Karim Lenhard

Airborne Prism Experiment Calibration Information System

The calibration of remote sensing instruments is a crucial step in the generation of products tied to international reference standards. Calibrating imaging spectrometers is particularly demanding due to the high number of spatiospectral pixels and, consequently, the large amount of data acquired during calibration sequences. Storage of these data and associated meta-data in an organized manner, as well as the provision of efficient tools for the data analysis and fast and repeatable calibration coefficient generation with provenance information, is key to the provision of traceable measurements. The airborne prism experiment (APEX) calibration information system is a multilayered information technology solution comprising a database based on the entity-attribute-value (EAV) paradigm and software written in Java and Matlab, providing data access, visualization and processing, and handling the data volumes over the expected lifetime of the system. Although developed in the context of APEX, the system is rather generic and may be adapted to other pushbroom-based imagers with little effort.

Characterisation methods for the hyperspectral sensor HySpex at DLR’s calibration home base

The German Aerospace Center’s (DLR) Remote Sensing Technology Institute (IMF) operates a laboratory for the characterisation of imaging spectrometers. Originally designed as Calibration Home Base (CHB) for the imaging spectrometer APEX, the laboratory can be used to characterise nearly every airborne hyperspectral system. Characterisation methods will be demonstrated exemplarily with HySpex, an airborne imaging spectrometer system from Norsk Elektro Optikks A/S (NEO). Consisting of two separate devices (VNIR-1600 and SWIR-320me) the setup covers the spectral range from 400 nm to 2500 nm. Both airborne sensors have been characterised at NEO. This includes measurement of spectral and spatial resolution and misregistration, polarisation sensitivity, signal to noise ratios and the radiometric response. The same parameters have been examined at the CHB and were used to validate the NEO measurements. Additionally, the line spread functions (LSF) in across and along track direction and the spectral response functions (SRF) for certain detector pixels were measured. The high degree of lab automation allows the determination of the SRFs and LSFs for a large amount of sampling points. Despite this, the measurement of these functions for every detector element would be too time-consuming as typical detectors have 105 elements. But with enough sampling points it is possible to interpolate the attributes of the remaining pixels. The knowledge of these properties for every detector element allows the quantification of spectral and spatial misregistration (smile and keystone) and a better calibration of airborne data. Further laboratory measurements are used to validate the models for the spectral and spatial properties of the imaging spectrometers. Compared to the future German spaceborne hyperspectral Imager EnMAP, the HySpex sensors have the same or higher spectral and spatial resolution. Therefore, airborne data will be used to prepare for and validate the spaceborne system’s data.

Independent Laboratory Characterization of NEO HySpex Imaging Spectrometers VNIR-1600 and SWIR-320m-e

The Remote Sensing Technology Institute (Institut fur Methodik der Fernerkundung) of the German Aerospace Agency (DLR) operates two sensors for airborne hyperspectral imaging, i.e., a Norsk Elektro Optikk A/S (NEO) HySpex VNIR-1600 and a NEO HySpex SWIR-320m-e. Since these sensors are used for the development of physically based inversion algorithms, atmospheric correction algorithms and for calibration/ validation activities, their properties need to be characterized in detail, and an accurate calibration is mandatory. The characterization is performed at the calibration laboratory of DLR for imaging spectrometers in Oberpfaffenhofen. Key results of the characterization are assessments of the radiometric, spectral, and geometric performances, including the typical optical distortions prevalent in pushbroom imaging spectrometers, keystone and smile, and the associated measurement uncertainties. Potential sources of systematic error, the detector nonlinearity and the polarization sensitivity are discussed. The radiometric calibration is traceably performed to the German national metrology institute Physikalisch-Technische Bundesanstalt, whereas the spectral measurements can be traced back to the spectral properties of atomic line lamps. The implemented level 0 to level 1 calibration procedure is presented as well.

Detection and Correction of Radiance Variations During Spectral Calibration in APEX

The airborne prism experiment (APEX) is an imaging spectrometer developed by a joint Swiss-Belgian consortium composed of institutes (University of Zurich, Flemish Institute for Technological Research) and industries (RUAG, OIP, Netcetera), supported by the European Space Agencys PRODEX programme. APEX is designed to support the development of future space-borne Earth observation systems by simulating, calibrating or validating existing or planned optical satellite missions. Therefore, periodic extensive calibration of APEX is one major objective within the project. APEX calibration under laboratory conditions is done at its dedicated calibration and characterization facility at the German Aerospace Center (DLR) in Oberpfaffenhofen, Germany. While environmental influences under laboratory conditions are reduced to a minimum, the effects of atmospheric absorption and the properties of the underlying calibration infrastructure may still influence the measurements and subsequently the accuracy of the sensor spectral response estimations. It is demonstrated that even a lightpath of ~2 m through the atmosphere or the monochromator grating can have significant impact on the spectral response estimation of the sensor. A normalization approach described in this letter is able to compensate for these effects. The correction algorithm is exemplarily demonstrated on actual measurements for the short wavelength-IR range channel.

Determination of combined measurement uncertainty via Monte Carlo analysis for the imaging spectrometer ROSIS.

