Remote Sensing EnMAP related Cal/Val Activities of the User · of the UAV is below 10 kg, thus...
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Remote Sensing Technology Institute Experimental Methods The User Service OpAiRS The user service Optical Airborne Remote Sensing & Calibration Home Base (OpAiRS) is an ISO 9001 certified entity within the DLR Remote Sensing Technology Institute (IMF). Together with the German Remote Sensing Data Center (DFD), the IMF forms the DLR Earth Observation Center (EOC). In close cooperation with the DLR Flight Experiments Facility, OpAiRS provides comprehensive hyperspectral remote sensing expertise covering a wide range of services from sensor operation to the generation of various thematic products (figure 3). Thereby OpAiRS builds on the expertise and infrastructure of various EOC departments. OpAiRS focuses its activities on the development of innovative remote sensing methods through the provision of high quality sample data. Furthermore we support the validation of current and upcoming satellite missions (EnMAP, Sentinel-2) with airborne & ground based campaigns. Our services are available to third-party institutions through cooperative research projects. The HySpex Sensor System OpAiRS operates a combination of two imaging HySpex spectrometers manufactured by the Norwegian company Norsk Elektro Optikk (NEO) [1]. The push broom sensors (see figure 2) of type VNIR 1600 and SWIR 320m-e (figure 1) are used for the airborne acquisition of spectrally and spatially highly resolved hyperspectral data. Due to a similar spectral resolution and the almost identical spectral range, the system is perfectly suited for the simulation and later validation of data recorded by the German EnMAP [3] satellite. This is the reason why the system is also referred to as DLR EnMAP simulator. HySpex can optionally be operated in combination with a very high resolution stereo camera system for experimental data fusion applications. The HySpex system is available for Transnational Access (TNA) through EUFAR. EnMAP related Cal/Val Activities of the User Service OpAiRS C. H. Köhler, A. Baumgartner, J. F. S. Brachmann Figure 3: Flowchart of OpAiRS core services (blue background) with contributions of various EOC departments (gray background). Claas H. Köhler Deutsches Zentrum für Luft- und Raumfahrt (DLR) Institut für Methodik der Fernerkundung (IMF) Oberpfaffenhofen [email protected] EOC/ User IMF-EXV DFD-INF IMF-PBA DLR Flight Experiments IMF-ATP Campaign Organization Airworthiness Certification Data Acquisition Calibration System Correction Sensor Model Orthorectification Raw Data (L0) Atmospheric Correction Radiance (L1B) Reflectance (L2A) Radiance (L1C ) D-SDA Archive Product Generation Water Depth Vegetation Parameters … www.dlr.de/opairs Ground Based and UAV borne Instruments Apart from HySpex, OpAiRS operates various field spectrometers and a UAV borne snapshot imager (figure 4). Handheld spectrometers such as the SVC 1024i have proven to be useful tools for reference measurements at the surface. They facilitate the in- situ identification of distinct spectral properties for various materials, which form the basis of most remote sensing applications. Recently OpAiRS also acquired a Cubert UHD-185 Firefly snapshot spectrometer, which can be mounted on one of several Unmanned Aerial Vehicles (UAVs) available at IMF. The light weight Firefly imager can be flown in configurations, where the maximum take-off weight of the UAV is below 10 kg, thus minimizing legal constraints on airborne operations. UAVs are expected to play a crucial role in bridging the gap between ground based measurements and aircraft observations. However, a reliable calibration is a prerequisite for a meaningful comparison of measurements acquired by different instruments from different platforms. We are currently investigating, in how far commercially available instruments such as the Firefly are capable of delivering the required data quality. Table 1 shows selected key properties, which were determined in the CHB for Cubert Firefly and NEO HySpex. As the snapshot imaging technology allows to acquire entire hyperspectral scenes without scanning, we are also examining the potential regarding hyperspectral remote sensing of highly dynamic scenes, where common push broom scanners deliver unsatisfactory results. focal plane array grating lenses slit mirror IFOV FOV footprint Figure 2: Principle setup of a push broom scanner Figure 