CALIBRATION OF VEGETATION CAMERAS ON-BOARD SPOT4

Transcription

1 CALIBRATION OF VEGETATION CAMERAS ON-BOARD SPOT4 Patrice Henry, Aimé Meygret CNES (Centre National d'etudes Spatiales) 18 avenue Edouard Belin TOULOUSE CEDEX 4 - FRANCE Tel: 33 (0) , Fax: 33 (0) E.mail:Patrice.Henry@cnes.fr ABSTRACT This paper presents the results of the VEGETATION radiometric calibration obtained with different methods based on experience with SPOT and POLDER in: - pre-launch measurements - onboard calibration systems - vicarious calibration over test sites - calibration over molecular scattering - calibration over sun glint - calibration over clouds - VEGETATION/HRVIR inter-calibration. The accuracy of these measurements is discussed. The measurements are combined in a model which gives the cameras sensitivity as a function of time for the ground processing algorithms. INTRODUCTION The accurate characterization of the conversion of image digital counts to radiance values, or absolute calibration, is important in remote sensing as it enables to compare the responses of different satellite sensors, to monitor their changes over time and to provide users with data which do not depend on the sensor. Since the launch of the first SPOT satellite in 1986, CNES has placed particular emphasis on the development of calibration techniques in order to improve preflight and in-flight radiometric characterization of sensors. The experience gained with onboard absolute calibration systems, the difficulty of ensuring their stability under conditions in orbit as well as their high cost has led CNES to develop complementary vicarious calibration techniques. Within this framework, POLDER, a wide field-of-view imaging radiometer with no onboard calibration systems was launched in 1996 on board the Japanese ADEOS satellite. The ambitious calibration scheme based on ground targets such as molecular scattering, clouds, glitter and desert areas (Hagolle et al., 1999) has enabled very high accuracy to be achieved (4%). However, onboard calibration systems give easy access to calibration data that cannot be provided by ground methods. The techniques are complementary and have to continued. Thus, VEGETATION flies an onboard calibration system but also takes advantage of SPOT1 to 3 and POLDER experience in calibration over ground targets. This paper presents calibration for SPOT4 VEGETATION cameras using several techniques and describes the operational procedure chosen for routine calibration. An error budget based on comparison of the different results enables to evaluate the obtained accuracy. -1-

2 RADIOMETRIC MODELLING OF THE VIEWING SYSTEM It is assumed that, after correcting for sensitivity differences between detectors, the VEGETATION cameras output Xk is directly proportional to the input radiance Lk and may be written: Xk = Ak Gmk Lk (1) where: k is the spectral band, Ak is the absolute calibration coefficient, Gmk is the electronic gain: Gmk = m-2, m [1,6] Lk is the normalized radiance: L k = L(λ)S k(λ)dλ S k (λ)dλ (W.m -2.sr -1.µm -1 ) (2) with L(λ) the spectral radiance and Sk(λ) the spectral sensitivity of the sensor. The normalization appearing in Eq.(2) is such that Sk(λ) is defined to within a multiplication factor; only its shape is of any importance. The absolute calibration consists in estimating and monitoring the parameter Ak (W -1.m 2.sr.µm). Actually, for many applications, the most important thing is not the absolute calibration but the relative calibration between images of the same instrument at different dates (multidate calibration), images acquired simultaneously in different spectral bands (interband calibration), images acquired by two different sensors (sensors intercalibration). The calibration requirements for VEGETATION are expressed as follows:. Absolute calibration accuracy 5%. Multidate calibration accuracy 3%. Interband calibration accuracy 3%. HRVIR/VGT intercalibration accuracy 3% DESCRIPTION OF CALIBRATION METHODS VEGETATION takes benefit of more than 10 years of SPOT instruments calibration and on the effort undertaken to achieve the accurate calibration of POLDER. Consequently, a lot of calibration methods are available and can be used concurrently. Pre-Flight Measurements Before the launch, the cameras are calibrated by viewing a large aperture integrating sphere. The sphere is regularly calibrated and provides a first estimate of the camera s sensitivity; this measurement is completed by estimating the spectral sensitivity variation under vacuum conditions. For VGT, ground calibration was performed using three different spheres (including those used for HRVIR and POLDER) with some discrepancies between the measurements. The accuracy of the pre-launch calibration is estimated to approximately 8% (Leroy et al., 1990). Onboard Calibration Lamp VEGETATION onboard calibration system consists of a lamp and associated optical devices, mounted on a carbon fiber bar which moves in front of the optics. About 100 detectors of each of the four cameras are simultaneously lighted. The scanning of the entire field of view, obtained by rotating the bar, ensures full coverage of the detector array. The complete calibration lasts about 3 min. and is performed once a month. The in-orbit behavior of the VEGETATION lamp is excellent and it is used to monitor the cameras sensitivity. The lamp stability is however validated by comparing it with the temporal evolution of the measurements provided by all the other methods. -2-

