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Application of GNSS to
Environmental Studies
PENINA AXELRAD
Colorado Center for Astrodynamics Research
University of Colorado at Boulder
ABSTRACT
GNSS signals are influenced by the transmission media and interaction with
surfaces near the receiving antenna. Observations of the modified signals from the
ground and from airborne and spaceborne platforms allow for scientific study of
the ionosphere, atmosphere, and Earth surface. Methods using standard ground-
based receivers provide estimates of atmospheric water vapor and soil moisture.
Specialized receivers measuring occulted signals enable high-resolution estimates
of atmospheric density and temperature. Receivers measuring reflected signals
are used to infer surface roughness and reflectivity, which can be related to sur-
face conditions like ocean winds, soil moisture, and ice type. Modern receivers
that can use multiple GNSS constellations will provide a rich global data set for
environmental study.
INTRODUCTION
Intended primarily for position, navigation, and timing (PNT), Global Navi -
gation Satellite Systems (GNSS) bathe Earth with a multitude of highly stable
ranging signals that are readily available for use in probing the atmosphere and
surface of Earth. Furthermore, GNSS receivers deployed in diverse environments
(land, marine, air, space) for conventional PNT purposes can also be utilized to
make environmental observations with these same signals. All GNSS signals
received on the ground or in low Earth orbit traverse the ionosphere. At L-band,
frequency dependent dispersion of modulation and carrier phase allows total
179
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180 GLOBAL NAVIGATION SATELLITE SYSTEMS
electron content to be observed by measuring range and/or phase at two or more
frequencies, or through the dispersion of code and carrier. Atmospheric attenu-
ation and delay observed by ground stations can be modeled/observed by esti -
mating parameters associated with empirical models. Measuring the phase delay
and amplitude variations of occulting signals from aircraft or satellites enables
high-resolution retrieval of atmospheric density, which also exposes temperature
and water vapor content. Signals reflected by Earth contain information on surface
properties including roughness and reflectivity. Measuring reflected signals from
ground, aircraft, or spacecraft enables retrieval of surface conditions including
soil moisture, sea ice type, and ocean surface winds.
ATMOSPHERIC SENSING
Ground-based receivers at known locations measuring pseudorange and
phase to all GNSS satellites in view provide useful observations for estimating
precipitable water vapor (PWV) in the atmosphere. The integrated atmospheric
effect along the satellite to receiver ray path can be isolated by removing all other
sources of error. The technique was established in the early 1990s (Bevis et al.,
1992; Rocken et al., 1993) and is done fairly routinely today (Rocken et al., 2005),
although the estimates are not sufficiently widespread to be operationally incorpo-
rated in the weather prediction models. Ground-based GNSS PWV sensing relies
on a network of geodetic-quality dual-frequency receivers at known locations,
precise orbit models, and accurate models relating temperature and moisture
content in the atmosphere. High-precision software like Bernese (Dach et al.,
2007) must be used to ensure that all other error sources have been eliminated.
A more powerful worldwide approach to exploit GNSS for atmospheric
sensing relies on radio occultations measured by orbiting satellites. Researchers
have studied the atmospheres of Mars, Venus, and Jupiter since the 1970s through
radio occultation. The use of GPS for radio occultation (GPS-RO) measurements
from LEO was first initiated in 1995 by the GPS/MET experiment (Schreiner et
al., 1998; Yunck et al., 2000). This satellite, placed in a 735 km orbit at 70 degree
inclination, flew a modified TurboRogue receiver designed to track signals from
above for precise orbit determination and from a separate antenna, signals pass -
ing close to the limb of Earth. Now, in 2011, there are about a dozen satellites
flying operational or experimental occultation payloads including the six-satellite
COSMIC constellation (Anthes et al., 2008), CHAMP, SAC-C, GRAS/Metop,
C/NOFS, GRACE-A, and TerrSAR-X. These platforms use forward and backward
facing antennas to measure GNSS signals as they rise or set and are occulted by
Earth’s atmosphere as shown in Figure 1. A vertical profile of the bending angle
through the ionosphere, stratosphere, and troposphere is determined from the
excess phase measurements. Refractivity is derived from the bending angles and
then further analyzed to determine electron density profiles, temperature, pres -
sure, and water vapor. At its peak performance, COSMIC provided approximately
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181
APPLICATION OF GNSS TO ENVIRONMENTAL STUDIES
FIGURE 1 Illustration of GPS radio occultation. Sources: The Earth image is provided
by the SeaWiFS Project, NASA Goddard Space Flight Center, and ORBIMAGE. The Leo
satellite is “Courtesy of Orbital Corporation.” The image composition is © University
Corporation for Atmospheric Research. Courtesy University Corporation for Atmospheric
Axelrad-1 NEW
Research.
2,500 soundings per day. These observations are already being used as key inputs
to numerical weather prediction (Cucurull et al., 2007), atmospheric studies, and
climate monitoring, and their impact and significance are expected to grow in the
future (Anthes et al., 2008). Compelling results have also been presented (Huang
et al., 2010) illustrating the positive benefit of GPS occultation observations on
severe weather prediction. New applications being developed include measure -
ment of the planetary boundary layer, temperature inversions, and turbulence. It
is notable that a National Research Council decadal survey in 2007 recommended
that GPS-RO be made operational by NOAA and that additional GPS-RO instru -
ments be put on NASA science platforms whenever appropriate (NRC, 2007).
