Desert Targets Research Articles

INSAT-3D and -3DR relative radiometric performance over bright target

The Geostationary Earth Orbit (GEO) satellite INSAT-3D and -3DR derived data products plays a vital role in helping to develop long-term trends of many climate variables and to improve our understanding of earth's environmental processes and dynamics occurring on the land, in the oceans and in the atmosphere. In order to provide high quality, consistent and well calibrated measurements, the radiometric calibration of the sensor must be consistent, stable and traceable to its calibration standards. Due to absence of an onboard calibration mechanism for the visible (VIS) and Short Wave Infra-Red (SWIR) bands, accuracy of Top-of-atmospheric radiance of these bands data depend on vicarious calibration methods to provide post launch calibration coefficients to compensate for the degradation responsivity. In this study, a methodology has been implemented to perform degradation assessment and seasonal variation pattern for the visible and SWIR band of INSAT-3D and -3DR satellite based on clear-sky observations. The current methodology relies on clear sky apparent reflectance which are derived from level-1 images over pseudo-invariant desert target. In this study, authors are focused to investigate visible and SWIR band performance based on clear-sky observations over desert target and the instrument long-term drift traced by a polynomial fitting function. A trending algorithm is proposed here, which consists of a polynomial function (for the description of instrument degradation performance) with two sine terms for quashing the impact of the seasonal variations of the solar zenith angle and atmospheric constituents, is applied to fit the time series of apparent reflectance.

Determination of the Key Comparison Reference Value from Multiple Field Calibration of Sentinel-2B/MSI over the Baotou Site

Field calibration is a feasible way to evaluate space-borne optical sensor observations via natural or artificial sites on Earth’s surface with the aid of synchronous surface and atmospheric characteristic data. Since field calibration is affected by the coupled effects of surface and atmospheric characteristics, the single calibration results acquired under different surface and atmospheric conditions have different biases and different uncertainties, making it difficult to determine the consistency of these multiple calibration results. In view of this, by assuming that the radiometric performance is invariant during field calibration and the calibration samples are independent of each other, the surface–atmosphere invariant Key Comparison Reference Value (KCRV) is essentially derived from various calibration results. As the number of calibration samples increases, the uncertainty in the KCRV should decrease, and the KCRV should approach the “true” value. This paper addresses a novel method for estimating a weighted average value from multiple calibration results that can be used to compare each calibration result, and this value is accepted as the KCRV. Furthermore, this method is preliminarily applied to the field calibration of the Multispectral Instrument (MSI) onboard the Sentinel-2B satellite via the desert target at the Baotou site, China. After employing a chi-squared test to verify that 12 calibration samples are independent from each other, the KCRV of the 12 calibration samples at the Baotou site is derived, which exhibits much lower uncertainty than a single sample. The results show that the KCRVs of the relative differences between the simulated and observed at-sensor reflectance are 3.75%, 5.11%, 6.09%, and 5.03% for the four bands of Sentinel-2B/MSI, respectively, and the corresponding uncertainties are 1.84%, 1.87%, 1.90%, and 1.93%. It is noted that the KCRV uncertainty obtained with only 12 calibration samples is reduced significantly, and in the future, more samples in other instrumented sites will be used to validate this method thoroughly.

Cross-Calibration of MODIS Reflective Solar Bands With Sentinel 2A/2B MSI Instruments

