4 ms·

This reminds me of certain credit card signature pads that generate an error if you don't begin your signature in a particular area on the pad. It's all too ea

This reminds me of certain credit card signature pads that generate an error if you don't begin your signature in a particular area on the pad. It's all too easy for designers to consider input constraints that (a) don't actually improve the user experience, and (b) don't actually make the downstream part of the system more robust. If the user's signature is a chicken scratch and the GPS is reporting a trusted, verified altitude of -750 feet, who cares?

shakna 9y ago
> verified altitude of -750 feet, who cares? Ah, altitude. I'll use an unsigned long.
- csours 9y ago
  Q: What are the internal units in the GPS system (overall, not necessarily an arbitrary receiver unit)? Like mm? radians?
  - moftz 9y ago
    GPS data from a receiver is usually output using the NMEA format. The lat,long is reported in degrees, minutes, seconds, and the altitude is reported in meters. After the actual receiver calculates position, I'm sure this same data is just handed over to the part of the device so it can display your coordinates or pin point on a map. You can buy chips that will do all the GPS signal processing and positional math for you and just spit out NMEA data sentences for a computer to process.
  - pjc50 9y ago
    http://www.gps.gov/technical/ps/1995-SPS-signal-specification.pdf http://www.gps.gov/technical/ps/1995-SPS-signal-specificatio... "GPS operates in a well-de - fined set of coordinate systems, and all performance standard definitions assume their usage. The satellite position and geometric range computations must be accomplished in the World Geo - detic Survey 1984 (WGS-84) Earth-Centered, Earth-Fixed (ECEF) coordinate system." Later tables say that the ephemeris (not the user position) is broadcast in units of 2^-29 radians. Other bits of the table use "2^-31 semi-circles" - effectively a custom coordinate system that spreads the 2^32 bits evenly around a circle.

shakna 9y ago

> verified altitude of -750 feet, who cares? Ah, altitude. I'll use an unsigned long.

csours 9y ago
Q: What are the internal units in the GPS system (overall, not necessarily an arbitrary receiver unit)? Like mm? radians?
- moftz 9y ago
  GPS data from a receiver is usually output using the NMEA format. The lat,long is reported in degrees, minutes, seconds, and the altitude is reported in meters. After the actual receiver calculates position, I'm sure this same data is just handed over to the part of the device so it can display your coordinates or pin point on a map. You can buy chips that will do all the GPS signal processing and positional math for you and just spit out NMEA data sentences for a computer to process.
- pjc50 9y ago
  http://www.gps.gov/technical/ps/1995-SPS-signal-specification.pdf http://www.gps.gov/technical/ps/1995-SPS-signal-specificatio... "GPS operates in a well-de - fined set of coordinate systems, and all performance standard definitions assume their usage. The satellite position and geometric range computations must be accomplished in the World Geo - detic Survey 1984 (WGS-84) Earth-Centered, Earth-Fixed (ECEF) coordinate system." Later tables say that the ephemeris (not the user position) is broadcast in units of 2^-29 radians. Other bits of the table use "2^-31 semi-circles" - effectively a custom coordinate system that spreads the 2^32 bits evenly around a circle.