RapidEye Earth Observation Constellation
RapidEye is a full end-to-end commercial Earth Observation system comprising a constellation of five minisatellites, a dedicated SCC (Spacecraft Control Center), a data downlink ground station service, and a full ground segment designed to plan, acquire and process up to 5 million km2 of imagery every day to generate unique land information products.
The system is owned and operated by BlackBridge. MDA (MacDonald, Dettwiler and Associates Ltd) was the mission prime contractor and was responsible for the delivery of the space and ground segments, launch of the constellation, and on-orbit commissioning and camera calibration. The two major subcontractors to MDA were SSTL (Surrey Satellite Technology Ltd.) for the spacecraft bus, SCC and spacecraft AIT (Assembly, Integration and Test) services, as well as Jena Optronik GmbH (JOP) who provided the 5-band multispectral imager (RGB, red edge, and near IR bands).
The RapidEye constellation represents a major milestone in the Earth observation industry. It is the first fully commercial operational class Earth observation system using a constellation of 5 satellites that provides unparalleled performance. 1) 2)
• Intelligence and Defense
• Security & Emergency
• Visual Simulation.
Table 1: Overview of key mission parameters
Background: The RapidEye business concept was initiated in 1996 by Kayser-Threde GmbH of Munich with support from the German Space Agency (DLR). The overall goal was to provide end-to-end solutions to clients whose geospatial information needs require large-area coverage, repetitive monitoring and frequent revisits. RapidEye was established as an independent company in December 1998 once the concept had matured enough to receive seed financing from the German Space Agency (DLR) and Vereinigte Hagelversicherung (VH), the largest German agro-insurance company, as well as a few private investors.
In September 2002, RapidEye (known as BlackBridge since November 2013) and MDA of Richmond, BC, Canada, signed a partnership agreement. In this arrangement, MDA is the general contractor for the mission, responsible for the implementation of the space and ground segments, launch and in-orbit commissioning and calibration. 12) 13)
On November 6, 2013, the former RapidEye unit of Berlin, Germany, has officially changed its name to BlackBridge. This change is the result of a two-year process of uniting all BlackBridge owned companies as one presence in the marketplace. It will empower the BlackBridge group to harness its combined resources to offer its partners and customers end-to-end solutions across the entire geospatial value chain at a global level. 16)
The satellite platforms (MicroSat-150 - also referred to as SSTL-150) are being developed and built by SSTL (Surrey Satellite Technology Ltd., Guildford, Surrey, UK) based on the enhanced platform, used for the following remote sensing missions: TopSat, Beijing-1 (both of which were launched on Oct. 27, 2005). Each RapidEye spacecraft is three-axis stabilized, featuring a box-like shape of approximate dimensions: 0.78 m x 0.938 m x 1.17 m. The overall design has the S/C divided into three separate functional volumes. At the base of the S/C is the launch vehicle separation system which is baselined as a four-point release system with integral deployment springs along with some attitude sensors. In the mid-deck are the majority of the bus subsystems and the PEU (Payload Electronics Unit), while the optical imager and the star camera are located at the top end of the spacecraft.
Figure 1: Illustration of the RapidEye spacecraft (image credit: SSTL, MDA, BlackBridge)
The SSTL-150 bus is built around a “stack” of core avionics modules (e.g. computers and data recorders) contained in microtrays, and providing mechanical strength. This supports other components of the spacecraft. Attached to the stack are further small panels which provide extra support for the payload panel and the separation panel. The battery is mounted on the stack, between small panels, which also support the star tracker electronics modules (Ref. 2).
The S/C launch mass is ~ 156 kg, of which 12 kg is propellant and 43 kg is the optical payload.
Figure 2: Exploded view of the spacecraft bus and internal configuration (image credit: SSTL)
The spacecraft architecture and its various subsystems are shown in Figure 9.
• The electric power subsystem consists of three GaAs solar panels located on the +x, -z, and -x faces of the S/C. The solar panels are based on single junction GaAs/GE cells. The +x and –x panels are larger and have a nominal maximum power of 110 W; the smaller –z panel has a nominal power of 55 W. The orbit average power generated by the panels is 64 W at BOL (Beginning of Life). A 15 Ah Li-ion battery (from ABSL, UK) is used consisting of BCRs (Battery Charge Regulators).
