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

Michael Gorbunov,Christian Rocken,Vladimir Irisov

doi:10.3390/rs14030691

Michael Gorbunov, Christian Rocken + Show 1 more

Open Access

https://doi.org/10.3390/rs14030691

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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.

Highlights

There are several active radio occultation (RO) missions
We investigated the statistics of the average Signal-toNoise Ratio (SNR) at altitudes 60–80 km and the statistics of the Noise Floor (NF) for different missions (COSMIC, METOP-A,B,C, COSMIC-2, and Spire) for different constellations (GPS, GLONASS, Galileo, and QZSS)
Quality Control (QC) was reduced to a minimum: it was only requested that the specified characteristics of the SNR record were possible to be evaluated

Summary

Introduction

There are several active radio occultation (RO) missions. First of all, COSMIC2 should be mentioned [1,2,3,4]. SNR is nearly constant at large ray perigee heights (above 30 km), where it only indicates relatively small scintillations due to ionospheric propagation effects and measurement noise. A general assumption is that the measurements in the shadow zone represent the additive white noise affecting the whole profile record [18]. The effect of this noise depends on the signal strength that is estimated from the SNR at high perigee altitude. In order to compare different missions with different SNR, a common calibration is necessary In this respect, we have to take a close look at such an important characteristic, as the background noise level [18], or the Noise Floor (NF). The use of inter-comparable SNRs may help to reduce of the number of mission-specific tuning parameters in RO processing and to arrive at a better definition of the profile bottom

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