Moderate resolution imaging spectroradiometer (MODIS) is a 36-band spectroradiometer that measures the Earth's surface from 0.4 to 14.4 μm at three spatial resolutions, 250 m (two bands), 500 m (five bands), and 1000 m (29 bands). The wide-scale use of the science products derived from the two MODIS instruments on the Terra and Aqua spacecrafts is a result of their excellent on-orbit performance, calibration stability, and accuracy through the life of the mission, making them a benchmark against which the performance of newer instruments is frequently evaluated. The recently launched multispectral instrument (MSI) aboard the Sentinel 2A and Sentinel 2B spacecrafts are part of the European Union's Copernicus program designed to acquire high spatial resolution imagery in the reflective spectrum from 0.4 to 2.2 μm. One of the popular techniques to evaluate the on-orbit calibration is by comparing the top-of-atmosphere (TOA) reflectance with an independent (well-calibrated) sensor while viewing a pseudoinvariant desert target, such as Libya 4. In this work, the TOA reflectances from Terra and Aqua MODIS and Sentinel 2A and Sentinel 2B MSI are compared using the same-day scenes from Libya 4. The corrections for spectral response function mismatch and the bidirectional reflectance distribution function (BRDF) are formulated and applied to obtain an effective TOA reflectance difference between the spectrally matching bands of these sensors. The availability of off-nadir MODIS overpass pairs with MSI facilitates the comparison across the entire MODIS scanangle range and in turn an on-orbit evaluation of the response versus scan-angle (RVS) corrections is performed. Additionally, each MODIS instrument is used as a transfer mechanism to evaluate the calibration differences between the two MSIs, with an agreement to within 1% observed between the two MSIs. The radiometric calibration differences between Terra MODIS and the two MSIs at nadir is generally within 4%, with only the red-band pair (Terra MODIS band 1 and MSI band 4) showing disagreement beyond 4%. In general, the reflectance ratios are in better agreement with Aqua MODIS than with Terra MODIS. A better agreement between MODIS and the two MSIs is observed at nadir, indicating some residual effects associated with the BRDF correction that are observed in the off-nadir scene pairs. Also included in this article is a description of the various uncertainties associated with this cross-calibration.

Selection and Characterization of Glaciers on the Tibetan Plateau as Potential Pseudoinvariant Calibration Sites

Vicarious calibration using pseudoinvariant calibration sites (PICS) has become increasingly popular, as it is the most cost-effective method for on-orbit calibration of space-borne optical sensors. Glaciers on the Tibetan Plateau (TP) are located at more than 4500 m above sea level. These snow targets, exhibiting much brighter reflectance and less affected by aerosol and water vapor than desert targets, can help to reduce the uncertainties associated with vicarious calibration. In contrast to polar snow, the TP is not limited by polar night, thus ensuring continuous calibration monitoring. This study investigated the potential role of TP glaciers as PICS for the first time. To accomplish this, the coefficient of variation (CV) was applied to characterize the spatial uniformity and temporal stability of five relatively larger and flatter TP glaciers based on several independent Landsat Thematic Mapper and Operational Land Imager datasets over 20 years. The criteria of CV < 3% (both spatial and temporal) through Landsat visible and near-infrared channels were used to identify potential PICS across the candidate glaciers. Finally, three snow PICS with areas of at least 7.2 km2 and two snow PICS with areas of 1.2–2.1 km2 were identified. The area of the snow PICS is smaller in autumn and winter due to the snow cover. The feasibility of using these sites for long-term calibration monitoring of satellite sensors is also discussed. Results indicate that although the snow PICS exhibits stronger BRDF than the invariant desert target, the MODIS calibration trends detected from snow PICS are comparable to those from the desert target.

Cross-calibration of reflective bands of major moderate resolution remotely sensed data

The major global biogeophysical products obtained from remote sensing are usually composites of 8 or 16days, and are almost always retrieved from a single sensor. Subsequently, the applications of these products are limited in cases requiring higher temporal frequency. With the increasing number of freely available moderate resolution remote sensing datasets, multi-sensor synergy to increase temporal sampling is warranted. However, radiometric consistency of multi-sensor data is not as good as expected; therefore, first the constituency should be evaluated. A new cross-calibration approach of reflective bands for moderate resolution remotely sensed data is proposed in this paper. The new approach uses a time series of MODIS data from both Terra and Aqua to retrieve both the bidirectional reflectance distribution function (BRDF) and aerosol optical depth (AOD) simultaneously over an invariant desert target. The MODIS retrieved BRDF and AOD are then used to cross-calibrate the medium resolution spectral imager (MERSI) and visible and infra-red radiometer (VIRR) onboard FengYun3-A and B satellites (FY3A/B) and advanced very high resolution radiometer (AVHRR) onboard NOAA-16, 17, and 18. The cross-calibration method is validated in two ways: 1) by comparing with the vicarious calibration; and 2) the top of atmosphere (TOA) reflectance before and after calibration, which show that the calibration error of this new approach is consistent within 5%. Compared with most cross-calibration methods, our new method is generic and better consider on BRDF models; furthermore, the well-built BRDF of the calibration site and simultaneously retrieved AOD make the method fully automatic and cost effective for the calibration of most moderate resolution remote sensors, especially Chinese sensors.

