Vol49_1_2006def 65 ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006 Key words hyperspectral – airborne scanner – MIVIS – radiometric calibration – data quality 1. Introduction The uncertainty introduced by the retrieval methods of remotely sensed measurements is re- lated both to sensor calibration and its stability, as well as to atmospheric effects. As a matter of fact, the problem of data calibration has become a key problem for many remote sensing applica- tions. The main task of the calibration process is to experimentally determine the relationship be- tween the measured signals, expressed as instru- mental counts, and the corresponding physical values of radiance of the investigated targets, and it has to guarantee the reliability and repro- ducibility of the measured object radiances not only in the laboratory but also throughout the mission. The methods used for sensor calibration can be grouped into three domains (Dinguirard and Slater, 1999): pre-launch, in-flight and indi- rect or vicarious approaches. As many authors stated (Teillet et al., 1997; Thome, 2002), to derive high quality data from remote sensed images, sensors must be time-sta- ble instruments with well-understood characteris- tics. Therefore, it is always a good norm to in- clude calibration approaches that are independent from pre-flight in research programs. Traditional- ly, the radiometric calibration of satellite systems has been accomplished by on-board calibration A new methodology for in-flight radiometric calibration of the MIVIS imaging sensor Marco Gianinetto and Giovanmaria Lechi Laboratorio di Telerilevamento, Dipartimento di Ingegneria, Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento (DIIAR), Politecnico di Milano, Italy Abstract Sensor radiometric calibration is of great importance in computing physical values of radiance of the investigated targets, but often airborne scanners are not equipped with any in-flight radiometric calibration facility. Consequent- ly, the radiometric calibration or airborne systems usually relies only on pre-flight and vicarious calibration or on indirect approaches. This paper introduces an experimental approach that makes use of on-board calibration tech- niques to perform the radiometric calibration of the CNR’s MIVIS (Multispectral Infrared and Visible Imaging Spectrometer) airborne scanner. This approach relies on the use of an experimental optical test bench originally de- signed at Politecnico di Milano University (Italy), called MIVIS Flying Test Bench (MFTB), to perform the first On-The-Fly (OTF) calibration of the MIVIS reflective spectral bands. The main task of this study is to estimate how large are the effects introduced by aircraft motion (e.g., e.m. noise or vibrations) and by environment condi- tions (e.g., environment temperature) on the radiance values measured by the MIVIS sensor during the fly. This pa- per describes the first attempt to perform an On-The-Fly (OTF) calibration of the MIVIS reflective spectral bands (ranging from 430 nm to 2.500 nm). Analysis of results seems to point out limitations of traditional radiometric cal- ibration methodology based only on pre-flight approaches, with important implications for data quality assessment. Mailing address: Dr. Marco Gianinetto, Laboratorio di Telerilevamento, Dipartimento di Ingegneria, Idraulica, Ambientale, Infrastrutture Viarie, Rilevamento (DIIAR), Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Mila- no, Italy; e-mail: gianinetto@polimi.it 66 Marco Gianinetto and Giovanmaria Lechi sources (such as lamps, diffusers, etc.) supported by vicarious calibration activities using natural targets, while nowadays several sensors are using the moon (Barnes et al., 1999; Kieffer and Wildey, 1996) or stellar targets (Bowen, 2002) as calibration references. Unfortunately, most of the airborne sensors are not equipped with any in- flight radiometric calibration device. For this rea- son, the calibration procedure of airborne sys- tems usually relies only on pre-flight and indirect approaches that cannot assess the real perform- ance of the sensor during the fly. This paper introduces an experimental ap- proach that makes use of on-board calibration techniques to perform the radiometric calibration of CNR’s MIVIS (Multispectral Infrared and Visible Imaging Spectrometer) airborne scanner. This approach relies on the use of an experimen- tal optical test bench originally designed at the Remote Sensing Laboratory of the Politecnico di Milano University (Italy), called MIVIS Flying Test Bench (MFTB), to perform the first on-the- fly (OTF) calibration of the MIVIS reflective spectral bands. The OTF of the MIVIS sensor was performed with a 1-h in-flight ATP (Accep- tance Test Procedure), using the MFTB as refer- ence