CHAPTER 11 SECTIONS > Level 0R Product | Level 1R Product | Level 1G Product | Level 1 Differences

11.1 Level 0R Product

Figure11.1 - Simulated Level 0R Image

Unlike earlier Landsat programs, the Landsat 7 system was not originally designed to produce high level (i.e. Level 1) products for users. The baselined program philosophy was to provide raw data only which would leave the value added domain for commercial companies. A prevailing "wait and see" position by commercial vendors prompted NASA to add a systematic correction capability to ensure product availability. The primary product for users and vendors seeking higher level processing, however, is 0R data - an essentially raw data form that is marginally useful prior to radiometric and geometric correction. This is readily apparent when viewing a simulated 0R image. A Landsat 7 0R product, however, does contain all the ancillary data required to perform these corrections including a calibration parameter file (CPF) generated by the Landsat 7 IAS.

LPS spatially reformats earth imagery and calibration data into Level 0R data. This involves shifting pixels by integer amounts to account for the alternating forward-reverse scanning pattern of the ETM+ sensor, the odd-even detector arrangement within each band, and the detector offsets inherent to the focal plane array engineering design. All LPS 0R corrections are reversible; the pixel shift parameters used are documented in the IAS CPF.

The LPS 0R output is HDF-EOS formatted and archived. Details of the archival format can be found in the Landsat 7 System Wideband DFCB, Vol. 4.

Figure 11.2 - Level 0R Product Alternatives

Three options, depicted in Figure 11.2, exist when defining the size or spatial extent of a Landsat level 0R product ordered from the LP-DAAC.

• Standard Worldwide Reference System (WRS) Scene. The standard WRS scene as defined for Landsats 4 and 5 was preserved as a product for Landsat 7. The WRS indexes orbits (paths) and scene centers (rows) into a global grid system comprising 233 paths by 248 rows. The path/row notation was originally employed to provide a standard designator for every nominal scene center and allow straight forward referencing without using longitude and latitude coordinates.
  

  The distance between WRS center points along a path is 161.1 kilometers. A path distance of 90 kilometers before and after a WRS center point defines the standard scene length of 180 km. This length includes 20 scans of overlap with neighboring scenes. The standard WRS scene overlaps neighboring scenes along a path by 5% at the equator and has a width or cross track distance of 185 kilometers.

  Landsat 7 browse is framed according to WRS scenes. An ordered scene will cover the same geographic extent observed in the browse with the following caveat. Standard WRS scenes have 375 scans. Partial scenes (less than 375 scans) may exist at the beginning or end of a subinterval due to the fact that imaging events do not always start and end on scene boundaries. Browse and scene metadata for these occurrences accurately reflect their partial scene nature and geographic extent although partials are currently not offered due to complexities associated with level 1 processing.



• Subinterval. An interval is a scheduled ETM+ image period along a WRS path, and may be from one to 90 scenes in length. A subinterval is a contiguous segment of raw wideband data received during a Landsat 7 contact period. Subintervals are caused by breaks in the wideband data stream due to communication dropouts and/or the inability of the spacecraft to transmit a complete observation (interval) within a single Landsat 7 contact period. The largest possible subinterval is 35 scenes long. The smallest possible subinterval is a single ETM+ scene.


• Partial Subinterval A partial Landsat 7 subinterval can also be ordered. The partial subinterval is dimensioned according to standard WRS scene width, is at least one WRS scene in length, and can be up to 10 scenes in length if ordered in 0R form or 3 scenes in length in 1G form. A partial subinterval can float or be positioned at any scan line starting point within a subinterval. Partial subintervals are defined by either contiguous WRS locations or a bounding longitude/latitude rectangle. In the latter case, all scan lines touched by the bounding rectangle are included in their entirety.

A complete scene-sized 0R product consists of 19 data sets derived from the wideband telemetry, an IAS-generated calibration parameter file, a product specific metadata file, a geolocation index generated by EOSDIS Core System (ECS), and an HDF directory. Therefore, if you order a complete (i.e. all bands) scene-based 0R product it will have 23 distinct files. A brief description of each follows.

• 1 - 9. Earth Image Data - The unique bands of ETM+ image data comprise nine of the data sets. The data is laid out in a scan line sequential format in descending detector order (i.e. detector 16 followed by detector 15 and so on for the 30 meter bands). Band 6 is captured twice - once in low and the other in high gain mode. Under nominal satellite configuration the low gain form of band 6 will be present in format 1. All image samples or pixels are 8 bits in size.