• Power is supplied from the battery to the redundant PCM/PDM units. These have the following functions:
• PCM (Power Conditioning Module): conversion of the unregulated supply from the battery to a regulated 5 V supply
• PDM (Power Distribution Module): switching of both the 28 V unregulated battery supply and the regulated 5 V supply to spacecraft subsystems.
• The AOCS (Attitude and Orbit Control Subsystem) relies on four reaction wheels for three-axis control with redundant magnetic torques (dual-wound) for momentum management. In addition, there is a xenon propulsion system. Attitude sensing is provided by redundant sun sensors and magnetometers (coarse attitude knowledge); in addition a redundant star camera is providing high-accuracy attitude information (model: Altair HB star tracker developed by SSTL; heritage of BilSat-1 and Beijing-1). The star camera is mounted directly to the payload optical bench to minimize alignment errors (FOV of 15.74º x 10.53º).
The spacecraft uses a redundant GPS receiver for orbit determination and on-board time provision. A body-pointing capability in the roll axis of the spacecraft exists which permits a ±20º FOR (Field of Regard) for camera observations into any direction. Data gathering and processing is provided by the ADCS (Attitude Determination and Control Subsystem) which is running on the OBC. The ADCS modules interface between the CAN bus and analog sensors and actuators.
Figure 3: RapidEye spacecraft structure overview (image credit: SSTL)
Figure 4: AOCS functional overview (image credit: SSTL, MDA)
Figure 5: Block diagram of the AOCS (image credit: SSTL, MDA)
Figure 6: The Altair HB star tracker, camera head module and baffle (image credit: SSTL)
• The propulsion subsystem is based on a warm gas blow down system utilizing a Xenon resistojet thruster with a single propellant tank and associated plumbing. The primary mission applications envisaged for the low-power resistojet are: a) drag compensation, and b) constellation orbit phasing (each satellite relative to rest of constellation). The resistojet provides an Isp in the 50-100 s range, a thrust of 10-100 mN, and is capable of imparting a ΔV of up to 35 m/s (for a S/C mass of 150 kg). A maximum of 12 kg gaseous Xe at 70 bar can be filled into the spherical tank (volume of 7.4 liter). The propulsion unit may require up to 30 W from the platform power system. The onboard propulsion system is being used for constellation maintenance. 17)
Figure 7: Illustration of the resistojet thruster (image credit: SSTL)
Figure 8: Illustration of the propulsion module with propellant tank for the resistojet thruster (image credit: SSTL)
• Onboard data handling (OBDH) and monitoring functions are provided by redundant OBCs (OBC 386). A dual-redundant CAN (Control Area Network) bus provides communication between all subsystems and the OBC, including the payload. The CAN platform is a resilient high-speed serial platform which runs at 388 kbit/s. A real-time operating system is used to support SSTL’s custom spacecraft software which controls and monitors all the on-board systems.
Figure 9: Block diagram of RapidEye and spacecraft architecture (image credit: SSTL, MDA)
• RF communications: The onboard storage capacity of imagery is 48 Gbit. RF communication of imagery is provided in X-band (8.25-8.40 GHz), while TT&C communications are in S-band. The S-band system consists of: 2 uplink receivers (9.6 kbit/s), 2 patch antennas, 2 downlink transmitters (38.4 kbit/s), and 2 monopole antennas (downlink), modulation of QPSK.
The X-band downlink has a payload data rate of 80 Mbit/s. The data downlink uses commercial data receive stations operating in the X-band. The mission control center is located at BlackBridge headquarters, Berlin, Germany. - Each RapidEye spacecraft employs a new compact X-band antenna design of SES (Saab Ericsson Space) as shown in Figure 10. The X-band transmitters are controlled and monitored via the CAN platform. The X-band antennas use a helical radiating element that provides a wide gain pattern with a peak gain around 70º off the antenna bore sight to allow for downlinking with large roll angles in either direction.
