Global Positioning System Reference
In-Depth Information
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6.1 OVERVIEW
The propagation media affect electromagnetic wave propagation at all frequencies,
resulting in a bending of the signal path, time delays of arriving modulations, ad-
vances of carrier phases, scintillation, and other changes. In GPS positioning one is
primarily concerned with the arriving times of carrier modulations and carrier phases.
Geometric bending of the signal path causes a small delay that is negligible for eleva-
tion angles above 5°. The propagation of electromagnetic waves through the various
atmospheric regions varies with location and time in a complex manner and is still
the subject of active research. The relevant propagation regions are the troposphere
and the ionosphere. Whereas positioning with GPS requires careful consideration of
the impacts of the propagation media, GPS, in turn, has become a tool for studying
the atmosphere. The subject of propagation of electromagnetic signals in the GPS
frequency range, which is approximately the microwave region, is discussed but only
to the extent required for GPS positioning.
Most of the mass of the atmosphere is located in the troposphere. We are concerned
with the tropospheric delay of pseudoranges and carrier phases. For frequencies
below 30 GHz, the troposphere behaves essentially like a nondispersive medium;
i.e., the refraction is independent of the frequency of the signals passing through it.
This tropospheric refraction includes the effect of the neutral, gaseous atmosphere.
The effective height of the troposphere is about 40 km. The density in higher regions
is too small to have a measurable effect. Mendes (1999) and Schüler (2001) recently
studied the details of tropospheric refractions. Typically, tropospheric refraction is
treated in two parts. The first part is the hydrostatic component that follows the
laws of ideal gases. It is responsible for a zenith delay of about 240 cm at sea level
locations. It can be computed accurately from pressure measured at the receiver
antenna. The more variable second part is the wet component, or more precisely
labeled the nonhydrostatic wet component, which is responsible for up to 40 cm of
delay in the zenith direction. Computing the wet delay accurately is a difficult task
because of the spatial and temporal variation of water vapor. Figure 6.1 shows the
ZWD every 5 minutes for eleven consecutive days, beginning on July 10, 1999, at
Lamont, Oklahoma, as determined by GPS, and the difference between the GPS and
WVR determination. Both determinations agree within 1 cm. The gaps indicate times
when suitable observations were not available.
Figure 6.2 demonstrates the impact water vapor variation can have over a 43 km
baseline. The observations were taken over eleven days and processed with the precise
ephemeris. Essentially, two cases are compared: (a) measuring the ZWD with the
WVR and reducing the measured value to the slant delay using a mapping function
that has no azimuth dependency; and (b) measuring the slant wet delay (SWD) with
the WVR pointed in the direction of the satellite. In both cases, the hydrostatic
delay was computed from the Saastamoinen model using barometric pressure. The
largest ellipse in Figure 6.2 shows the repeatability over eleven days using the zenith
radiometer corrections; the second largest (closest to spherical shape) shows the
repeatability over eleven days using the pointed radiometer corrections. The next to
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