4 ms·

All the imperial units in this PDF hurt my head. I'm an American, so I'm used to them, but seeing them in the context of precision aerospace hardware is so jarr

All the imperial units in this PDF hurt my head. I'm an American, so I'm used to them, but seeing them in the context of precision aerospace hardware is so jarring. As a student it seemed like math re: chemistry and physics was so much easier (read: less error-prone) with metric units than with imperial. Mars Climate Orbiter indeed.

Koshkin 4y ago
I mean, sure, converting grams to kilograms is easier (for a human) than converting ounces to pounds, but how often do we have to do that in our heads? Computers, on the other hand, do not care (and they "prefer" the binary system anyway).
- perilunar 4y ago
  Converting grams to kilograms not so much, but mm to m, mL to L, even g to mL and L to kg (of water) — all the time, and of course in our heads — it's so easy you barely need to think about it.
  - chriswarbo 4y ago
    I don't even consider those to be "conversions" at all; in the same way that "two dozen metres" and "twenty four metres" and are both just some number of metres (not a conversion from a separate "dozen-metre" unit). Technically, the SI standard does consider millimetres, centimentres, kilometres, etc. to be separate ("derived") units from the base unit of "metre". That matters when we have multiple interacting multiples, e.g. "one cubic centimetre" is not the same as "one centi cubic metre"; but of course, that's avoided if we stick to base units like cubic metre. (see http://www.chriswarbo.net/projects/units/improving_our_units.html#problems-with-si http://www.chriswarbo.net/projects/units/improving_our_units... )
sbradford26 4y ago
The biggest lessons learned on the Mars Climate Orbiter wasn't exactly that everything should be metric. It was the unit conversion is dangerous and needs to be handled very carefully. Just saying everything will be metric doesn't avoid unit conversion. You might have one thing measuring fuel burn rate in grams/second and another in kg/second. That conversion can still lead to issues if not handled correctly. In the context of the space shuttle it was all designed in imperial units starting in the 70s so it would be very risky to convert everything to metric.
- fnordpiglet 4y ago
  I mean what issues would power of 10 fractions have in floating point after all?
  - sbradford26 4y ago
    If I have data going out on a data bus in grams/second, but someone reading that data in a different sub system reads it and thinks it is kilograms/second because that is the default unit they use then you have an issue. The actual conversion of the bits isn't the issue it is that certain systems might use different units internally and making sure those conversions are done correctly. It is much more an engineering design/human interaction issue than a computation issue.
    - fnordpiglet 4y ago
      Yes, I understood that. However repeated conversions can lead to rounding errors such as catastrophic cancellation. This is even more esoteric than “grams here kilograms there” and thus easier to fall prey to. Depending on how precise your measurements are you could easily drop significant digits or add random noise, especially if you do large scale changes. https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.h...
      - sbradford26 4y ago
        Yeah we tended to utilize fixed point for many of our variables and documented the precision for each one. Then an analysis was done verifying that we were maintaining precision through the calculations from input from a sensor to output.
        fnordpiglet 4y ago
        The problem is almost nothing today accelerates fixed point and everything supports floating point accelerations, which will speed up your ability to calculate the wrong number by many orders of magnitude.

Koshkin 4y ago

I mean, sure, converting grams to kilograms is easier (for a human) than converting ounces to pounds, but how often do we have to do that in our heads? Computers, on the other hand, do not care (and they "prefer" the binary system anyway).

perilunar 4y ago
Converting grams to kilograms not so much, but mm to m, mL to L, even g to mL and L to kg (of water) — all the time, and of course in our heads — it's so easy you barely need to think about it.
- chriswarbo 4y ago
  I don't even consider those to be "conversions" at all; in the same way that "two dozen metres" and "twenty four metres" and are both just some number of metres (not a conversion from a separate "dozen-metre" unit). Technically, the SI standard does consider millimetres, centimentres, kilometres, etc. to be separate ("derived") units from the base unit of "metre". That matters when we have multiple interacting multiples, e.g. "one cubic centimetre" is not the same as "one centi cubic metre"; but of course, that's avoided if we stick to base units like cubic metre. (see http://www.chriswarbo.net/projects/units/improving_our_units.html#problems-with-si http://www.chriswarbo.net/projects/units/improving_our_units... )

sbradford26 4y ago

The biggest lessons learned on the Mars Climate Orbiter wasn't exactly that everything should be metric. It was the unit conversion is dangerous and needs to be handled very carefully. Just saying everything will be metric doesn't avoid unit conversion. You might have one thing measuring fuel burn rate in grams/second and another in kg/second. That conversion can still lead to issues if not handled correctly. In the context of the space shuttle it was all designed in imperial units starting in the 70s so it would be very risky to convert everything to metric.

fnordpiglet 4y ago
I mean what issues would power of 10 fractions have in floating point after all?
- sbradford26 4y ago
  If I have data going out on a data bus in grams/second, but someone reading that data in a different sub system reads it and thinks it is kilograms/second because that is the default unit they use then you have an issue. The actual conversion of the bits isn't the issue it is that certain systems might use different units internally and making sure those conversions are done correctly. It is much more an engineering design/human interaction issue than a computation issue.
  - fnordpiglet 4y ago
    Yes, I understood that. However repeated conversions can lead to rounding errors such as catastrophic cancellation. This is even more esoteric than “grams here kilograms there” and thus easier to fall prey to. Depending on how precise your measurements are you could easily drop significant digits or add random noise, especially if you do large scale changes. https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.h...
    - sbradford26 4y ago
      Yeah we tended to utilize fixed point for many of our variables and documented the precision for each one. Then an analysis was done verifying that we were maintaining precision through the calculations from input from a sensor to output.
      - fnordpiglet 4y ago
        The problem is almost nothing today accelerates fixed point and everything supports floating point accelerations, which will speed up your ability to calculate the wrong number by many orders of magnitude.