Figure 10: Illustration of small-size quadrifilar helix antenna with radome attached (image credit: SES)
Figure 11: RapidEye spacecraft showing internal systems (image credit: SSTL, MDA)
Figure 12: View of the five spacecraft at SSTL prior to launch site shipment (image credit: BlackBridge)
Launch: A single launch of the RapidEye minisatellite constellation on a Dnepr launch vehicle took place on August 29, 2008. The launch provider was ISC (International Space Company) Kosmotras; the launch site was the Baikonur Cosmodrome, Kazakhstan. Fifteen minutes after liftoff, the tracking station in Oman received confirmation that all 5 satellites had been released from the rocket fairing on schedule. All spacecraft were being placed into slightly different orbits to allow constellation phasing. 18)
Figure 13: Photo of the RapidEye Dnepr launch vehicle at Baikonur (image credit: SSTL, MDA, BlackBridge)
Orbit: Sun-synchronous orbit (all five satellites are evenly spaced in a single orbital plane), altitude = 630 km , inclination =~ 98º, local equator crossing time at 11:00 hours (± 1 hour) on the descending node (LTDN), period = 96.7 min, revolutions/day = 14.89, spacing/orbit = 24.202º.
The S/C follow each other in their orbital plane at about 19 minute intervals. The constellation approach in a single orbital plane permits a cumulative swath to be built up (the spacecraft view adjacent regions of the ground, with image capture times separated by only a few minutes). A revisit time of one day can be obtained anywhere in the world (±84º latitude) with body pointing techniques. The average coverage repeat period over mid-attitude regions (e.g., Europe and North America) is 5.5 days at nadir.
The RapidEye constellation requires its constituent spacecraft to be at the same altitude and phased around the orbit in order to meet the imaging requirements. To achieve this, SSTL has implemented a CMS (Constellation Management System) based on previous work on the DMC (Disaster Monitoring Constellation). The CMS supports the analysis of orbit data and planning of orbit correction maneuvers, and has a simple and robust user interface. It also provides orbit data which may be used for simple visualization of the state of the constellation (Ref. 1).
Figure 14: High-level system view of CMS (image credit: SSTL)
Figure 15: RapidEye constellation in one orbital plane (image credit: BlackBridge)
Figure 16: Artist's view of the RapidEye constellation (image credit: BlackBridge)
• The RapidEye constellation spacecraft and their payloads are operating nominally in 2014. In August 2013, the constellation was 5 years on orbit. 19)
Figure 17: Imaging frequency of the RapidEye archive data between the start of operations in August 2009 and the end of 2013 (image credit: BlackBridge)
• The constellation is adding ~ 1 billion km2/year of imagery into their archives. This corresponds to about seven times the Earth’s land surface/year. Subsequently, potential AOIs (Areas of Interest) are not only covered once, but with a high temporal resolution. Figure 17 summarizes the imaging frequency of archived data. The Earth’s populated land mass is covered multiple times. 20)
• BlackBridge undertook serious efforts to adapt to changing market and boundary conditions. The capabilities of the constellation have evolved and continue to evolve. To keep up with evolving market demands, BlackBridge has continually worked at enhancing the mission concept beyond its original vision and capabilities, extending the mission concept to include data sales, and emergency response.
• In 2013/2014, BlackBridge is studying and evaluating the requirements for data continuity, allowing the seamless provision of RapidEye data to the community beyond the lifetime of the current satellite system.
