A Novel Approach to Relative Radiometric Calibration on Spatial and Temporal Variations for FORMOSAT-5 RSI Imagery.

Tang-Huang Lin,Fuan Tsai,Hai-Po Chan,Min-Chung Hsiao

doi:10.3390/s18071996

Abstract

Radiometric calibration for imaging sensors is a crucial procedure to ensure imagery quality. One of the challenges in relative radiometric calibration is to correct detector-level artifacts due to the fluctuation in discrepant responses (spatial) and electronic instability (temporal). In this paper, the integration of the empirical mode decomposition (EMD) with Hilbert–Huang transform (HHT) in relative radiometric calibration was explored for a new sensor, FS-5 RSI (remote sensing instrument onboard the FORMOSAT-5 satellite). The key intrinsic mode functions (IMFs) analyzed by HHT were examined with the pre-flight datasets of the FS-5 RSI in temporal and spatial variations. The results show that the EMD–HHT method can stabilize and improve the radiometric quality of the FS-5 imagery as well as boost its application ability to a new level. It is noticed that the IMFs of the spatial variation would be disturbed by the instability of the temporal variation. The relative response discrepancies among detector chips can be well calibrated after considering the temporal effect. Taking a test imagery dataset of gain setting G2 as an example, the standard deviation (STD) of the discrepancy in the digital number after calibration was dramatically scaled down compared to the original ones (e.g., PAN: 66.31 to 1.85; B1: 54.19 to 1.90; B2: 36.50 to 1.49; B3: 32.43 to 1.56; B4: 37.67 to 1.20). The good performance of pre-flight imagery indicates that the EMD–HHT approach could be highly practical to the on-orbit relative radiometric calibration of the FS-5 RSI sensor and is applicable to other optical sensors. To our knowledge, the proposed EMD–HHT approach is used for the first time to explore relative radiometric calibration for optical sensors.

Highlights

Sensors of modern digital imagers onboard satellites must be assured of accuracy by pre-flight radiometric calibration
The good performance of pre-flight imagery indicates that the empirical mode decomposition (EMD)–Hilbert–Huang Transform (HHT) approach could be highly practical to the on-orbit relative radiometric calibration of the FS-5 remote-sensing instrument (RSI) sensor and is applicable to other optical sensors
The FS-5 RSI is the successor to the FS-2 RSI, providing high spatial-resolution imagery at the global scale for environmental resources and disaster-monitoring services

Summary

Introduction

Sensors of modern digital imagers onboard satellites must be assured of accuracy by pre-flight radiometric calibration. This task is relatively easy in the space environment simulation laboratory prior to a satellite’s launch. After launch, sensors are apt to experience degradation over time which may occur in optical, electrical or supportive components during launch; or due to satellite orbital drifts and perturbations, cosmic ray bombardment, etc. FORMOSAT-2 (FS-2) onboard sensors’ degradation causes the temporal variation of mean conversion factor between the output digital number (DN) and the input radiation intensity of the observed imagery. Sensors must be regularly calibrated and corrected with the radiometric conversion factor to ensure high-quality imagery.

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Conclusion