Effects of Landsat 7’s Orbit Drift and a Solution to Preserve Its Science Capabilities

Abstract:

Landsat 7’s orbit began to drift to an earlier Mean Local Time (MLT) since 2017 because on-board fuel resources prohibit maintenance of its nominal orbit, making quantitative analysis from Landsat 7 potentially unreliable for many scientific studies. To comprehensively understand the impacts of orbit drift on Landsat 7 data ahead of time, we analyzed Earth Observing-1 (EO-1) historical data, which shared similar orbit drift but occurred earlier. This analysis is based on the Top-Of-Atmosphere (TOA) reflectance data from different parts of the world. Results show that Landsat 7’s orbit drift would globally increase Blue band TOA reflectance and decrease the reflectance of Green, Red, Near Infrared (NIR), and two Short Wave Infrared (SWIR) spectral bands. The influence of orbit drift is more substantial for Red, NIR, and two SWIR bands but less for Blue and Green bands. On April 2023, we estimated that the TOA reflectance of Blue band would increase to an absolute value of respectively. Moreover, for different latitudes and/or over different land cover types, the influence is variable. To reduce this orbit drift effect and make future Landsat 7 data usable, we proposed a Time-series-based Reflectance Adjustment for Orbit Drift (TRAOD). Specifically, for each pixel, we used clear-sky pairs of Landsat 7 observations with orbit drift (simulated from EO-1 satellite) and the observations without orbit drift within a short interval (e.g., ± 1 day) to build a linear regression model to correct the effects. A cross-validation assessment indicates TRAOD can substantially reduce the orbit drift effects especially for Red and two SWIR bands. This would provide a solution to correct the effects of Landsat 7 orbit drift using Landsat 8 (and Landsat 9 if successfully launched) time series as the reference and preserve Landsat 7’s nominal mission science capabilities till 2023.