33 lines
5.0 KiB
Markdown
33 lines
5.0 KiB
Markdown
---
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title: "Advanced Very-High-Resolution Radiometer"
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chunk: 2/4
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source: "https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer"
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category: "reference"
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tags: "science, encyclopedia"
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date_saved: "2026-05-05T09:44:05.717614+00:00"
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instance: "kb-cron"
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---
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==== Loeb ====
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In another similar method using surface targets, Loeb [1997] uses spatiotemporal uniform ice surfaces in Greenland and Antarctica to produce second-order polynomial reflectance calibration curves as a function of solar zenith angle; calibrated NOAA-9 near-nadir reflectances are used to generate the curves that can then derive the calibrations for other AHVRRs in orbit (e.g. NOAA-11, -12, and -14).
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It was found that the ratio of calibration coefficients derived by Loeb [1997] and Rao and Chen [1995] are independent of solar zenith angle, thus implying that the NOAA-9-derived calibration curves provide an accurate relation between the solar zenith angle and observed reflectance over Greenland and Antarctica.
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==== Iwabuchi ====
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Iwabuchi [2003] employed a method to calibrate NOAA-11 and -14 that uses clear-sky ocean and stratus cloud reflectance observations in a region of the NW Pacific Ocean and radiative transfer calculations of a theoretical molecular atmosphere to calibrate AVHRR Ch. 1. Using a month of clear-sky observations over the ocean, an initial minimum guess to the calibration slope is made. An iterative method is then used to achieve the optimal slope values for Ch. 1 with slope corrections adjusting for uncertainties in ocean reflectance, water vapor, ozone, and noise. Ch. 2 is then subsequently calibrated under the condition that the stratus cloud optical thickness in both channels must be the same (spectrally uniform in the visible) if their calibrations are correct [Iwabuchi, 2003].
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==== Vermote and Saleous ====
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A more contemporary calibration method for AVHRR uses the on-orbit calibration capabilities of the VIS/IR channels of MODIS. Vermote and Saleous [2006] present a methodology that uses MODIS to characterize the BRDF of an invariant desert site. Due to differences in the spectral bands used for the instruments' channels, spectral translation equations were derived to accurately transfer the calibration accounting for these differences. Finally, the ratio of AVHRR observed to that modeled from the MODIS observation is used to determine the sensor degradation and adjust the calibration accordingly.
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==== Others ====
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Methods for extending the calibration and record continuity also make use of similar calibration activities [Heidinger et al., 2010].
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=== Long-term calibration and record continuity ===
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In the discussion thus far, methods have been posed that can calibrate individual or are limited to a few AVHRR sensors. However, one major challenge from a climate point of view is the need for record continuity spanning 30+ years of three generations of AVHRR instruments as well as more contemporary sensors such as MODIS and VIIRS. Several artifacts may exist in the nominal AVHRR calibration, and even in updated calibrations, that cause a discontinuity in the long-term radiance record constructed from multiple satellites [Cao et al., 2008].
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==== International Satellite Cloud Climatology Project (ISCCP) method ====
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Brest and Rossow [1992], and the updated methodology [Brest et al., 1997], put forth a robust method for calibration monitoring of individual sensors and normalization of all sensors to a common standard. The International Satellite Cloud Climatology Project (ISCCP) method begins with the detection of clouds and corrections for ozone, Rayleigh scatter, and seasonal variations in irradiance to produce surface reflectances. Monthly histograms of surface reflectance are then produced for various surface types, and various histogram limits are then applied as a filter to the original sensor observations and ultimately aggregated to produce a global, cloud free surface reflectance.
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After filtering, the global maps are segregated into monthly mean SURFACE, two bi-weekly SURFACE, and a mean TOTAL reflectance maps. The monthly mean SURFACE reflectance maps are used to detect long-term trends in calibration. The bi-weekly SURFACE maps are compared to each other and are used to detect short-term changes in calibration.
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Finally, the TOTAL maps are used to detect and assess bias in the processing methodology. The target histograms are also examined, as changes in mode reflectances and in population are likely the result of changes in calibration.
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==== Long-term record continuity ====
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Long-term record continuity is achieved by the normalization between two sensors. First, observations from the operational time period overlap of two sensors are processed. Next, the two global SURFACE maps are compared via a scatter plot. Additionally, observations are corrected for changes in solar zenith angle caused by orbital drift. Ultimately, a line is fit to determine the overall long-term drift in calibration, and, after a sensor is corrected for drift, normalization is performed on observations that occur during the same operational period [Brest et al., 1997]. |