ISS Utilization: MUSES (Multi-User System for Earth Sensing) with DESIS instrument
Teledyne Brown Engineering of Huntsville AL, USA, is developing MUSES (Multiple User System for Earth Sensing), an Earth imaging platform, as part of the company's new commercial space-based digital imaging business. MUSES hosts earth-viewing instruments (Hosted Payloads), such as high-resolution digital cameras, hyperspectral imagers, and provides precision pointing and other accommodations. It hosts up to four instruments at the same time, and offers the ability to change, upgrade, and robotically service those instruments.
MUSES, a commercial Earth-sensing platform on the ISS, will further increase the Space Station's research capabilities. MUSES is is a precision-pointing platform that will mount externally to the ISS. The instruments installed on the platform – including high-resolution digital cameras – are oriented towards Earth. The platform can host up to four Earth observation instruments and offers the ability to change, upgrade, and robotically service those instruments.
On 1 October 2013, Teledyne Brown Engineering, Inc. (TBE), a subsidiary of Teledyne Technologies Inc., Thousand Oaks, CA, and DLR (German Aerospace Center — Deutsches Zentrum für Luft- und Raumfahrt) signed a memorandum of agreement to develop an instrument for MUSES (Multi-User System for Earth Sensing), which will be mounted on the ISS (International Space Station). 1)
On May 20, 2014 at the ILA Berlin Air Show, DLR and the US corporation TBE (Teledyne Brown Engineering, Inc.) signed an agreement to install and operate the imaging spectrometer, DESIS (DLR Earth Sensing Imaging Spectrometer) on board the ISS. This DLR-built instrument will be one of four camera systems for remote sensing fitted to the MUSES (Multi-User System for Earth Sensing) instrument carrier to be installed by TBE on the ISS. DESIS will be able to detect changes in the land surface, oceans and atmosphere; it will contribute to the development of effective measures to protect the environment and climate. 2) 3)
Under the agreement, DLR will build the DESIS (DLR Earth Sensing Imaging Spectrometer), a hyperspectral instrument which Teledyne will integrate onto its ISS imaging platform, the MUSES (Multi-User System for Earth Sensing). Teledyne is planning to operate the instrument and retrieve remote sensing data which the company will use for commercial applications and DLR will apply the data to scientific research in atmospheric physics and Earth sciences.
Development status: 6)
• Planned launch in Q2 2017.
• On June 27-29, 2016, CDR (Critical Design Review) in Berlin.
Figure 1: Overview of the MUSES mission capabilities (image credit: TBE, DLR)
MUSES itself is not a sensing instrument; it is a platform for pointing a range of instruments that could include spectrometers, multi and hyperspectral imagers, high resolution panchromatic imagers, lidar, radar, magnetic sensors and more. The platform can host four instruments simultaneously and offers the ability to change, upgrade, and robotically service each individually. One of the first instruments will be DESIS, a VNIR (Visible/Near-Infrared) imaging spectrometer built by the DLR (German Aerospace Center) to gather information on atmospheric physics and Earth sciences. Ironically, the instrument will itself become an object of study—DLR scientists will investigate the influence of the space environment on remote sensing instruments once DESIS is returned to Earth at the end of its mission. The fact that it can be returned to Earth after its mission emphasizes once again some of the ISS's unique advantages. 7) 8) 9) 10)
MUSES will be the first commercial Earth-sensing platform on the ISS—commercial in the sense that it will be designed, built, operated and managed by a commercial entity TBE (Teledyne Brown Engineering), and research institutions and private sector companies outside NASA will have the opportunity to mount their instruments on the platform, specify where they would like to look and own rights to the data gathered.
The MUSES platform provides accommodations for two large and two small hosted payloads (Figure 2). MUSES is attached at the EXPRESS Logistics Carriers (ELC-4) starboard of the ISS. It is a space-based, Earth-pointing platform providing position sensing, data downlink, and other core services for each payload attitude control. 11)
Figure 2: MUSES platform with the four slots for different instruments. The hyperspectral sensor DESIS will be integrated in one of the large slots (TBE)
DESIS is one of the hosted instruments on the MUSES platform, it has a mass of ~88 kg and is integrated in one of the large containers.