• Nov. 20, 2013: As part of their long-standing relationship, BlackBridge AG has provided ESA ( European Space Agency) with a substantial time series of RapidEye imagery to support the Sentinel-2 preparatory project. The aim was to collect imagery at the same time frequency Sentinel-2 will, as well as support the R&D activities relevant to the Sentinel-2 mission (e.g. agriculture, wetlands, coastal, food security and forest monitoring). RapidEye imagery was selected by ESA because of its spatial resolution and revisit capabilities. 21)
• Sept. 30, 2013: RapidEye announced today that its North American agricultural imaging campaign has been completed successfully. The campaign, which ran monthly from May 15, 2013 through September 14, 2013, generated more than 16 million km2 of cloud-free imagery over 3 million km2 spanning twenty-eight US states and three Canadian provinces. 22)
• May 2013: RapidEye starts the 2013 North American Agricultural Imaging Campaign, to be imaged four times over four months. This is the second year for the campaign, which will task 2.9 million km2 covering most major agricultural areas in the US, including the ‘corn belt’, and Canada. The entire campaign, which runs through September 15, will cover these areas once each month, producing over 11.5 million km2 of imagery for agricultural monitoring. 23)
The customer base for this campaign are basically agronomy/agriculture companies providing value-added products to their customers (mostly farmers). 24)
• April 2013: All 5 satellites are fully-operational. Engineers and operators monitor systems trends and performance routinely. Improved calibration techniques developed to ensure image quality uniformity and stability. The space segment is utilized beyond initial design capacity. 25) 26)
- The original design of the RapidEye system aimed for a collection capacity of 4 million km2/ day and a processing capacity of around 2.1 million km2/day. At the start of operations in 2009, the initial capacity was about 3 million km2 per day. Steady improvements have augmented the collection capacity of the constellation to be able to now (2013) collect up to 5 million km2/ day. Among other measures, RapidEye applies seasonal data compression settings to optimize the usage of the on-board data storage capacity, keeping image quality at the highest standard.
- The power consumption of the constellation has turned out to be lower than initially estimated during the satellite design phase. A lower power consumption rate allows data to be acquired during more orbits, without the need for rest orbits. The original limitation of 10 imaging orbits per day has now been extended to 12 - 13 imaging orbits per day without compromising the battery charge-discharge lifetime performance. This extends the capabilities of each satellite and results in a larger cumulative amount of acquired data (Ref. 26).
- As of early April, 2013, RapidEye has an increased data downlink capacity through greater access to KSAT’s larger antenna network in Svalbard, Norway. These larger strategically placed Northern antennae offer RapidEye the ability to provide more reliable continuous data collection around the globe,particularly in Australia and the Far East.- In addition, RapidEye has recently upgraded its existing dedicated KSAT antenna with S-band capability to support TT&C contact with the constellation. The improved communication service allows RapidEye contact with the spacecraft on every orbit, which allows for more agile response and better maintenance of the satellites’ health and lifespan. 27)
• In January 2013, the RapidEye constellation is over 4 years on orbit and continues to operate effectively, with all five satellites fully operational. Based on the current performance of the satellites and the abundance of consumables such as propellant and power, RapidEye now expects the constellation to perform into 2019 or later. 28)
- As of mid-December 2012, RapidEye AG relocated its HQs from Brandenburg an der Havel (~ 80 southwest of Berlin) to the city of Berlin. The spacecraft control center is located in Berlin, and the S-band and X-band downlink stations are located in Svalbard, Norway.
• In November 2012, BlackBridge joined the ESA TPM (Third Party Mission) program. Through this program, ESA provides data to the European and international scientific user community, as well as to supplement its own internal projects for application development and research. RapidEye's archive data is now available to the scientific user community from ESA member states (including Canada), the European Commission Member States, as well as Africa and China (as part of the Dragon program) through project proposal submission via the RapidEye information area on ESA's Earthnet Online Portal. 29)
Figure 18: RapidEye image of Alaska, USA, collected in September 11, 2012 (image credit: BlackBridge)
• On August 29, 2012, the constellation was four years on orbit.
• Summer 2012: All 5 spacecraft of the constellation are operating nominally. 30)
- A healthy power margin allows to drive the constellation almost to its imaging capacity
- Cameras are well calibrated: Relative calibration shows that all 5 cameras are well within ± 1% of each other
- Absolute calibration indicates accuracy within 6%
- Based on the current status of the consumables, the lifetime could extend until 2020.
• The RapidEye constellation is operating nominally in 2012.
