Landsat Missions
The Long Term Acquisition Plan (LTAP) is used to direct the acquisition of Landsat 7 scenes. These scenes are archived at the United States Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS). A Landsat scene covers approximately 100 square miles and is defined by a grid system called the Worldwide Reference System (WRS).
Five parameters are associated with the LTAP:
1. Seasonality File – (Excel .xls (3.68 MB) -or- ASCII Text File - .lst (921 KB))
This file
specifies which WRS scenes are to be acquired during which periods of time (request period), and the frequency of acquisition during those periods. The results of ingesting this file into the scheduler and developing the daily schedule are posted daily on the web at this server location. See the "warning label" below for important information.
The column headings for the seasonality file are:
Important points about this file:
Here is the warning label associated with the seasonality file:
The scheduler has many resources and priorities that it juggles during consideration of requests for scheduling, including:
The result of this juggling is that scenes marked with an "all" opportunities frequency are usually acquired every 4-5 cycles (64-80 days) instead of every cycle (16 days). Another outcome of this is that scenes marked with a "once" frequency may be acquired multiple times within the request period in an attempt to better the cloud cover results, should the first acquisition's cloud cover be considerably worse than the nominal. So please treat the frequency assignment in the seasonality as a guide, not a rule.
2. Land Definition – (Excel - .xls (1.5 MB))
The WRS Land Database lists all of the WRS path/rows that cover land and are considered part of the global archive.
The worksheet is formatted with three columns:
The version posted is sorted by definition, then path, then row. Definition can be any of the following:
3. Historical Cloud Cover – (Word text file - .doc (5.7 MB) and Word text file - .doc (5.7 MB)
The historical cloud cover reports the average cloud cover for each WRS scene for each month of the year. The average cloud cover is derived from the International Satellite Cloud Climatology Project D2 data set, years 1989-1993. There are two files: one for rows 1-122 and for rows 123-248, which may be in daylight during some part of the year and therefore may be imaged.
The columns in the nominal cloud cover files are separated by "|" and include:
For each month, cloud cover values for row 1 across all paths are given, then for row 2, etc.
4. Gain Settings – (ASCII text file - .lst (4.7 MB)
The gain settings file identifies the gain settings that will be used as defaults for each WRS scene for each day of the year. The file spans one generic year. The file addresses both descending (daytime) and ascending (nighttime) rows of interest to the U.S. archive. The file entries show when the default gain setting has changed from its previous setting.
The default gain settings were generated using rules that take land cover type and sun angle into account. To understand these rules, read the file "gain setting rules - .doc (42.0 KB)" Other files reference by those rules are:
Important points about the Band Gain column:
The maximum solar zenith angle is used as a threshold during scheduling. Daylight imaging will not be scheduled if the solar zenith angle is equal to or greater than the maximum solar zenith angle value.
As of 24 July 2002, there are two maximum solar zenith angle values, one for the Northern Hemisphere and one for the Southern Hemisphere. The values are:
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://landsat.usgs.gov
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Page Contact Information: landsat@usgs.gov
Page Last Modified: May 12, 2008
