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by mysterydip 1419 days ago
There was a spot I stood on in Japan probably 20 years ago that made my (handheld Garmin) GPS think I was traveling at something like 700mph at some angle across the globe. Once I left the area by maybe 10 feet, everything returned to normal. Went back to the spot, same thing. Always wondered what caused that.
3 comments
the_biot 1419 days ago
The "travelling" part sounds like it could be an artifact of your GPS receiver interpolating between two vastly different location points, and interpreting that as moving between them -- at impossible speed/angle. So a single wildly wrong location could be enough to cause this.

Do GPS packets have a checksum?

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martyvis 1419 days ago
Id be fairly confident that they would. They also are encoded so that receivers that literally have no idea where they are can train on the received stream. Also don't forget that the receiver needs to hear signal from at least 3 or 4 satellites to be able to triangulate their position and altitude.
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the_biot 1419 days ago
The wikipedia page has some info, looks like various CRC and parity checks. Maybe the receiver was just ignoring them, and receiving faulty data.

I dunno, it seems to me like that GPS receiver had fairly bad software. If you get packets from a bunch of different satellites but one is way off, it seems like good sense to throw that one away.

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mysterydip 1419 days ago
It worked fine everywhere else I went for weeks, so while I'm sure it was a bug of some sort, I wouldn't say it was overall a bad receiver.
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Maxion 1419 days ago
I have the same experience with a handheld garmin from like 20 years ago. This was in Finland while boating, the gps suddenly decided we were going around 700mph heading north north west. It lasted for around 30min or so before it returned us to where we were. I remember using a Garmin GPS 45.

https://panbo.com/my-garmin-gps-45-was-amazing-in-1994-and-i...

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Rebelgecko 1419 days ago
Was it in a big city? Because parts of Japan are so vertical, they have a lot of trouble with urban canyons and multipath from signals getting reflected off of buildings (which is why they built the high-elevation QZSS mini-constellation to augment GPS)
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mysterydip 1419 days ago
IIRC it was walking through a park, near some kind of sculpture.
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