Global Positioning System Reference
In-Depth Information
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TABLE 7.1 IGS Products in 2002
Product
Accuracy
Latency
Updates
GPS Satellite Ephemeris
and Satellite Clocks
Predicted (Ultra Rapid)
∼
25 cm;
∼
5 ns
real time
Twice daily
Rapid
5 cm; 0.2 ns
17 hours
daily
Final
<
5 cm; 0.1 ns
∼
13 days
weekly
Geocentric Coordinates
of IGS Tracking Stations
Final horizontal and
3 mm&6mm
12days
weekly
vertical positions
F
inal horizontal and
[23
2 mm/yr & 3 mm/yr
12 days
weekly
vertical velocities
Earth Rotation Parameters
Lin
—
-
——
No
PgE
Rapid polar motion
0.2 mas
Polar motion rates
0.4 mas/day
17 hours
daily
Length-of-day
0.030 ms
Final polar motion
0.1 mas
Polar motion rates
0.2 mas/day
∼
13 days
weekly
Length-of-day
0.020 ms
Atmospheric Parameters
[23
Final tropospheric
4 mm zenith path delay
<
4 weeks
weekly
Ionospheric TEC grid
Under development
So
urce:
IGS (2002).
7.
2.1 PhaseWindup Correction
One must go back to the electromagnetic nature of GPS transmissions in order to
understand this correction. In short, the GPS carrier waves are right circularly polar-
ized (RCP). The electromagnetic wave may be visualized as a rotating electric vector
field that propagates from the satellite antenna to the receiver antenna. The vector
rotates 360° every spatial wavelength or every temporal cycle of the wave. The ob-
served carrier phase can be viewed as the geometric angle between the instantaneous
electric field vector at the receiving antenna and some reference direction on the an-
tenna. As the receiving antenna rotates in azimuth, this measured phase changes. The
same is true if the transmitting antenna changes its orientation with respect to the
receiver antenna. Since the phase is measured in the plane of the receiving antenna,
its value depends on the direction of the line of sight to the satellite, in addition to the
orientation of the antenna.