3 Calibration With In-Situ Measurements The radiometry of the SPOT high resolution images is regularly compared to measurements over test sites. Both the ground reflectance and the atmosphere are characterized simultaneously with the satellite pass so as to estimate the observed radiance. These in-situ measurements have been performed over White Sands (New Mexico, US) and, more recently, Railroad Valley (Nevada, US) by the Remote Sensing Group of the University of Arizona (Biggar et al., 1991). A complementary site has been set up at La Crau (France) by the Laboratoire d Optique Atmosphérique of Lille and The University of Littoral (Santer et al., 1992). This method is not really applicable to VEGETATION because of its 1-km resolution: the in-situ measurement area is too small. Nevertheless, some results can be obtained over the White Sands site, since it offers a very large homogeneous area of bright desert. For the other sites a VGT/HRVIR cross calibration is used to transfer HRVIR calibration results to VEGETATION. Calibration Over Rayleigh Scattering Rayleigh calibration is based on the idea that apparent radiance at the top of the atmosphere (TOA) observed over a clear ocean, at short wavelengths, mainly comes from atmospheric molecular scattering. It provides a standard radiance for calibrating sensors without human intervention. The molecular scattering, also named Rayleigh scattering, is well mastered and is modeled as a function of the pressure and the viewing angle. However, the measured signal also comes from atmospheric scattering by aerosols and from reflections over the sea surface. The difficulty of the method lies in correctly evaluating these terms, as they contribute greatly to the error budget. The viewing conditions are defined so as to reduce the perturbing part of the signal due to water radiance: - deep ocean targets in the south hemisphere with well known weak and stable chlorophyll content, - low wind speed to minimize foam radiance, - large viewing angle and high solar zenith angle in order to increase the atmospheric path, - viewing in a westerly direction so as to avoid specular reflection. The aerosol optical thickness is estimated using the B3 channel for which water and Rayleigh scattering radiances are low. Atmospheric conditions namely water vapor content, wind speed and surface pressure are given by meteorological data, ozone content is given by TOMS, and climatology provides the oxygen content. The method was firstly proposed to calibrate the green channel on SPOT cameras (Vermote et al., 1992), and then was improved so as to calibrate the B0 and B2 channels of VEGETATION (Briottet et al., 1997). Calibration Over Sun Glint The principle is similar to that of the Rayleigh scattering method, except that the geometrical viewing is set to observe the sun s reflection over the sea. The sun glint reflectance, greater than 0.2, represents the most important part of the signal which means that the errors in the estimation of the other contributions (water, aerosols, etc) can be minimized. If there were no atmosphere, the sun glint over the sea would not depend on the wavelength as it obeys Fresnel s laws and as the water refraction index almost does not vary in the visible and near infrared spectrum. However, the reflected signal strongly depends on the wind speed, and the wind speed supplied through meteorological data is not accurate enough for use as a standard radiance. Consequently, this method has only been used as an interband calibration method. A reference band (B2), assumed to be calibrated, is used to estimate the sun glint reflectance at sea level. The atmosphere in the channel to be calibrated is then taken into account to provide the TOA radiance. This technique enables calibration of the B0, B3 and SWIR channels. Calibration Over Deserts It has been shown that some North African deserts provide stable and lambertian targets (Cosnefroy et al., 1996). A calibration method over these deserts has been developed, based on POLDER measurements (Cabot et al., 1999). The ground bi-directional reflectance of 20 African deserts is estimated using POLDER images taken in Atmospheric corrections applied to these reflectances provided the TOA radiance viewed by the VEGETATION -3-

