Party like it’s 1995

Garmin GPS-III on table
My GPS-III Pilot getting to know the 21st century, again

A couple of days ago, I fired up my trusty Garmin GPS-III Pilot to take on a ham-radio trip. After it initialized and found itself, it declared the date to be Dec 2, 1995. I let it sit for a while, thinking that it might just be slow in waking up. After several hours, however, it remained convinced that Christmas 1995 was only just a few weeks away. I had another GPS (or 6) to use, so grabbed another one for the trip and I didn’t get a chance to investigate this temporal lapse until this morning.

It turns out, that this flashback was the result of a date-rollover error. Basically, the GPS uses a 10-bit number to count the number of weeks since January 6, 1980, when time began for GPS units. With 10 binary bits, you can represent 1,024 different things–weeks, in this case. After 1,024 weeks, after the GPS’ calendar returns to the beginning, back in 1980. This occurred in Aug, 1999, but was easily anticipated and GPS units manufactured shortly before that date could be programmed to accommodate the event by correcting for values that would result in a date that predated its manufacture. But, it seems that my GPS has lived long enough for that correction to no longer work as it did 15 years ago (i.e. the corrections to the current week values result in a date that’s reasonable to the GPS: 1995).

It turns out that many vintage GPS receivers manufactured in the mid to late 90s have similar problem, so it’s one that’s easily fixed by using a program written to update even older GPS units in preparation for the 1999 event. After running this program, my GPS-III Pilot is now living in the 21st century and is enjoying a morning in the sun on the back deck as it catches up with what’s been going on in GPS circles recently (i.e. it’s downloading the current satellite information from the GPS satellites).

This is interesting in that it’s a reminder of how even well designed software can surprise you, and it makes me wonder why I’m so attached to a GPS unit that’s almost 20 years old. It could be because we have had a lot of adventures together or it could be that I’m just a pack rat. Either way, we’re both sync’d up to the correct time.

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