Appendix D - L0R and L1R Products (Historical)
The 2008 single-product data policy changes at EROS made the L0R product option obsolete. The following paragraphs are only relevant from a historical perspective.
D.1 Level 0R Product
Unlike earlier Landsat satellite systems, the Landsat 7 system was not originally designed to produce high level (i.e. Level 1) products for users. The baseline 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 L0R data - an essentially raw data form that is marginally useful prior to radiometric and geometric correction. This is readily apparent when viewing a simulated L0R image. A Landsat 7 L0R product (see Figure D-1), however, does contain all the ancillary data required to perform these corrections including a Calibration Parameter File (CPF) generated by the Landsat 7 Image Analysis System (IAS).
Figure D-1. Simulated ETM+ Level 0R Image
LPS spatially reformats Earth imagery and calibration data into L0R 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 L0R corrections are reversible; the pixel shift parameters used are documented in the IAS CPF. The LPS L0R output is HDF-EOS formatted and archived. Details of the archival format can be found in the Landsat 7 System Wideband Data Format Control Book (DFCB).
D.1.1 Product Size
Three options, depicted in Figure D-2, existed when defining the size or spatial extent of a Landsat L0R product ordered prior to 2008 when Landsat data became freely available.
Figure D-2. Level 0R Product Alternatives
Standard Worldwide Reference System-2 (WRS-2) Scene
The standard WRS-2 scene as defined for Landsat 4 and Landsat 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 straightforward referencing without using longitude and latitude coordinates. The distance between WRS center points along a path is 161.1 kilometers (km). A path distance of 90 km before and after a 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 percent at the equator and has a width or cross track distance of 185 km. Landsat 7 browse is framed according to WRS-2 scenes. An scene covers 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 because the 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.
An interval is a scheduled ETM+ image period along a WRS path, and may be from 1 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.
A partial Landsat 7 subinterval could 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 L0R 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.
D.1.2 Product Components
A complete scene-sized L0R product consisted of 19 data sets derived from the wideband telemetry, an IAS-generated CPF, a product specific metadata file, a geolocation index generated by EOSDIS Core System (ECS), and an HDF directory. Therefore, if a complete (i.e. all bands) scene-based L0R product was ordered, it had 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 gain mode and the other in high gain mode. Under nominal satellite configuration the low gain form of Band 6 is 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 are 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 are 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 L0R product ordered. Each logical record consists of three MSCD data values - the first half scan error, the second half scan error, and the scan line direction. This information, which applies to the previous scan, is used to compute deviations from nominal scan mirror profiles as measured on the ground and reported in the CPF. 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 because 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 as 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 kept with the product in the event Format 1 is lost or corrupted.
16. Scan line offsets - Format 1 - During LPS processing image data are 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 Band 1 through Band 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 are 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 Band 6 high gain through Band 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 Band 1 through Band 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 Band 6 - high gain, Band 7, and Band 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 to provide 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 EROS 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 L0R product using the HDF library and interface routines.
A user may order a subset of the available bands that affects the actual file count in a L0R 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 IC, scan line offset, and Earth image file counts are affected by a product possessing less than the full complement of bands.
D.1.3 Product Format
The product delivered to Landsat 7 data users is packaged in HDF - an open standard file format 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 L0R 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 Zero-R Distribution Product Data Format Control Book for details regarding the HDF design used for the 0R product. Included are references to National Center for Supercomputing Applications (NCSA)-authored documentation.
D.2 Level 1R Product
The 2008 single-product data policy changes at EROS made the L1R product option obsolete. The following paragraphs are only relevant from a historical perspective.
The L1R product is a radiometrically corrected L0R product. Radiometric correction is performed using either the CRaM gains in the CPF or gains computed on the fly from the IC data. The choice is available to a user when the product is ordered. The biases used are always calculated from the IC data. Image artifacts such as banding, striping, and scan- correlated shift are removed prior to radiometric correction. Radiometric corrections are not reversible. The L1R product geometry is identical to the input L0R data.