Alternative method of on-orbit response-versus-scan-angle characterization for MODIS reflective solar bands

The moderate resolution imaging spectroradiometer (MODIS) has 20 reflective solar bands (RSB), covering a spectral range from 0.41 to 2.2 μm, which are calibrated on-orbit using its onboard calibrators, which include a solar diffuser, a solar diffuser stability monitor, and a spectroradiometric calibration assembly. A space view (SV) port is used to provide a background reference and also facilitates near-monthly lunar observations through a spacecraft roll. In every scan, the Earth’s surface, SV, and onboard calibrators are viewed via a two-sided scan mirror, the reflectance of which depends on the angle of incidence (AOI) as well as the wavelength of the incident light. Response-versus-scan-angle (RVS) is defined as a dependence function of the scan mirror’s reflectance over AOI. An initial RVS for each RSB was measured prelaunch for both Terra and Aqua MODIS. Algorithms have been developed to track the on-orbit RVS variation using the measurements from the onboard calibrators, supplemented with the earth view (EV) trends from pseudoinvariant desert targets obtained at different AOI. Since the mission beginning, the MODIS characterization support team (MCST) has dedicated efforts in evaluating approaches of characterizing the on-orbit RVS. A majority of the approaches focused on fitting the data at each AOI over time and then deriving the relative change at different AOI. The current version of the on-orbit RVS algorithm, as implemented in the collection 6 (C6) level-1B (L1B), is also based on the above rationale. It utilizes the EV response trends from the pseudoinvariant Libyan desert targets to supplement the gain derived from the onboard calibrators. The primary limitation of this approach is the assumption of the temporal stability of these desert sites. Consequently, MCST developed an approach that derives the on-orbit RVS change using measurements from a single desert site, combined with the on-orbit lunar measurements. In addition, the EV and onboard responses are fit first as a function of AOI before fitting temporally in order to eliminate the dependence on the stability of the desert site. Comprehensive comparisons are performed with current C6 RVS results for both Terra and Aqua MODIS. Results demonstrate that this alternative method provides a supplemental means to monitor the on-orbit RVS for MODIS RSB.

Investigation and Mitigation of the Crosstalk Effect in Terra MODIS Band 30

It has been previously reported that thermal emissive bands (TEB) 27–29 in the Terra (T-) MODerate resolution Imaging Spectroradiometer (MODIS) have been significantly affected by electronic crosstalk. Successful linear theory of the electronic crosstalk effect was formulated, and it successfully characterized the effect via the use of lunar observations as viable inputs. In this paper, we report the successful characterization and mitigation of the electronic crosstalk for T-MODIS band 30 using the same characterization methodology. Though the phenomena of the electronic crosstalk have been well documented in previous works, the novel for band 30 is the need to also apply electronic crosstalk correction to the non-linear term in the calibration coefficient. The lack of this necessity in early works thus demonstrates the distinct difference of band 30, and, yet, in the same instances, the overall correctness of the characterization formulation. For proper result, the crosstalk correction is applied to the band 30 calibration coefficients including the non-linear term, and also to the earth view radiance. We demonstrate that the crosstalk correction achieves a long-term radiometric correction of approximately 1.5 K for desert targets and 1.0 K for ocean scenes. Significant striping removal in the Baja Peninsula earth view imagery is also demonstrated due to the successful amelioration of detector differences caused by the crosstalk effect. Similarly significant improvement in detector difference is shown for the selected ocean and desert targets over the entire mission history. In particular, band 30 detector 8, which has been flagged as “out of family” is restored by the removal of the crosstalk contamination. With the correction achieved, the science applications based on band 30 can be significantly improved. The linear formulation, the characterization methodology, and the crosstalk effect correction coefficients derived using lunar observations are once again demonstrated to work remarkably well.

