Landsat 7 is unique in having a long term plan driving its acquisition strategy. The plan is an outgrowth of recommendations from the scientific community. The purpose of the plan is to optimize the use of the Landsat instrument to create an archive of the processes on the planet's land surfaces.

The acquisition strategy of data from satellites is perhaps one of the most critical aspects of project planning.  How often and under what conditions the satellite will acquire its imagery is essential information for understanding the kinds of science questions that can be answered by the data set.  At the University of Maryland we are developing the long term acquisition plan (LTAP) that will serve as the basic Landsat-7 operational scheme.

High priority needs can, at any given time, overrule the LTAP specifications (e.g., natural disasters, national security needs, specific user requests). In the absence of high priority demands, for each 24-hour period, sensor operations will be determined by the specifications contained in the LTAP.  The LTAP will be queried to determine which of the pending WRS scenes should be acquired, based on the following criteria:
 

1.  Seasonality is the critical driver of the plan's acquisition strategy. The aim is to record the annual cycle of vegetation dynamics over the life of the mission. This will create a valuable archive that can be used as to understand current distribution and changes in vegetaiton, and act as a benchmark in the future as global change proceeds.

The plan puts a premium on documenting change thus periods of significant change are assigned multiple acquisitions while periods without significant change are documented with single acquistions. In the annual cumulative acquisition image the areas with the most seasonal change have the highest number of acquisitions (red). For example, temperate areas of North America and Europe are assigned many acquisitions as are areas such as the Sahel. Areas with little change such as deserts and rainforests are assigned fewer acquisitions (blue). Areas of high latitude are assigned fewer acquisitions because of sun angle constraints.

In the actual operation of the satellite areas of interest superceed the seasonally based acquistion requests. The continental U.S. and Alaska are acquired at every opportunity as are tropical rainforests. Other specific research interests are also incorporated into the satellite as additional requests, such as imaging glaciers and coral reefs, as well as responding quickly to image volcanic events.

The broad range of interests addressed in the long range plan and its coherent use of vegetation dynamics add substantially to the value of the satellite record that Landsat7 will generate.

2. Cloud Conditions: The predicted cloud conditions originate from NOAA and consist of predicted percentage cloud cover derived from forecasts 6 to 24 hours (TBD) prior to acquisition. This prediction is compared with a cloud climatology derived, for each WRS, from the ISSCP (International Satellite Cloud Climatology Project) data set (Rossow et al. 1989). If the predicted cloud cover is lower than the ISSCP-generated cloud cover threshold, then the acquisition priority of that WRS scene is increased.  See Figures Below.
 

 

 

3. Gain Settings: For pending WRS scenes which are designated for acquisition by both the seasonality index and cloud conditions, the per-band sensitivity of the instrument is set by adjusting the gain to conditions expected for that location at that time of year.