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AVHRR Vicarious Calibration

LTDR Product Documents

LTDR Version History

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Quality Assessment

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The LTDR V5 forward production will be switched from the AVHRR instrument onboard N19 to the same from MetOp-B, in a month, and this will be followed up by a re-processing of all the LTDR records from the beginning of the MetOp-B mission, in V5. (click here for the details)

V5 and V4 LTDR data now available. V3 LTDR has been decommissioned

Version History

LTDR Version 5 Transition to MetOp-B (August 2020)

The LTDR Version 5 forward processing will start using the AVHRR instrument, on-board the MetOp-B platform, soon. The forward processing will be switched first, followed by a re-processing of a portion of the Version 5 data record, since the beginning of mission for MetOp-B. The switch to MetOp-B in the forward processing is expected to happen soon and all MetOp-B specific data records from Version 5 are expected to be released to users in about a month's time from now, by 09/2020.

The forward processing, using the AVHRR from N19, will also continue in parallel, for some time but will eventually fully switch over to MetOp-B.

Please note that the users may notice some discrepancy between the N19 and comparable MetOp-B data records, after the switch to MetOp-B. This is expected to be seen, as the later years of N19 records were not that well calibrated and MetOp-B should be much better calibrated in all these cases.

Version 5 product (August 2017)

V5 reprocessing of LTDR has been completed for the period 1981 through Current, and forward processing from N19 is at the leading edge. Forward processing of V4 LTDR will continue along with V5 until the end of this year where onwards only V5 will be continued. V4 LTDR data for the period 1981 through 2017 could be available to the user for some more time until it is decommissioned later in 2018. Improvements in V5 LTDR include

  • Improved flagging of bad scans in the QA
  • Improvements of BRDF correction
      1) subtracting NDVI value from NDVI min
      2) doing BRDF correction of RHO3 only if RHO1 was BRDF-corrected
  • Improvements of calibration of KLM data
      1) several changes to calibration of KLM data
      2) fix of problem with calibration of the first orbit (it wasn't being calibrated correctly)
  • Improvement to compositing (minimum view angle compositing) Uses MODIS-derived ancillary data (water vapor and ozone) for data days from 2002 to 2017-179. For days 2017-180 onward, being NRT the processing uses NCEP ancillary data.
  • Improvements to water vapor, ozone and other-gas transmission correction coefficients Uses 'NOAAXX.resets' files for clock correction

Version 4 product (June 2014)

1. Includes records from processing of data from NOAA-16, NOAA-18 and NOAA-19 extending the LTDR records from AVHRR to current data day.

2. Improved geolocation accuracy from use of OLE used in data processing. V3 data used TLE.

3. Center of the grid is used as the reference. V3 used the top left corner.

4. Data value at a grid cell is computed as average of available good observations. V3 used one output sample.

5. TOA product AVH02C1 is now available.

6. User can request for spatial and temporal subsets of products.

Known Issues

  • The dataset relative azimuth contains values in the range (-360, 360) instead of being in the range (-180, 180). The routine to restrict the range to (-180, 180) was accidentally missed in the current reprocessing and will be corrected in the next reprocessing. Until then users are requested to derive the relative azimuth (GOOD_REL_AZ) from the current file value (REL_AZ) using the following pseudocode.

      SIN_REL_AZ = sin(REL_AZ)
      COS_REL_AZ = cos(REL_AZ)

  • LTDR from N16 are not available for period beyond 2007001. Data records were negatively impacted by significant data gap and noise in the acquired L1 data from scan motor problems.
  • Processing and distribution of LTDR from N18 was discontinued due to lack of global coverage in the data records resulting from constant drift in the satellite orbit.
  • Stripes of fill or invalid values from a software bug (See Details). These retrievals are flagged as invalid.
  • Ch3 reflectance is still experimental and not validated.
  • Data for some of the days in the series may be missing for one or more following reasons:
    • Input GAC data not available.
    • The data day has been found to contain significant geolocation residual erroe.
    • Input data noisy.

The version 4 products can be ordered from the LTDR Data Products page.

Version 3 MODIS Continuation Records (March 2013)

The Continuation data records of the LTDR series for years 2000 and beyond are created by processing the daily data acquired by MODIS on-board NASA's Terra and Aqua satellite. This record also includes the daily global Surface Reflectance and NDVI generated in a geographic projection at a resolution of 0.05 degree similar to the records generated from the processing of the AVHRR data for the period 1981 - 1999.

The MODIS instrument has 36 very narrow channels with spatial resolution varying from 250m at nadir for the 0.67 um and 0.87 um bands to 500m in the visible.middle infrared land bands, and 1 km for the longwave infrared bands. MODIS Adaptive Processing System (MODAPS) generates the daily surface reflectance Climate Modeling Grid (CMG) product by processing the MODIS L1B calibrated reflectance using correction approaches similar to those used in generation of the LTDR series from processing of the AVHRR data. In addition, the MODIS algorithm retrieves and corrects for the atmospheric aerosol.

The LTDR continuation record is created by applying BRDF correction to the spectral subset of bands in the MODIS Surface Reflectance CMG product that are heritage of the bands in the LTDR record created from the AVHRR. The LTDR continuation records contain land surface reflectance from MODIS bands 1 (0.645 um), 2 (0.858 um), and 7 (2.13 um), brightness temperature from bands 31 (11.03 um) and 32 (12.02 um), NDVI and quality flag. Separate records are produced from Terra MODIS for the period 2000-2012, and Aqua MODIS for the period 2002-2012.

The LTDR continuation records provided here as part of the Version 3 LTDR series are labeled as of provisional quality

  • Product quality may not be optimal
  • Incremental product improvements are still occurring
  • General research community is encouraged to participate in the QA and validation of the product, but need to be aware that product validation and QA are ongoing
  • Users are urged to contact science team representatives prior to use of the data in publications may be replaced in the archive when the validated product becomes available

The MODIS Continuation Records can be ordered from the LTDR Data Products page.

Version 3 product (April 2010)

1. Improved calibration: Revised calibration coefficient for NOAA-7, NOAA-9, NOAA-11 & NOAA-14.

2. Improved geolocation accuracy. Uses new ephemeris file.

3. New & improved cloud algorithm. See the AGU-Fall 2009 for details. Note that there is no partly cloud flag in V3.

4. Corrected QA flags. See the V3 file format.

5. BRDF correction applied.

6. Ch3 reflectance is produced.

Known Issues

  • Detection of cloud shadow & dense vegetation may not be reliable.
  • Ch3 reflectance is still experimental and not validated.
  • Data for some of the days in the series may be missing for one or more following reasons:
    • Input GAC data not available.
    • The data day has been found to contain significant geolocation residual erroe.
    • Input data noisy.

The version 3 products can be ordered from the LTDR Data Products page.

Version 2 Products (October 2007)

Improvements and fixes include:

1. Full implementation of water vapor correction for NOAA-7, NOAA-9, and NOAA-14 products. In version 1, only the NOAA-11 data were correctly adjusted for water vapor.

2. Correct implementation of the CLAVR screening algorithm. Critical bugs were fixed.

3. Correct implementation of Quality Assessment (QA) flags.

4. Improved geolocation accuracy.

5. Revised calibration coefficients for NOAA-14.

The version 2 products have been decommissioned.


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