4 ms·

Im confused...were 10-bit timers being used in recent history? Also they're using weeks as their atomic unit? This seems like something that never should have

Im confused...were 10-bit timers being used in recent history? Also they're using weeks as their atomic unit? This seems like something that never should have existed.

andyjohnson0 4y ago
The GPS system uses 10 bits for the week number part of the broadcast timestamp. This causes a rollover [1] every 19.6 years, and devices that aren't designed to anticipate it will report the current date/time as being two decades (or multiples of that) in the past. I guess the ultimate cause is data framing in the transmission protocol. The timestamp contains the number of weeks since 00:00:00 1980-01-06, together with the number of seconds since the start of the current week. The number of seconds in a week won't fit into 16 bits, so my supposition is that the designers had to also use some of the bits that could otherwise have been used for a wider week counter. I'd like to think that nowadays we'd use self-describing, upgradeable protocols. But GPS was designed in the 70s for the constrained technology of that era. And I'm pretty sure nobody anticipated how widely deployed it would be 45+ years later. [1] https://en.m.wikipedia.org/wiki/GPS_week_number_rollover https://en.m.wikipedia.org/wiki/GPS_week_number_rollover
- GekkePrutser 4y ago
  > I'd like to think that nowadays we'd use self-describing, upgradeable protocols. That would open a whole new can of security worms though. Being able to modify a protocol in-band is something we're starting to move away from. Things are becoming more static as a precaution, like stored procedures on SQL so an attacker can't inject a change.
  - ezconnect 4y ago
    Stored procedure was the norm when I was a young programmer. Dynamic SQLs are recent and was the cause of SQL injection vulnerabilities.
- toyg 4y ago
  > And I'm pretty sure nobody anticipated how widely deployed it would be 45+ years later. Or maybe they thought by now we would have replaced it with something better. Spacefaring was still riding high in the '70s, the brakes were only applied after '89 (sadly one of the many indirect effects of glasnost, RIP Gorbachev).
  - LukeShu 4y ago
    GPS was originally only for military use; opening it up to civilians didn't come until later. They didn't anticipate all of us pesky civilians coming up with creative new uses for it (like time keeping in our networks) or putting it in millions of devices that don't have strict maintenance schedules.
- defrost 4y ago
  You've missed two key points: * GPS depends upon acurate timing and positions. * GPS broadcasts a data packet. The length of the packet is limited by the broadcast frequency and the time to update .. so the packet was kept tight. The satellites drift (orbital decay, magnetic torque, solar wind, random particle pressure) and live in a different gravity (aka relativistic time) .. their 'self' data (position | time) is regularly updated from ground stations and their internal epochs only need to operate for some low N number of expected update cycles .. if they lack an accurate sense of 'self' they lack function as waypoints. Adding clock data widths that count nanoseconds for the lifetime of the sun would be pointless: it makes the data packet longer and it doesn't change the hard requirement for regular time|position updates from the ground. They were designed with short epoch counters with an understanding of functional constraints.
  - usrusr 4y ago
    Still surprised, every time I see it mentioned, to have a cultural construct like a week in the data model. Likely a deliberate break from SI to avoid ambiguity between monotonously increasing counters and calendaric time, but surprising nonetheless.
    - withinboredom 4y ago
      They were fighting the good fight of whether a week begins on Sunday or Monday… /s
  - deleted 4y ago
    [deleted]
deleted 4y ago
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montroser 4y ago
> This seems like something that never should have existed The engineers who designed and implemented the GPS system many decades ago were pioneering a fundamental technology, and worked within the constraints at the time to balance efficiency, cost, and complexity deliberately and as best they could. Now we get to drive around in our luxurious cars, decked out with supercomputers listening to satellite communications, charting our course for us, all so we don't have to read our own maps.
Rebelgecko 4y ago
What do you consider recent history? The design work for GPS started about 50 years ago. Shaving bits off of the L1 transmission also leads to a small but real improvement in acquisition time. Every bit you waste on a counter that only changes every ~20 years means receivers have to wait that much longer to get the latest ephemerides (this is less of an issue nowadays since cell phones can just download almanac and ephemerides from the internet). Weeks worked nicely as a time unit due to the periodicity of the p-code signal. If you have any doubts about the competency of the people who designed GPS, IMO it's worth checking out the Woodford/Nakamura report that considered a lot of alternative implementations. From an engineering perspective, they had a lot of tradeoffs to consider but typically took the direction that made the system more user friendly but harder to implement (like not requiring receivers to have atomic clocks or transmitters). It's also interesting to see how that sort of info was presented in the pre-powerpoint days

