Potential and Challenges of Harmonizing 40 Years of AVHRR Data: The TIMELINE Experience

Abstract

Earth Observation satellite data allows for the monitoring of the surface of our planet at predefined intervals covering large areas. However, there is only one medium resolution sensor family in orbit that enables an observation time span of 40 and more years at a daily repeat interval. This is the AVHRR sensor family. If we want to investigate the long-term impacts of climate change on our environment, we can only do so based on data that remains available for several decades. If we then want to investigate processes with respect to climate change, we need very high temporal resolution enabling the generation of long-term time series and the derivation of related statistical parameters such as mean, variability, anomalies, and trends. The challenges to generating a well calibrated and harmonized 40-year-long time series based on AVHRR sensor data flown on 14 different platforms are enormous. However, only extremely thorough pre-processing and harmonization ensures that trends found in the data are real trends and not sensor-related (or other) artefacts. The generation of European-wide time series as a basis for the derivation of a multitude of parameters is therefore an extremely challenging task, the details of which are presented in this paper.

Satellite-Derived Land Surface Temperature Dynamics in the Context of Global Change—A Review

“…The AVHRR on the NOAA satellites combines the high temporal resolution of MODIS with a long time series (the first LST products have existed since 1981), providing a valuable data source to quantify long-term processes, such as climate change. However, the 16 generations of satellites have different overpass times and experience orbits, meaning that it is difficult to create a harmonized LST time series [157,[187][188][189]. Fifteen of the reviewed studies (9%) used a combination of sensors, whereby more than half of them used the combination of MODIS and Landsat.…”

“…Because the Terra and Aqua satellites carrying the MODIS sensor were launched in 1999 and 2002, the MODIS LST time series today is limited to 22 years. However, study periods should be ideally longer than 30 years to make climate-relevant statements [187].…”

Opportunities and Challenges of Spaceborne Sensors in Delineating Land Surface Temperature Trends: A Review

“…NOAA's AVHRR, having the longest data records of the polar-orbiting TIR since 1979 with consistent spatial (1.1 km) and temporal (twice/day) resolutions, should probably the best option. However, we found a very limited number of studies in literature since it would be quite challenging to generate well calibrated and harmonized time series observations of AVHRR instruments (AVHRR, AVHRR/2, and AVHRR/3) that were flown on 14 different platforms [115]. Because AVHRR sensors, onboard multiple platforms that have been active over the years, could have sensor degradation, scanline defects, satellite orbit drift and channel calibration drift of the different AVHRRs [115].…”

“…However, we found a very limited number of studies in literature since it would be quite challenging to generate well calibrated and harmonized time series observations of AVHRR instruments (AVHRR, AVHRR/2, and AVHRR/3) that were flown on 14 different platforms [115]. Because AVHRR sensors, onboard multiple platforms that have been active over the years, could have sensor degradation, scanline defects, satellite orbit drift and channel calibration drift of the different AVHRRs [115]. Though a study successfully calibrated the observations of different AVHRR instruments during 1981-2015 over the peninsular Spain and determined the LST trends [116]; however, such calibrated dataset across the globe is unavailable to our knowledge.…”

First Analyses of the TIMELINE AVHRR SST Product: Long-Term Trends of Sea Surface Temperature at 1 km Resolution across European Coastal Zones

“…The TIMELINE project conducted at the German Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) aims to generate well-calibrated and harmonized time series spanning four decades from the early 1980s based on 1 km Local Area Coverage (LAC) AVHRR data over Europe, which are exclusively received and archived at the DLR. Besides SST, the TIMELINE product suite includes LST, NDVI, Hot Spot/Burnt Area, Cloud Probability, and Snow Cover [14]. First analyses of the higher-level (Level 3) products are on the way, and the products are to be released on the DLR Geoservice (https://geoservice.dlr.de/web/) in the near future.…”