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4. Land-monitoring aspects

This section discusses the issues related to monitoring from AVHRR in general. Only some, but not all, of these aspects have been accounted for in the GVI data products at present. Some are still in a research and development stage. The present status of the GVI data production and the issues to be resolved are described in the next section.

a. Standardized anomalies
We define monitoring as detection of anomalies , that is, deviations of the observed quantities V from their multiannual means <V>. To quantitatively estimate the extent (reality and magnitude) of the anomalies, one has to have a reference noise level, with which the anomalies should be compared (Malingreau 1986) and that is provided by the climatological variance around the mean, . Assessment of statistical significance of the anomalies as compared to the level of inherent local year-to-year fluctuations is done by considering standardized anomalies . The above approach is hardly new and has been used in climate studies for decades. The lack of long-term satellite datasets has prevented this method from being widely utilized in remote sensing.

b. Multiparameter monitoring
AVHRR has five spectral bands, allowing retrieval of many land surface geophysical parameters. Although a comprehensive monitoring system should use all of them, usually only one monitoring parameter is used (e.g. NDVI) which fails to characterize the surface fully. In aggregating NDVI, individual information from solar channels is lost. Furthermore, the thermal IR channels provide additional information. For example, both droughts and floods are associated with lowered NDVI but have different IR signatures. Using several monitoring parameters derived from AVHRR reduces the ambiguity in interpretation of statistically significant anomalies.

c. Using top-of-atmosphere parameters
Monitoring of land surface implies the use of surface geophysical parameters, such as LAI or fraction of vegetation (i.e. CX.5 data in our nomenclature). However, retrieval algorithms for surface parameters are not always available or reliable. An equivalent approach to monitoring can be based on the standardized anomalies of surface , NDVI, T₄, and T₅ normalized to a common observation-illumination geometry and time of day (). If the latter effects contribute negligibly to the variance of remote signal, then may be used. If most year-to-year variability in the observed data comes from the surface (since atmospheric window regions are used, and surface contribution is expected to dominate), even standardized anomalies of top-of-atmosphere (TOA) parameters () can be utilized for most practical purposes. Screening of cloud contamination is, however, mandatory. The currently developed monitoring system within the GVI project, based on calibrated and cloud-screened TOA composite data (C3.2), is described in Section 5d.

d. Limiting factors
The use of AVHRR for monitoring is seriously hampered because of nonuniformity of the satellite time series. This includes satellite/sensor discontinuities due to satellite/sensor change, and trends caused by sensor instability (Kaufman and Holben 1993) and by satellite orbit drift (Price 1991). The AVHRR solar channels are not calibrated in flight and degrade with time, which is taken care of by applying postlaunch calibration (Rao and Chen 1994). The satellite-to-satellite change and orbital drift affect all variables because of changing illumination and diurnal surface/atmosphere variability. Large-scale atmospheric perturbations from volcanic eruptions like Mt. Pinatubo contribute to nonuniformity of time series because stratospheric aerosols produced by these eruptions have a strong and persistent impact on observed radiances, mostly in solar channels (Stowe et al. 1992). All the above factors present difficulties in using AVHRR data if atmospheric/angular corrections are not applied at the B level.