.

I found that a navigation fix on the GPS is not necessary to get the time signal. Every satellite puts out the time data, so acquiring just one satellite, even with a weak signal, suffices. This is almost always available, even indoors, near a window.

P.S. I still would love to find a solution using one of the cheap WWVB clocks. If anyone comes across one please let the rest of us know how you do it!

Jim Mandaville (A-90), zygo@dakotacom.net