Geoscience Reference
In-Depth Information
GPS Time System
GPS has a dedicated time system for precision navigation and positioning. The GPS
time system, abbreviated as GPST, is provided by atomic clocks in GPS monitoring
stations.
GPST belongs to the atomic time system. It has the same interval unit of one SI
second as TAI (International Atomic Time), but a different point of origin from
TAI, so there is an integer-second offset of 19 s in any instant of time between
GPST and TAI, such that TAI GPST¼19 s. GPST was consistent with UTC
(Coordinated Universal Time) at 0 h on January 6, 1980. Over time there is an offset
of integral multiples of 1 s.
Features and Functions of GPS
With the employment of high-orbiting ranging systems, GPS defines its basic
observed quantity as the distance between stations and the satellites. There are
two main GPS measurement strategies for obtaining the observed quantity. One is
the pseudo-range measurement, which measures the propagation time taken for the
pseudo-random code to travel from the satellite to the user's receiver. The other is
the carrier phase measurement, which records the phase difference between the
carrier signals from the GPS satellites with Doppler frequency shift and the
reference carrier signals produced by receivers. Pseudo-range measurement has
the highest speed in positioning whereas carrier phase measurement has the highest
precision in positioning. The three-dimensional position of a receiver can be
deduced through the simultaneous pseudo-range or phase measurements of four
or more satellites.
With the appearance of GPS, electronic navigation technology has entered a
brilliant period. Compared with other navigation systems, GPS is mainly distin-
guished by the following (see Xu et al. 1998):
Continuous Global Coverage. Since there are enough GPS satellites in a reason-
able distribution, at least four satellites can theoretically be observed continuously
and synchronously from any point on the globe, which guarantees all-weather
global continuous navigation and positioning in real-time.
Multifunction and High Precision. GPS can provide three-dimensional position,
velocity, and time information continuously with high precision for all kinds of
users.
High Speed in Real-Time Positioning. One-time positioning and velocity mea-
surement of GPS receivers can be done within 1 s or even less, which is especially
important for high dynamic users.
Remarkable Anti-interference Capacity and Adequate Confidentiality.
Because of the employment of pseudo-noise spread spectrum technology in GPS,
the signals from GPS satellites have remarkable anti-interference capacity and
sufficient confidentiality.
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