Comparison of PROBA-V, METOP and eMODIS NDVI anomalies over the Horn of Africa : evaluation of PROBA-V satellite data quality

After 15 years of operational service, the VGT instrument has reached its end-of-life in May 2014. Before this date, the PROBA-V satellite (PV) was launched in May 2013 by ESA, as a "gap filler", i.e., between the SPOT VGT and the future Sentinel 3 satellite, foreseen to be launched in mid-2015. In order to evaluate the quality and usability for operational crop monitoring of the new PV satellite NDVI data we compared it to other moderate to low spatial resolution instruments. The analysis has been performed over the Horn of Africa using TAMSAT rainfall estimates and NDVI from METOP and MODIS instruments as term of comparison. We found large disagreement (up to more than 40% according to the proposed classification scheme) between NDVI anomaly derived from PV the one hand, and METOP or eMODIS on the other hand. The disagreement between METOP and eMODIS is lower and normally within 30%. The analysis also shows that in case of disagreement, the difference between paired anomalies is biased when PV is compared with one of the other two sensors. That is, when PV and one of the two sensors do not agree on the sign of the anomaly, it is PV indicating a positive anomaly and the other sensor a negative one in 80% of the cases. Visual inspection of anomalies support these findings and highlight the fact that this agreement has indeed operational implications. That is, PV appears to deliver a contrasting information regarding vegetation status as compared to METOP and eMODIS, especially in some specific dekads (e.g., dekads 25 to 28). Performing the same comparison for the year 2013 when NDVI data were provided by VGT instead of PV, we confirm that major differences among the three sensors were not present and we corroborate the hypothesis that the cause of the problem is related to the use of PV NRT data with VGT LTA data to compute the anomalies. Finally, rainfall estimates time series of 2014 were used as an additional and qualitative check for the observed differences. The visual inspection of rainfall anomalies suggests that the anomalies shown by PV are generally too positive while METOP and eMODIS are showing a better convergence with the rainfall estimates. The results of this preliminary comparison call for further analysis to understand the cause of the observed seasonality of the agreement between the PV anomalies and those of other comparable sensors (METOP and eMODIS). Also it is not known at the moment whether this situation is specific for the East African window or if similar patterns can also be observed in other regions of the world. Before this clarification is made and corrective actions are taken, we recommend to follow a conservative approach and use eMODIS or METOP for the operational monitoring of FOODSEC.