Comment by kijin | Hacker News Reader

Comment by kijin | Hacker News Reader

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My dad used to have a crappy aftermarket GPS in his car that did the same thing. It would get lost dozens of miles away, and then hundreds of miles away.

The explanation I found online at the time was that a GPS receiver needs to download data about the exact orbits of all GPS satellites from time to time. Satellites slowly lose altitude and change their orbits. Up-to-date information is constantly broadcasted by every satellite, but it takes about 15 minutes for a device on the ground to download this dataset.

Most GPS devices do this automatically whenever they get the chance. But if your GPS is somehow unable to stay online for 15 consecutive minutes (bad firmware, faulty memory, tunnels, underground parking lots, etc), it will be relying on increasingly outdated info and drift far off its actual location.

There's no way a modern smart phone or car relies on those ephemeris transmissions. They all just get it from the internet, which takes less than a second. That's one of the reasons why a smart phone has a reliable GPS fix basically instantly after being booted up, while old-school offline GPS units needed minutes to get a fix.

That's only the case for non-internet connected GPS units. Throwback to the early 2000s when my family car had such a unit, which, after having been turned on, would require ~15 minutes of waiting before it became functional. Funnily, I remember the mapping app would refuse to use the device clock and only use the time from GPS satellites. So at least you would know you need to wait if it didn't know the current time.

_deleted incorrect statement_

Well, that's certainly not the case. Each satellite transmits a low-precision almanac to the receiver that helps it lock onto the others, as well as a higher-resolution ephemeris that provides the necessary pseudorange accuracy for that particular SV.

But it's true that neither of those factors accounts for miles of error. That has to come down to either poor sky coverage/signal strength, poor software, or (more likely) both.

You are correct, there's several words of data at the end of the packet that is a low precision almanac

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