Characterizing and Mitigating Digital Sampling Effects on the CYGNSS Level 1 Calibration

This paper presents a detailed examination of fluctuating input noise power levels on the analog to digital convertor (ADC) sampling hardware of the NASA CYGNSS instruments and the associated impacts on the level 1 NBRCS estimation performance. The impact of external noise variations on both the CYGNSS science and navigation channels are quantified with respect to how the fluctuating received power impacts the low level ADC sampling distribution and subsequent NBRCS estimation. This work demonstrates that there are clear quantifiable geo-spatially dependent noise variations linked to GNSS space based augmentation systems (notably QZSS and WAAS) and that these additional noise sources significantly alter the digital sampling distribution of the CYGNSS instruments, actively degrading the level 1 NBRCS estimation if not corrected. A derivation of the theoretical correction for both the science and navigation channel ADC sampling variations is presented which is later tuned based on empirical performance metrics in an effort to minimize the induced calibration errors. The impacts of the enhancements outlined in this work on CYGNSS level 1 calibration are evaluated using 1 year of observations before and after the digital sampling corrections, using model ECMWF wind and Wavewatch III MSS surface validation data sets.