The solar reflectance bands (SRB) of the Advanced Very High Resolution Radiometers (AVHRR) flown onboard NOAA satellites are often referred to as non-calibrated in-flight. In contrast, the Earth emission bands (EEB) are calibrated using two reference points, deep space and the internal calibration target. In the SRBs, measurements of space count (SC) are also available, however, historically they are not used to specify the calibration offset ("zero count", ZC), which does not even appear in the calibration equation. A regression calibration formulation is used instead, equivalent to setting the ZC to a constant, whose value is specified from pre-launch measurements. Our analyses supported by a review of the instrument design and a wealth of historical SC information show that the SC varies in-flight and it differs from its pre-launch value. We therefore suggest that (1) the AVHRR calibration equation in the SRBs be re-formulated to explicitly use the ZC, consistently with the EEBs, and (2) the value of ZC be specified from the onboard measurements of SC. This study emphasizes the importance of clear discrimination between the SC (which is a measured quantity and therefore takes on a range of values, characterized by the empirical probability density function, PDF), from the ZC (which is a parameter in the calibration equation, i.e. a number whose value needs to be estimated from the measured SC as a mean, median or other statistic of the measured PDF). The ZC-formulation of the calibration equation is physically solid, and it minimizes human-induced calibration errors resulting from the use of a regression formulation with an un-constrained intercept. Specifying the calibration offset improves radiances, most notably at the low end of radiometric scale, and subsequently provides for more accurate vicarious determinations of the calibration slope (inverse gain). These calibration improvements are important for the products derived from the AVHRR low-radiances, such as aerosol over ocean, and particularly critical when generating their long-term climate data records.

The solar reflectance bands (SRB; centered at lambda(1) = 0.63, lambda(2) = 0.83, and lambda(3A) = 1.61 mum) of the Advanced Very High Resolution Radiometers (AVHRR) flown on board NOAA satellites are often referred to as noncalibrated in-flight. In contrast, the Earth emission bands (EEBs; centered at lambda(3B) = 3.7, lambda(4) = 11, and lambda(5) = 12 mum) are calibrated using two reference points: deep space and the internal calibration targets. In the SRBs, measurements of space count (SC) are also available;, however, historically they are not used to specify the calibration offset [zero count (ZC)], which does not even appear in the calibration equation. A regression calibration formulation is used instead, equivalent to setting the ZC to a constant, whose value is specified from prelaunch measurements.The analyses below, supported by a review of the instrument design and a wealth of historical SC information, show that the SC varies in-flight and differs from its prelaunch value. It is therefore suggested that 1) the AVHRR calibration equation in the SRBs be reformulated to explicitly use the ZC, consistently with the EEBs; and 2) the value of ZC be specified from the onboard measurements of SC. The ZC formulation of the calibration equation is physically solid, and it minimizes human-induced calibration errors resulting from the use of a regression formulation with an unconstrained intercept. Specifying the calibration offset improves radiances, most notably at the low end of radiometric scale, and subsequently provides for more accurate vicarious determinations of the calibration slope (gain). These calibration improvements are important for the products derived from the AVHRR low radiances, such as aerosol over ocean, and are particularly critical when generating their long-term climate data records (CDRs).

Experiment demonstrates feasibility of using recently calibrated airborne radiometer to calibrate satellite-borne radiometer monitoring Earth and not calibrated since before launch. Calibration technique helps to assure Earth scientists of accuracy of satellite radiometric measurements taken during long time. Optical train of orbiting radiometer degraded slowly and sensitivities of detectors and gains of amplifiers changed slowly in outer-space environment, but frequent calibrations by airborne-radiometer technique make it possible to compensate for these changes.

Present and future visible and near-infrared wavelength sensors mounted on operational satellites do not have on-board absolute calibration devices. One means of establishing an in-orbit calibration for a satellite sensor is to make simultaneous measurements of a bright, relatively uniform scene along the satellite view vector from a calibrated instrument on board a high altitude aircraft. Aircraft data were recorded over White Sands, New Mexico, and the coincident aircraft and orbiting satellite data is compared for the visible and near-infrared wavelength channel of the GOES-6 Visible Infrared Spin-Scan Radiometer.

A method for calibrating satellite radiometers is investigated. A calibrated spectral radiometer carried aboard a U2 aircraft at an altitude of 60 000 ft was aligned with White Sands. New Mexico along the same view vector as the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-9 spacecraft at the time of the spacecraft's overpass on 26 August 1985. Both sets of data have been transformed into best estimates of the radiance at satellite altitude inside the footprint of the aircraft radiometer, allowing an estimate of radiance calibration changes in the AVHRR to be made. It is assumed that both instrument systems are linear, that the spectral response function of AVHRR has not changed from its prelaunch value, and that the zero radiance responses of both instruments are accurately known. Extrapolation of the radiances measured from the aircraft to those expected at satellite altitude is achieved by modeling the experimental conditions at White Sands and calculating the ratio of radiances at the two altitudes through the LOWTRAN VI computer program. Results from data taken within 2 minutes either side of the satellite overpass indicate a 98.9% correlation between the two sets of data, and a change in gain relative to the prelaunch calibration of +2 ± 5% for channel 1 and −2 ± 5% for channel 2 of the NOAA-9 AVHRR. Analysis of other coincident data for the NOAA-9 AVHRR and the aircraft spectral radiometer, including a large dataset from October and November 1986, is now in progress and will establish the day-to-day repeatability of results using this method.

Radiances obtained from the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) have been compared with those derived from a coaligned aircraft-mounted double-pass Fastie-Ebert spectrometer when both instruments were simultaneously observing the dunes region of White Sands, New Mexico. The radiance calibration of the airborne spectrometer was arranged to be traceable to NBS standards through the procedures that were used for the prelaunch calibration of the AVHRR. Data from August 1985 and October/November 1986 have been analyzed, and suitable corrections made for the effects of the atmosphere between the U-2 aircraft (flying at a pressure level near 6 N/m2 (60 mb)) and the satellite. For this purpose, atmospheric altitude profiles of temperature, pressure and water vapor were measured from a radiosonde, the most probable ozone profile was derived from statistically inter-preted Nimbus-7/SBUV measurements, and likely stratospheric aerosol conditions were assumed to calculate the ratio (as a function of wavelength) of the radiances expected at each instrument. The calculated ratio was used to adjust the measured U-2 spectra to their equivalent at the altitude of the NOAA-9 AVHRR. The calculations indicated an absolute accuracy of +5% in the derived sensitivity of the AVHRR visible channels relative to the sensitivity measured prelaunch, and that some improvement in absolute accuracy is possible with modest additional effort. The analysis showed that in August, 1985, the sensitivities of channel 1 (570-700 nm) and channel 2 (710-1000nm) of the NOAA-9 AVHRR were indistinguishable from their prelaunch values. In October/November, 1986, sensitivity losses were calculated to be approximately 12% for channel 1, and 19% for channel 2. Flights on three days during the period 24 October to 5 November 1986 reproduced these results to within +1.5% in sensitivity loss. Results for the GOES-6 VISSR visible channel showed a 9-14% loss in sensitivity after 42 months in orbit. For LANDSAT-5 TM a 12% loss in band 4 after 17 months in orbit was indicated, with no discernable change in the sensitivities of bands 1-3.