Geoscience Reference
In-Depth Information
molecules in the atmosphere, and the wet
refractivity due to the polar nature of the water molecules.
In general, empirical models, which are functions of temperature, pressure, and relative humidity,
can eliminate 90 to 95 percent of the tropospheric effect from the GPS observables.
9.5.1.2
Relativistic propagation error
The gravitational field causes space-time curvature of the signal; thus, propagation correction has
to be applied to the GPS carrier-phase observable. This effect strongly depends on the geometry
between station, satellite, and geocenter, but it is small in magnitude. The maximum effect is 19 mm,
and it virtually cancels out in differential GPS (see Section 9.7).
9.5.1.3
Multipath
Multipath is a result of an interaction of the upcoming signal with the objects in the antenna sur-
rounding, such as buildings and rocks. It causes multiple reflections and diffractions, and as a result,
the signal arrives at the antenna via direct and indirect paths, increasing the travel time of the signal.
These signals interfere with each other, resulting in an error in the measured pseudorange or carrier
phase, degrading the positioning accuracy. The magnitude of multipath effect tends to be random
and unpredictable, varying with satellite geometry, location, and type of reflective surfaces in the
antenna surrounding, and can reach 1 to 5 cm for the carrier phases and 10 to 20 m for the code
pseudoranges (Hofman-Wellenhof et al., 2001). Properly designed choke ring antennas can almost
entirely eliminate this problem for the surface waves and signals reflected from the ground.
9.5.2 s
a t e l l i t e
a n d
R
e c e i v e R
c
l o c k
e
R R o R s
Even though the satellite onboard clock is a high-accuracy atomic clock, it may have a slight tim-
ing error that will affect the GPS observable if not corrected. This error is, however, monitored and
estimated by the GPS
Control Segment
; the satellite clock correction (predicted) is provided in the
satellite navigation message and can be used to correct the GPS observables. The receiver's built-in
clock is not as accurate as the atomic clocks onboard the GPS satellites; therefore, it may be subject
to timing error, defined as the difference between the clock's time and the true GPS time. This error
is either eliminated by differential (relative) positioning (see Section 9.7.1.2) or must be estimated in
the positioning solution if the point positioning mode is used.
It should also be mentioned that the relativistic effect on GPS receiver clock, due to the fact that
the receiver is placed in the gravitational field and rotates with Earth, is corrected by the receiver
software; it amounts to ~1 ns = 30 cm after 3 hours. Moreover, the constant drift, which is a part of
the total correction due to relativistic time difference between the receiver and the satellite, is com-
pensated for before the launch time by reducing the frequency of the satellite clock by 0.00455 Hz
from its nominal value of 10.23 MHz. However, the periodic term has to be modeled; for GPS
altitude, it has the maximum amplitude of ~30 ns in time or 10 m in distance. The periodic part can
be canceled by performing between-stations differencing, but for point positioning it is still prob-
lematic if not properly accommodated. For more information about relativistic effects on GPS, see,
e.g. (Schwarze et al., 1993; Zhu & Groten, 1988).
9.5.3 o
R b i t a l
e
R R o R s
Orbital errors are also known as broadcast ephemeris errors. These are inaccuracies of the satel-
lite's location (predicted orbit) reported in the broadcast ephemeris (navigation message). The sat-
ellite orbits are subject to perturbation forces, such as solar radiation, Earth and planetary gravity
fields, tidal effects, relativistic effects, and others, that cause departure from the predefined elliptical
orbit. These departures from the predefined orbit are continuously monitored by the
Control Seg-
ment
, allowing for accurate orbit prediction that is uploaded to the satellites' onboard computers for