source mounted onto the MIVIS scan head. By rationing the digital numbers (DNs) output from the MIVIS with the laboratory measured radiance of the optical test bench, the absolute OTF radiometric calibration of the MIVIS hy- perspectral scanner was achieved. The main task of this study was to estimate how large were the effects introduced by air- craft motion (e.g., noise, vibrations) and by op- erational conditions (e.g., environment temper- ature) on radiance values measured by the MIVIS hyperspectral scanner, and also to veri- fy the temporal stability of the MIVIS sensor during the fly. All this to improve the radiomet- ric calibration of MIVIS data. A brief overview of the MFTB calibrator and first results of the MIVIS OTF calibration tests are presented. 2. The MIVIS sensor The MIVIS is an airborne hyperspectral scanner for remote sensing applications belong- ing to Italian CNR-IIA, Roma. The MIVIS op- tical system consists of a scanner and four spec- trometers. Line array detectors are used, requir- ing scanning to be performed mechanically in whisk broom mode (Bianchi et al., 1995). One spectrometer line detector array exists for each of the visible (VIS), near infrared (NIR), short- wave infrared (SWIR) and thermal infrared (TIR) portions of the spectrum, for a total of 102 spectral bands ranging from 430 nm to 12 . 680 nm. 3. Radiometric calibration of the MIVIS sensor and the MFTB The radiometric calibration of the MIVIS hyperspectral scanner is usually done prior to the flight with the optical test bench Daedalus AB532 which was supplied from the manufac- turer (Daedalus Enterprises, Ann Arbor, MI, U.S.A.). For VIS, NIR and SWIR calibration (ranging from 430 nm to 2.500 nm) the Daeda- lus AB532 optical bench uses a white Spec- tralon (polytetrafloroethylene) panel illuminat- ed under a fix geometrical configuration, while for TIR calibration (ranging from 8.200 nm to 12 .700 nm) it uses a blackbody flat plate. The DN to at-sensor-radiance conversion is per- formed using a self calibration procedure called Acceptance Test Procedure (ATP). In particular, for the 92 reflective bands (spectral bands from no. 1 to no. 92), the ATP measures, for every MIVIS scanning frequency (from 8.3 Hz to 25 Hz), the radiance of the Daedalus AB532 test bench illuminated at constant power (fig. 1). The limit in using the Daedalus AB532 test bench for deriving the MIVIS radiometric cali- bration parameters (gains and offsets) is that the Daedalus AB532 test bench is not a transportable device (dimensions: 890 mm × 640 mm × 1200 mm, weight: 160 kg), thus it is not possible to test the scanner’s performance during the flight. To estimate how large are the effects introduced by the aircraft motion and environment condi- tions (e.g., temperature effects) on the radiance values measured by the MIVIS sensor, a very compact and light calibration device was de- signed to be used for MIVIS OTF radiometric calibration. 67 A new methodology for in-flight radiometric calibration of the MIVIS imaging sensor The MFTB was designed at Politecnico di Milano University (Italy) and built with the technical support of Compagnia Generale Ri- preseaeree (CGR), Parma, Italy (now a Blom ASA Company). The project of the MFTB con- cerned the definition of physical, optical and ra- diometric characteristics of the new test bench system, followed by the building of a prototype. The MFTB is composed of two different sub systems: the test bench and the power supply. The test bench was shaped to fit the MIVIS scan head during the flight and was designed to fit in the aircraft Casa 212C used to carry the MIVIS (140 mm × 340 mm × 210 mm). It was built using a 3mm aluminium sheet for the ex- ternal chassis and a 10mm white Teflon coat for the internal Lambert diffuser. As light sources, four 10 W halogen lamps (with colour temper- ature 2900 K) were used, disposed with a sym- metrical geometry. To supply the MFTB both using the 220 V AC and the aircraft 28 V DC, a current controlled power supply with constant output (3.32 A at 12.5 V DC) was built. 4. Field experiments and preliminary results After a laboratory check, the radiance of the MFTB was measured using a laboratory Analyt- ical Spectral Device (ASD) FieldSpec FR spec- troradiometer. The ASD was chosen because of its spectral range similar to that of the MIVIS re- flected bands (ranging from 350 nm to 2.500 nm) and its higher sampling frequency (1.4 nm at 350-1.000 nm and 2 nm at 1.000-2.500 nm). Moreover, for the VIS and the NIR bands, the Noise Equivalent Radiance (NER) of the ASD is of one order of magnitude smaller than the NER of the MIVIS, and is of two order of magnitude Fig. 1. Example of Acceptance Test Procedure (ATP) self calibration procedure output file for the first 20 MIVIS spectral bands. 