Abstract
Abstract. A 1-year data set of ground-based GPS signal observations aiming at geometric elevation angles below +2° is analysed. Within the "GLESER" measurement campaign about 2600 validated setting events were recorded by the "OpenGPS" open-loop tracking receiver at an observation site located at 52.3808° N, 13.0642° E between January and December 2014. The measurements confirm the feasibility of open-loop signal tracking down to geometric elevation angles of −1 to −1.5° extending the corresponding closed-loop tracking range by up to 1°. The study is based on the premise that observations of low-elevation events by a ground-based receiver may serve as test cases for space-based radio occultation measurements, even if the latter proceed at a significantly faster temporal scale. The results support the conclusion that the open-loop Doppler model has negligible influence on the derived carrier frequency profile for strong signal-to-noise density ratios above about 30 dB Hz. At lower signal levels, however, the OpenGPS receiver's dual-channel design, which tracks the same signal using two Doppler models differing by 10 Hz, uncovers a notable bias. The repeat patterns of the GPS orbit traces in terms of azimuth angle reveal characteristic signatures in both signal amplitude and Doppler frequency with respect to the topography close to the observation site. Mean vertical refractivity gradients, extracted from ECMWF meteorological fields, correlate weakly to moderately with observed signal amplitude fluctuations at geometric elevation angles between +1 and +2°. Results from multiple phase screen simulations support the interpretation that these fluctuations are at least partly produced by atmospheric multipath; at negative elevation angles diffraction at the ground surface seems to contribute.
Highlights
For more than a decade the existing Global Navigation Satellite System (GNSS) infrastructure is exploited in meteorological applications and climate studies
Beyerle et al (2011) claimed, on the basis of GNSS-RO observations recorded by the “IGOR” receiver aboard the TerraSAR-X spacecraft, that the O/L Doppler model may influence the derived refractivity values for low signal amplitudes below about 25 V/V and potentially contribute to the negative refractivity bias
For more than a decade the OpenGPS receiver is used at GFZ in several ground-based and airborne measurement campaigns
Summary
For more than a decade the existing Global Navigation Satellite System (GNSS) infrastructure is exploited in meteorological applications and climate studies. With a typical sampling rate of fs = 50 Hz this requirement translates into a maximum frequency deviation of 25 Hz. Recently, Beyerle et al (2011) claimed, on the basis of GNSS-RO observations recorded by the “IGOR” receiver aboard the TerraSAR-X spacecraft, that the O/L Doppler model may influence the derived refractivity values for low signal amplitudes below about 25 V/V and potentially contribute to the negative refractivity bias. Whilst ground-based observations of low-elevation setting events do not allow to derive bending angle profiles for ray tangent points above the receiver altitude (Zuffada et al, 1999; Haase et al, 2014; Healy, 2002; Sokolovskiy et al, 2001), these measurements are useful to investigate receiver tracking behaviour under multipath conditions with strongly fluctuating SNRs. In addition, the signal excess phase paths have been shown to be sensitive to the local refractivity field The OpenGPS hardware and software are described in the Appendix
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