Figure 19: Image of Moscow acquired in August 2012 (image credit: BlackBridge)
• In December 2011, RapidEye was awarded a contract from China's Ministry of Land and Resources (MLR) to cover almost five million square kilometers of China over the next few months. This represents the third consecutive year that RapidEye was a successful bidder to cover China for the MLR. 31)
• In September 2011, RapidEye was awarded an Indefinite Delivery Indefinite Quantity (IDIQ) contract with NGA (National Geospatial-Intelligence Agency) of Washington D.C. 32)
• The RapidEye constellation is operating nominally in 2011. Two years of operational service have proven RapidEye's capability to react fast and efficiently to varying customer requests. 33)
- BlackBridge undertook serious efforts to adapt to changing market and boundary conditions. The capabilities of the constellation have evolved and continue to evolve. To keep up with evolving market demands, RapidEye has continually worked at enhancing the mission concept beyond its original vision and capabilities, extending the mission concept to include data sales, and emergency response. The service of direct downlink capabilities to customer stations will be added in 2011 -corresponding studies were conducted in 2010 (Ref. 33).
- Since becoming operational in February 2009, RapidEye acquired and archived imagery of more than 1.7 billion km2 of the Earth's surface, among them 1.3 billion km2 with cloud cover below 20% (about 10 times the area of the solid globe). 34)
- To provide the highest quality data products possible, the space and ground segments are constantly being monitored and fine-tuned, including extensive calibration and validation campaigns. 35) 36) 37)
• The RapidEye constellation is operating nominally in 2010. In August 2010, the constellation was two years in orbit. 38)
Figure 20: RapidEye World Coverage from Jan 1 – Aug 31, 2010, with less than 10% cloud cover (image credit: BlackBridge)
• On February 27, 2010 a heavy 8.8 magnitude earthquake struck the vicinity of Concepción, Chile. The area of Concepción was hit the hardest and was the most affected area in the region hit by the quake. Immediately after the news had spread to Europe, RapidEye, now BlackBridge), imaged the area next to the epicenter of the earthquake, which covered a total area of 13,125 km2. RapidEye used its own before and after imagery of the city to record the changes that were caused by the earthquake. RapidEye delivered these images to relief organizations who were in need of the most current and reliable Earth Observation information when trying to assess where the greatest efforts should be concentrated or to evaluate the full extent of a disaster. - Fast data acquisition and analysis of the impact of earthquakes are important issues when mapping natural disasters. 39)
- Imagery of China: In March 2010, RapidEye completed a contract to image 7.8 Million km2 of the country of China, ahead of schedule. The contract between RapidEye and the Ministry of Land and Resources (MLR) for the People's Republic of China was coordinated through the Chinese distributor, Beijing Earth Observation (BEO). 40)
• On August 28, 2009, the RapidEye team celebrated its first year of the constellation in space. 41)
Figure 21: Palm Jumeirah in Dubai acquired by CHOMA (RapidEye 5) on August 9, 2009 (image credit: BlackBridge)
• The RapidEye constellation started nominal operations on February 4, 2009 when all testing and system calibrations were successfully completed. This opens the era of commercial provision of imagery to the global EO community. 42) 43)
• MDA handover of the system to RapidEye, now BlackBridge, in January 2009.
• As of late fall 2008, the constellation was in the commissioning phase with all spacecraft functioning as expected. The commissioning phase will take approximately 3 months until the system is expected to be ready for full operations (Ref. 2).
• SSTL is controlling the constellation throughout the two week LEOP (Launch and Early Operation Phase) in a coordinated effort between their mission control facilities in the UK and the ground station supplied to RapidEye, now BlackBridge, in Brandenburg, Germany. Commissioning will be performed from both the RapidEye ground station in Brandenburg and the SSTL mission control centre in Guildford. The 5 satellites will gradually disperse from each other following separation from the launch vehicle, allowing 3 satellites to be tracked and operated from Brandenburg with the remainder under the control of the Guildford operators. This method of operation offers the most efficient route to achieving the maximum amount of contact time per satellite during the early days of commissioning following launch. As the satellites are gradually maneuvered into position around the orbit during the commissioning phase, the Brandenburg ground station will assume full control over the constellation.44)
Figure 22: First RapidEye public image of El Bolsón in Argentina (released on Oct. 22, 2008), image credit: BlackBridge
Sensor complement: (REIS)
REIS (RapidEye Earth Imaging System):
REIS is a multispectral imaging system designed and developed by JOP (Jena-Optronik GmbH), a subsidiary of the Photonics Division of Jenoptik) Jena, Germany. The instrument is also referred to as JSS-56 (Jena-Optronik Spaceborne Scanner-56) as well as MSI (Multispectral Imager) in the literature.
