Drifting Effects of NOAA Satellites on Long-Term Active Fire Records of Europe

Helga Weber,Stefan Wunderle

doi:10.3390/rs11040467

Helga Weber, Stefan Wunderle

Open Access

https://doi.org/10.3390/rs11040467

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Abstract

Explicit knowledge of different error sources in long-term climate records from space is required to understand and mitigate their impacts on resulting time series. Imagery of the heritage Advanced Very High Resolution Radiometer (AVHRR) provides unique potential for climate research dating back to the 1980s, flying onboard a series of successive National Oceanic and Atmospheric Administration (NOAA) and Meteorological Operational (MetOp) satellites. However, the NOAA satellites are affected by severe orbital drift that results in spurious trends in time series. We identified the impact and extent of the orbital drift in 1 km AVHRR long-term active fire data. This record contains data of European fire activity from 1985–2016 and was analyzed on a regional scale and extended across Europe. Inconsistent sampling of the diurnal active fire cycle due to orbital drift with a maximum delay of ∼5 h over NOAA-14 lifetime revealed a ∼90% decline in the number of observed fires. However, interregional results were less conclusive and other error sources as well as interannual variability were more pronounced. Solar illumination, measured by the sun zenith angle (SZA), related changes in background temperatures were significant for all regions and afternoon satellites with major changes in −0.03 to −0.09 K deg − 1 for ▵ B T 34 (p ≤ 0 . 001). Based on example scenes, we simulated the influence of changing temperatures related to changes in the SZA on the detection of active fires. These simulations showed a profound influence of the active fire detection capabilities dependent on biome and land cover characteristics. The strong decrease in the relative changes in the apparent number of active fires calculated over the satellites lifetime highlights that a correction of the orbital drift effect is essential even over short time periods.

Highlights

Unpredictable random and systematic errors are the two main types of errors, which can be quantitively assessed to analyze the quality of a satellite-based time series [1]
While we identified a prominent orbit drift signal on the European scale, effects on the regional scale (1◦ grid cells) were less conclusive and other factors might have been more dominant on the active fire time series of each National Oceanic and Atmospheric Administration (NOAA) satellite
Our study presents an example of how severely the impact of orbital drift affects Advanced Very High Resolution Radiometer (AVHRR) active fire records of Europe, which have resulted in non-homogenous time series and biased trends

Summary

Introduction

Unpredictable random and systematic errors are the two main types of errors, which can be quantitively assessed to analyze the quality of a satellite-based time series [1]. Time series of the Advanced Very High Resolution Radiometer (AVHRR) are affected by both. Changing sensor technology [2], radiometric calibration uncertainty (e.g., [3,4]), and navigational errors like satellite attitude, satellite clock, and orbital drift (e.g., [5,6]) result in systematic errors, which affect measurement accuracy. The orbital drifting, as already described by [7], can result in the detection of false trends or masking of actual trends in AVHRR retrieved climatological time series (e.g., [8]).

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