68 Marco Gianinetto and Giovanmaria Lechi smaller for the SWIR. Therefore, the uncertain- ties in using the ASD to validate the MFTB cal- ibrator is at least one order of magnitude small- er than the MIVIS precision. The construction phase of the MFTB was followed by a period of validation tests at CGR, where the intercalibration between the Daedalus AB532 and MFTB optical test benches was achieved on the basis of a traditional laboratory ATPs. To realize the OTF calibration for the MIVIS sensor, the MFTB was mounted onto the MIVIS scan head and the sensor was flown to perform a 1-h in-flight ATP under controlled conditions (fig. 2). To control the MFTB cali- brator stability during the tests, the OTF ATP was carried out controlling the current flowing from the MFTB power supply to the MFTB in- ternal lamps (3.32 A at 12.5 V DC). Concerning the use of the MFTB during the flight, unpredictable differences in at-sensor ra- diance were found comparing the results of the laboratory calibration tests with those per- formed on-the-fly (fig. 3). While differences in at-sensor radiance measured in the VIS (rang- ing from 430 nm to 830 nm) are small and lim- ited to 1-2%, in the NIR (ranging from 1.150 nm to 1.550 nm) they grow up to more than 6.5% for MIVIS spectral band no. 27 (1.450 nm-1.500 nm wavelength) and no. 28 (1.500 nm-1.550 nm wavelength). Our OTF experiments are in accordance with the differences found in the MIVIS survey of Grado (Italy) in winter 2000 (Giardino et al., 2001), where, after performing on the MIVIS data atmospheric correction with the Second Simulation of Satellite Signal in the Solar Spec- trum (6S) radiative transfer code (Vermote et al., 1997), researchers found that the MIVIS NIR data and the ground reference measurements taken with the ASD during the survey diverged of about 10%. Analysis of results points out the limitations of traditional calibration techniques for airborne sensors. Using a traditional methodology (labora- tory test bench or laboratory integrating sphere) Fig. 2. Relationship between DN and at-sensor-radiance computed with the OTF procedure (OTF) and with the traditional laboratory ATP (laboratory). Comparison of results for the MIVIS spectral bands no. 26 and no. 27 using the MFTB calibrator. 69 A new methodology for in-flight radiometric calibration of the MIVIS imaging sensor for computing the instrumental gain and offset pa- rameters, it is not possible to test the sensor in re- al conditions. For this reason, a precise data con- version from instrumental counts into radiance values can be only derived in post-processing us- ing on OTF procedure as that here described. As shown in our tests, sometimes these differences can be not negligible and also seems to be wave- length dependant, not only a constant bias. 5. Conclusions For many remote sensing applications the use of calibrated data has become one of the most important points. Besides, the use of a large number of narrow spectral bands forces hyperspectral sensors to have SNR that de- creases with increasing wavelength, and this can restrict the real use of hyperspectral data to only visible and near-infrared data, if not ade- quately calibrated (Colombo et al., 2002). A new optical test bench for the MIVIS air- borne hyperspectral scanner (MFTB) was orig- inally developed at Politecnico di Milano Uni- versity to improve the use of remote sensed MIVIS data. The new approach to airborne OTF calibration proposed in this paper and the use of the experimental MFTB has shown that MIVIS data may be affected by considerable error if the radiometric conversion procedure is not based on real in-flight conditions, as Giardi- no et al. (2001) showed with an independent study for Grado (Italy) MIVIS survey. Fig. 3. Comparison between ground tests and OTF tests. Differences in at-sensor-radiance stands out between 1% and 2% in the VIS (ranging from 430 nm to 830 nm wavelength) and more than 6.5% in the NIR (ranging from 1.150 nm to 1.550 nm wavelength). (*) = spectral band not working during the tests. 70 Marco Gianinetto and Giovanmaria Lechi Acknowledgements This work has been carried out under a re- search framework founded by Italian National Research Council (CNR), contract title «Cali- brazione radiometrica su 102 canali distribuiti fra le bande dal visibile all’infrarosso termico dello scanner iperspettrale aerotrasportato MIVIS del CNR» and by the Italian Space Agency (ASI) within the project «Certificazione di qualità delle mappe tematiche e dei Modelli Digitali delle El- evazioni (DEM) ottenute con tecniche di teleril- evamento e standardizzazione di legende, classi- ficatori e caratteristiche spettrali radiometriche e geometriche» contract number I/R/132/01. 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