Advanced very-high-resolution radiometer

The Advanced Very-High-Resolution Radiometer (AVHRR) 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 (NOAA) family of polar orbiting platforms (POES) and European MetOp satellites.

The AVHRR has been succeeded by the Visible Infrared Imaging Radiometer Suite, carried on the Joint Polar Satellite System spacecraft.

These sensors have proven useful for a number of other applications, however, including the surveillance of land surfaces, ocean state, aerosols, etc.

However, the highly accurate prelaunch calibration will degrade during launch and transit to orbit as well as during the operational life of the instrument [Molling et al., 2010].

One major design constraint of AVHRR instruments is that they lack the capability to perform accurate, onboard calibrations once on orbit [NOAA KLM].

Rao and Chen [1995] 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.

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 [Rao and Chen, 1995], and any sensor degradation is corrected for by adjusting the slope (as a function of days after launch) between the albedo and digital count signal recorded [Rao and Chen, 1999].

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.

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 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].

The first step involves using a radiative transfer model that will convert observed MODIS scenes into those that a perfectly calibrated AVHRR would see.

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.

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.

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.

Operational experience with the MODIS[5] sensor onboard NASA's Terra and Aqua led to the development of AVHRR's follow-on, VIIRS.