RNAV Training Manual - Keilir

How does GPS work?
5. International time and the GPS time system
International Atomic Time
(TIA)
The standard international scientific
time scale
The length of a second is defined by
a frequency property of the cesium-
133 atom, and atomic clocks are
used to count or accumulate
seconds
TIA is derived from 230 atomic
clocks in 65 sites around the world,
and 11 different laboratory caesium
ffrequency standards t d d
The data is collated by the BIPM
(Bureau International des Poids et
Mesures) in Paris, who calculate
TIA and promulgate the results to
various international centres
Universal Coordinated Time
(UTC)
Earth time ( UT1 ) defines the earth s
angular position with respect to the
celestial sphere; p ; this is the most useful
time scale for navigation and
astronomy
Fluctuations in the earth s spin mean
that UT1 deviates from the precise TIA
reference
GPS Time
P A
The GPS system uses a time
reference ( GPS Time ) maintained
by y the Master Control gground
station s atomic clocks
GPS time uses the TIA second, and was set equal to
UTC in 1980. It does not introduce leap seconds, and
today is 14s ahead of UTC (the difference between TIA
& UTC was 19s in 1980, , hence today y s 33s-19s=14s). )
The Navigation Message transmits a correction for
UTC, so that GPS receivers can display UTC and local
time zones.
UTC, the official world time is a
compromise between Earth time and
TIA TIA; it uses the th TIA second, d bbut t
introduces leap seconds to account for
changes in the earth s spin and
Each satellite carries its own atomic
clock clock, which will have a small error or
offset from GPS Time. This is
known as SV (Satellite Vehicle) time
maintain a useful consistency with UT1 SV clock offset information is
At any given time, UTC equals TIA
minus an integer number of seconds.
broadcast in each satellite s
Navigation Message
In January 2008, UTC was 33s behind. The GPS calendar is a counter of
Typically, a leap second is subtracted weeks and days of the week. Date
once a year zero was 6 January 1980
Time measurement is the basis for GPS navigation, because the range from a
satellite to a receiver can be determined by y the time delay y in receiving g a signal, g ,
and with multiple range fixes, a position can be calculated
Source: The Science of Timekeeping ; DW Allen, N Ashby, C Hodge (Hewlett Packard Application Note 1289) www.allanstime.com/Publications/DWA/Science_Timekeeping
See also the US Naval Observatory Time Service Department website, http://tycho.usno.navy.mil/
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