The performance of the visible(channel 1: ÷0.58-0.68 æm) and near-infrared(channel 2: ÷0.72-1.1 æm) channels of the Advanced Very High Resolution Radiometer is observed to degrade in orbit, initially because of the outgassing (e.g., water vapor from the filter interstices) and launch associated contamination (e.g., rocket exhaust and outgassing), and subsequently because of the continued exposure to the harsh space environment (e.g., Brest and Rossow 1992; Kaufman and Holben 1993). It is therefore necessary to correct the measured signals in these two channels for the in orbit degradation to obtain correct values of the upwelling radiance at the top of the atmosphere (e.g., Rao et al.,1993; Teillet and Holben 1994). Use of the pre-launch calibration coefficients would lead to erroneous values of the upwelling radiances, and hence, of the geophysical products derived from these radiances such as the Normalized Difference Vegetation Index, Earth's short wave radiation budget, and the columnar aerosol burden over the global oceans. The absence of on board calibration devices renders the task of post-launch characterization of the performance of the two AVHRR channels rather difficult.
The work presented here was performed as part of the NOAA/NASA AVHRR Pathfinder program which has for its main objective the reprocessing and rehabilitation of the long-term records of AVHRR- derived geophysical products for the Pathfinder period, 1981 - present. We summarize here some of the results of the AVHRR Pathfinder Calibration Activity (Rao et al. 1993), an integral part of the Pathfinder program. For programmatic reasons, we have confined our attention to the AVHRRs on the NOAA-7, -9, and -11 spacecraft. Formulae for the calculation of radiances and albedos (AVHRR usage) measured in the two channels, after the in-orbit degradation has been duly accounted for, are given, and their derivation briefly explained. Several applications of these formulae to date indicate that they do minimize, if not remove altogether, spurious trends in the long term records of AVHRR-derived geophysical products, and do serve to establish linkages among the segments of records of the same(geophysical products) obtained with the AVHRRs on the NOAA-7, -9, and -11 spacecraft.
The relative degradation rates, which essentially characterize the response of the given AVHRR channel when viewing an Earth scene on any given day in orbit in reference to its response on the day of launch of the spacecraft, all other conditions remaining unaltered, were determined using statistical procedures on the International Satellite Cloud Climatology(ISCCP) B3 data (Brest and Rossow 1992) for the southeastern part of the Libyan desert (210-230N latitude; 280-290E longitude). It was assumed that the desert site was a radiometrically stable target, and that the decay of the sensors was exponential in time. Details of the surface reflection model used, and of the numerical techniques to determine the daily degradation rate, k, are found in Staylor (1990) and Rao et al.(1993).
The relative annual degradation rates, given by 100[1 - exp(-365k)] in per cent, for the two channels are respectively: 3.6 and 4.3 for the NOAA-7 AVHRR; 5.9 and 3.5 for the NOAA-9 AVHRR; and 1.2 and 2.0 for the NOAA-11 AVHRR. The variation in time of the albedo or reflectance factor of an Earth scene according to the above degradation rates is shown in Fig.1 (left panel) where the albedo or reflectance factor has been set equal to 100 per cent on the day of launch of the spacecraft.
In view of the length of records of the geophysical products derived from radiances measured with the AVHRRs on different spacecraft, it becomes necessary to establish linkages between the calibrations of the different AVHRRs to ensure the quality and continuity of the long-term records. We have used the AVHRR on NOAA-9 as the standard of comparison to establish inter-satellite linkages since its post-launch performance has been studied extensively and since its effective operational life covered multi-agency, multi-platform experiments such as the First International Satellite Cloud Climatology Regional Experiment (FIRE), and the First International Land Surface Climatology Project Field Experiment (FIFE).
The relative degradation rates for the visible and near-infrared channels of the AVHRR on NOAA-9 were rendered absolute by comparison with absolute calibrations based on congruent path aircraft/satellite radiance measurements made over White Sands, New Mexico, U.S.A. during October/November 1986 (Smith et al., 1988). The absolute calibrations are based on establishing correspondence between simultaneous measurements of upwelling radiance by a well-calibrated spectrometer on board a high flying U-2 aircraft, and the signal issuing from the AVHRR when both instruments are looking at the same White Sands target under identical conditions of illumination and observation.
