AVHRR/3 Operational Calibration Jörg Ackermann, Remote Sensing and Products Division 1 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
AVHRR/3 Operational Calibration Basic Considerations Operational Calibration of Visible (VIS) Channels Operational Calibration of Infrared (IR) Channels Limitations Summary 2 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Basic Considerations: History and Channel Properties First AVHRR in-orbit was a 4-channel radiometer, carried on TIROS-N (launched 13 October 1978). The first 5-channel instrument (AVHRR/2) was carried on NOAA-7 (launched 23 June 1981). The actual instrument version is AVHRR/3, with 6 channels, first carried on NOAA- 15 launched on 13 May 1998. Out of 16 launched AVHRR instruments, five are presently operating (NOAA-15, 18, 19; Metop-A/B) Last AVHRR instrument will be launched onboard Metop-C in October 2018 VIS IR 3 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Basic Considerations: Facts AVHRR/3 has a 10-bit radiometric resolution only AVHRR/3 has six channels only (five are transmitted to ground) AVHRR/3 has a resolution of >1km at nadir and about 2x6km at scan edge AVHRR/3 has no internal VIS calibration target but AVHRR/3 is embarked on both, US and European operational weather satellites, thus contributing to the Initial Joint Polar System, a long-term agreement between NOAA and EUMETSAT AVHRR/3 and its predecessors are operated in space since more than 30 years AVHRR/3 products are processed in Near-Real Time with an availability of more than 98%. AVHRR/3 is used as reference for multi sensor projects (eg. ISCCP) AVHRR/3 has many NWP and climate monitoring applications (cloud detection, vegetation indices, snow coverage, sea-ice monitoring, surface temperatures, forest fires, atmospheric motion vectors) 4 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Basic Considerations: Definitions Calibration: The process of quantitatively defining the instrument response to known, controlled signal inputs => Instrument response: Measured Earth view counts => Known, controlled signal inputs: 1. Deep space radiance 2. Deep space reflectivity 3. On-board calibration target radiance 4. Vicarious calibration target radiance 5. Vicarious calibration target reflectivity 5 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Product Generation: Input Data: Raw AVHRR/3 Chs. #1, #2, and #3A Earth View Counts (Actual Measurements); Calibration coefficients derived from on-ground target measurements Computation of the Target Reflectance Factors using dual-gains Conversion of the Target Reflectance Factors into In- Band Radiances Writing out In-Band radiances into the EPS AVHRR Level 1B Product 6 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Count-to-Target Reflectance Conversion (Metop-A AVHHR/3) Two slopes (gain) b and two intercepts a for each channel Due to changes in detector sensitivity, a and b (and the intersection point) are regularly (~ 1-2 months) updated IJPS requires consistency between NOAA and EUMETSAT derived products Updates of VIS calibration coefficients is performed by NOAA and forwarded to EUMETSAT 7 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Courtesy: Tiejun Chang and Fred Wu, NOAA STAR 8 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels History of Calibration Information (Metop-A) Pre-Launch October 2007 January 2009 Rel. Change during Operations Ch 1 (0.63µm) a 1 a 2 b 1 b 2-2.172-54.78 0.05371 0.1579-2.117-53.40 0.05236 0.1547-2.147-54.16 0.0531 0.1569-1.4 % -1.4 % +1.4 % +1.4 % Ch 2 (0.84 µm) a 1 a 2 b 1 b 2-2.167-55.91 0.05452 0.1619-2.500-64.49 0.06288 0.1868-2.516-64.92 0.06329 0.1880-0.64 % -0.66% +0.65% +0.64 % 9 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Long-term drift of the VIS channels (Metop-A) Loss of sensitivity is channel dependent: #1: ~0.6 % / year #2: ~1.12 % / year #3A: ~0.75 % / year 10 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Target Reflectance A to In-Band Radiance R Conversion A F 100 2 1 R Wm sr with F being the extraterrestrial solar irradiance at normal incidence at the top of the atmosphere, i.e. F 2 F 1 0, d 0, : window response function F : extraterrestrial solar irradiance at wavelength λ ( 2 1 Wm m ) Metop-A Values for F : 139.873 (Ch. 1) 232.920 (Ch. 2) 14.016 (Ch. 3A) 2 Wm Values are appended to the header of the AVHRR-L1B product 11 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Validation of AVHRR/3 Ch.1 using co-located GOME-2 measurements: principle Generation of