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

Beyerle, G.; Zus, Florian

doi:10.5194/amt-10-15-2017

Cited by 5 publications

(4 citation statements)

References 74 publications

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“…Due to the geometry of the GPS orbits, the reflection of a specific satellite occurs roughly over the same area of the Earth every day with small variations in the azimuth domain, as shown in Beyerle and Zus (2017). Moreover, as the satellite ascends or descends in its orbit, the specular reflection point moves closer to, or farther away from, the receiver's position along a track that covers different areas.…”

Section: Monitoring Resolution and Systematic Effectsmentioning

confidence: 99%

Analysis of Grazing GNSS Reflections Observed at the Zeppelin Mountain Station, Spitsbergen

et al. 2017

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A reflectometry station has been set up in 2013 near Ny‐Ålesund, Svalbard, at 78.9082°N, 11.9031°E. The main goal of the setup is to resolve the spatial and temporal variations in snow and ice cover, based on reflection power observations at grazing elevations. In this study, we develop a method to map the recorded signal power to the main reflection contributions while also discussing the spatial characteristics of the observations. A spectral analysis resolving differential Doppler between direct and reflected signals is presented to identify reflection contributions for a complete year (2014). Strong water reflections are identified with power ratios higher than 70 dB/Hz and constant Doppler shifts of 0.5–0.6 Hz for all elevations. Contributions with ratios higher than 40 dB/Hz can be related to specular land or glacier reflections, for which Doppler shift usually increases with the elevation angle and the distance between reflection point and receiver. Reflections nearby, around 3–5 km, show differential Doppler of 0.4–0.5 Hz, while for reflections farther than 16 km away, Doppler shift is usually larger than 0.8 Hz. Azimuth variations cause cross‐track drift of up to 4° during the observation year. Topography‐induced shadowing of very low lying satellites limits the extent of the monitoring area. However, the amount of satellites tracked daily, up to 30, allows the reflectometry station to constantly record reflections over areas with thick snow cover and glaciers. This offers the possibility to compare the derived reflected power with local meteorological data to resolve snow and ice variations on the area.

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Section: Monitoring Resolution and Systematic Effectsmentioning

confidence: 99%

Analysis of Grazing GNSS Reflections Observed at the Zeppelin Mountain Station, Spitsbergen

et al. 2017

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“…Thousands of daily profiles are now available from these missions, making GNSS-RO one of the top contributors to global weather modeling and forecasting [8]. Airborne and ground-based GNSS-RO are used to improve the regional availability of RO observations and to provide raw intermediatefrequency (IF) data to support study of RO signal structures to enable development of robust receiver algorithms [9]- [12].…”

Section: Introductionmentioning

confidence: 99%

“…For COSMIC, if the impact height is below 10 km, the mean difference between the Doppler observation and the Doppler model is about 8 Hz and the standard deviation of the differences is about 7 Hz [15]. According to [12], [16], an error of 5 Hz in the Doppler model will lead to significant bias in the retrieved signal bending angles. Therefore, the retrieval of lower troposphere parameters is a serious challenge for OL-based RO receivers.…”

Section: Introductionmentioning

confidence: 99%

Kalman Filter-Based Robust Closed-Loop Carrier Tracking of Airborne GNSS Radio-Occultation Signals

Wang

Yang

Morton

2020

IEEE Trans. Aerosp. Electron. Syst.

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GNSS radio occultation (RO) signals have been demonstrated as a viable means to retrieve atmospheric profiles. Current GNSS-RO observations rely on open-loop (OL) processing of the signals, especially for signals propagating through the lower troposphere. The reason is that GNSS signals at low elevations are adversely affected by multipath effects due to propagation through lower troposphere structures and reflections and scattering from the Earth surface. The lowelevation RO signals are characterized by deep and fast amplitude fading and rapid signal carrier phase fluctuations, collectively referred to as signal scintillation. The conventional phase-lock loop (PLL) may lose lock of these signals. While OL tracking is known for its robustness, its accuracy is determined by the climatological models used to create the reference for the GNSS signal carrier tracking loop. The wide bandwidth typically associated with OL tracking also introduces large errors in signal parameters estimations. In this paper, we present an adaptive Kalman filterbased closed-loop (KFC) tracking method, which takes into consideration the tropospheric scintillation, platform vibration, and real-time C/N 0 estimation of the RO signals. The KFC method has comparable robustness with and improved accuracy over the OL tracking, which are demonstrated through comparison using real GPS RO data collected on an airborne platform. Analysis of the excess Doppler estimation, retrieved bending angles and impact parameters also confirms the improved performances of the proposed algorithm over OL tracking.