4 cameras. All the acquisitions over the 20 sites are systematically processed providing more than 200 measurements a month. This technique is not applicable to the SWIR band. Calibration Over Clouds Under certain conditions, thick clouds provide stable radiance that can be used to calibrate spatial sensors (Vermote and Kaufman, 1995). It is not possible to accurately compute cloud radiance, but their reflective properties are spectrally flat, in the visible and near infrared, which is favorable for inter-band calibration. The most suitable clouds are located in the sub-tropical convective system, over the sea. They are high altitude very reflective clouds and are thick enough for the radiative effect of the surface to be negligible. The aerosol and water vapor also have a negligible influence on the signal since they are located mainly in the lower layers of the atmosphere. The only contributions to the observed signal are from the cloud reflectance, molecular scattering and absorption by ozone. The measurements have thus to be corrected for atmospheric scattering and absorption. With regard to VEGETATION, no information is available for estimating cloud altitude. A climatological study of selected zones has led to the choice of an altitude of 13-km for performing this atmospheric correction. The method allows for inter-calibrating channel B2, taken as the reference, with channels B0 and B3. HRVIR/VGT Calibration Cameras cross calibration is important because it ensures the radiometric homogeneity of the two instruments onboard SPOT4. Moreover, it means that advantage can be taken of the specific calibration of each camera. The systematic simultaneous images taken by HRVIR and VEGETATION provide numerous opportunities for intercalibration. The main difficulty lies in the geometric registration of the two images which is performed with an accuracy of 300 m, using the accurate geometrical model of the instruments. The results given in table 1 were computed for 20 image pairs. The higher standard deviation for B2 channels is due to the slight spectral sensitivity difference between HRVIR-1 and VGT cameras. Ak(HRVIR-1)/Ak(VGT) B2 B3 SWIR In flight measurement σ = 3.5% σ = 1% σ = 2% Table 1: HRVIR-1/VGT Inter-Calibration (August 1998) These results were used to transfer the La Crau measurement from HRVIR to VGT. VEGETATION CALIBRATION PROCEDURE All the results obtained during the in-flight commissioning phase have been carefully analyzed. Some methods appear to be more reliable and present less dispersion than others do, that drives the choice for the reference calibration. Choice Of The Reference Calibration The good in-orbit behavior of the onboard lamp, which shows perfect reproducibility along the detectors line from one acquisition to the next one, led to it being selected as the reference for monitoring changes in the cameras sensitivity over time. This choice is reinforced by the very good consistency of lamp measurement in relation to other calibration measurements (Rayleigh scattering, sun glint and deserts) that proved to evolve in the same way over time. However, the lamp source does not yield absolute calibration since no reliable pre-launch reference is available (due to a last minute modification before the launch), and since it is difficult to accurately assess the changing under orbital conditions. Consequently, the lamp calibration has to be adjusted with absolute calibration by an other -4-

