Bernd Harnisch

FLORIS: phase A status of the fluorescence imaging spectrometer of the Earth Explorer mission candidate FLEX

The Fluorescence Explorer (FLEX) mission is currently subject to feasibility (Phase A) study as one of the two candidates of ESA’s 8th Earth Explorer opportunity mission. The FLuORescence Imaging Spectrometer (FLORIS) will be an imaging grating spectrometer onboard of a medium sized satellite flying in tandem with Sentinel-3 in a Sun synchronous orbit at a height of about 815 km. FLORIS will observe vegetation fluorescence and reflectance within a spectral range between 500 nm and 780 nm. It will thereby cover the photochemical reflection features between 500 nm and 600 nm, the Chlorophyll absorption band between 600 and 677 nm, and the red-edge in the region from 697 nm to 755 nm being located between the Oxygen A and B absorption bands. By this measurement approach, it is expected that the full spectrum and amount of the vegetation fluorescence radiance can be retrieved, and that atmospheric corrections can efficiently be applied. FLORIS will measure Earth reflected spectral radiance at a relatively high spectral resolution of ~0.3 nm around the Oxygen absorption bands. Other spectral band areas with less pronounced absorption features will be measured at medium spectral resolution between 0.5 and 2 nm. FLORIS will provide imagery at 300 m resolution on ground with a swath width of 150 km. This will allow achieving global revisit times of less than one month so as to monitor seasonal variations of the vegetation cycles. The mission life time is expected to be at least 4 years. The fluorescence retrieval will make use of information coming from OLCI and SLSTR, which are onboard of Sentinel-3, to monitor temperature, to detect thin clouds and to derive vegetation reflectance and information on the aerosol content also outside the FLORIS spectral range. In order to mitigate the technological and programmatic risk of this Explorer mission candidate, ESA has initiated two comprehensive bread-boarding activities, in which the most critical technologies and instrument performance shall be investigated and demonstrated. The breadboards will include representative optics and dispersive elements in a configuration, which is expected to be very close to the instrument flight configuration. This approach follows the guideline to reach, before it goes into the implementation phase, a technology readiness level of at least 5. It thereby requires a demonstration of predicted performance in a configuration, where the basic technological components are integrated with reasonably realistic supporting elements such that it can be tested in a simulated environment. We will report, within the limits of the competitive nature of the industrial studies, on the currently running or planned preparatory activities. We will present the mission configuration, the imposed instrument requirements and the identified instrument concepts as derived by the Phase A studies.

On the demands on imaging spectrometry for the monitoring of global vegetation fluorescence from space

Vegetation fluorescence when measured from space contributes only a tiny fraction of the signal coming on top of the reflected radiance by the Earth surface and the atmosphere. As a consequence, imaging spectrometers have to provide sufficient throughput and radiometric accuracy to enable accurate global monitoring of the daily to seasonal variations of the Earths vegetation breath, which is particularly challenging if ground resolutions of a few hundred meters are targeted. Since fluorescence retrieval algorithms have to make corrections for atmospheric effects, it is necessary to provide sufficient spectral resolution, so that signal alterations due to the main parameters such as surface pressure, atmospheric temperature profile, vertical distribution of aerosols concentration, and water vapour content can be accurately modelled. ESA’s Earth Explorer 8 candidate mission FLEX carries a Fluorescence Imaging Spectrometer (FLORIS), which has been designed and optimised to enable such measurement. The spectrometer will measure in a spectral range between 500 and 780 nm and provide high spectral resolution of 0.3 nm in particular at the Oxygen-A and -B bands. It will also cover the photochemical reflection features between 500 and 600 nm, the Chlorophyll absorption region between 600 and 677 nm, and the red-edge in the region of 697 to 755 nm. FLEX will fly in formation with Sentinel-3 in order to further enhance the spectral coverage from measurements made by the Sentinel-3 instruments OLCI and SLSTR, particularly for cloud screening and proper characterization of the atmospheric status.