a pseudo-avhrr measurement using colocated GOME-2 measurements and the actual window response function Lang, 2013 12 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of VIS Channels Validation of AVHRR/3 Ch.1 using co-located GOME-2 measurements: results Homogeneous Scenes, 3. November 2012: Metop-B Metop-A Lang, 2013 13 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of IR Channels Product Generation: Input Data: Raw AVHRR/3 Chs. 3B, 4, and 5 Earth View counts, Space View counts, and Warm Target View counts (Actual Measurements); Thermistor counts of the internal black body (ICT) Computation of a black body temperature from averaged (55 scan lines) thermistor count values Per scan line, computation of the coefficients for the Earth view count-to-radiance conversion (second order polynomial for Chs. 4 and 5, linear regression for Ch. 3B) Conversion of actual Earth view counts into radiances Writing out radiances into the EPS AVHRR Level 1B Product 14 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Calibration of IR Channels 15 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Validation of IR Channels Validation of AVHRR/3 Ch.4 and 5 using co-located IASI measurements: results Coppens, 2014 16 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Validation of IR Channels Channel 4 Channel 4 Validation of AVHRR/3 Ch.4 and 5 using co-located IASI measurements: results Channel 5 Channel 5 Metop-A: Temporal evolution of radiance bias No significant change of channel performance Coppens, 2014 17 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Operational Validation of IR Channels Validation of AVHRR/3 Ch.4 and 5 using co-located IASI measurements: overall results R and ΔR T and ΔT 18 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Detector Sensitivity AVHRR/3 Ch. 3B, Ch.4 and 5: 1 measurement count, expressed in brightness temperature changes Breakdown of channel 3B sensitivity for low scene temperatures 19 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Detector Sensitivity AVHRR/3 Ch. 3A saturation for sunglint conditions: Metop-A AVHRR/3, 7. March 2017, 4:22 to 4:25 UTC Level 1B Level 0 Measurement counts of channel 3A reach value of 1023 20 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Moon Intrusion Space Views: Metop-B AVHRR/3, 17. March 2014, 21:01:12 to 21:03:36UTC Channel 4: 21 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Moon Intrusion Warm Target Views: Metop-B AVHRR/3, 17. March 2014, 21:01:12 to 21:03:36UTC Channel 4: 22 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Moon Intrusion Earth views: Metop-B AVHRR/3, 17. March 2014, 21:01:12 to 21:03:36UTC 23 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Limitations: Spatial Resolution http://maps-africa.blogspot.de/2012/05/map-of-africaphysical-picture.html http://www.lib.utexas.edu/maps/ Information content is drastically reduced when approaching towards the scan edges Remapped data should be interpreted with caution Metop AVHRR/3, 21. June 2013 Center Pixel: 4 Center Pixel: 904 Center Pixel: 1974 24 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)
Summary AVHRR data have many surface and atmosphere applications and allow to generate time series of geophysical variables with lengths of 30 years covering the full globe. Operational AVHRR/3 sensor calibration is different for the three VIS and the three IR channels. VIS channels rely on vicarious calibration (Libyan Desert) updated in 1 to 2 months intervals by NOAA. IR channels use measurements of space and of an internal warm target sampled during each scan. Metop-A AVHRR/3 VIS channels exhibit a channel-dependent loss of sensitivity of 0.6%/year to 1.1%/year; AVHRR/3 IR channels 4 and 5 show no long-term change of sensitivity for both Metops. For channels 4 and 5, radiance biases are quantified using IASI measurements as reference. The accuracy of retrieved channel 3B brightness temperatures decreases significantly for cold targets Under certain circumstances (optically thick cloud deck observed under sunglint conditions), saturation of channel 3A can occur. When the moon is in the field of view of the space view, this also impacts the warm target and Earth view measurements Due to the extension of the field of view from nadir (1x1km) towards the scan edge (2x6km) and the pixel overlapping, the information content varies strongly with the scanning angle. 25 Workshop`Radiometric Calibration for European Missions, 30/31 Aug. 2017`,Frascati (EUM/RSP/VWG/17/936014)