5 method. The pre-flight assessment and the measurement precision obtained in orbit have confirmed the usefulness of the Rayleigh scattering method for B0 and B2 calibration and the sun glint method for B3 and SWIR calibration. All the other methods are used to validate and assess the accuracy of the reference calibration. Procedure For Reference Calibration As stated previously, the Rayleigh scattering method requires prior calibration of the B3 channel as it is used for retrieving the aerosol optical thickness. At the same time, the sun glint method uses the B2 band as a reference for calibrating the B3 band. It thus appears necessary to iterate with the two methods: B3 calibration over sun glint and B2 calibration over Rayleigh scattering so as to respectively refine the calibration of each one. This operation, which has already been performed on POLDER (Hagolle et al., 1999), consists in first calibrating B2 over Rayleigh scattering using the B3 pre-launch calibration. Then, the two band cross calibration over sun glint provides a new estimation for channel B3. This new calibration is used to again calibrate channel B2 over Rayleigh scattering. A second B3/B2 cross calibration over sun glint provides another estimation of channel B3 sensitivity and so on. Iteration is performed until the process converges. Table 2 shows the evolution of the VEGETATION calibration coefficient for the different iterations and the corresponding camera sensitivity variation. It may be noted that a 5.8% variation of B3 sensitivity respectively induces a 3.1% variation of B2 sensitivity and a 0.6% variation of B0 sensitivity. B3 calibration affects the different wavelengths according to the relative aerosol contribution to the TOA signal. This table also shows that the process converges after a few iterations. Only three iterations were performed. Rayleigh Sun glint B0 B0 variation B2 B2 variation B3 B3 variation Ground Flight iteration % % % Flight iteration % % % Flight iteration % % % Table 2: B0 and B2 Rayleigh scattering calibration sensitivity to B3 calibration over sun glint This procedure is now applied routinely. The lamp is used once a month, while three campaigns a year (duration: 1.5 month) are programmed for Rayleigh scattering and sun glint calibration. RESULTS All the results are given in figures 1 to 4 for the 4 spectral bands. The results are normalized by the pre-flight measurement. The plain line stands for the reference calibration. For the Rayleigh scattering, sun glint and cloud methods, each calibration point corresponds to the average of all the selected measurements performed over the processed image. For the desert method, each calibration point corresponds to the average of all the measurements over one month. One may notice: - the small dispersion of Rayleigh and sun glint measurements (< 1.5%) - the very good consistency of Rayleigh and sun glint measurements with the reference curve (< 2%) - the good consistency of Rayleigh, sun glint and clouds measurements for B0 band (bias < 3%) - the very good consistency of clouds and sun glint for B3 band (< 0.5%) - a bias with POLDER over deserts (5 to 10% according to the band) - a bias with in-situ measurements for B0, B2 and SWIR bands (6 to 10%) Table 3 presents the root mean square deviation between the VGT reference calibration and all the other methods. The calibration accuracy (last line of the table) is estimated by the root mean square of these deviations. -5-

6 Deviation/reference calibration B0 B2 B3 SWIR Sphere 0.1 % 8.1 % 7.3 % 10.8 % White Sands 10.9 % 6.8 % 3.7 % 9.3 % Rayleigh 0.8 % 2.0 % / / Sun glint (ref.=b2) 1.8 % / 0.5 % 0.6 % Clouds (ref.=b2) 3.1 % / 0.9 % / Deserts / POLDER 4.6 % 5.9 % 9.5 % / La Crau HRVIR / 7.7 % 0.1 % 6.3 % Rayleigh HRVIR / 1.7 % / / RMS Calibration Performance 5.1 % 5.9 % 5.1 % 7.8 % Table 3: Comparison of calibration results with the reference calibration -6-

7 VGT channel B0 calibration VGT channel B2 calibration 1,1 1,0 estimed Ak Pre-flight Rayleigh Sun glint Clouds Deserts White Sands 1,1 1,0 estimated Ak Pre-flight Rayleigh Rayleigh HRVIR Deserts White Sands La Crau 0,9 0,9 0,8 0,8 0,7 24/03/98 02/07/98 10/10/98 18/01/99 28/04/99 06/08/99 14/11/99 22/02/00 Days 0,7 24/03/98 02/07/98 10/10/98 18/01/99 28/04/99 06/08/99 14/11/99 22/02/00 Days VGT channel B3 calibration VGT channel SWIR calibration estimated Ak 1,1 Pre-flight Sun glint 1,1 estimated Ak Pre-flight Clouds Sun glint Deserts White Sands White Sands La Crau La Crau 1,0 1,0 0,9 Ak 0,9 0,8 0,8 0,7 24/03/ /07/ /10/ /01/ /04/ /08/ /11/ /02/2000 Days 0,7 24/03/ /07/ /10/ /01/ /04/ /08/ /11/ /02/2000 Days Fig. 1. to 4.: VGT calibration results for the four spectral bands compared with the reference calibration displayed in plain line The absolute calibration results are normalized by the pre-flight -7- measurement

