Exploring the validity of the long term data record V4 database for land surface monitoring

The last (and final) version of the Long Term Data Record (LTDR) — Version 4 — has been released recently by NASA. This database includes daily information for all AVHRR (Advanced Very High Resolution Radiometer) channels, as well as ancillary data, since July 1981 up to present. This database is the longest available record of remotely sensed data useful for land surface monitoring, since it allows the estimation of vegetation indices at daily resolution, as well as the daily estimation of land surface temperature (LST). Here, we analyze the fitness of this database for land surface monitoring. To that end, we first estimated NDVI (Normalized Difference Vegetation Index), LST, as well as extracted SZA (Solar Zenith Angle) from the ancillary data. Then, we reconstructed the yearly temporal profiles of NDVI and LST using the IDR approach, from which we extracted different parameters, such as minimum and maximum values and corresponding dates, as well as the dates of mid-amplitude crossing. We also retrieved SZA values at all previous dates. In the following step, we checked for the presence and estimated values for trends using the Mann-Kendall framework for all retrieved dates as well as for minimum and maximum values. We then compared the retrieved trends and values to independent ground data. As a conclusion, the LTDR-V4 dataset seems adequate for regional to global land surface monitoring, provided time series reconstruction techniques are applied to time series, although LST-derived parameters should be first corrected from the orbital drift effect.