[time-nuts] ublox NEO-M8T improved by insulated chamber?
leo at leobodnar.com
Wed Nov 8 05:16:46 EST 2017
> Subject: Re: [time-nuts] ublox NEO-M8T improved by insulated chamber?
I have not read IS-GPS for quite awhile so out of curiosity found what I believe is the latest one https://www.gps.gov/technical/icwg/IRN-IS-200H-001+002+003_rollup.pdf
It states "The OCS shall control the GPS time scale to be within one microsecond of UTC (modulo one second).
The NAV data contains the requisite data for relating GPS time to UTC. The accuracy of this data during the transmission interval shall be such that it relates GPS time (maintained by the MCS of the CS) to UTC (USNO) within 20 nanoseconds (one sigma)."
My understanding is: the target difference between UTC and GPS time TOS is 1µs or less. This is just an operational target and does not affect conversion accuracy.
The end user is provided required GPS to UTC correction data - in legacy NAV and [reasonably] new CNAV messages to perform conversion locally with expected accuracy to below 20ns (1 sigma.)
Time conversion accuracy is further diluted by uncertainty of GPS time itself - by design (accuracy of data in ephemeris), by receiver implementation (correlator resolution, etc) and environment (ionospheric disturbances, multipath, etc.)
GPS to UTC correction data contains constant offset and its rate of change for LNAV messages and adds extra 2nd order correction (rate of rate of change) for CNAV messages.
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