CHAPTER 9 SECTIONS > File Description | File Content

9.1 File Description

The IAS is responsible for the sustained radiometric and geometric calibration of the Landsat 7 satellite and ETM+ and passing this knowledge to the user community. This is achieved by assessing new imagery on a daily basis, performing both radiometric and geometric calibration when needed, and developing new processing parameters for creating level 1 products. Processing parameters are stored in the Calibration Parameter File (CPF) which is stamped with applicability dates and sent to the LP-DAAC for storage and eventual bundling with outbound Level 0R products. The CPF is also sent to international ground stations via the Landsat 7 Mission Operations Center.

IAS updates and distributes the calibration parameter file at least every 90 days. Updates will likely be more frequent during early orbit checkout and will also occur between the regular 90-day cycles whenever necessary. Irregular updates, however, will not affect the regular 90 day schedule. The timed release of a new calibration parameter file must be maintained because of the UT1 time corrections and pole wander predictions included in the file. These parameters span a 180 day interval time centered on the effective start date of the new IAS CPF. A CPF archive is maintained by the IAS. At this web site, you can download and view all CPFs since launch.

Time Stamps.
The calibration parameter file is time stamped by IAS with an effective date range. The first two parameters in the file, Effective_Date_Begin and Effective_Date_End, designate the range and are of the form YYYY-MM-DD. The Effective_End_Date for the most recent parameter file is its Effective_Date_Begin plus 90 days. After this date the file is without applicable UT1 time predictions. The parameter file that accompanies an order has an effective date range that includes the acquisition date of the image ordered.

File Naming Conventions
Through the course of the mission, a serial collection of CPFs is generated and sent to the LP-DAAC for coupling to 0R products. A distinct probablity exists that a CPF will be replaced due to improved calibration parameters for a given periord or perhaps due to file error. The need for unique file sequence numbers becomes necessary as file contents change. The following file naming procedure is used by IAS to name the CPF:

As an example, suppose four calibration files were created by the IAS on 90-day intervals and sent to the LP-DAAC during the first year of the mission. Further suppose that the first file was updated twice and the second and third files were updated once. The assigned file names would be as follows:

It is worth noting the 00 sequence number assigned to the original CPF. This reserve sequence number uniquely identifies the pre-launch CPF. Sequence numbers for subsequent time periods all begin with 01. New versions or updates are incremented by one.

This example assumes the effectivity dates do not change. The effectivity date range for a file can change, however, if a specific problem (e.g. detector outage) is discovered somewhere within the nominal 90-day effectivity range. Assuming this scenario, two CPFs with new names and effectivity date ranges are spawned for the time period under consideration. The effective_date_end for a new pre-problem CPF would change to the day before the problem occurred. The effective_date_begin remains unchanged. A post-problem CPF with a new file name would be created with an _effective_date_begin corresponding to the imaging date the problem occurred. The effective_date_end assigned would be the original effective_date_end for the time period under consideration. New versions of all other CPFs affected by the erroneous parameter also would be created.

Using this example, suppose a dead detector is discovered to have occurred on January 31, 1999. Two new CPFs are created that supersede the time period represented by file number three, version 2, and a new version of file number four is created. The new file names and sequence numbers become:

All calibration parameters are stored as American Standard Code for Information Interchange (ASCII) text using the ODL syntax developed by JPL. ODL is a tagged keyord language developed to provide a human-readable data structure to encode data for simplified interchange. The body of the file is composed of two statement types:

1. Attribute assignment statement used to assign values to parameters.
   
2. Group statements used to aid in file organization and enhance parsing granularity of parameter sets.

To illustrate consider the first three parameters in the file: Effective_Date_Begin, Effective_Date_End, and the CPF_File_Name. These three parameters form their own group which is called FILE_ATTRIBUTES. The syntax employed for this collection of parameters in the CPF appears as: