> >> 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.
But A0, A1 is not related to SV clocks. They are for translating t_GPS to t_UTC. A_f0, A_f1 and friends are used to get t_GPS from t_SV. It seems they wasted a few bits, since the USNO goal is having t_GPS within 1us. Actual performance is as told in the some ns range. -- Björn _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.