3 ms·
> was it so expensive to add several bits If this is the GPS week number, there is very limited space for it in the GPS navigation message; every extra bit use
by cesarb 4y ago
> was it so expensive to add several bits
If this is the GPS week number, there is very limited space for it in the GPS navigation message; every extra bit used for the counter means one less bit for everything else. From what I've read, the navigation message is transmitted at only 50 bits per second, and the time including the week number repeats every 6 seconds, so they had only 300 bits to play with. Given that a 10-bit week number already allows for nearly 20 years, a receiver only needs to know which decade it is today for that value not to be ambiguous, and that should be easy for a receiver with a few bits of local storage unless it's been powered off for more than a decade.
- nullc 4y ago> and that should be easy for a receiver with a few bits of local storage unless unless it gets a single spoofed or corrupted signal, then it's stuck in the future. Of course you can implement countermeasures, like requiring months of observation to advance the flag, but that makes it much less easy (and harder to test). > it's been powered off for more than a decade. That's also a concern. You don't want a receiver to get hit by lightning then get swapped out with a spare on the shelf and suddenly get the wrong date. Now you need a provisioning step that requires remote access to a normally completely embedded component. Again, not insurmountable-- but it's getting away from easy.