Intercalibration of GOES Imager visible channels over the Sonoran Desert

AbstractThe Geostationary Operational Environmental Satellites (GOES) have been observing the Western Hemisphere since the late 1970s, providing valuable information for weather forecast and climate change studies. Due to the lack of an onboard calibration device for the visible channel, accurate reflectance of the visible channel data depends on vicarious calibration methods to provide postlaunch calibration coefficients to compensate for the degraded responsivity. In this study, the Sonoran Desert, which can be viewed by both GOES‐East and GOES‐West satellites, is used to intercalibrate the visible channels on board the three‐axis stabilized GOES satellite Imagers traceable to the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 (C6) calibration standard. It was found that when the anomalous reflectance in 2004 and 2005 are excluded, the Sonoran Desert is radiometrically, spatially, and spectrally stable at the GOES viewing geometries and thus can be considered as a pseudo‐invariant calibration site to develop long‐term GOES Imager visible data set. To characterize the desert target reflectance with the MODIS data, GOES observations over 1 year period are used to convert the MODIS reflectance to the GOES viewing and solar illumination geometries. The spectral band adjustment factor for each GOES Imager visible channel is generated with a set of clear‐sky Hyperion measurements. A trending algorithm, which consists of a polynomial function for the description of instrument degradation performance and two sine terms for the impacts of the seasonal variations of the solar zenith angle and atmospheric components, is applied to fit the time series of prelaunch calibrated reflectance. The combined calibration uncertainty of the desert calibration method is less than 4% at the Aqua MODIS C6 calibration standard. The difference of the postlaunch calibration coefficients between the desert calibration and the current GOES visible operational calibration methods is mainly within 5%.

Climatological assessment of desert targets over East Asia — Australian region for the solar channel calibration of geostationary satellites

Desert targets for solar channel calibration of geostationary satellites in the East Asia — Australian region were selected and their qualities were assessed with aid of Moderate Resolution Imaging Spectroradiometer data (i.e., white-sky surface albedo, aerosol optical thickness, and cloud fraction) from 2002 to 2008. The magnitude, spatial uniformity, and temporal stability of the white-sky surface albedo are examined in order to select bright and stable targets. Subsequently those selected targets over China, India, and Australia are further checked for their qualities in terms of data yielding ratio, aerosol optical thickness, cloud fraction, satellite viewing angle, and solar zenith angle. Results indicate that Chinese targets are found to be not adequate as calibration targets in spite of excellent surface conditions because of high percentage of cloud, possibly heavy aerosol loading, and lower solar elevation angle in particular during winter time. Indian site should be take care about relatively high temporal variation of surface condition and heavy aerosol loading. On the other hand, Australian desert targets are considered to be best when surface brightness, spatial and temporal stability, data yielding ratio, aerosol, and cloud are counted.

Use of simulated reflectances over bright desert target as an absolute calibration reference

This letter presents the improvements of an absolute calibration reference system based on simulated top-of-atmosphere bidirectional reflectance factor time series over bright desert targets. The current work highlights a case study performed over Committee on Earth Observation Satellites (CEOS) calibration target Libya4, demonstrating that it is possible to achieve a mean accuracy of 3% when simulation is compared with calibrated observations acquired by polar orbiting satellites.

Solar Channel Calibration Using Desert Targets in Australia: Application to the MTSAT-1R Visible Sensor

Focusing on the use of desert targets for solar channel calibration of geostationary satellites located in the west Pacific region, a desert target in Simpson Desert, Australia, was selected and used for the development of a calibration algorithm based upon radiative transfer modeling. The surface spectral reflectance required for radiative transfer calculation was obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS)-bidirectional reflectance distribution functions after tuning against the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spectral data. Top of the atmosphere radiance simulations were conducted over the desert target, and comparisons with MODIS- and Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-measured visible channel radiances revealed that the developed calibration method using the target over the Simpson Desert can be used to calibrate the geostationary visible channel within a 5% level of uncertainty. Application of the developed algorithm to Multi-Functional Transport Satellite (MTSAT-1R) visible channel measurements suggested that the MTSAT-1R operational calibration for the visible sensor appeared to be within a 5% error, but over the lower count value range.

