[time-nuts] TBolt: UTC PPS

[Hal Murray]

> Referring to the GPS ICD, the size of A0 in subframe 4 is a signed scaled 32
> bit integer so if a receiver chooses to convert it to floating point it
> needs a double precision format to contain it without loss of precision. A1,
> on the other hand, is a signed scaled 24 bit integer and so it just fits in
> a single precision floating point number. So Trimble did the right thing
> here. 

Then why do they need 32 bits in the GPS packet format?

A single precision floating point would work fine until the 24th bit gets 
turned on.  That's 16,000,000 ns or 16 ms.  So back in the days when they 
were designing the packet formats, somebody must have thought the satellite 
clocks might drift a long way from UTC.

Maybe their control loop wouldn't be good enough to keep the clocks locked 
but the clocks would be stable enough for navigation.



-- 
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