The Influence of the Signal-to-Noise Ratio upon Radio Occultation Retrievals

Abstract

We study the dependence of radio occultation (RO) inversion statistics on the signal-to-noise ratio (SNR). We use observations from four missions: COSMIC, COSMIC-2, METOP-B, and Spire. All data are processed identically using the same software with the same settings for the retrieval of bending angles, which are compared with reference analyses of the National Oceanic and Atmospheric Administration (NOAA) Global Forecast System. We evaluate the bias, the standard deviation, and the penetration characterized by the fraction of events reaching a specific height. In order to compare SNRs from the different RO missions, we use the results of our previous study, which defined two types of SNR. The statically normalized SNR is defined in terms of the most probable value of the noise floor for the specific mission and global navigation satellite system. The dynamically normalized SNR uses the noise floor value for the specific profile. This study is based on the dynamical normalization. We also evaluate the latitudinal distributions of occultations for different missions. We show that the dependence of the retrieval statistics on the SNR is not very strong, and it is mostly defined by the variations of latitudinal distributions for different SNR. For Spire, these variations are the smallest, and here, the bias and standard deviation reach saturated values for a relatively low SNR.

Assessment of Operational Non-Time Critical Sentinel-6A Michael Freilich Radio Occultation Data: Insights into Tropospheric GNSS Signal Cutoff Strategies and Processor Improvements

“…As discussed in the works of Gorbunov et al (2022a) and Gorbunov et al (2022b), higher SNRs improve the tropospheric penetration or RO profiles. The effect of SNR to the random error when RO profiles are compared against Numerical Weather Prediction data is mission dependent, showing some saturation features larger for some missions and smaller for others.…”

Global Stratospheric Properties of Gravity Waves From 1 Year of Radio Occultations

“…It was already shown that RO T from different missions can be combined to produce climatologies. Signal‐to‐noise‐ratio (SNR) is mainly important in the lower troposphere but not in the stratosphere (Gorbunov et al., 2022), which is the atmospheric layer considered in this study. For example, those authors evaluated four RO missions (COSMIC, COSMIC‐2, METOP‐B, and Spire) and have shown that higher SNR does not necessarily lead to higher statistical quality of the RO profiles if the ratio is above a given threshold.…”

Comparative Assessment of Spire and COSMIC-2 Radio Occultation Data Quality

“…Currently, Spire and COSMIC-2 have developed new-generation GNSS-RO payloads, i.e., STRATOS and the Tri-GNSS Radio-occultation System (TGRS), respectively, to improve their retrieval quality in the low troposphere. It should be noted that, compared to other payloads with high-gain antennas (e.g., the TGRS and CION [27]), STRATOS is equipped with relatively low-gain antennas to track GNSS signals, thus directly leading to relatively low SNR values in their measurements.…”

“…The 60-80 km altitude range is optimal for evaluating signal strength, unaffected by atmospheric interference. It's sufficiently high to render attenuation from typical atmospheric refraction negligible, yet it doesn't extend to the E-layer where disturbances are more pronounced [21,27]. As a result, the average L1-signal SNR within the 60-80 km altitude range (hereafter referred to as the SNR average) is related to the signal strength of the RO event.…”

“…Based on the research by Gorbunov et al (2022) [27], it has been revealed that the SNR serves as an indicator of signal strength and holds a crucial role in penetration. Penetration depth was found to be influenced by SNR, GNSS, RO modes, topography, and latitude, as revealed by combined results obtained in Sections 3.2 and 3.3.…”