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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Advanced Very-High-Resolution Radiometer | 3/4 | https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer | reference | science, encyclopedia | 2026-05-05T09:44:05.717614+00:00 | kb-cron |
==== Calibration using the moderate-resolution imaging spectroradiometer ==== Another recent method for the absolute calibration of the AHVRR record makes use of the contemporary MODIS sensor onboard NASA's TERRA and AQUA satellites. The MODIS instrument has high calibration accuracy and can track its own radiometric changes due to the inclusion of an onboard calibration system for the VIS/NIR spectral region [MCST]. The following method utilizes the high accuracy of MODIS to absolutely calibrate AVHRRs via simultaneous nadir overpasses (SNOs) of both MODIS/AVHRR and AVHRR/AVHRR satellite pairs as well as MODIS-characterized surface reflectances for a Libyan Desert target and Dome-C in Antarctica [Heidinger et al., 2010]. Ultimately, each individual calibration event available (MODIS/AVHRR SNO, Dome C, Libyan Desert, or AVHRR/AVHRR SNO) is used to provide a calibration slope time series for a given AVHRR sensor. Heidinger et al. [2010] use a second-order polynomial from a least-squares fit to determine the time series. The first step involves using a radiative transfer model that will convert observed MODIS scenes into those that a perfectly calibrated AVHRR would see. For MODIS/AVHRR SNO occurrences, it was determined that the ratio of AVHRR to MODIS radiances in both Ch1 and Ch2 are modeled well by a second-order polynomial of the radio of MODIS reflectances in channels 17 and 18. Channels 17 and 18 are located in a spectral region (0.94mm) sensitive to atmospheric water vapor, a quantity that affects the accurate calibration of AVHRR Ch. 2. Using the Ch17 to Ch 18 ratio, an accurate guess at the total precipitable water (TPW) is obtained to further increase the accuracy of MODIS to AVHRR SNO calibrations. The Libyan Desert and Dome-C calibration sites are used when MODIS/AVHRR SNOs do not occur. Here, the AVHRR to MODIS ratio of reflectances is modeled as a third-order polynomial using the natural logarithm of TWP from the NCEP reanalysis. Using these two methods, monthly calibration slopes are generated with a linear fit forced through the origin of the adjusted MODIS reflectances versus AVHRR counts. To extend the MODIS reference back for AVHRRs prior to the MODIS era (pre-2000), Heidinger et al. [2010] use the stable Earth targets of Dome C in Antarctica and the Libyan Desert. MODIS mean nadir reflectances over the target are determined and are plotted versus the solar zenith angle. The counts for AVHRR observations at a given solar zenith angle and corresponding MODIS reflectance, corrected for TWP, are then used to determine what AVHRR value would be provided it had the MODIS calibration. The calibration slope is now calculated.
==== Calibration using direct AVHRR/AVHRR SNOs ==== One final method used by Heidinger et al. [2010] for extending the MODIS calibration back to AVHRRs that operated outside of the MODIS era is through direct AVHRR/AVHRR SNOs. Here, the counts from AVHRRs are plotted and a regression forced through the origin calculated. This regression is used to transfer the accurate calibration of one AVHRRs reflectances to the counts of an un-calibrated AVHRR and produce appropriate calibration slopes. These AVHRR/AVHRR SNOs do not provide an absolute calibration point themselves; rather they act as anchors for the relative calibration between AVHRRs that can be used to transfer the ultimate MODIS calibration.
== Next-generation system == Operational experience with the MODIS sensor onboard NASA's Terra and Aqua led to the development of AVHRR's follow-on, VIIRS. VIIRS is currently operating on board the Suomi NPP and NOAA-20 satellites. Whereas EUMETSAT MetOp satellites with AVHRR instruments will be succeeded by MetOp-SG satellites with a European MetImage instrument.
== Launch and service dates ==
== See also == Ocean temperature
== References ==