Water vapor correction to improve the operational calibration for NOAA AVHRR/3 channel 2 (0.85 µm) over a desert target

The Advanced Very High Resolution Radiometers (AVHRRs) flown on the National Oceanic and Atmospheric Administration (NOAA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) polar satellites have been providing invaluable data for Earth system science and global change studies. However, the use of AVHRR solar reflectance products (e.g., normalized difference vegetation index (NDVI)) is constrained by calibration uncertainty, largely owing to the lack of an on-orbit calibration device for solar reflectance channels. Since the mid-1990s, NOAA has been applying operational calibration to these channels using the time series of top-of-atmosphere (TOA) measurements of the Libyan Desert site. However, the assumed radiometric stability of Libyan Desert TOA measurements can be disrupted by short-term variations in atmospheric components, especially for AVHRR channel 2, which has a spectral response function covering water vapor absorption lines in the near-infrared (NIR) spectra. This study aims to improve the calibration accuracy of AVHRR channel 2 data by applying water vapor correction over the homogeneous desert target surface using the water vapor content (W) derived from the linear relationships between the AVHRR split-window temperature difference (ΔT) and the Moderate Resolution Imaging Spectroradiometer (MODIS) NIR water vapor product. Results show that the linear ΔT–W relationship is affected by the temperature lapse rate at the satellite over-passing time and the spectral response functions of AVHRR split-window channels. Water vapor correction reduces the calibration uncertainty from 2.6%–3.5% to 1.7%–1.8%, and about 1.0% surface-based relative calibration accuracy is independent of the water vapor uncertainty introduced by the ΔT–W regression. Furthermore, the short-term variations in the channel 2 operational degradation rate are largely reduced after water vapor correction. Thus, implementing water vapor correction can improve the operational calibration accuracy for AVHRR channel 2.

Comparing Operational MSG/SEVIRI Land Surface Albedo Products From Land SAF With Ground Measurements and MODIS

This study is devoted to an assessment of the quality performance of the Satellite Application Facility on Land Surface Analysis (LSA-SAF) albedo products, which are generated from the high frequency of observations now offered by the Meteosat Second Generation (MSG) geostationary satellite series. Land surface albedo products are compared against in situ measurements and the analogous products from the Collection 4 of Moderate Resolution Imaging Spectroradiometer (MODIS) in domains in European and Northern African regions. We find a good consistency with MODIS broadband shortwave and near-infrared directional-hemispherical reflectance (DHR) over midlatitude regions. Comparative statistics indicate biases below 0.01 in absolute and 5% in relative terms, and an average root-mean-square error (RMSE) around 0.03 for all situations, including in snow cases. The comparison for shortwave bihemispherical reflectance (BHR) shows an even better match over herbaceous cover with respective values for bias and RMSE of 0.007 and 0.012. Broadband visible BHR from LSA-SAF overestimates MODIS equivalent albedo product by 20%, which seems to be caused by remaining uncertainties in narrowband-to-broadband conversion, residual aerosol signals, and the use of a different bidirectional reflectance distribution function (BRDF) model. The commendable statistical results obtained over bright desert targets such as Northern Africa are worth noticing. Assuming that the MODIS product is a good reference, a relative bias of 10% can be judged satisfactory for the LSA-SAF snow-free broadband DHR values. In addition, quarter-hourly MSG observations better capture snow episodes, although some discrepancies appear for areas with high vegetation cover.