Increasing the number of GNSS transmitters and the constellation properties
with Compass and Galileo will directly increase coverage of Earth’s atmosphere
and the timeliness of the observations. This will have a tremendous impact on
understanding of Earth in the long term (climate change) and short term (numeri -
cal weather prediction, severe storm monitoring). Adding new LEO observing
satellites able to track multiple GNSS systems will contribute thousands of
observations daily.
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182 GLOBAL NAVIGATION SATELLITE SYSTEMS
SURFACE SENSING
When GNSS is used for conventional PNT purposes, reflections from sur-
faces near the receiving antenna, termed multipath, introduce errors in the solu -
tion by distorting the composite signal tracked by the receiver. By deliberately
monitoring and modeling the effects of the reflections, surface and near-surface
properties can be inferred. Two basic approaches are being used. The first, pio -
neered by Kristine Larson (Larson et al., 2008a, 2008b, 2010; Small et al., 2010)
uses standard geodetic ground-based receiving equipment and models the effect
of the surface reflections on the composite signal tracked by the receiver. The
second approach, initially investigated by Martin-Neira (1993) and separately by
Katzberg and Garrison (1996) uses an airborne or spaceborne receiver with both
a standard upward-facing and specialized downward-facing antenna to separately
track direct and reflected signals.
Figure 2 shows the oscillations in the signal-to-noise ratio that are typical
of GPS tracking in the presence of ground reflections at GPS tracking sites like
the Plate Boundary Observatory (PBO) site in Marshall, Colorado (Figure 3).
By relating the frequency and phase shift of these oscillations to the reflectivity
and location of the reflection point on the ground, researchers have been able to
FIGURE 2 Time history of C/No from Marshall, Colorado, PBO site. Oscillations near the
start and end of the pass are due to multipath from the ground. Note: This plot is based on
data provided by the Plate Boundary Observatory operated by UNAVCO for EarthScope
(http://www.earthscope.org) and supported by the National Science Foundation (No. EAR-
Axelrad-2 NEW
0350028 and EAR-0732947).
Bitmapped
low res
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183
APPLICATION OF GNSS TO ENVIRONMENTAL STUDIES
observe changes in soil moisture content (Figure 4) (Larson et al., 2008a, 2008b,
2010), crop growth (Small et al., 2010), and snow depth (Larson et al., 2009).
Because this approach relies on standard geodetic receivers, the proliferation
of such receivers for other geodetic purposes including earthquake monitoring,
will make it possible to coincidentally observe Earth surface conditions with no
additional expense or installations.
FIGURE 3 GPS Antenna at Marshall, CO PBO site used for soil moisture and snow
experiments. Source: Larson et al., 2008a. Courtesy UNAVCO.
Axelrad_Fig3.eps
bitmap
FIGURE 4 Daily precipitation (blue), water content reflectometer range (gray), and GPS
Axelrad_Fig4.eps
soil moisture measurements (colors) for PBO GPS site at Marshall, Colorado. Source:
Larson et al., 2008b, Figure 3. Courtesybitmap Larson,
of Kristine
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184 GLOBAL NAVIGATION SATELLITE SYSTEMS
Figure 5 shows an airborne configuration for surface sensing using GNSS
reflections, and Figure 6 (Esterhuizen, 2006) illustrates the receiver elements
for tracking direct and reflected signals for this scenario. For this delay-mapping
approach, a zenith antenna receives direct GPS signals, and a nadir antenna (with
opposite polarization) receives the same GPS signals after reflection from surface
below. Figure 7 compares the signal correlation function for a direct and reflected
signal. The delay, attenuation, and spreading of the reflected signals provide the
observability of surface properties. Various researchers have demonstrated the abil-
ity to detect ocean surface winds (Garrison et al., 2002), soil moisture (Katzberg
et al., 2005; Masters et al., 2004), ice and frozen surface roughness (Rivas et al.,
2010), and land cover from these observations. While initial results look promising,
this technique has not yet been as firmly established as occultation measurements
of the atmosphere.
Measurements from space of GPS signals reflected from Earth’s surface are
also of interest. The concept was first demonstrated from space using data from
the Shuttle SIR-C radar (Lowe et al., 2002). Gleason (2006) showed observations
of ocean, ice, and land from GPS reflection data collected onboard the UK-DMC.
CONCLUSIONS
Innovative uses for GNSS signals for probing Earth’s atmosphere, iono-
sphere, and surface will certainly continue to develop as more signals and more
advanced signals become available. This will enable an ever-increasing improve -
ment in our ability to measure and predict changes in Earth’s environment.
FIGURE 5 Airborne surface sensing based on reflected GNSS signals.
Axelrad_Fig5.eps
bitmap
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185
APPLICATION OF GNSS TO ENVIRONMENTAL STUDIES
FIGURE 6 Delay mapping receiver used for characterization of surface height, roughness,
Axelrad_Fig6.eps
and reflectivity. Source: Esterhuizen, 2006, Figure 2.5. Courtesy of Stephan Esterhuizen.
bitmap
FIGURE 7 Model of effect of surface conditions on correlation function for reflected
signals.
Axelrad_Fig7.eps
bitmap
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186 GLOBAL NAVIGATION SATELLITE SYSTEMS
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