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“…The occultation signal from GNSS satellites has low and negative elevation angles [9][10][11]. Many approaches to data processing in the presence of multipath propagation (methods based on Fourier Integral Operators, like Canonical Transform, Full Spectrum Inversion and Phase Matching) can be found in references [12][13][14][15][16][17][18].…”

Section: Introductionmentioning

confidence: 99%

Mountain Top-Based Atmospheric Radio Occultation Observations with Open/Closed Loop Tracking: Experiment and Validation

Hou

Kan

et al. 2020

Remote Sensing

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Through Global Navigation Satellite System (GNSS) occultation measurement, the global ionosphere and atmosphere can be observed. When the navigation satellites’ signal passes through the lower atmosphere, the rapid change of the atmospheric refractive index gradient will cause serious multipath phenomena in radio wave propagation. Atmospheric doppler frequency shift and amplitude signal fluctuations increase drastically. Due to the attenuation of signal amplitude and the rapid change of the Doppler frequency, the general phase locked loop (PLL) cannot work properly. Hence, a more stable tracking technology is needed to track the occultation signal passing through the lower atmosphere. In this paper, a mountain-top based radio occultation experiment is performed, where we employ an open-loop receiver and remove the navigation bits by the internal demodulation. In the process of the experiment, we adopt the open-loop tracking technique and there is no feedback between the observed signal and the control model. Specifically, taking the pseudo-range and doppler information from models as input, three key parameters, i.e., accurate code phase, carrier doppler and code doppler, can be obtained, and furthermore, the accurate accumulation is determined by them. For the full open-loop occultation data, a closed-loop observation assisted strategy is presented to compare the tracking results between open-loop and closed-loop occultation data. Through the compared results, we can determine whether the initial phase has been reversed or not, and obtain the high consistency corrected open-loop data that can be directly used for subsequent atmospheric parameters inversion. To verify the effect of open-loop tracking and open-loop inversion, we used the company’s self-developed occult receiver system for verification. The company’s self-developed occult receiver system supports Global Position System (GPS)/Beidou satellites constellation (BD, the 2nd and 3rd generations) dual systems. We have verified GPS and BD open-loop tracking and inversion, carried out in a three-week mountain-based experiment. We used closed-loop and open-loop strategies to track and capture the same navigation star to detect its acquisition effect. Finally, we counted the results of a week (we only listed the GPS data; BD’s effect is similar). The experimental results show that the open-loop has expanded the signal-cut-off angle by nearly 20% under the condition of counting all angles, while the open-loop has increased the signal-cut-off angle value by nearly 89% when only calculating the negative angle. Finally, the atmosphere profiles retrieved from observations in open-loop tracking mode are evaluated with the local observations of temperature, humidity and pressure provided by the Beijing Meteorological Bureau, and it is concluded that the error of open-loop tracking method is within ~4% in MSER (mean square error of relative error), which meets the accuracy of its applications (<5%, in MSER).

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

Cited by 5 publications

References 74 publications

Analysis of Grazing GNSS Reflections Observed at the Zeppelin Mountain Station, Spitsbergen

Analysis of Grazing GNSS Reflections Observed at the Zeppelin Mountain Station, Spitsbergen

Kalman Filter-Based Robust Closed-Loop Carrier Tracking of Airborne GNSS Radio-Occultation Signals

Mountain Top-Based Atmospheric Radio Occultation Observations with Open/Closed Loop Tracking: Experiment and Validation

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