Removal of Contaminated Pixels from the Desert Target for AVHRR Vicarious Calibration

Abstract Desert-based vicarious calibration plays an important role in generating long-term reliable satellite radiances for the visible and near-infrared channels of the Advanced Very High Resolution Radiometer (AVHRR). Lacking an onboard calibration device, the AVHRR relies on reflected radiances from a target site, for example, a large desert, to calibrate its solar reflective channels. While the radiometric characteristics of the desert may be assumed to be stable, the reflected radiances from the target can occasionally be affected by the presence of clouds, sand storms, vegetation, and wet surfaces. These contaminated pixels must be properly identified and removed to ensure calibration performance. This paper describes an algorithm for removing the contaminated pixels from AVHRR measurements taken over the Libyan Desert based on the characteristics of consistent normalized difference vegetation index (NDVI) land-cover stratification. An NDVI histogram-determined threshold is first applied to screen pixels contaminated with vegetation in each individual AVHRR observation. The resulting analyses show that the vegetation growth inside the desert target has a negligibly small impact on the AVHRR operational calibration results. Two criteria based on the maximum NDVI compositing technique are then employed to remove pixels contaminated with clouds, severe sand storms, and wet sand surfaces. Compared to other cloud-screening methods, this algorithm is capable of not only identifying high-reflectance clouds, but also removing the low reflectance of wet surfaces and the nearly indifferent reflectance of severe dust storms. The use of clear pixels appears to improve AVHRR calibration accuracy in the first 3–4 yr after satellite launch.

Variability of biome reflectance directional signatures as seen by POLDER

Reflectance measurements acquired with the spaceborne POLDER instrument are used to analyze the variability of land surface directional signatures as a function of vegetation cover type. The reflectance directional signatures are quantified by the three parameters of a modified version of the Ross–Li reflectance model. The variability of the estimated parameters with respect to the seven MODIS biome classes was found to be higher within the classes than between classes, with the exception of the desert targets that show more isotropic reflectances. A limited number of standard BRDFs (typically 5 in the red and near infrared) capture most of the variability of the directional reflectance measurements, supporting the idea that different land surfaces have similar directional signatures. Over vegetation targets, they are characterized by a strong increase toward backscattering and much smaller variations in forward directions. The results express the diversity in structural situations within a given biome class and indicate that, at the resolution of the POLDER sensor, i.e. a few kilometers, the BRDF contains little information on the dominant vegetation type. We show that standard directional signatures may be used to correct the reflectance measurements for directional effects with an RMS error on the order of 0.011 in the red and 0.015 in the near infrared.

Meteosat and GOES-East Imager Visible Channel Calibration

Abstract As a preliminary step to solar irradiance calculations, the Centre de Météorologie Spatiale (CMS) has developed a pragmatic approach to calibrate the visible channels of Meteosat and GOES-East imagers. The responsivity of the Meteosat visible channel has been monitored with three desert targets from 1989 to 2002. The annual degradation rate has been estimated to 1.8% for Meteosat-4, 1.4% for Meteosat-5, and 1.9% for Meteosat-7. A reference calibration coefficient for Meteosat-7 has been derived from a comparison with Clouds and Earth's Radiant Energy System (CERES) data in summer 1998. Meteosat and GOES-East data corresponding to homogenous pixels along longitude 37.5°W and around 1200 LST have been compared on a monthly basis, leading to a calibration of GOES-East visible channel. GOES-8 data have been processed from June 1998 to December 2002 and the annual degradation rate obtained during this period is 4.0%. GOES-12 data have been processed from April to August 2003. During this short period, no degradation rate can be estimated but only a mean value of the calibration coefficient, which corresponds to a 7% increase of the prelaunch coefficient.

Using Moderate Resolution Imaging Spectrometer (MODIS) to calibrate advanced very high resolution radiometer reflectance channels