8 CONCLUSION A significant effort has been made to achieve accurate calibration of the VEGETATION cameras. A number of concurrent methods are applied and an official and operational calibration procedure has now been established, combining onboard and vicarious calibration results. The consistency of the results provided by such different methods (lamp, Rayleigh scattering, sun glint, clouds, deserts, test sites) increases their reliability. Nevertheless, research is being done to explain the calibration bias noted with the test sites measurements and with the POLDER calibration. All calibration requirements have been met, except for absolute calibration of the SWIR band, which is more difficult to assess. The estimated calibration accuracy is: - absolute calibration: around 5% for visible and NIR bands - multidate calibration: better than 2% - interband calibration: B2/B3: 2% B2/B0 and B2/B1: 3% - HRVIR/VGT calibration: B2: 3% B3 and SWIR: 2% ACKNOWLEDGEMENTS We are grateful to all members the VEGETATION calibration team at CNES and ONERA/DOTA : Xavier Briottet, François Cabot, Magdeleine Dinguirard, Bertrand Fougnie, Olivier Hagolle, Bruno Lafrance, Marie- Christine Laubies, Frédérique Meunier, Paul Soule. REFERENCES Briottet, X., E. Diligeard, R. Santer, J.L. Deuze, VEGETATION Calibration of the Blue and Red Channels Using Rayleigh Scattering Over Open Oceans, EUROPTO, European Symposium on Aerospace Remote Sensing, London, Biggar, S.F., M. Dinguirard, D. Gellman, P. Henry, R. Jackson, M.S. Moran, P.N. Slater, Radiometric calibration of SPOT 2 HRV A comparison of three methods, Proc. SPIE 1943: , Cabot, F., O. Hagolle, C. Ruffel, P. Henry, Remote sensing data repository for in-flight calibration of optical sensors over terrestrial targets, SPIE Proc., 3750, Denver, Cosnefroy, H., M. Leroy and X. Briottet, "Selection and characterization of Saharan and Arabian desert sites for the calibration of optical satellite sensors", Remote Sens. Environ., 58, , Hagolle, O., Ph. Goloub, P.Y. Deschamps, H. Cosnefroy, X. Briottet, T. Bailleul, J.M. Nicolas, F. Parol, B. Lafrance, M. Herman, "Results of POLDER in-flight calibration", IEEE Transactions on Geoscience and Remote Sensing, 37, 03, Henry, P., M. Dinguirard and M. Bodilis, " SPOT multi-temporal calibration over stable deserts areas", Proc. SPIE, International Symposium on Optical Engineering and Photonics, 1938, Orlando, Leroy, M., P. Henry, B. Guenther and J. McLean, Comparison of CNES spherical and NASA hemisphere large aperture integration sources, Remote Sens. Environ., 31:97-104, 1990 Meygret, A., X. Briottet, P. Henry and O. Hagolle, "Calibration of SPOT4 HRVIR and VEGETATION cameras over Rayleigh scattering", Proc. SPIE s International Symposium on Optical Science and Technology, San Diego, Santer, R., X.F. Gu, G. Guyot, J.L. Deuze, C. Devaux, E. Vermote, M. Verbrugghe, SPOT calibration at the La Crau test site, Remote Sens. Environ., 41: , Vermote, E., R. Santer, P.Y. Deschamps, M. Herman, In-flight calibration of large field-of-view sensors at short wavelengths using Rayleigh scattering, Int. J. of Remote Sensing, 13, No18,

9 Vermote, E. and Y.J. Kaufman, Absolute calibration of AVHRR visible and near infrared channels using ocean and cloud views, Int. J. Remote Sensing, 16, ,