The collector optics utilizes a TMA (Three Mirror Anastigmatic) design - permitting generally larger FOVs (in the range of about 2-12º) than those of Cassegrain or Ritchey-Chrétien systems (FOV of about 2º max). The TMA telescope aperture diameter is 145 mm. The TMA design is based on all-aluminium telescopes. The necessary optical surface quality for applications in the visible range is achieved with ultra-precision milling and polishing techniques. The aluminium mirrors are Ni coated to achieve a suitable surface polishing quality. REIS is a pushbroom instrument which images the Earth's surface in 5 spectral bands over a swath width of 78 km (corresponding to a FOV of ± 6.75º about nadir) at a spatial resolution of 6.5 m at nadir. The collector optics image onto five parallel linear 12 k pixel CCD detectors. Filters, placed in close proximity to each CCD line array, separate the spectral imaging bands. 46) 47) 48) 49)
Table 3: Spectral parameters of REIS
Table 4: Overview of REIS instrument parameters
Figure 23: Elements of the REIS payload configuration (image credit: MDA, SSTL, JOP)
• PEU (Payload Electronics Unit): The dedicated PEU, in close proximity to the focal plane assembly (FPA), provides support of all REIS data handling functions. For each spectral channel, a dedicated signal chain module sends the required CCD clocks and voltages, and reads out the CCD data. It includes two analog-to-digital converters (ADC digitization) for odd and even CCD video output, one FPGA, as well as data and command interfaces. The signal chains also include gain amplification and CDS (Correlated Double Sampling). Optional pixel binning is performed in the data processing and control electronics. The JSS-56 PEU design is based on technology developed by DLR in Berlin.
After digitization, the image data pass a typical processing flow comprising data compression in COU (Compression Unit), data storage in MMU (Mass Memory Unit), data formatting in DFU (Data Formatting Unit), and the downlink (Figure 28).
• Telescope and mirrors. This telescope configuration is very compact as it uses a folded optical path that has no optical obstruction so the aperture can be kept small. The telescope is an all-aluminum design that is telecentric which minimizes plate-scale changes due to residual thermal effects.
Figure 24: Layout of the telescope (image credit: JOP)
• FPM (Focal Plane Module): The focal plane used 2 CCD packages from e2v (France) where each package has 3 linear CCD lines. These CCD’s are similar to those used for the Spot-5 program. As the payload only has 5 bands, only 2 CCD lines are used from one of the packages. Each CCD line has 12,000 pixels with a 6.5 µm square pixel pitch. There are 4 CCD outputs per line with a read-out rate of 6.5 MHz per line. The CCD dies are mounted directly to the ceramic substrate that is in the FPM. The bandpass filters are mounted directly in front of the CCD lines.
Figure 25: Illustration of the FPM (image credit: JOP)
Legend to Figure 25: The left photo shows the two triple-line CCDs with 3 x 12 k pixels in a ceramic plate, the right photo shows five metal oxide interference filter stripes integrated in a filter plate are mounted directly on the FPA.
• FEE (Front End Electronics): The FEE reads out each of the CCD lines and converts the analog signals to a 12 bit digital number and sends this to the PEU. There are 5 data inputs from the FEE that correspond to each band. This data goes directly into a real-time compression unit (COU) dedicated to each channel (Figure 28). From the compression unit, the data is moved into one of 3 MMU (Mass Memory Unit) boards.
For downlink operations, the data from the mass memory is sent to the DFU (Data Formatter Unit) which performs the CCSDS formatting, Reed-Solomon encoding, and sends the data directly to the X-band downlink transmitters. The payload operation is controlled by the CIU (Control and Interface Unit) which has a CAN interface to the bus and also a PPS interface to the GPS to allow for time synchronization.