MUSES is equipped with two gimbals, thus allowing rotations around two axes up to ±25° forward-backward, 45° backboard view and a 5° starboard view. The pointing accuracy is better than 30 arcseconds, which corresponds to about 60 m on ground at 400 km altitude. Together with the POI (Pointing Unit) of the DESIS instrument, a ±40º along track viewing is possible. The MUSES platform is also equipped with a star tracker and a miniature inertial measurement unit providing attitude measurement.
The MUSES platform is equipped with a star tracker (sampling rate 10 Hz) and a miniature IMU (Inertial Measurement Unit ) with a sampling rate 50 Hz providing a 10 Hz attitude measurement after filtering. ISS GPS data provide position and velocity vectors and time tags (sampling rate 1 Hz) serving as a master time for the MUSES instruments with an accuracy of ±250 µs. The predicted viewing capability of MUSES, when operating at the ISS orbit inclination of 51.6°, will enable the DESIS instrument to scan about 90% of the populated Earth with a 3-5 day average cadence. The daily download capacity is 225 GB.
MUSES will be the first commercial instrument platform on the ISS. The MUSES platform has a size of 85 cm x 85 cm and can accommodate four instruments. It will be attached to a pivot arm on the side of the ISS facing Earth during an astronaut spacewalk.
MUSES Facility Operations:
TBE will operate the DESIS hyperspectral sensor on the ISS and will cooperate with DLR in using the data in various areas, for instance systematic and applied research. Nominal MUSES commanding is accomplished from the TBE TSC (Telescience Support Center) in Huntsville, Alabama. A nominal weekly file upload window is planned to allow regular scheduled updates to be in sync with POIF (Payload Operations and Integration Function) normal upload cycles. Any commands considered "critical" are issued from the POIF, which manages all commands classified as "critical".
The end-user data products of MUSES can be used for: Maritime Domain Awareness, Agricultural Awareness, Food Security, Disaster Response, Air Quality, Oil/Gas Exploration, and Heritage Preservation.
Figure 3: Artist's rendition of where MUSES will attach to the ISS (image credit: TBE, NASA)
Launch: MUSES was launched to the ISS on June 3, 2017 on the SpaceX CRS-11 Falcon-9 vehicle as part of a Dragon capsule cargo resupply mission from Launch Complex LC-39A, Cape Canaveral, Florida. The external payloads manifested for this flight are NICER (Neutron star Interior Composition ExploreR), MUSES (Multi-User System for Earth Sensing) and ROSA (Roll-Out Solar Array). 12) 13)
Orbit: Near circular orbit, altitude of ~ 400 km, inclination = 51.6º.
DESIS (DLR Earth Sensing Imaging Spectrometer)
DESIS is a hyperspectral camera that records image data using an array of up to 235 closely spaced channels, covering the visible and near infrared portions of the spectrum (450 - 1000 nm) with a ground resolution of 30 m. This multifaceted information allows scientists to detect changes in ecosystems and to make statements on the condition of forests and agricultural land. Among other things, its purpose is to secure and improve the global cultivation of food. The data from the ISS instruments will be available quickly in the event of a catastrophe and can help rescue teams operating on the ground to org anise their deployment. DLR and TBE seek to combine the data from other MUSES instruments to develop advanced methods for remote sensing of the Earth. Cooperation in this scientific and commercial use will also promote hyperspectral technologies for future satellites. 14) 15)
Installation on the ISS will also mean that the instruments can be brought back to Earth after a service life of between three and five years to analyze the influence of the space environment on the remote sensing instruments. The platform with the DLR DESIS instrument is scheduled to be mounted on the ISS by mid of 2017 and will, after a four month commissioning phase, enter its operational phase at least until 2020.