1: Photo of the HySpex sensor system (left) with an RGB image created from data acquired over DLR‘s Oberpfaffenhofen site (right). The OpAiRS Calibration Facility (CHB) Within the scope of OpAiRS the IMF maintains a laboratory specially dedicated to the characterization of push broom sensors (figure 2). This so-called Calibration Home Base for APEX (CHB, [2]) has been designed for the airborne imaging spectrometer APEX funded by ESA. One distinct feature of the laboratory is its high level of automation, which enables the encompassing characterization of the OpAiRS sensor suite on a regular basis including their spectral, geometric and radiometric properties. The CHB is also used for the development of novel calibration techniques. Based on the experience gained with the CHB and HySpex, OpAiRS supports the preflight calibration of EnMAP in close cooperation with the EnMAP prime contractor OHB. References [1] DLR Remote Sensing Technology Institute (IMF) (2016). Airborne Imaging Spectrometer HySpex. J. large-scale res. fac., 2(A93) [2] DLR Remote Sensing Technology Institute (IMF) (2016). The Calibration Home Base for Imaging Spectrometers. J. large-scale res. fac., 2(A82) [3] http://www.enmap.org spatial stray light spectral stray light Figure 5: Schematic depiction of a stray light measurement of the central pixel of the “redmost” channel. Towards Improved Remote Sensing Products Our ultimate goal at OpAiRS is to support the development of innovative remote sensing methods. Current and upcoming instruments designed for optical earth observation offer a combination of high spatial resolution and fine spectral resolution. The information content of the measured data allows to combine retrieval techniques typically applied to atmospheric remote sensing with those developed for traditional earth observation applications. In combination with an accurate and reliable instrument calibration recent developments may lead to significantly improved remote sensing products, especially in challenging applications such as remote sensing of inland waters or the determination of atmospheric aerosols and trace gases. Figure 6: Stray light correction for the idealized case of mono- chromatic illumination of a single geometric pixel: Contributions erroneously detected by other channels/pixels have to be reattributed to the correct channel & pixel. The plot uses a logarithmic intensity scale. VNIR- 1600 SWIR- 320me UHD-185 Firefly Spectral range [μm] 0.41–1 0.97–2.5 0.45–0.95 Spectral res. [nm] 3.5–6 5.6–7 5–30 Num. of channels 160 256 125 Num. of pixels 1600 320 50x50 Min. spatial res. [m] 0.3/0.6* 0.6/1.2* 0.1 Figure 4: SVC 1024i field spectrometer (left) and Cubert Firefly snapshot spectrometer mounted beneath an ASCTEC UAV (right). Table 1: Characteristic properties of the HySpex VNIR & SWIR sensors compared to those of the UAV-borne Firefly snapshot camera. The asterisk (*) indicates usage of an expander lens, which approximately doubles the HySpex field of view. EnMAP Calibration Support Based on the expertise gathered with multiple airborne instruments over the last 25 years, OpAiRS supports the calibration of the EnMAP satellite instrument in a joint OHB-DLR project. Apart from consulting OHB regarding the measurement of the geometric and spectral instrument properties, OpAiRS develops a method for the experimental determination (figure 5) and subsequent algorithmic correction of stray light (figure 6). Stray light is defined as radiation detected under a wrong wavelength and/or a wrong angle of incidence due to imperfect optical imaging inside the instrument. Stray light can cause significant problems if strong gradients occur inside a single frame. These gradients can be caused by scenes with large intensity variations (e.g. white sand close to dark water bodies) or by scenes containing strong spectral variations (consider red light with high intensity scattered into a blue channel with faint illumination). The experimental challenge consists in the exposure of a small detector region to radiation with an intensity sufficient to cause measurable stray light on the remaining detector pixels while avoiding unpredictable behavior of the detector due to overexposure . Additional challenges arise due to the large number of measurements required to completely characterize the stray light properties of an optical system.
Transcript of Remote Sensing EnMAP related Cal/Val Activities of the User · of the UAV is below 10 kg, thus...