Airborne test results for smart pushbroom imaging system with optoelectronic image correction

Smart pushbroom imaging system (SMARTSCAN) solves the problem of image correction for satellite pushbroom cameras which are disturbed by satellite attitude instability effects. Satellite cameras with linear sensors are particularly sensitive to attitude errors, which cause considerable image distortions. A novel solution of distortions correction is presented, which is based on the real-time recording of the image motion in the focal plane of the satellite camera. This allows using such smart pushbroom cameras (multi-/hyperspectral) even on moderately stabilised satellites, e.g. small sats, LEO comsats. The SMARTSCAN concept uses in-situ measurements of the image motion with additional CCD-sensors in the focal plane and real-time image processing of these measurements by an onboard Joint Transform Optical Correlator. SMARTSCAN has been successfully demonstrated with breadboard models for the Optical Correlator and a Smart Pushbroom Camera at laboratory level (satellite motion simulator on base of a 5 DOF industrial robot) and by an airborne flight demonstration in July 2002. The paper describes briefly the principle of operation of the system and gives a description of the hardware model are provided. Detailed results of the airborne tests and performance analysis are given as well as detailed tests description.

Optical SAR processor for space applications

Synthetic Aperture Radar (SAR) systems typically generate copious amounts of data in the form of complex values difficult to compress. Processing this data provides real-valued images that are easier to compress, however comprehensive processing capabilities are required. Optical processor architectures provide inherent parallel computing capabilities that could be used advantageously for SAR data processing. Onboard SAR image generation would provide local access to processed information paving the way for real-time decisions. This could also provide benefits to navigation strategy or automatic instruments orientation. Moreover, for interplanetary missions or unmanned aerial vehicles (UAVs), onboard analysis of images could provide important feature identification clues and could help select the appropriate images to be transmitted to the ground (Earth). This would reduce the data throughput requirements and the related transmission bandwidth. This paper reviews the preliminary work performed for the analysis of SAR image generation using an optical processor and describes the set-up of an optical SAR processor prototype. Results of optical reconstruction of SAR signals acquired with a state-of-the-art SAR satellite are presented. Real-time processing capabilities and dynamic range calculations for a tracking optical processor architecture are also discussed.

Compact fourier-transform imaging spectrometer for small satellite massions

A compact Fourier-Transform-Imaging-Spectrometer (FTIS) for small satellite remote sensing missions is currently being studied under ESA contract with the objective to demonstrate its feasibility by breadboarding. Compared to classical hyperspectral imagers using dispersive spectrometers the major advantages of the FTIS are the very compact optics module and the tolerable higher detector temperature, thus reducing the overall mass and easing the instrument thermal design. The feasibility of this instrument concept, payload requirements to the satellite attitude stability and the data handling concept will be discussed. REQUIREMENTS AND INSTRUMENT CONCEPT The reference mission for this Fourier-transform imaging spectrometer is a Moon orbiting remote sensing satellite. The altitude of the orbit varies between 100km and 10km, where the ground pixel resolution shall be 1m at the altitude of 10km. The imaging data cube shall cover both the VNIR and the SWIR wavelength region with moderate spectral resolution. The image size shall be 1024 x 1024 pixels and the radiometric accuracy better than 1%. Baseline technology for the FTIS instrument is a Michelson interferometer with fixed tilted mirrors, which generate a spatially modulated interferogram. Each line of the detector is connected to a defined optical path difference (OPD) introduced by the interferometer. In contradiction to a Michelson interferometer with moving mirrors, the OPD change for a ground pixel is generated by the imaging of a ground line subsequently on different detector lines with different OPD’s during the flight. The FTIS instrument consists of two modules operating in the VNIR and SWIR spectrum. Some important instrument requirements are compiled in the following table: VNIR SWIR Spectral Range [μm] 0.45 – 0.90 0.90 – 2.40 Spectral Resolution [cm-1] 120 60 Spatial Resolution [mrad] 0.1 x 0.1 0.1 x 0.1 Detector Array Size 1024 (spatial) x 440 (spectral) Table 1 FTIS Requirements The FTIS optical design is shown in Figure 1. It consists of a Three-MirrorAnastigmat (TMA) lens with three conical mirrors, the interferometer cube in the intermediate image and the Offner Relay system in an all spherical mirror configuration and a corrector lens for controlling spherical aberration over the complete wavelength range. The aperture stop of the system is placed at the TMA secondary mirror. W&W + Offner F/6 f=250 Scale: 0.46 HLT 23-Nov-00 54.35 MM Figure 1 Optical Design The interferometer cube layout is shown in Figure 2. The optical path difference in the detector plane will be generated by a glass wedge in one of the two interferometer arms. The glass wedge and mirror 2 have a reflective coating on their rear side. The intermediate image is located in the mirror plane. In Figure 3 the mechanical layout of FTIS is shown. The structure and housing material is aluminum. The spaceborne FTIS shall be mounted on a baseplate. In the centre of this baseplate the instrument support structure (optical bench) is mounted Virtual image plane inside glass after reflection Image plane 2 Without OPD

