Abstract

Ocean altimetry with Global Navigation Satellite Systems signals (GNSS) signals is a remote sensing technique that measures the height of the sea surface through the difference in path length of the direct and reflected signal. Code altimetry estimates this parameter by tracking the code delay after performing correlations with a GNSS signal replica. It is of limited precision due to the low signal-to-noise ratio (SNR) and narrow bandwidth of the ocean-reflected GNSS signal. However, the potential advantages of the GNSS-R systems such as high temporal resolution and spatial coverage are a motivation to improve its altimetric precision. In this article, we present a performance assessment of the Likelihood Map Waveform tracking technique, a method based on Maximum Likelihood Estimation theory that exploits the available reflected power in a more efficient way than the single tracking point methods. We use a modification of the theoretical optimal solution that achieves a better performance than previous methods. We estimate it, in terms of SNR gain, using Monte Carlo method with a detailed stochastic model of the signal, and with actual signals from the Cyclone Global Navigation Satellite System. The gain values obtained were between 1.64 and 3.66 dB in the theoretical analysis, and between 1.69 and 2.62 dB with the real data, confirming the potential of the proposed approach.

Highlights

  • R EFLECTOMETRY with Global Navigation Satellite Systems signals (GNSS-R) has enabled many new Earth observation techniques

  • We focus on the processing of ocean-reflected GNSS signals received on board Low Earth Orbit (LEO) satellites, where the delay-Doppler map (DDM) presents a spread distribution of the correlation power in the delay-Doppler plane due to the diffuse reflection process

  • In [6], we presented the derivation of the Maximum Likelihood Estimator (MLE) of the specular reflection point (SP) delay based on a signal model that takes into account the delay and Doppler spread of the reflected signal

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Summary

INTRODUCTION

R EFLECTOMETRY with Global Navigation Satellite Systems signals (GNSS-R) has enabled many new Earth observation techniques. We focus on the processing of ocean-reflected GNSS signals received on board LEO satellites, where the DDM presents a spread distribution of the correlation power in the delay-Doppler plane due to the diffuse reflection process. We analyze a modified version of this algorithm designated as modified LM (MLM) that avoids the difficulties that faces the optimal solution This modification uses the already averaged DDM values for the calculation of each point in the map, making MLM a postprocessing approach in contrast to the theoretically optimal method. Significant contributions that make up the total reflected signal and use the subscript m to refer to each individual return These are characterized by their delay-Doppler pair (τm, fm) defined by the position of their corresponding cell relative to the SP, and a complex amplitude αm that models their magnitude and phase.

OCEAN-REFLECTED SIGNAL MODEL AND ALTIMETRY
Delay Doppler Map
Code Altimetry With DDM
LM WAVEFORM
LM Limitations in GNSS Ocean Altimetry
PERFORMANCE ASSESSMENT
Stochastic DDM Model
Simulation Results
Findings
CONCLUSION

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