 |
| Titel |
Open-loop GPS signal tracking at low elevation angles from a ground-based observation site |
| VerfasserIn |
Georg Beyerle, Florian Zus |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250125251
|
| Publikation (Nr.) |
 EGU/EGU2016-4810.pdf |
|
|
|
|
|
| Zusammenfassung |
| For more than a decade space-based global navigation satellite system (GNSS) radio
occultation (RO) observations are used by meteorological services world-wide for their
numerical weather prediction models. In addition, climate studies increasingly rely on
validated GNSS-RO data sets of atmospheric parameters. GNSS-RO profiles typically
cover an altitude range from the boundary layer up to the upper stratosphere; their
highest accuracy and precision, however, are attained at the tropopause level. In the
lower troposphere, multipath ray propagation tend to induce signal amplitude and
frequency fluctuations which lead to the development and implementation of open-loop
signal tracking methods in GNSS-RO receiver firmwares. In open-loop mode the
feed-back values for the carrier tracking loop are derived not from measured data, but
from a Doppler frequency model which usually is extracted from an atmospheric
climatology.
In order to ensure that this receiver-internal parameter set, does not bias the carrier phase
path observables, dual-channel open-loop GNSS-RO signal tracking was suggested.
Following this proposal the ground-based “GLESER” (GPS low-elevation setting event
recorder) campaign was established. Its objective was to disproof the existence of
model-induced frequency biases using ground-based GPS observations at very low elevation
angles.
Between January and December 2014 about 2600 validated setting events, starting at
geometric elevation angles of +2∘ and extending to −1∘… − 1.5∘, were recorded by the
single frequency “OpenGPS” GPS receiver at a measurement site located close to Potsdam,
Germany (52.3808∘N, 13.0642∘E). The study is based on the assumption that these
ground-based observations may be used as proxies for space-based RO measurements, even if
the latter occur on a one order of magnitude faster temporal scale. The “GLESER” data
analysis shows that the open-loop Doppler model has negligible influence on the derived
frequency profile provided signal-to-noise density ratios remain above about 30 dB Hz. At
low signal levels, however, the dual-channel open-loop design, which tracks the same signal
using two Doppler models separated by a 10 Hz offset, reveals a notable bias.
A significant fraction of this bias is caused by frequency aliasing. The receiver’s
dual-channel setup, however, allows for unambiguous identification of the affected
observation samples. Finally, the repeat patterns in terms of azimuth angle of the
GPS orbit traces reveals characteristic signatures in both, signal amplitude and
Doppler frequency with respect to the topography close to the observation site.
On the other hand, mean vertical refractivity gradients extracted from ECMWF
meteorological fields exhibit moderate correlations with observed signal amplitude
fluctuations at negative elevation angles emphasizing the information content of
low-elevation GPS signals with respect to the atmospheric state in the boundary
layer. |
|
|
|
|
|
|