Analysis of reflections in GNSS radio occultation measurements using the phase matching amplitude

Fengyun-3C (FY-3C) is a Global Navigation Satellite Systems (GNSS) Radio Occultation (RO) mission founded which was by China on 23 September 2013. In this study, under a specific temporal and spatial domain, we systematically compare FY-3C refractivity profiles with Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) refractivity profiles for the year 2015. The COSMIC profiles used in this study contain reflections, as identified in the Radio Occultation Meteorology Satellite Application Facility (ROM SAF) flag database. From 0 to 25 km altitude, the mean biases and relative standard deviations of the comparisons between FY-3C and COSMIC are less than 1% and 2% when COSMIC profiles present reflected signals. Radio holographic analysis is used to visualize and identify the spectra of FY-3C-reflected signals in the time-frequency domain. It is confirmed that the reflected signals in the lower troposphere and near the surface can be tracked by an FY-3C receiver. Further, most of the FY-3C events that matched with COSMIC reflected events show reflection patterns at a lower height, especially above the ocean’s surface. Under Bouguer’s rule and spherical symmetry assumptions, we reconstructed the reflected bending angle models by Abel transformation, which are valuable for reducing N-bias in the ducting layer. Three examples of FY-3C events show that the reflected bending branch is near the surface. Overall, the reflected signal of FY-3C could be used as a supplementary data portion for FY-3C atmospheric products.

Section: Introductionmentioning

confidence: 86%

Preliminary Validation of Surface Reflections from Fengyun-3C Radio Occultation Data

Chen

Xiong

et al. 2021

“…The roughness of the sea surface has limited impact in this case since the grazing angle is relatively small (∼ 1 • when the impact height is 1 km below the surface). The bending angle of each reflected path can be directly calculated from the received signal amplitude and phase via the current radio-holographic method, like phase matching (PM), by extending the impact parameter range below the surface [27]. Applying PM to the MPS, simulated signals result in both direct and reflected bending angles for the corresponding refractivity profile.…”

Section: Reflecting Signal and Ductingmentioning

confidence: 99%

GNSS-RO Refractivity Bias Correction Under Ducting Layer Using Surface-Reflection Signal

Wang

Juárez

2020

Due to its high vertical resolution and cloud-penetrating capability, GNSS-Radio Occultation (RO) remote sensing technique has been utilized to observe the vertical structure of the Planetary Boundary Layer (PBL) in recent years. However, the critical refraction, or ducting, caused by large refractivity gradients usually associated with the top of the stratocumulus clouds, can negatively bias the retrieved refractivity and humidity within the PBL. Previous research has shown that combining RO retrievals and the external information, such as collocated precipitable water (PW) estimates, can effectively reduce the negative bias and enhance the retrieval quality. Nevertheless, the requirement of collocated observations from other techniques limits the applicability of this reconstruction method in practice. Here, we describe an alternative approach that uses the coherent grazing signals from the same RO event that are reflected by the Earth’s surface to remove the bias due to ducting. Additional observations are no longer necessary in this approach because the reflected signals provide the extra constraint. A least squares framework is used to select the candidate from a family of solutions wherein reflected bending angle best matches the corresponding observation. This new method was validated by both multiple phase screen (MPS) simulation and the simulated RO bending angle via forward Abel transform, and it was tested with the actual GPS-RO measurements. While, in general, the reflected bending retrieved from the current mission was noisy, the results show that this approach can potentially reduce the negative bias and improve RO observation within the PBL without assistance by the external information, such as PW.

“…After inverting the spectrogram back to RO signals, they could retrieve the bending angles of reflected rays. The use of one-sided windows to remove the deeper direct rays while keeping reflected rays in the signal was proposed by Sievert et al [15]. Since reflected rays are received at a higher point in orbit than some direct rays, they were able to show that reflected rays could potentially be separated from the direct rays by truncating the RO signal.…”

Section: Introductionmentioning

confidence: 99%

Using A Sliding Window Phase Matching Method for Imaging of GNSS Radio Occultation Signals

Sievert

Rasch²,

Carlström

et al. 2021

Self Cite

Global Navigation Satellite System Radio Occultation (GNSS-RO) is a technique used to sound the atmosphere and derive vertical profiles of refractivity. Signals from GNSS satellites are received in a low-Earth orbit, and they are then processed to produce bending angle profiles, from which meteorological parameters can be retrieved. Generating two-dimensional images in the form of spectrograms from GNSS-RO signals is commonly done to, for instance, investigate reflections or estimate signal quality in the lower troposphere. This is typically implemented using, e.g., the Short-Time Fourier Transform (STFT) to produce a time-frequency representation that is subsequently transformed to bending angle (BA) and impact height (IH) coordinates by non-linear mapping. In this paper, we propose an alternative method based on a straightforward extension of the Phase Matching (PM) operator to produce two-dimensional spectral images in the BA-IH domain by applying a sliding window. This Sliding Window Phase Matching (SWPM) method generates the spectral amplitude on an arbitrary grid in BA and IH, e.g., along the coordinate axes. To illustrate, we show both SWPM and STFT methods applied to operational MetOp-A data. For SWPM we use a constant window in the BA-dimension, whereas for STFT we use a conventional constant time window. We show that the SWPM method produces the same result as STFT when the same window length is used for both methods. The sample points in impact parameter and bending angle are those generated by and the main advantage is that SWPM offers the user a convenient way to freely sample the BA-IH space. The cost for this is processing time that is somewhat longer than implementations based on the Fast Fourier Transform, such as the STFT method.