Does the value 252 seconds (or 1/2400 week) ring a bell for anyone in the context of GPS? I'm trying to reverse-engineer data written by a GPS device, I was able to make sense of most of the fields, but there's something which seems to be a counter wrapping every 252s. 🤨 •1/8
Context on what I'm trying to do is here:
https://twitter.com/gro_tsen/status/1305951245862162436— but not really important here. What's relevant is that I have data from a GPS device and I'm trying to understand its format. I was able to decode most of it (date, time, lat, lon, speed, bearing…), … •2/8
… but a few fields still escape me. Irritatingly, there doesn't seem to be a sub-second timestamp in my data. On the other hand, there's this weird set of values, time counters of sorts, which I'll call: a (4 bytes), b (1 byte) and c (2 bytes) — in this order: … •3/8
… the value b is incremented every second, mod 252, i.e., it wraps around every 252s. (At least over short time periods: over longer ranges its period doesn't seem to be exactly 252s.) WHY 252s? I have no idea. This is 1/2400 week, but this doesn't really help. •4/8
(I mean, I know weeks are used by GPS, but I don't know about 1/2400 week, and I wasn't able to find anything which goes up to 2400.) As for the value c, it is incremented every time b wraps from 251 to 0, so it keeps count of 252s increments. •5/8
But c seems to wrap to 0 every day or so (not every week which would sort of have made sense), at approximately but not exactly 20:00 UTC, and not even exactly the same time each day. This makes absolutely no sense to me. •6/8
As for a, it seems to count days, except not really. I would like to think that a is incremented when c wraps back to 0, but I don't have proof. Over short periods, a seems to increment every day, but over more than a year, it increased by more than it should have. •7/8
The whole thing is extremely weird, and I have no idea why anyone would want to include a 252s counter in GPS data but not include a subsecond timestamp. (OTOH, I also wonder why anyone would want to XOR the entire record with a seemingly random byte. Makes no sense!) •8/8
• • •
Missing some Tweet in this thread? You can try to
force a refresh
