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

claas.koehler@dlr.de

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.