DESIS was developed by DLR in a partnership with La Trobe University in Melbourne, Australia. 16)
Teledyne owns the platform, determines pointing schedules, and retains data rights in cooperation with partners. Teledyne and DLR have partnered to build, integrate, & operate the DLR Earth Sensing Imaging Spectrometer (DESIS) from the Teledyne-owned MUSES Platform on the ISS (Ref. 6).
DESIS is a hyperspectral instrument is a pushbroom imaging spectrometer in a spectral range of 400 nm up to 1000 nm (VNIR) and based on a modified Offner design for the spectrometer. The telescope is based on a TMA design. Beyond the scientific mission goals, Teledyne will use the instrument for commercial applications. The main difference between DESIS and most of the hyperspectral design is that DESIS is equipped with a steering mirror for BRDF (Bidirectional Reflectance and Distribution Function) measurements; the minimum spectral resolution will be 2.55 nm which is realized over 235 channels in an spatial resolution of 30 m GSD (Ground Sample Distance). A 2D back illuminated CMOS (Complementary Metal Oxide Semiconductor) detector array from BAE (CIS2001) is employed.
Features of the DESIS instrument:
• Offner spectrometer, compact structure with a minimal number of optical components (lens, slit, primary mirror, convex grating and CCD array detector)
• Corrected, telecentric lens objective (robust, easy adjustment, low mass) with flat field and minimal aberrations, no complicated multi-mirror system
• Spherical primary mirror, which combines collimating and imaging optics
• Convex grating as dispersion element, the appropriate optimization of the spectrometer at high spatial and spectral resolution up to a flat focal plane (a prerequisite for using plane CCD detectors)
• A specially designed groove profile of the grating leads to:
- Almost complete suppression of the second order spectrum
- Almost complete polarization insensitivity of the spectrometer over the entire spectral range
• In-orbit calibration with internal lamps besides the spectrometer slit and with a LED screen
• Pointing unit (POI).
Table 1: Key parameter of DESIS
On-board programmable binning up to a factor of four is possible. The SNR (Signal-to-Noise Ratio) for the corresponding spectral sampling distances is shown in Figure 5. The GSD (Ground Sampling Distance) depends on the flight altitude and is about 30 m at nadir. This results in a swath width of about 30 km. The electronic shutter mechanism is based on a rolling shutter: each channel collects light during the same period of time, but the time light collection starts and ends is slightly different for each channel. As a result, each spectral channel integrates light over slightly different surface areas on ground.
Figure 5: SNR for the spectral sampling of 2.55 nm (lower curve) and the 4x binning 10. 10.21 nm spectral sampling (upper curve). The simulation is based on MODTRAN (standard mid-latitude summer atmosphere; albedo= 0.3; sampling of 0.2 nm), image credit: DLR Optical Systems
Figure 6: Photos of the Offner Spectrometer (image credit: DLR)
Figure 7: Offner spectrometer: Focal plane, linear dimensions, second order spectrum suppressed (image credit: DLR)
DESIS is equipped with a POI (Pointing unit) consisting of two fixed and one rotating mirror in front of the entrance slit that allows, by rotating one mirror, a forward and backward viewing change up to ±15° w.r.t. the nominal (e.g. nadir) view. The POI can be operated in a static mode with 3º angle steps for the viewing direction and in a dynamic mode with up to 1.5º change in viewing direction/s. This change in viewing direction allows – besides normal Earth data takes – acquiring experimental data to produce e.g. stereo or BRDF (Bidirectional Reflectance Distribution Function) products and continuous observations of the same targets on ground (FMC forward motion compensation). The POI can also be operated in calibration mode to minimize the external light fields and allowing on-board calibration measurements.
Besides a pre-flight spectral, radiometric and geometric calibration and characterization of the instrument in laboratory, the instrument contains an on-board calibration unit comprising different monochromatic and white light LEDs between 400 and 1000 nm. The calibration unit is located close to the POI mirror in front of the DESIS instrument, and allows the illumination of the full spectrometer FOV with color and white light. The calibration unit is characterized in laboratory and temperature controlled in orbit within 1 K.
