Advanced Very-High-Resolution Radiometer
The Advanced Very-High-Resolution Radiometer instrument is a space-borne sensor that measures the reflectance of the Earth in five spectral bands that are relatively wide by today's standards. AVHRR instruments are or have been carried by the National Oceanic and Atmospheric Administration family of polar orbiting platforms and European MetOp satellites. The instrument scans several channels; two are centered on the red and near-infrared regions, a third one is located around 3.5 micrometres, and another two the thermal radiation emitted by the planet, around 11 and 12 micrometres.
The first AVHRR instrument was a four-channel radiometer. The final version, AVHRR/3, first carried on NOAA-15 launched in May 1998, acquires data in six channels. The AVHRR has been succeeded by the Visible Infrared Imaging Radiometer Suite, carried on the Joint Polar Satellite System spacecraft.
Operation
Prior to 2025, NOAA had at least two polar-orbiting meteorological satellites in orbit at all times, with one satellite crossing the equator in the early morning and early evening and the other crossing the equator in the afternoon and late evening. The primary sensor on board both satellites was the AVHRR instrument. Morning-satellite data were most commonly used for land studies, while data from both satellites were used for atmosphere and ocean studies. Together they provided twice-daily global coverage, and ensured that data for any region of the earth are no more than six hours old. The swath width, the width of the area on the Earth's surface that the satellite can "see", is approximately 2,500 kilometers. The satellites orbit between 833 or 870 kilometers above the surface of the Earth.The highest ground resolution that can be obtained from the current AVHRR instruments is per pixel at the nadir.
Data from AVHRR has been collected continuously since 1981.
The primary purpose of these instruments is to monitor clouds and to measure the thermal emission of the Earth. These sensors have proven useful for a number of other applications, however, including the surveillance of land surfaces, ocean state, aerosols, etc. AVHRR data are particularly relevant to study climate change and environmental degradation because of the comparatively long records of data already accumulated. The main difficulty associated with these investigations is to properly deal with the many limitations of these instruments, especially in the early period. Whereas, the follow-on VIIRS instruments on JPSS and METimage in MetOp-SG satellites have on-board calibration mechanisms.
The AVHRR instrument also flies on the MetOp series of satellites and as of 2025 these are the only remaining AVHRR instruments. The Metop-B&C MetOp satellites are part of the EUMETSAT Polar System run by EUMETSAT, which will be succeeded by MetOp-SG.
Calibration and validation
applications of the AVHRR sensor are based on validation techniques of co-located ground observations and satellite observations. Alternatively, radiative transfer calculations are performed. There are specialized codes which allow simulation of the AVHRR observable brightness temperatures and radiances in near infrared and infrared channels.Pre-launch calibration of visible channels (Ch. 1 and 2)
Prior to launch, the visible channels of AVHRR sensors are calibrated by the instrument manufacturer, ITT, Aerospace/Communications Division, and are traceable to NIST standards. The calibration relationship between electronic digital count response of the sensor and the albedo of the calibration target are linearly regressed:where S and I are the slope and intercept of the calibration regression . However, the highly accurate prelaunch calibration will degrade during launch and transit to orbit as well as during the operational life of the instrument . Halthore et al. note that sensor degradation is mainly caused by thermal cycling, outgassing in the filters, damage from higher energy radiation, and condensation of outgassed gases onto sensitive surfaces.
One major design constraint of AVHRR instruments is that they lack the capability to perform accurate, onboard calibrations once on orbit . Thus, post-launch on-orbit calibration activities must be performed to update and ensure the accuracy of retrieved radiances and the subsequent products derived from these values . Numerous studies have been performed to update the calibration coefficients and provide more accurate retrievals versus using the pre-launch calibration.
On-orbit individual/few sensor absolute calibration
Rao and Chen
Rao and Chen use the Libyan Desert as a radiometrically stable calibration target to derive relative annual degradation rates for Channels 1 and 2 for AVHRR sensors on board the NOAA -7, -9, and -11 satellites. Additionally, with an aircraft field campaign over the White Sands desert site in New Mexico, USA , an absolute calibration for NOAA-9 was transferred from a well calibrated spectrometer on board a U-2 aircraft flying at an altitude of ~18 km in a congruent path with the NOAA-9 satellite above. After being corrected for the relative degradation, the absolute calibration of NOAA-9 is then passed onto NOAA −7 and −11 via a linear relationship using Libyan Desert observations that are restricted to similar viewing geometries as well as dates in the same calendar month , and any sensor degradation is corrected for by adjusting the slope between the albedo and digital count signal recorded .Loeb
In another similar method using surface targets, Loeb 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.It was found that the ratio of calibration coefficients derived by Loeb and Rao and Chen 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.
Iwabuchi
Iwabuchi 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 if their calibrations are correct .Vermote and Saleous
A more contemporary calibration method for AVHRR uses the on-orbit calibration capabilities of the VIS/IR channels of MODIS. Vermote and Saleous 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.Others
Methods for extending the calibration and record continuity also make use of similar calibration activities .Long-term calibration and record continuity
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 .International Satellite Cloud Climatology Project (ISCCP) method
Brest and Rossow , and the updated methodology , 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 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.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.
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.