• FEE Power Convertors
• Payload supports
Figure 26: Illustration of the REIS (JSS-56) instrument (image credit: JOP)
Figure 27: Schematic of the TMA telescope design accommodated on RapidEye (image credit: JOP)
Figure 28: Data handling electronics of the REIS instrument (image credit: MDA, SSTL)
The corrected image data can be processed in a variety of ways to reduce data volume prior to transmission. Pixel binning in 2 x 2 or 3 x 3 sizes provides the most rudimentary data compression method (one axis is binned directly on the CCD to reduce readout noise). The PEU also supports both selectable lossless 2:1 compression and lossy (up to 10:1) compression ratios based upon DCT (Direct Cosine Transform) or wavelet algorithms. The compressed data, together with spacecraft GPS and attitude information, is stored in mass memory, which provides sufficient storage for a 5-band imaging scene length of up to 1200 km.
The REIS (or MSI) instrument does not contain an on-board calibration subsystem. A preliminary in-orbit vicarious calibration campaign was performed after launch to confirm pre-launch calibration results by assessing the spatial response non-uniformity of each sensor. Over time, detector sensitivity changes require new gain and offset values to correct the banding and striping artifacts in the image data. 50)
In an effort to improve the relative spatial calibration of the pushbroom MSI and demonstrate the feasibility of an independent spatial calibration methodology, a SSM (Side Slither Maneuver) was performed with the RapidEye constellation. During SSM, the spacecraft is oriented in a 90º yaw configuration while confining the roll and pitch angle to 0º. The detector array runs parallel to the direction of motion. Thus, each pixel on the detector will be excited by the same input level of radiance as they each cover the same tightly confined target areas. Specific target areas over deserts and homogeneous snow covered region allow for effective calibration at differing levels of radiance. While roll maneuvers about the along-track direction are part of the daily imaging activities, imaging activities in non-zero yaw configurations were never foreseen by the manufacturer.
Several side-slither campaigns were performed using all of the satellites in the RapidEye constellation. Pseudo-invariant and spatially uniform terrestrial scenes that included desert and snow/ice fields were imaged with the sensor in a 90º yaw orbital configuration. In this configuration, each detector on the focal plane was positioned parallel to the ground-track direction thereby imaging the same segment of ground and exposing each detector to the same target radiance. This maneuver produced a radiometrically flat-field input to the sensor so that the relative response of each detector was determined for the same exposure level and compared to the array average.
Ideally when a side-slither image is processed, a given point on the ground is seen as a 45º line in the resultant image. This is because, a given point on the ground is sampled by a detector and is then translated over one column and down one row (in the processed image) during each successive integration period as the array moves along the target. The side-slither scanning and the resulting image formation concept are illustrated in Figure 29.
Figure 29: Side slither sampling of a ground target and the resulting image (image credit: BlackBridge)
A number of Earth scene sites from North Africa were initially considered for acquiring side-slither data including the CEOS radiometric calibration sites in Libya, Algeria, and Egypt as well as the land-sea transition between Libya and the Mediterranean. The first set of side-slither data, acquired by RE5, was along a north-south line passing through Libya. These results were not promising due to scene content variations and pointing instability issues with the spacecraft that were later corrected. In addition to these sites, several regions on the Arabian Peninsula were investigated. These regions are in general uniform with minimal directional variations and were deemed suitable for side-slither imaging.
After inspecting all of the imagery from these regions, the sites over Saudi Arabia showed the most promise, and three target areas within Saudi Arabia were selected. Finally, high reflectance snow/ice regions in Greenland and Antarctica were also chosen. These sites produce radiance values that span the mid-to-high radiance portion of the MSI dynamic range. The Greenland and Antarctica sites are more uniform and excite the sensor at a higher radiance level than the Saudi Arabia sites and are therefore more desirable targets. By choosing sites in both Greenland and Antarctica, access is available to these preferable sites for most of the year, but for the few months out the year, where neither target is adequately illuminated, Saudi Arabia fills in nicely. When they are available, the large size and high/low latitude of the Greenland and Antarctica sites also allow for more imaging opportunities than Saudi Arabia. Figure 30 shows the locations of the three Saudi Arabia sites, the three Greenland sites, and the Dome C target (Antarctica), using overlays from GoogleEarthTM Mapping Services.