Figure 8: Optical scheme of the Offner spectrometer (image credit: DLR)
Figure 9: Illustration of the DESIS container instrument (image credit: DLR)
The basic idea of MUSES is that all parts of the instrument can be controlled and operated via TCP/IP in the normal ISS network environment. This includes an on-instrument mass memory unit which is able to store up to 512 Gbit of data without data compression and an internal instrument control which is able to fulfill all the imaging and calibration modes.
DLR's motivation to do the DESIS design together with Teledyne was to have the possibility to generate a new class of hyperspectral data. Of course, the design is able to generate standard data sets like most of the hyperspectral designs. But in addition the design is able to penetrate the atmosphere in different angles which allows a separation from volume to surface BRDF effects. This will be used to control the atmospheric correction to have a reproducibility of data results independent of the ISS inclination.
DLR will use the same technology to observe lightning by night not with the necessary special resolution but with the high spectral resolution. In this case DESIS will be operated in a forward motion compensation mode based on an analog steering mirror control. DESIS can also be operated in a forward motion compensation for other applications where the SNR shall be enhanced.
DLR is also interested to investigate work on florescence effects measurements in a surrounding of mega cities as well in the country side.
Figure 10: Artist's view of the DESIS instrument on the MUSES platform of the ISS (image credit: NASA)
Processing chain and products
The DESIS products are derived from tiled data takes of a size 1024 x 1024 pixels (i.e. about 30 x 30 km2), which are generated within an automatic processing chain. Two identical processing chains are implemented within DLR and the TBE Ground Segment. The processing levels are following the definitions of ESA (European Space Agency).
DESIS level 1A products – namely Earth image scenes, onboard calibration measurements, dark current measurements and experimental products - will be long-term archived together with the corresponding metadata, while level 1B products (systematically and radiometrically corrected data), level 1C products (geometrically corrected data), level 2A products (atmospherically corrected data) will be processed on demand before being delivered to the user (Ref. 11).
Table 2: DESIS product definitions
DESIS level 1A products – namely Earth image scenes, on-board calibration measurements, dark current measurements and experimental products - will be long-term archived together with the corresponding metadata, while level 1B products (systematically and radiometrically corrected data), level 1C products (geometrically corrected data), level 2A products (atmospherically corrected data) will be processed on demand and delivered to the user community (see Figure 11 for the DESIS processing chain and the products).
Figure 11: Processing chain and product generation (image credit: DLR)
Transcription Processor (L1A):
The Level 1A Processor collects information from the different data streams, extracts and interprets the information, and evaluates and derives additional information for long term storage. The functionality of the Level 1A Processor comprises the following tasks:
• Screening of the data includes the inspection (comparison to reference tables) of all status information, temperatures, currents and voltages, which are available in the VC (Virtual Channel) of the DESIS instrument data.
• Extraction and evaluation of DC (Dark Current) measurements before and after each datatake with 128 frames each. Mean and standard deviations of the DC frames are compared to reference values.
• Preparation of Earth data takes, which includes tiling of the data take (1024x1024 spatial pixels), annotating metadata for further processing and quicklook generation.
• Processing of calibration data takes and publishing to the off-line calibration process, which derives new calibration tables.
Systematic and Radiometric Conversion Processor (L1B):
The L1B processor corrects the data for systematic effects and converts them to physical at-sensor radiance or TOA values based on the calibration tables valid for the specific time period. The correction includes the following tasks:
• Dead pixel flagging
• Dark signal correction using linear interpolated dark signal values derived from the dark signal measurements before and after each datatake
• Non-linearity response correction using the valid calibration tables
• Spectral referencing using the spectral calibration database
• Radiometric referencing using the valid radiometric calibration table
• Generation of data quality indicators like abnormal pixel masks, band-to-band correlations, bad line and bad column mask.
• Radiometric conversion towards at-sensor radiance values
Orthorectification Processor (L1C):
Orthorectification is the process to generate map conform products by removing geometric distortions caused by the sensor internal geometry, the satellite motion during data acquisition and the terrain related influences. The DESIS L1C processor produces orthoimages employing the technique of the rigorous model of DG (Direct Georeferencing) . The LOS (Line-of-Sight) model forms the basis of DG and is derived from the collinearity equation. LOS in this context means the view direction of each pixel at any instance of time.
