Appendix Hpos-Gps Timing - Applanix POS MV V5 Installation And Operation Manual

Appendix H POS-GPS Timing
Background
Global Positioning System (GPS) time is a continuous measurement of time from an epoch started on
January 6, 1980 at midnight (0 hours 0 minutes 0 seconds) Universal Time Coordinated (UTC). GPS
Time is often stated in a number of weeks and seconds from this GPS Time epoch. GPS Time does not
introduce leap seconds and therefore, is ahead of UTC by a number of seconds.
The GPS Master Control Station, located at Schriever AFB in Colorado, steers GPS Time to within one
microsecond (less leap seconds) of UTC. Navigation messages transmitted by the Space Vehicles (SVs)
contains parameters that permit users to compute an estimate of the current GPS/UTC sub-microsecond
difference as well as the number of leap seconds introduced into UTC since the GPS epoch. GPS Time is
derived from the GPS Composite Clock (CC), consisting of the atomic clocks at each Monitor Station and
all of the GPS SV frequency standards.
The following identify a few of the many time standards:
• Local time is the date/time reported by your PC. Local time differs from UTC by the number of
hours for your time zone (plus local PC clock errors).
• UTC, popularly known as GMT (Greenwich Mean Time), or Zulu time.
• GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks
in the GPS ground control stations and the GPS satellites.
• TAI, Temps Atomique International, is the international atomic time scale based on a continuous
counting of the International System of Units (SI) second.
Because GPS and TAI time do not have leap seconds, they will change by one second with respect to
UTC whenever a leap second is inserted. GPS and UTC time scales were aligned when GPS Time
began on January 6, 1980. TAI and UTC time scales were aligned when TAI time began on January 1,
1958.
Copyright © Applanix Corporation, 2017
POS MV V5 Installation and Operation Guide
GPS to UTC
Date
Offset (sec)
Jan 6, 1980 0
Jul 1, 1981 1
Jul 1, 1982 2
Jul 1, 1983 3
Jul 1, 1985 4
TAI to UTC
Date
Offset (sec)
Jan 1, 1980 19
Jul 1, 1981 20
Jul 1, 1982 21
Jul 1, 1983 22
Jul 1, 1985 23
POS-GPS
Timing
H-1