During L1R product rendering image pixels are converted to units of absolute radiance using 32-bit floating-point calculations. Pixel values are then multiplied by 100 and converted to 16-bit integers prior to media output. Two digits of decimal precision are thus preserved. One merely divides each pixel value by 100 to convert the L1R image data back to radiance units. The 16-bit 1R product is twice the data volume of an alike 0R product. For Band 6, a bias was found in the pre-launch calibration by a team of independent investigators post launch. This was corrected for in the LPGS processing system beginning December 20, 2000. For data processed before this, the 16-bit image radiances are 0.31 (W/(m² * sr * µm)) too high.
In the fall of 2000, two Landsat 7 science team investigator groups discovered a Band 6 calibration bias in Level 1 ETM+ data products emanating from the LPGS at EROS. This bias apparently results from limitations in the pre-launch calibration of the ETM+. The magnitude of the correction is estimated to be 0.31 (W/(m² * sr * µm)) or about 3-4 percent radiance error at typical surface temperatures. This apparent systematic error in Band 6 radiance calibration translates into estimated temperatures derived from Landsat 7 ETM+ being about 3°C too high for typical Earth surface temperatures.
To remedy the situation, several changes were made to the product generation software and CPF. The Band 6 biases and instrument component view coefficients were changed in the October 1, 2000 release of the CPF. The calibration equation used by LPGS software was operationally updated on December 20, 2000. Users need to be aware of the impact these changes have on Level 1 products.
- LPGS Level 1 products - pre 12/20/00. Users should subtract the bias value (0.31 (W/(m² * sr * µm))) from the radiances obtained from Level 1R and Level 1G data products generated since launch by LPGS for both high and low gain Band 6 data. The changes made to the October 1, 2000 CPF effectively remove the temperature bias but only if the product generation software uses the changed CPF values. The LPGS software in place prior to December 20th does not.
- LPGS Level 1 products - post 12/20/00. Users can safely use temperatures derived from Band 6 radiance values. No bias correction is necessary.
- Other Systems - pre 10/01/00. If the CPF gains and biases are used then the Band 6 radiance values should be adjusted for the bias described in paragraph three above.
- Other Systems - post 10/01/00. If the CPF gains and biases are used then no adjustments are necessary after this date. Other systems that employ a Band 6-calibration equation are outside NASA/USGS configuration and control. For accurate processing, please consult with your product provider.
D.2.1 Product Size
Two options existed for users when defining the size or spatial extent of a Landsat L1R product ordered from EROS.
- Standard WRS-2 Scene. The standard WRS-2 scene, as defined above for the L0R product, could be ordered in L1R form. Partial scenes that may exist at the beginning and end of subintervals could also be ordered.
- Partial Subinterval. A partial subinterval could also be ordered in L1R form. Unlike the L0R product the L1R was limited to a maximum of three WRS scenes in size. The variably sized L1R product could float or be positioned at any scan line starting point within a subinterval. Alternatively, the product could be defined by up to three contiguous WRS-2 locations.
D.2.2 Product Components
A complete scene-sized L1R product consists of 17 data sets derived from the wideband telemetry, an IAS-generated CPF, a product specific metadata file, a geolocation index generated by ECS, and an HDF directory. Therefore, if you order a complete (i.e., all bands) scene-based L0R product it has 21 distinct files. There are two fewer data files than an alike L0R product because the multiple PCD and MSCD files are merged into single consensus files. Please reference the L0R file product for individual file descriptions.
A user could order a subset of the available bands, which affected the actual file, count in a L1R product. In all cases, however, every product included one consensus PCD file, one consensus MSCD files, three metadata files, the CPF, and the HDF directory. Only the IC, scan line offset, and Earth image file counts were affected by a product possessing less than the full complement of bands
D.2.3 Product Format
The L1R product was delivered to users only in the HDF format. The HDF 0R and 1R formats are nearly identical. Exceptions include the united PCD and MSCD files and an enhanced product specific metadata file that reflects L1R correction characteristics. Please refer to the Landsat 7 Zero-R Distribution Product DFCB for details regarding HDF specifics.