Figure 30: Desert sites on the Arabian Peninsula, Snowfield sites in Greenland and the Dome C region in Antarctica (image credit: BlackBridge)
The improvement in image quality shows that adopting the side slither calibration technique into routine operations was worth the work. All spacecraft in the RapidEye fleet perform the side slither maneuver over one target once every three months. The method has been used successfully to provide an improvement in the relative spatial calibration of remote sensing payloads and was deemed a viable adjunct approach for the RapidEye mission (Ref. 50).
The RapidEye ground segment provides the following functional spectrum:
• A customer order interface capability
• Satellite data acquisition planning function that takes into account satellite constraints, weather and cloud predictions, the underlying data acquisition plan, and special image tasking requests for stereo data acquisitions and acquisition of specific targets
• Satellite monitoring and control to task the constellation and maintain its health and safety
• Image processing capability to convert raw imagery into ortho-products
• A capability to ensure the sensor performance and processing system
• An interface to the value-added information product processing facility
• A data archive for level 1B data, and level 3A data (ortho-products)
• Support to other data providers to obtain weather forecasts, cloud cover predictions, DEMs and other information.
Figure 31: Ground segment architecture of the RapidEye system (image credit: MDA, BlackBridge)
The ground segment features commercial off-the-shelf hardware and MDA proprietary software that has been selected for its performance, maintainability and expendability. The ground based equipment and facilities consist of:
• A dedicated Spacecraft Control Centre to control the spacecraft constellation
• A ground segment that provides the data processing, archiving facilities and customer interfaces
• Use of commercial data downlink sites
• An interface to RapidEye's product processing facility that uses the image data from the ground segment to generate the information products needed by customers.
Figure 32: Overview of the BlackBridge system (image credit: BlackBridge)
After establishing a long term service contract with Kongsberg Satellite Services AS (KSAT), BlackBridge has the possibility of downloading the imagery data to an X-band ground station in Svalbard, Norway providing the necessary bandwidth as well as an advantageous position at high latitude. Thus, every orbit of the RapidEye satellites can be supported by KSAT. Besides imagery, also received Payload Ancillary Files (PAF) and Bus Ancillary Files (BAF) are forwarded by KSAT using terrestrial communication lines to Berlin. In order to prepare the receipt of these files from the constellation, KSAT evaluates the reception schedules, provided by BlackBridge, which also contain the latest two-line elements (TLE).
Figure 33: RapidEye system structure (image credit: BlackBridge)
RapidEye image products:
RapidEye image products are provided in different processing levels to be directly applicable to customer needs. The table below summarizes the various processing levels of image products.
Table 5: Image processing levels of RapidEye products
1) Jonathan Gebbie, Mark Pollard, Haval Kadhem, Lee Boland, Alex da Silva Curiel, Philip Davies, Phil Palmer, Joe Steyn, George Tyc, “Spacecraft Constellation Deployment for the RapidEye Earth Observation System,” Proceedings of the 60th IAC (International Astronautical Congress), Daejeon, Korea, Oct. 12-16, 2009, IAC-09.B4.3.11
2) George Tyc, Joe Steyn, Norman Hannaford, Jonathan Gebbie, Ben Stocker, Adam Baker, Michael Oxfort, “RapidEye- A cost-effective Earth Observation Constellation,” Proceedings of the 59th IAC (International Astronautical Congress), Glasgow, Scotland, UK, Sept. 29 to Oct. 3, 2008, IAC-08-B4.3.03
3) G. Tyc, K. Ruthman, J. Steyn, ”The RapidEye Low Cost Mission Design,” Proceedings of IAC 2004, Vancouver, Canada, Oct. 4-8, 2004, IAC-04-IAA.4.11.4.02
4) G. Tyc, K. Ruthman, D. Schulten, M. Krischke, M. Oxfort, P. Stephens, A. Wicks, T. Butlin, M. Sweeting, ”RapidEye - A Cost Effective Small Satellite Constellation for Commercial Remote Sensing,” Proceedings of the 54th IAC, Bremen, Germany, Sept. 29 - Oct. 3, 2003
5) M. Krischke, M. Oxfort, D. Schulten, G, Tyc, ”RapidEye - Business Oriented, Dedicated Earth Observation,” Proceedings of 54th IAC, Bremen, Germany, Sept. 29 - Oct. 3, 2003
6) G. Tyc, J. