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| Titel |
Simulations of direct and reflected wave trajectories for ground-based GNSS-R experiments |
| VerfasserIn |
N. Roussel, F. Frappart, G. Ramillien, J. Darrozes, C. Desjardins, P. Gegout, F. Perosanz, R. Biancale |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 7, no. 5 ; Nr. 7, no. 5 (2014-10-02), S.2261-2279 |
| Datensatznummer |
250115732
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| Publikation (Nr.) |
 copernicus.org/gmd-7-2261-2014.pdf |
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| Zusammenfassung |
| The detection of
Global Navigation Satellite System (GNSS) signals that are reflected off the
surface, along with the reception of direct GNSS signals, offers a unique
opportunity to monitor water level variations over land and ocean. The time
delay between the reception of the direct and reflected signals gives access
to the altitude of the receiver over the reflecting surface. The field of
view of the receiver is highly dependent on both the orbits of the GNSS
satellites and the configuration of the study site geometries. A simulator
has been developed to determine the location of the reflection points on the
surface accurately by modeling the trajectories of GNSS electromagnetic waves
that are reflected by the surface of the Earth. Only the geometric problem
was considered using a specular reflection assumption. The orbit of the GNSS
constellation satellites (mainly GPS, GLONASS and Galileo), and the position
of a fixed receiver, are used as inputs. Four different simulation modes are
proposed, depending on the choice of the Earth surface model (local plane,
osculating sphere or ellipsoid) and the consideration of topography likely to
cause masking effects. Angular refraction effects derived from adaptive
mapping functions are also taken into account. This simulator was developed
to determine where the GNSS-R receivers should be located to monitor a given
study area efficiently. In this study, two test sites were considered: the
first one at the top of the 65 m Cordouan lighthouse in the Gironde estuary,
France, and the second one on the shore of Lake Geneva (50 m above the
reflecting surface), at the border between France and Switzerland. This site
is hidden by mountains in the south (orthometric altitude up to
2000 m), and overlooking the lake in the north (orthometric altitude of
370 m). For this second test site configuration, reflections occur until
560 m from the receiver. The planimetric (arc length) differences (or
altimetric difference as WGS84 ellipsoid height) between the positions of the
specular reflection points obtained considering the Earth's surface as an
osculating sphere or as an ellipsoid were found to be on average 9 cm (or
less than 1 mm) for satellite elevation angles greater than 10°, and
13.9 cm (or less than 1 mm) for satellite elevation angles between 5 and
10°. The altimetric and planimetric differences between the plane and
sphere approximations are on average below 1.4 cm (or less than 1 mm) for
satellite elevation angles greater than 10° and below 6.2 cm (or
2.4 mm) for satellite elevation angles between 5 and 10°. These
results are the means of the differences obtained during a 24 h simulation
with a complete GPS and GLONASS constellation, and thus depend on how the
satellite elevation angle is sampled over the day of simulation. The
simulations highlight the importance of the digital elevation model (DEM)
integration: average planimetric differences (or altimetric) with and without
integrating the DEM (with respect to the ellipsoid approximation) were found
to be about 6.3 m (or 1.74 m), with the minimum elevation angle equal to
5°. The correction of the angular refraction due to troposphere on
the signal leads to planimetric (or altimetric) differences of an
approximately 18 m (or 6 cm) maximum for a 50 m receiver height above the
reflecting surface, whereas the maximum is 2.9 m (or 7 mm) for a 5 m
receiver height above the reflecting surface. These errors increase deeply
with the receiver height above the reflecting surface. By setting it to
300 m, the planimetric errors reach 116 m, and the altimetric errors reach
32 cm for satellite elevation angles lower than 10°. The tests
performed with the simulator presented in this paper highlight the importance
of the choice of the Earth's representation and also the non-negligible
effect of angular refraction due to the troposphere on the specular
reflection point positions. Various outputs (time-varying reflection point
coordinates, satellite positions and ground paths, wave trajectories, first
Fresnel zones, etc.) are provided either as text or KML files for
visualization with Google Earth. |
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