A matched data set was established consisting of measurements made over the Libyan desert calibration target site by the AVHRRs on NOAA-7 and -11 and by the reference AVHRR on NOAA-9 with the requirements that the solar and satellite zenith angles for measurements made with the AVHRR on NOAA-7(-11) should be within one degree of the corresponding angles for the measurements made with the AVHRR on NOAA-9, and that the measurements should have been made in the same month of the year. It is thus likely that measurements made in any month, say March, during the ith year of operation of NOAA-7(-11) could be matched with measurements made by the AVHRR on NOAA-9 in March during the jth year of operation of NOAA-9. The signals (counts) issuing from the AVHRR on either NOAA-7 or NOAA-11 were then corrected for sensor degradation using the relative degradation rates determined earlier, and regressed against the radiances measured by the AVHRR on NOAA-9 to obtain the "slopes" (AVHRR usage), expressed in units of Watt/(m2 æm sr count), on the day of launch of the respective sensors. The "slope" (i.e., the reciprocal of the gain of the instrument) essentially gives the amount of radiant energy required to generate unit response from the AVHRR and increases in time because of the deterioration of the sensors. The results are graphically displayed in Fig.1 (right panel). Greater details of the normalization of the AVHRRs on NOAA-7 and -11 spacecraft to the AVHRR on NOAA-9 are found in Rao and Chen(1994).
We give in Table 1 the formulae for the calculation of the upwelling radiances in AVHRR channels 1 and 2 from the measured 10-bit counts on any given day in orbit, with elapsed time expressed in days(d) reckoned from the day of launch of the spacecraft; the dates of launch are shown in parentheses after the spacecraft identification in the table. The corresponding formulae for the calculation of the albedos(AVHRR usage) are found in Table 2; the albedo, A, in either channel is given by A = 100(ãIb/Fb) where Ib and Fb are the in-band upwelling radiance and the extraterrestrial solar irradiance respectively; we use the Neckel and Labs (1984) extraterrestrial solar irradiance values in the calculation of Fb. Details of conversion of radiance to albedo are found in Rao (1987) and Price (1987).
The calibrated radiance and albedo formulae have been used in the generation of the AVHRR Pathfinder Land Data Set; in the generation of global vegetation index climatology; and in the validation of the NOAA aerosol product with encouraging results.
Brest, C. and W.R. Rossow, 1992: Radiometric calibration and monitoring of NOAA AVHRR data for ISCCP. Int. J. Remote Sensing, 13, 235-273.
Kaufman, Y.J. and B.J. Holben, 1993: Calibration of AVHRR visible and near-IR bands by atmospheric scattering, ocean glint, and desert reflection, Int. J. Remote Sensing, 14, 21-52.
Neckel, H. and D. Labs, 1984: The solar radiation between 3300 and 12500A, Solar Physics, 90, 205-258.
Price, J.B., 1987: Calibration of satellite radiometers and comparison of vegetation indices. Remote Sens. Environ., 21, 15-27.
Rao, C.R.N., 1987: Prelaunch calibration of channels 1 and 2 of the Advanced Very High Resolution Radiometer, NOAA Technical Report NESDIS 36, Department of Commerce, Washington, D.C.
Rao, C.R.N., J. Chen, F.W. Staylor, P. Abel, Y.J. Kaufman, E.Vermote, W.R. Rossow, and C. Brest, 1993: Degradation of the visible and near-infrared channels of the Advanced Very High Resolution Radiometer on the NOAA-9 spacecraft: Assessment and recommendations for corrections, NOAA Technical Report NESDIS 70, U.S. Department of Commerce, Washington, D.C.
Rao, C.R.N., and J. Chen, 1994: Post-launch calibration of the visible and near-infrared channels of the Advanced Very High Resolution Radiometer on NOAA-7, -9, and -11 spacecraft, NOAA Technical Report NESDIS 78, U.S. Department of Commerce, Washington, D.C.
Smith, G.R., R.H. Levin, P. Abel, and H. Jacobowitz, 1988: Calibration of the solar channels of the AVHRR on using high altitude aircraft measurements, J. Atmos. Ocean. Tech., 5, 631-639.
Staylor, W.F., 1990: Degradation rates of the AVHRR visible channel for the NOAA 6, 7, and 9 spacecraft, J. Atmos. Ocean. Tech., 7, 411-423.
Teillet, P.M. and B.J. Holben, 1994: Towards operational radiometric calibration of NOAA AVHRR imagery in the visible and near-infrared channels, Canadian Journal of Remote Sensing, 20, 1-10.
Figure 1. Relative degradation of the two channels(left) and the variation in time of the "slopes" of the two channels(right)
Table 1. Formulae for calibrated radiances
Table 2. Formulae for calibrated albedos