A series of 10 advanced very high resolution radiometers (AVHRRs) flown on National Oceanic and Atmospheric Administration (NOAA)'s polar‐orbiting satellites for over 20 years has provided data suitable for many quantitative remote sensing applications. To be useful for geophysical research, each radiometer must be accurately calibrated, which poses problems in the AVHRR reflectance channels because they have no onboard calibration. Previous studies have shown that values of the reflectance channel calibrations, accurately measured during preflight, change abruptly immediately after launch and then change slowly during the satellite's lifetime. The presence of the dual‐gain reflectance channels on the current series of AVHRRs also complicates the application of previous calibration techniques. A technique is presented here for calibrating the AVHRR dual‐gain reflectance channels using Moderate Resolution Imaging Spectrometer (MODIS) data. This method employs selective criteria to reproduce a laboratory type calibration where instrument counts observed by AVHRR are matched to reflectances measured by MODIS on a pixel by pixel basis for coincident and co‐located scenes. Unlike AVHRR, MODIS employs onboard calibration of its reflectance channels. The goal here was to explore the utility of using MODIS to calibrate the new dual‐gain reflectance channels of the AVHRR. The AVHRRs in the NOAA‐KLM series of spacecraft employ a dual‐gain approach to increase the sensitivity to dark scenes. Traditional methods using radiometrically stable targets to calibrate the reflectance channels of AVHRR typically do not provide data for both gain settings. The data from two scenes that met the over‐pass criteria are analyzed. The regression of the MODIS reflectances versus the AVHRR counts for these scenes were able to produce calibration slopes and intercepts in both the low and high gain regions. The reflectance differences using the MODIS‐derived calibration compared to preflight calibration are well within the expected behavior of the AVHRR during its first year in orbit. Comparison with reference ch1 and ch2 reflectance values from NOAA 9 for a Libyan Desert Target were within 5% of those using the MODIS‐derived calibration. While the determination of the absolute accuracy this approach needs further study, it clearly offers the potential for calibration of the AVHRR dual‐gain reflectance channels.

Estimation of the ratio of sensor degradation between NOAA AVHRR channels 1 and 2 from monthly NDVI composites

Vegetation monitoring, based on the normalized difference vegetation index (NDVI) calculated from the Advanced Very High Resolution Radiometer (AVHRR) channels 1 and 2 data, requires continuous updates of calibration coefficients to correct for the sensor degradation in these channels. A method was developed to estimate calibration coefficients with monthly composited NDVI data from desert targets without recourse to the component channels 1 and 2 data. The method was tested on NDVI data from the AVHRR onboard the NOAA-7, -9, and -11 satellites for the period from 1982 until 1993. The results of the method outlined in this paper correlated high r, between 0.94 and 0.95, with the results from other studies that estimated sensor degradation for the individual AVHRR bands.

Signal degradation of the AVHRR shortwave channels of NOAA 11 and NOAA 14 by daily monitoring of desert targets

The only indication of sensor degradation affecting the shortwave channels of the AVHRR is the changing response of targets which are assumed to be invariant in time. Using daily data of the whole lifetime of NOAA 11 three different long term trends in the signal response could be identified and quantitatively separated: real sensor degradation, atmospheric influence caused by the changing local time of observation, and surface anisotropy by increasing solar zenith distance. Together they cause the decrease of the measured signal and should be called effective signal degradation. If atmospheric corrections are applied to get surface reflectances it is important to avoid duplicate corrections that would lead to long term trends caused by changing atmospheric attenuation. The degradation coefficients were calculated by postulating a constant NDVI of the desert area. Corrections for surface ansisotropy to the time series of the reflectances were applied. Thus a precise quantitative inter-calibration of the NOAA 14 to the NOAA 11 time series could be performed. The resulting degradation coefficients can be updated and applied in real time.

Long term (1983-1994) calibration of the Meteosat solar (VIS) channel using desert and ocean targets

Abstract The successive operational Meteosat solar (VIS) sensors have shown significant variations of their spectral band which affected their relative sensitivities to the light scattering by aerosols, molecules and Earth surface. This paper provides a calibration of the various Meteosat VIS sensors used between June 1983, the beginning of data archiving for the International Satellite Cloud Climatology Project (ISCCP), and December 1994, taking into account the characteristics of their individual spectral band thanks to an atmospheric radiative transfer model. The method is based on a monitoring of the reflectance of desert and ocean targets free of clouds and aerosols, and relies on the absolute vicarious calibration of Meteosat-2 performed in late 1981. We find different sensitivities and temporal evolutions for Meteosat-2, -3, -4, and -5, and our results compare well with other more limited calibration studies