To enable traceability of imaging spectrometer data, the associated measurement uncertainties have to be provided reliably. Here a new tool for a Monte-Carlo-type measurement uncertainty propagation for the uncertainties that originate from the spectrometer itself is described. For this, an instrument model of the imaging spectrometer ROSIS is used. Combined uncertainties are then derived for radiometrically and spectrally calibrated data using a synthetic at-sensor radiance spectrum as input. By coupling this new software tool with an inverse modeling program, the measurement uncertainties are propagated for an exemplary water data product.

Impact of Improved Calibration of a NEO HySpex VNIR-1600 Sensor on Remote Sensing of Water Depth

This paper investigates at the example of bathymetry how much an application can profit from comprehensive characterizations required for an improved calibration of data from a state-of-the-art commercial hyperspectral sensor. A NEO HySpex VNIR-1600 sensor is used for this paper, and the improvements are based on measurements of sensor properties not covered by the manufacturer, in particular, detector nonlinearity and stray light. This additional knowledge about the instrument is used to implement corrections for nonlinearity, stray light, spectral smile distortion and nonuniform spectral bandwidth and to base the radiometric calibration on a SI-traceable radiance standard. Bathymetry is retrieved from a data take from the lake Starnberg using WASI-2D. The results using the original and improved calibration procedures are compared with ground reference data, with an emphasis on the effect of stray-light correction. For our instrument, stray-light biases the detector response from 416-500 nm up to 8% and from 700-760 nm up to 5%. Stray-light-induced errors affect bathymetry mainly in water deeper than Secchi depth, whereas in shallower water, the dominant error source is the calibration accuracy of the light source used for radiometric calibration. Stray-light correction reduced the systematic error of water depth by 19% from Secchi depth to three times Secchi depth, whereas the relative standard deviation remained stable at 5%.

The Radiance Standard RASTA of DLR's calibration facility for airborne imaging spectrometers

The German Aerospace Center (DLR) operates the Calibation Home Base (CHB) as a facility for the calibration of airborne imaging spectrometers and for field spectrometers. Until recently, absolute radiometric calibration was based on an integrating sphere that is traceable to SI units through calibration at the German Metrology Institute PTB. However, the stability of the radiance output was not monitored regularly and reliably. This was the motivation to develop a new radiance standard (RASTA) which allows monitoring in the wavelength range from 380 to 2500 nm. Radiance source is a diffuse reflector illuminated by a tungsten halogen lamp. Five radiometers mounted in a special geometry are used for monitoring. This setup improves twofold the uncertainty assessment compared to the previously used integrating sphere. Firstly, lamp irradiance and panel reflectance have been calibrated at PTB additionally to the radiance of the complete system. This calibration redundancy allows to detect systematic errors and to reduce calibration uncertainty. Secondly, the five radiometers form a redundant control system to measure changes of the spectral radiance. This enables long-time monitoring of the radiance source including assessment of the uncertainty caused by aging processes. Further advantages concern the reduction of periods of non-availability, applicability to sensors with larger field of view, and the possibility to alter intensity and spectral shape in a well-known way by exchanging the reflector. RASTA has been calibrated at PTB in November 2011 in the wavelength range from 350 to 2500 nm.

Calibration procedures for imaging spectrometers: improving data quality from satellite missions to UAV campaigns

The Calibration Home Base (CHB) at the Remote Sensing Technology Institute of the German Aerospace Center (DLR-IMF) is an optical laboratory designed for the calibration of imaging spectrometers for the VNIR/SWIR wavelength range. Radiometric, spectral and geometric characterization is realized in the CHB in a precise and highly automated fashion. This allows performing a wide range of time consuming measurements in an efficient way. The implementation of ISO 9001 standards ensures a traceable quality of results. DLR-IMF will support the calibration and characterization campaign of the future German spaceborne hyperspectral imager EnMAP. In the context of this activity, a procedure for the correction of imaging artifacts, such as due to stray light, is currently being developed by DLR-IMF. Goal is the correction of in-band stray light as well as ghost images down to a level of a few digital numbers in the whole wavelength range 420-2450 nm. DLR-IMF owns a Norsk Elektro Optikks HySpex airborne imaging spectrometer system that has been thoroughly characterized. This system will be used to test stray light calibration procedures for EnMAP. Hyperspectral snapshot sensors offer the possibility to simultaneously acquire hyperspectral data in two dimensions. Recently, these rather new spectrometers have arisen much interest in the remote sensing community. Different designs are currently used for local area observation such as by use of small unmanned aerial vehicles (sUAV). In this context the CHBs measurement capabilities are currently extended such that a standard measurement procedure for these new sensors will be implemented.

Calibration of a monochromator using a lambdameter

The standard procedure for wavelength calibration of monochromators in the visible and near infrared wavelength range uses low-pressure gas discharge lamps with spectrally well-known emission lines as primary wavelength standard. The calibration of a monochromator in the wavelength range of 350 to 2500 nm usually takes some days due to the huge number of single measurements necessary. The useable emission lines are not for all purposes sufficiently dense and at the appropriate wavelengths. To get faster results for freely selectable wavelengths, a new method for monochromator characterization was tested. It is based on measurements with a lambdameter taken at equidistant angles distributed over the gratings entire angular range. This method provides a very accurate calibration and needs only about two hours of measuring time.