On-Ground Retracking to Correct Distorted Waveform in Spaceborne Global Navigation Satellite System-Reflectometry

Feng Wang,Weiqiang Li,Wei Yang,Dongkai Yang

doi:10.3390/rs9070643

Abstract

Spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) has been the research focus of Earth observation because of its unique advantages; however, there are still many challenges to be resolved. The reduction of the impact of the satellite motion on the GNSS-R waveform is the one of key technologies for spaceborne GNSS-R. The proposed delay retracking methods in existing literatures require too many instrument resources and too much priori information to refresh correlation window on each coherent integration time period. This paper aims to propose an on-ground alternative in which less frequency tracking refresh on board is needed. The model of dynamic delay waveform, which is expressed as the convolution of the pure waveform and the point spread function, are described. Based on this, the new methodology, which utilizes the least squares fitting to make the residual error between the dynamic model and measured waveform minimum, is employed to reconstruct the pure waveform. The validity of proposed method is verified using UK-DMC, UK-TDS-1 and simulated data. Moreover, the performances of sea surface height and wind speed retrieval using retracked and non-retracked waveforms are compared. The results show that (1) the MSEs between aligned and retracked waveform reduce to 0.026 and 0.044 from 0.110 and 0.156 between aligned and non-retracked waveform with the TRP of 1 s and 3 s for UK-DMC data, and for UK-TDS-1 data, the MSEs decrease from 161.02 and 227.34 to 70.10 and 61.80; (2) the standard deviation of sea surface height using retracked waveform is lower 5 times than the one using non-retracked waveform; (3) the retracked waveform could lead to a better measurement performance in wind speed retrieval. Finally, the relationship between the performance of retracking and Signal-to-Noise Ratio (SNR) is analyzed. The results show that when the SNR of the waveform is lower than 3 dB, the retrieval accuracies rapidly become worse.

Highlights

Global Navigation Satellite System-Reflectometry (GNSS-R) which was proposed by Martin-Neira et al in 1993 [1] has been used to retrieve sea wind speed and significant wave height (SWH)
This distortion results in a degradation of the retrieval performance as analyzed in [21] in which the results show that the performance of spaceborne GNSS-R altimeter is seriously degraded without a proper alignment of the waveform samples
The UK-DMC satellite was launched into a 680 km sun synchronous orbit as part of the disaster monitoring constellation in October 2003 and the first successful data collection was on 22 October 2005 [20]

Summary

Introduction

Global Navigation Satellite System-Reflectometry (GNSS-R) which was proposed by Martin-Neira et al in 1993 [1] has been used to retrieve sea wind speed and significant wave height (SWH). The spaceborne GNSS-R is especially attractive because of its low power-cost, low mass and high spatial-temporal resolution. Lowe [2] presented the first spaceborne observation of Global Positioning System (GPS) signals reflected from the Earth’s surface, which opened the gates for spaceborne GNSS-R research. First spaceborne retrieval of wind speed was reported by S.

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Conclusion