Radio occultation data analysis by the radioholographic method

Hocke, Klemens; Pavelyev, A. G.; Yakovlev, O. I.; Barthès, Laurent; Jakowski, N.

doi:10.1016/s1364-6826(99)00080-2

Cited by 79 publications

(73 citation statements)

References 14 publications

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“…The main steps include: (1) quality control and extrapolation of missing L2 where RS mode is active, (2) combination of bending (refraction) angle retrieval based on geometric optics (GO) above 25 km and wave optics below 25 km using the CT2 algorithm introduced by Gorbunov and Lauritsen (2004), the lowest altitude of a retrieved bending angle profile is determined as the maximum of the correlation of the CT2 amplitude with the step function (Gorbunov et al, 2006), (3) ionospheric correction combined with the statistical optimization (Gorbunov, 2002a), (4) standard refractivity retrieval by the Abel inversion, and (5) dry temperature retrieval. Figure 1 shows an example of the spectrogram of RO data (Hocke et al, 1999;Gorbunov, 2002b) indicating a reflection from the ocean surface combined with minor atmospheric multipath propagation effects. Reflected rays form the almost horizontal branch of the bending angle profile near the impact height of 2 km.…”

Section: The Data Processingmentioning

confidence: 99%

Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes

et al. 2011

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Abstract. Instrument GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) on-board of the Metop-A satellite was activated on 27 October 2006. Currently, Metop-A is a fully operational satellite with GRAS providing from 650-700 occultations per day. We describe our processing of GRAS data based on the modification of our OCC software, which was modified to become capable of reading and processing GRAS data. We perform a statistical comparison of bending angles and refractivities derived from GRAS data with those derived from ECMWF analyses. We conclude that GRAS data have error characteristics close to those of COSMIC data. In the height range 10-30 km, the systematic refractivity difference GRAS-ECMWF is of the order of 0.1-0.2 %, and the standard deviation is 0.3-0.6 %. In the lower troposphere GRAS refractivity and bending angle indicate a negative bias, which reaches its maximum value in the tropics. In particular the retrieved refractivity is biased by up to 2.5 %. The negative bias pattern is similar to that found in the statistical validation of COSMIC data. This makes it probable that the bias should not be attributed to the instrument design or hardware.

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Section: The Data Processingmentioning

confidence: 99%

Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes

et al. 2011

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“…At low altitudes the GNSS signals experience multipath beam propagation (see e.g. Gorbunov, 2002b;Hocke et al, 1999). The resulting optical path length differences lead to signal scintillations and these amplitude fluctuations increase the probability of an early loss of tracking lock.…”

Section: Introductionmentioning

confidence: 99%

Open-loop GPS signal tracking at low elevation angles from a ground-based observation site

Beyerle

Zus

2017

Atmos. Meas. Tech.

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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.

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“…This compels studies on actual phase measurements that are sampled under diverse atmospheric conditions. Previous studies showed that random components of the measurement error can be estimated dynamically based on a signal's spectral contents (Hocke et al, 1999;Gorbunov et al, 2006) or in terms of measured signalto-noise ratio (SNR) (Lohmann, 2007;Wee and Kuo, 2013). However, typical quality indicators relating to inter-sample variations, e.g., SNR and Allan deviation (Allan, 1966), give not much information about systematic errors.…”

Section: Introductionmentioning

confidence: 99%

A perspective on the fundamental quality of GPS radio occultation data

Wee

Kuo

2015

Atmos. Meas. Tech.

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Abstract. Radio occultation (RO) is a promising source of observation for weather and climate applications. However, the uncertainties arising from imperfect retrieval algorithms may weaken the overall confidence in the data and discourage their use. As an alternative approach of assessing the quality of RO data while avoiding the nuisance of retrieval errors, this study proposes to use minimally processed data (measurement) instead of derived RO data. This study compares measured phase paths with their model counterparts, simulated with an effective ray tracer for which the refractive indices along the complete ray path linking the transmitter and the receiver are realistically specified. The comparison of phase measurements with the European Centre for Medium-Range Weather Forecasts (ECMWF) data made in the observation space shows that the RO measurements are of sufficient accuracy to uncover regional-scale systematic errors in ECMWF's operational analysis and the 45-year reanalysis (ERA40), and to clearly depict the error growth of short-term ERA40 forecasts. In the southern hemispheric stratosphere, in particular, the RO measurements served as a robust reference against which both of the two analyses were significantly biased in opposite directions even though they were produced by the same center using virtually the same set of data. The measurement and ECMWF analyses showed a close agreement in the standard deviation except for the regions and heights that the quality of the ECMWF data is controversial. This confirms the high precision of RO measurements and also indicates that the main problem of the ECMWF analyses lies in their systematic error.

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Radio occultation data analysis by the radioholographic method

Cited by 79 publications

References 14 publications

Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes

Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes

Open-loop GPS signal tracking at low elevation angles from a ground-based observation site

A perspective on the fundamental quality of GPS radio occultation data

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