Im confused...were 10-bit timers being used in recent history? Also they're using weeks as their atomic unit? This seems like something that never should have existed.

andyjohnson0 4y ago

The GPS system uses 10 bits for the week number part of the broadcast timestamp. This causes a rollover [1] every 19.6 years, and devices that aren't designed to anticipate it will report the current date/time as being two decades (or multiples of that) in the past. I guess the ultimate cause is data framing in the transmission protocol. The timestamp contains the number of weeks since 00:00:00 1980-01-06, together with the number of seconds since the start of the current week. The number of seconds in a week won't fit into 16 bits, so my supposition is that the designers had to also use some of the bits that could otherwise have been used for a wider week counter. I'd like to think that nowadays we'd use self-describing, upgradeable protocols. But GPS was designed in the 70s for the constrained technology of that era. And I'm pretty sure nobody anticipated how widely deployed it would be 45+ years later. [1] https://en.m.wikipedia.org/wiki/GPS_week_number_rollover https://en.m.wikipedia.org/wiki/GPS_week_number_rollover

GekkePrutser 4y ago
> I'd like to think that nowadays we'd use self-describing, upgradeable protocols. That would open a whole new can of security worms though. Being able to modify a protocol in-band is something we're starting to move away from. Things are becoming more static as a precaution, like stored procedures on SQL so an attacker can't inject a change.
- ezconnect 4y ago
  Stored procedure was the norm when I was a young programmer. Dynamic SQLs are recent and was the cause of SQL injection vulnerabilities.
toyg 4y ago
> And I'm pretty sure nobody anticipated how widely deployed it would be 45+ years later. Or maybe they thought by now we would have replaced it with something better. Spacefaring was still riding high in the '70s, the brakes were only applied after '89 (sadly one of the many indirect effects of glasnost, RIP Gorbachev).
- LukeShu 4y ago
  GPS was originally only for military use; opening it up to civilians didn't come until later. They didn't anticipate all of us pesky civilians coming up with creative new uses for it (like time keeping in our networks) or putting it in millions of devices that don't have strict maintenance schedules.
defrost 4y ago
You've missed two key points: * GPS depends upon acurate timing and positions. * GPS broadcasts a data packet. The length of the packet is limited by the broadcast frequency and the time to update .. so the packet was kept tight. The satellites drift (orbital decay, magnetic torque, solar wind, random particle pressure) and live in a different gravity (aka relativistic time) .. their 'self' data (position | time) is regularly updated from ground stations and their internal epochs only need to operate for some low N number of expected update cycles .. if they lack an accurate sense of 'self' they lack function as waypoints. Adding clock data widths that count nanoseconds for the lifetime of the sun would be pointless: it makes the data packet longer and it doesn't change the hard requirement for regular time|position updates from the ground. They were designed with short epoch counters with an understanding of functional constraints.
- usrusr 4y ago
  Still surprised, every time I see it mentioned, to have a cultural construct like a week in the data model. Likely a deliberate break from SI to avoid ambiguity between monotonously increasing counters and calendaric time, but surprising nonetheless.
  - withinboredom 4y ago
    They were fighting the good fight of whether a week begins on Sunday or Monday… /s
- deleted 4y ago
  [deleted]

deleted 4y ago

[deleted]

montroser 4y ago

> This seems like something that never should have existed The engineers who designed and implemented the GPS system many decades ago were pioneering a fundamental technology, and worked within the constraints at the time to balance efficiency, cost, and complexity deliberately and as best they could. Now we get to drive around in our luxurious cars, decked out with supercomputers listening to satellite communications, charting our course for us, all so we don't have to read our own maps.

Rebelgecko 4y ago

What do you consider recent history? The design work for GPS started about 50 years ago. Shaving bits off of the L1 transmission also leads to a small but real improvement in acquisition time. Every bit you waste on a counter that only changes every ~20 years means receivers have to wait that much longer to get the latest ephemerides (this is less of an issue nowadays since cell phones can just download almanac and ephemerides from the internet). Weeks worked nicely as a time unit due to the periodicity of the p-code signal. If you have any doubts about the competency of the people who designed GPS, IMO it's worth checking out the Woodford/Nakamura report that considered a lot of alternative implementations. From an engineering perspective, they had a lot of tradeoffs to consider but typically took the direction that made the system more user friendly but harder to implement (like not requiring receivers to have atomic clocks or transmitters). It's also interesting to see how that sort of info was presented in the pre-powerpoint days