Remote Sensing Technology Institute Experimental Methods
The User Service OpAiRS
The user service Optical Airborne Remote Sensing &Calibration Home Base (OpAiRS) is an ISO 9001certified entity within the DLR Remote SensingTechnology Institute (IMF). Together with the GermanRemote Sensing Data Center (DFD), the IMF formsthe DLR Earth Observation Center (EOC). In closecooperation with the DLR Flight Experiments Facility,OpAiRS provides comprehensive hyperspectralremote sensing expertise covering a wide range ofservices from sensor operation to the generation ofvarious thematic products (figure 3). Thereby OpAiRSbuilds on the expertise and infrastructure of variousEOC departments. OpAiRS focuses its activities onthe development of innovative remote sensingmethods through the provision of high qualitysample data. Furthermore we support the validationof current and upcoming satellite missions (EnMAP,Sentinel-2) with airborne & ground based campaigns.Our services are available to third-party institutionsthrough cooperative research projects.
The HySpex Sensor System
OpAiRS operates a combination of two imagingHySpex spectrometers manufactured by theNorwegian company Norsk Elektro Optikk (NEO) [1].The push broom sensors (see figure 2) of type VNIR1600 and SWIR 320m-e (figure 1) are used for theairborne acquisition of spectrally and spatially highlyresolved hyperspectral data. Due to a similar spectralresolution and the almost identical spectral range,the system is perfectly suited for the simulation andlater validation of data recorded by the GermanEnMAP [3] satellite. This is the reason why the systemis also referred to as DLR EnMAP simulator. HySpexcan optionally be operated in combination with avery high resolution stereo camera system forexperimental data fusion applications. The HySpexsystem is available for Transnational Access (TNA)through EUFAR.
EnMAP related Cal/Val Activities of the User Service OpAiRS
C. H. Köhler, A. Baumgartner, J. F. S. Brachmann
Figure 3: Flowchart of OpAiRS core services (blue background) with contributions of various EOC departments (gray background).
Claas H. Köhler
Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Institut für Methodik der Fernerkundung (IMF)
Oberpfaffenhofen
EOC/ User
IMF-EXV
DFD-INF
IMF-PBA
DLR Flight Experiments
IMF-ATP
Campaign Organization
AirworthinessCertification
Data Acquisition
Calibration
System Correction
Sensor Model
Orthorectification
Raw Data (L0)
Atmospheric Correction
Radiance (L1B)
Reflectance (L2A)
Radiance (L1C )
D-SDA
Archive
Product Generation
Water Depth
Vegetation Parameters
…
www.dlr.de/opairs
Ground Based and UAV borne Instruments
Apart from HySpex, OpAiRS operates various fieldspectrometers and a UAV borne snapshot imager(figure 4). Handheld spectrometers such as the SVC1024i have proven to be useful tools for referencemeasurements at the surface. They facilitate the in-situ identification of distinct spectral properties forvarious materials, which form the basis of mostremote sensing applications. Recently OpAiRS alsoacquired a Cubert UHD-185 Firefly snapshotspectrometer, which can be mounted on one ofseveral Unmanned Aerial Vehicles (UAVs) available atIMF. The light weight Firefly imager can be flown inconfigurations, where the maximum take-off weightof the UAV is below 10 kg, thus minimizing legalconstraints on airborne operations. UAVs areexpected to play a crucial role in bridging the gapbetween ground based measurements and aircraftobservations. However, a reliable calibration is aprerequisite for a meaningful comparison ofmeasurements acquired by different instrumentsfrom different platforms. We are currentlyinvestigating, in how far commercially availableinstruments such as the Firefly are capable ofdelivering the required data quality. Table 1 showsselected key properties, which were determined inthe CHB for Cubert Firefly and NEO HySpex. As thesnapshot imaging technology allows to acquire entirehyperspectral scenes without scanning, we are alsoexamining the potential regarding hyperspectralremote sensing of highly dynamic scenes, wherecommon push broom scanners deliver unsatisfactoryresults.
focalplanearray
grating
lenses
slit mirror
IFOV
FOV
footprintFigure 2: Principle setup of a push broom scanner
Figure 1: Photo of the HySpex sensor system (left) with an RGB image created from data acquired over DLR‘s Oberpfaffenhofen site (right).