Optical-correlator-based system for the real-time analysis of image motion in the focal plane of an Earth observation camera

The paper describes the design concept of an optoelectronic system for real time image motion analysis. The system is proposed to be used onboard an Earth observation satellite for the real time recording of the image motion in the focal plane of the camera. With this record available, it is possible to use pushbroom scan cameras onboard satellites with moderate attitude stability (possible geometric distortions, caused by the attitude instability, can be corrected posteirori on base of the records). New experimental results are presented, which have been derived from a real-time breadboard model of the optical processor, developed and manufactured under ESA- contract. The results of the tests are provided as well as the expected performances of a full scale system.

A real-time high-resolution optical SAR processor

An optical SAR processor prototype exhibiting real-time and fine sampling capabilities has been successfully developed and tested. Synthetic Aperture Radar (SAR) images are typically processed digitally applying dedicated Fast Fourier Transform (FFT) algorithms. These operations are time consuming and require a large amount of processing power and are often performed in one dimension at a time. A true two dimensional Fourier transform may be instead performed through optics, as optical processing provides inherent parallel computing capabilities. By processing the azimuth and slant range directions simultaneously, a reduction in processing time and power is achieved. In addition, the configuration of the optics is such that high resolution images may be obtained at no additional processing cost. The optical SAR processor is also designed to adapt to SAR system parameter changes. It has the capability to produce full Envisat / ASAR scenes from the various image mode swaths (IS1 - IS7) within tens of seconds. This paper reviews the design of the real-time high resolution optical SAR processor prototype and discusses the results of images reconstructed from simulated point targets as well as from Envisat / ASAR data sets.

Compensation of focal plane image motion perturbations with optical correlator in feedback loop

The paper presents a concept of a smart pushbroom imaging system with compensation of attitude instability effects. The compensation is performed by active opto-mechatronic stabilization of the focal plane image motion in a closed loop system with visual feedback on base of an auxiliary matrix image sensor and an onboard optical correlator. In this way the effects of the attitude instability, vibrations and micro shocks can be neutralized, the image quality improved and the requirements to satellite attitude stability reduced. To prove the feasibility and to estimate the effectiveness of the image motion stabilization, a performance model of the smart imaging system has been developed and a simulation experiment has been carried out. The description of the performance model and the results of the simulation experiment are also given.

Off-axis telescopes: the future generation of Earth observation telescopes

In 1991 Carl Zeiss started a program to develop powerful telescopes for airborne and spaceborne earth-observation telescopes. This article summarizes some of the main result of this program. To emphasize the importance of these activities a short historical review was added.

Optical concepts for high-resolution imaging spectrometers

A large variety of optical concepts for imaging spectrometers with high geometrical and spatial resolution have been studied at Dasa/Ottobrunn in various projects, which were funded by national agencies (DLR) and the European Space Agency (ESA). The imaging spectrometers emphasized herein measure spatial images of the upwelling spectral radiance from 400 to 2400 nm at 5 to 15 nm spectral intervals. All concepts are prism/grating designs based on pushbroom imaging, and are designed to fulfill stringent requirements on spatial and spectral registration accuracy. Such imaging spectrometers comprise several critical and challenging subunits such as frontend calibration, pointing, baffling, telescope and spectrometer optics, focal plane assembly, etc. Of these subunits, the paper emphasizes the driving requirements and constraints of the optics. In particular, methods to control and optimize the most critical parameters like polarization, spatial and spectral purity/accuracy, transmission, and image quality are presented. The achieved performances and design inherent properties of all concepts are given.