Abstract
Pseudo-Invariant Calibration Sites (PICS) are one of the most popular methods for in-flight vicarious radiometric calibration of Earth remote sensing satellites. The fundamental question of PICS temporal stability has not been adequately addressed. However, the main purpose of this work is to evaluate the temporal stability of a few PICS using a new approach. The analysis was performed over six PICS (Libya 1, Libya 4, Niger 1, Niger 2, Egypt 1 and Sudan 1). The concept of a “Virtual Constellation” was developed to provide greater temporal coverage and also to overcome the dependence limitation of any specific characteristic derived from one particular sensor. TOA reflectance data from four sensors consistently demonstrating “stable” calibration to within 5%—the Landsat 7 ETM+ (Enhanced Thematic Mapper Plus), Landsat 8 OLI (Operational Land Imager), Terra MODIS (Moderate Resolution Imaging Spectroradiometer) and Sentinel-2A MSI (Multispectral Instrument)–were merged into a seamless dataset. Instead of using the traditional method of trend analysis (Student’s T test), a nonparametric Seasonal Mann-Kendall test was used for determining the PICS stability. The analysis results indicate that Libya 4 and Egypt 1 do not exhibit any monotonic trend in six reflective solar bands common to all of the studied sensors, indicating temporal stability. A decreasing monotonic trend was statistically detected in all bands, except SWIR 2, for Sudan 1 and the Green and Red bands for Niger 1. An increasing trend was detected in the Blue band for Niger 2 and the NIR band for Libya 1. These results do not suggest abandoning PICS as a viable calibration source. Rather, they indicate that PICS temporal stability cannot be assumed and should be regularly monitored as part of the sensor calibration process.
Highlights
Remote sensing using orbiting satellite sensors is essential for detecting and monitoring changes in the Earth’s land surfaces, oceans, atmosphere and climate [1]
All of the Landsat ETM+ and Operational Land Imager (OLI) images used in this study were downloaded to the SDSU IPLAB archive from the United States Geological Survey (USGS) Earth Resources Observation and Science (EROS) Data Center
The temporal stability of these Pseudo-Invariant Calibration Sites (PICS) has been assumed, implying that any change in observed temporal stability is due to changes in sensor response; if a PICS is not temporally stable, long term temporal trend monitoring results obtained for the site will not provide proper useful insights into the sensor’s radiometric performance
Summary
Remote sensing using orbiting satellite sensors is essential for detecting and monitoring changes in the Earth’s land surfaces, oceans, atmosphere and climate [1]. Regular in-flight calibration assesses the sensor’s on-orbit performance throughout its operating lifetime. These can be performed on data acquired from an on-board calibration source, such as a solar diffuser panel, and/or acquisition of radiance measurements from the Earth’s surface through vicarious calibration methods. It is important to highlight that a significant portion of the cost saving is achievable with small EO satellite sensors by removing on-board calibration source. For these sensors, vicarious calibration is the preferred option.
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