The OpAiRS Calibration Facility (CHB)
Within the scope of OpAiRS the IMF maintains alaboratory specially dedicated to the characterizationof push broom sensors (figure 2). This so-calledCalibration Home Base for APEX (CHB, [2]) has beendesigned for the airborne imaging spectrometerAPEX funded by ESA. One distinct feature of thelaboratory is its high level of automation, whichenables the encompassing characterization of theOpAiRS sensor suite on a regular basis includingtheir spectral, geometric and radiometric properties.The CHB is also used for the development of novelcalibration techniques. Based on the experiencegained with the CHB and HySpex, OpAiRS supportsthe preflight calibration of EnMAP in closecooperation with the EnMAP prime contractor OHB.
References
[1] DLR Remote Sensing Technology Institute (IMF)(2016). Airborne Imaging Spectrometer HySpex.J. large-scale res. fac., 2(A93)
[2] DLR Remote Sensing Technology Institute (IMF)(2016). The Calibration Home Base for ImagingSpectrometers. J. large-scale res. fac., 2(A82)
[3] http://www.enmap.org
spatial stray light
spectral stray light
Figure 5: Schematic depiction of a stray light measurement of the central pixel of the “redmost” channel.
Towards Improved Remote Sensing Products
Our ultimate goal at OpAiRS is to support thedevelopment of innovative remote sensing methods.Current and upcoming instruments designed foroptical earth observation offer a combination of highspatial resolution and fine spectral resolution. Theinformation content of the measured data allows tocombine retrieval techniques typically applied toatmospheric remote sensing with those developedfor traditional earth observation applications. Incombination with an accurate and reliable instrumentcalibration recent developments may lead tosignificantly improved remote sensing products,especially in challenging applications such as remotesensing of inland waters or the determination ofatmospheric aerosols and trace gases.
Figure 6: Stray light correction for the idealized case of mono-chromatic illumination of a single geometric pixel: Contributions erroneously detected by other channels/pixels have to be reattributed to the correct channel & pixel. The plot uses a logarithmic intensity scale.
VNIR-1600
SWIR-320me
UHD-185Firefly
Spectral range [µm] 0.41–1 0.97–2.5 0.45–0.95
Spectral res. [nm] 3.5–6 5.6–7 5–30
Num. of channels 160 256 125
Num. of pixels 1600 320 50x50Min. spatial res. [m] 0.3/0.6* 0.6/1.2* 0.1
Figure 4: SVC 1024i field spectrometer (left) and Cubert Firefly snapshot spectrometer mounted beneath an ASCTEC UAV (right).
Table 1: Characteristic properties of the HySpex VNIR & SWIR sensors compared to those of the UAV-borne Firefly snapshot camera. The asterisk (*) indicates usage of an expander lens, which approximately doubles the HySpex field of view.
EnMAP Calibration Support
Based on the expertise gathered with multipleairborne instruments over the last 25 years, OpAiRSsupports the calibration of the EnMAP satelliteinstrument in a joint OHB-DLR project. Apart fromconsulting OHB regarding the measurement of thegeometric and spectral instrument properties,OpAiRS develops a method for the experimentaldetermination (figure 5) and subsequent algorithmiccorrection of stray light (figure 6). Stray light isdefined as radiation detected under a wrongwavelength and/or a wrong angle of incidence dueto imperfect optical imaging inside the instrument.Stray light can cause significant problems if stronggradients occur inside a single frame. These gradientscan be caused by scenes with large intensityvariations (e.g. white sand close to dark waterbodies) or by scenes containing strong spectralvariations (consider red light with high intensityscattered into a blue channel with faint illumination).The experimental challenge consists in the exposureof a small detector region to radiation with anintensity sufficient to cause measurable stray light onthe remaining detector pixels while avoidingunpredictable behavior of the detector due tooverexposure . Additional challenges arise due to thelarge number of measurements required tocompletely characterize the stray light properties ofan optical system.