• 10. Internal calibrator (IC) data - format 1 - IC data for format 1 consists of scan line ordered internal lamp and shutter data for bands 1-5 and blackbody radiance and shutter data for low gain band 6. The data is collected once per scan and structured in a band sequential format in descending detector order (e.g. detector 16 followed by detector 15 and so on for the 30 meter bands).


• 11. Internal calibrator (IC) data - format 2 - IC data for format 2 consists of scan ordered internal lamp and shutter data for bands 7 and 8 and blackbody radiance and shutter data for high gain band 6. The data is collected once per scan and structured in a band sequential format in descending detector order (e.g. detector 16 followed by detector 15 and so on for the 30 meter bands).


• 12. MSCD - format 1. A logical record of MSCD exists for each data scan present in the 0R product ordered. Each logical record consists of 3 MSCD data values - the first half scan error, the second half scan error, and the scan line direction. This information, which actually applies to the previous scan, is used to compute deviations from nominal scan mirror profiles as measured on the ground and reported in the calibration parameter file. Also included in the MSCD file are scan based values such as time code, gain status and processing errors encountered by LPS The MSCD is trimmed to fit the product ordered although one additional record is added to the file during the subsetting process due to the fact that scan error and direction information corresponds to the prior scan.


• 13. MSCD - format 2. A duplicate set of MSCD is generated when format 2 is processed and is kept with the product in the event format 1 MSCD is lost or corrupted.


• 14. PCD - format 1 The PCD for format 1 consists of attitude and ephemeris profiles as well high frequency jitter measurements. PCD for the entire subinterval is included with the 0R product regardless of the size of the data set ordered.


• 15. PCD - format 2 A duplicate set of PCD is generated when format 2 is processed and is kept with the product in the event format 1 is lost or corrupted.


• 16. Scan line offsets - format 1. During LPS processing image data is shifted in an extended buffer to account for predetermined detector and band shifts, scan line length, and possible bumper wear. The scan line offsets represent the actual starting and ending pixel positions for valid (non-zero fill) Earth image data on a data line by data line basis for bands 1 through 6 low gain. The left starting pixel offsets also apply to the IC data.


• 17. Scan line offsets - format 2. During LPS processing image data is shifted in an extended buffer to account for predetermined detector and band shifts, scan line length, and possible bumper wear. The scan line offsets represent the actual starting and ending pixel positions for valid (non-zero fill) earth image data on a data line by data line basis for bands 6 high gain through 8. The left starting pixel offsets also apply to the IC data.


• 18. Metadata - format 1. During LPS format 1 processing metadata is generated that characterizes the subinterval's spatial extent, content, and data quality for bands 1 through 6 low gain. This file, in its entirety and original form, accompanies the 0R product.


• 19. Metadata - format 2. Format 2 metadata is similar but not identical to format 1 metadata. The subinterval-related metadata contents are identical; the scene-related metadata is specific to bands 6 - high gain, 7, and 8. Also, the format 2 metadata does not include cloud cover assessment data or references to browse data products. This file, in its entirety and original form, accompanies the 0R product.


• 20. Metadata - ECS. A third metadata file generated by ECS during order processing. This file contains product specific information such as corner coordinates and number of scans.


• 21. Geolocation Index. The geolocation index is also produced by ECS. This table contains scene corner coordinates and their product-specific scan line numbers for bands at all three resolutions. Its purpose is provide for efficient subsetting of a 0R product.


• 22. Calibration parameters. The IAS regularly updates the CPF to reflect changing radiometric and geometric parameters required for level 1 processing. These are stamped with applicability dates and sent to the LP-DAAC for storage and bundling with outbound 0R products.


• 23. HDF Directory. A file containing all the pointers, file size information, and data objects required to open and process the 0R product using the HDF library and interface routines.

A user may order a subset of the available bands which will affect the actual file count in a 0R product. In all cases, however, every product includes two PCD files, two MSCD files, three metadata files, the CPF, and the HDF directory. Only the internal calibrator, scan line offset, and earth image file counts are affected by a product possessing less than the full complement of bands.

The product delivered to Landsat 7 data users is packaged in HDF - an open standard selected by NASA for Earth Observing System (EOS) data products. HDF is a self-describing format that allows an application to interpret the structure and contents of a file without outside information. HDF allows Landsat 0R products to be shared across different computer platforms without modification and is supported by a public domain software library consisting of access tools and various utilities.

Product users are directed to the Landsat 7 0R Distribution Product Data Format Control Book, Volume 5 (PDF) for details regarding the HDF design used for the 0R product. Included are references to NCSA-authored documentation. New users should begin with Getting Started with HDF while the HDF User's Guide and HDF Reference Manual are excellent resources for the HDF programmer.