The sensor internal geometry of the hyperspectral image will be extensively characterized in the laboratory by highly accurate measurements of the direction angles of single illuminated pixels (gravity center of the pixel) to the (adjusted) collimator axis. For each pixel the two angles on object side completely describe the internal camera geometry. This also includes (possible) geometric keystone effects. The mounting angles with respect to the attitude measurement system (body coordinate frame) are refined in the commissioning phase by geometric calibration procedures due to the gravity release and temperature influence. The star tracker measurements are combined by Kalman filtering with the angular measurements of the inertial measurement unit. These (unit) quaternions are finally transformed from the ECI (Earth Centered Inertial) frame to the ECR (Earth Centered Rotated) frame). A GPS provides the satellite position (and velocity) with a rate of 1 Hz. The attitude and position measurements are interpolated (e.g. Lagrange interpolation) for each scan line of the image
Within an iterative process the intersection points of the LOS vectors for each pixel and the DEM (Digital Elevation Model) is determined and leads to 3D points in object space.
The object points of the grid are expressed in an Earth bound Cartesian coordinate frame and are transformed to a user selectable map projection system [e.g. UTM (Universal Transverse Mercator) with the zone derived from the center coordinates of scene as well as the neighboring zones, geographic projection]. Within the map projection system image resampling is performed towards 30 m pixel spacing in case of UTM and 1 arcsec in case of Geographic projection. Different selectable resampling methods (e.g. nearest neighbor, bi-linear, cubic convolution) to generate the final orthorectified products are offered to the customer.
The geometric accuracy of the orthorectification is crucial for overlaying the data with existing data sets, maps, or in geographic information systems (GIS) and using them for evaluations like change detection, map updating, and others like enhanced atmospheric correction using terrain information (see chapter of Atmospheric Correction Processor). Therefore, an improvement of the LOS model shall be achieved by GCPs (Ground Control Points), which are extracted automatically from reference images of superior geometric quality using image matching techniques. As a global reference image database, the Landsat-8 panchromatic images with an absolute geometric accuracy of 12 m circular error at 90% confidence level and 14 m ground resolution is foreseen. 20)
In a first step, tie points between the uncorrected image and the reference image are determined by intensity based matching. 21) Supplementing the height values, interpolated from the DEM, to the found tie points a set of full qualified GCP are derived. In a second step, the GCP sets serve as input to improve the LOS model parameters within a least squares adjustment process. GCP outlier detection is included in the matching processes itself as well as in the least squares adjustment.
The feasibility of the approach to extract GCP from reference images for a refinement of the DG model within an operational environment has been successfully demonstrated at different projects. For example, the production of two European coverages by orthorectification of SPOT- 4 HRVIR, SPOT- 5 HRG and IRS P-6 LISS III scenes. Figure 12 shows the mosaic of the orthorectified scenes covering main parts of Europe. A relative geometric accuracy of 10 m RMSE w.r.t. the reference images have been achieved. About 450 GCP per 1000 km2 (about the size of one DESIS scene) have been extracted automatically.
Figure 12: Example of orthorectified products derived from SPOT-4 HRVIR, SPOT-5 HRG and IRS-P6 LISS III data using automatically extracted GCP information from reference images for sensor model refinement. The overall geometric accuracy w.r.t. the reference is ~10 m RMSE linear (image credit: DLR)
Atmospheric Correction Processor (L2A):
The DESIS L2A processor performs atmospheric correction of the images employing the well-known code of ATCOR (Atmospheric/Topographic Correction for Airborne Imagery). 22) 23) Input for the atmospheric correction processor is the L1C product. For the atmospheric correction over land a combined atmospheric and topographic processing is possible, provided the geometric absolute accuracy of the DESIS orthoimage is sufficient. A geometric accuracy better than one pixel size is required for this combined topographic / atmospheric correction in order to avoid artefacts caused by the inaccurate co-registered DEM and orthorectified image.
