Noise Floor and Signal-to-Noise Ratio of Radio Occultation Observations: A Cross-Mission Statistical Comparison

Abstract

Multiple radio occultation (RO) missions are currently providing observations that are assimilated by the world’s leading numerical weather prediction centers. These RO missions use the same signals originating from the Global Navigation Satellite Systems (GNSS), but they have different satellite designs and sizes with different antennas and receivers. This results in different noise levels for different missions. Although the amplitude data are characterized by the Signal-to-Noise Ratio (SNR), the noise, to which they are normalized, is not the real Noise Floor (NF) of the RO observations. We study the statistical distributions of the SNR and NF for RO missions including COSMIC, COSMIC2, METOP-A, METOP-B, METOP-C, and Spire. We demonstrate that different missions have different NF values and different NF and SNR distributions, sometimes multimodal. We propose to use the most probable NF value as an SNR normalization constant in order to compare the SNR values from different RO missions.

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.…”

Spire RO Thermal Profiles for Climate Studies: Initial Comparisons of the Measurements from Spire, NOAA-20 ATMS, Radiosonde, and COSMIC-2

“…Different missions significantly differ in the nominal values of SNR. The normalized values may have a smaller difference due to different noise floors [53]. Therefore, to compare RO missions with different SNR For GNSS RO instruments, the signal-to-noise ratio (SNR) is the RO signal amplitude divided by the noise level and is provided in the voltage-to-voltage unit (V/V).…”

“…Different missions significantly differ in the nominal values of SNR. The normalized values may have a smaller difference due to different noise floors [53]. Therefore, to compare RO missions with different SNR values, the influence of SNR on the comparison of CRTMsimulated BT based on Spire/COMSIC-2 soundings with respect to ATMS BT products was also analyzed.…”

Performance assessment of radio occultation data from GeoOptics by comparing with COSMIC data

“…Because the SNR includes noise in the original RO signal as well as noise contributions from the GNSS satellites, ray path, and receiver in LEO satellites, it is more suitable for characterizing the overall RO signal quality than carrier-to-noise ratio (CNR) which captures the noise in the RF transport path only. Previous studies indicate that SNR is important in several aspects: (a) quality of retrieval measurements, (b) detection of sharp atmospheric boundary layer (ABL) tops, and (c) penetration of soundings lower into the troposphere (Gorbunov et al 2022;Schreiner et al 2020). On this basis, we analyzed the influence of SNR on the following three parameters: standard deviation of the difference between the bending angles and a model climatology bending angles between 60-80 km (STDV), bending angle lapse (BAL), and penetration depth.…”