Evaluation of Orbital Drift Effect on Proba-V Surface Reflectances Time Series

Fabrizio Niro

doi:10.3390/rs13122250

Abstract

Multi-temporal consistency of space-borne observations is an essential requirement for studying inter-annual changes and trends of satellite-derived biophysical products. The Proba-V mission, launched in 2013, was designed to ensure the continuity of the SPOT-VEGETATION long-term data record of global daily observations for land applications. The suitability of Proba-V to provide a temporally consistent data record is, however, potentially jeopardized by the orbital drift effect, which is known to induce spurious trends in time series. The aim of this paper is therefore to evaluate, for the first time, the orbital drift effect on Proba-V surface reflectance time series at 1 km resolution. In order to reliably identify such an effect, a two-fold approach is adopted. A simulation study is first defined to predict the temporal anomalies induced by the drifting illumination conditions. The numerical simulations are used as a benchmark to predict the impact of the drift for a range of sun-viewing angles. Real observations are then analyzed over a large set of land sites, globally spread and spanning a wide range of surface and environmental conditions. The surface anisotropy is characterized using the Ross-Thick Li-Sparse Reciprocal (RTLSR) Bidirectional Reflectance Distribution Function (BRDF) model. Both the simulation and the analysis of real observations consistently show that the orbital drift induces distinct and opposite trends in the two sides of the sensor across-track swath. Particularly, a positive drift is estimated in backward and a negative one in the forward scattering direction. When observations from all angular conditions are retained, these opposite trends largely compensate, with no remaining statistically significant drifts in time series of surface reflectances or Normalized Difference Vegetation Index (NDVI). As such, the Proba-V archive at 1 km resolution can be reliably used for inter-annual vegetation studies.

Highlights

Multi-temporal consistency of satellite-based Earth Observation (EO) time series is a necessary condition to investigate inter-annual changes and trends of bio-geophysical parameters
The purpose of this paper is to quantitatively evaluate, for the first time, the impact of the orbital drift on the time series of Proba-V [13,14] surface reflectances at 1 km resolution
The statistical analysis presented in the previous paragraphs shows that the difference between Bidirectional Reflectance Distribution Function (BRDF)-normalized and TOC data often lies within the range of variability of the estimated drifts, when observations from all angular conditions are merged

Summary

Introduction

Multi-temporal consistency of satellite-based Earth Observation (EO) time series is a necessary condition to investigate inter-annual changes and trends of bio-geophysical parameters. Consistency in this context means stability of the estimated geophysical products’ uncertainties and this requirement is notably paramount for climate-related applications [1]. One of the potential sources of uncertainties is the instability of the orbital parameters during the satellite lifetime, such as orbit attitude, altitude or inclination This instability can cause temporal variations of the space-based observation conditions, with resulting spurious trends in the considered time series. Infrared (SWIR) over an across-track swath of approximately 2250 km. Proba-V platform lacks an on-board calibration device; the sensor radiometric calibration was entirely based on vicarious approaches [17]

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