The MODTRAN-5.3.3 (moderate resolution atmospheric transmission) code is employed to model properties of the solar reflective spectrum (from 400 to 2500 nm). It supports a sufficiently high accuracy for the absorption simulation (water vapor, ozone, oxygen, carbon dioxide etc.). It also includes a rigorous treatment of the coupled scattering and absorption processes. Moreover, it offers a set of representative aerosol models (rural or continental, urban, maritime, desert). Therefore, MODTRAN-5.3.3 is selected to compile a database of atmospheric correction LUTs (Look-up Tables) with a high spectral resolution of 0.4 nm to enable the processing of the 2.55 nm (binned 10.21 nm) channel bandwidths of DESIS. This "monochromatic" or fine spectral resolution database has to be resampled with the DESIS channel filter curves. The advantage of compiling a "monochromatic" database is twofold. First it gives the possibility of quickly resampling it with updated spectral channel filter functions avoiding the necessity to run time-consuming radiative transfer calculations for the solar and view geometry pertaining to the acquired scenes and second to account for spectral smile corrections.
The atmospheric correction accounts for flat and rugged terrain (Figure 13), and includes haze/cirrus detection algorithms.
Figure 13: Example of topographic correction. Left image: without topographic correction; Middle: slope image derived from a DEM; Right: with topographic correction. (Example SPOT-5 scene from Switzerland), image credit: DLR
Inflight spectral, radiometric and geometric calibration:
Inflight calibration refers to all measurements and data analyses aiming to assess radiometric, spectrometric and geometric characteristics of the DESIS hyperspectral instruments in orbit. DESIS will undergo extensive characterization and calibration measurements before launch and will be re-calibrated after launch by updating the calibration tables. 26)
The DESIS calibration unit (see Figure14 for the layout and Figure 4 for the location of the calibration unit) consists of a number of different monochromatic and white LEDs with wavelengths between 400 and 1000 nm. The shape of the emitted light beam of the calibration unit allows monochromatic and spectrally broad-band illumination within the full spectrometer FOV. Pre-defined combinations of LED's will be operated at the same time to simulate different illumination scenarios. The unit will be positioned in the spectrometer optical beam by the corresponding position of the pointing unit mirror. The switched-off calibration unit serves as dark reference.
Figure 14: Principal design of the DESIS Calibration Unit, which shows three lines with each eight monochromatic LED's and one white LED (image credit; DLR-OS)
• Rigorous pre-launch geometric & radiometric calibration (Ref. 6)
• On-board calibration unit:
- DSNU (Dark Signal Non-Uniformity) calibration before/after each collected scene.
- Periodic PRNU (Photo Response Non-Uniformity) calibration.
- Geometry Calibration using Single LEDs (Light-Emitting Diodes)
• Vicarious Calibration
- Planned every 30 – 60 days
- CEOS sites, RadCalNet autonomous instrumented sites
- Spectral diversity using ARSC (Alabama Remote Sensing Consortium) research fields and forests.
The DESIS instrument will be operated by Teledyne and only Teledyne will receive the raw data from the ISS. TBE has the exclusive right to license or transfer image data for commercial use. For scientific and humanitarian purposes, DLR has the right to task DESIS or request archived data. In these cases, and if no conflicts with the commercial activities appear, Teledyne will hand over the data to DLR. Therefore, Teledyne provides DLR a license to the instrument data DLR receives for scientific and humanitarian use. The parties will attempt to schedule DLR's tasking requests so that tasking is generally balanced throughout the calendar year.
For scientific purposes only, DLR can share DESIS scientific data with other scientific organizations. Any commercial use of these instrument data is prohibited without Teledyne's prior written permission. All end users of the instrument data provided to DLR for scientific use will be required to enter into a data license agreement among DLR and Teledyne. Scientific use includes:
• basic and application oriented research
• projects by national and international educational or research institutions or by governmental institutions
• development and demonstration of future applications for scientific and/or operational use and
• preparation and execution of government-funded education, research and development programs.
Figure 15: DESIS data access policy (image credit: DLR)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).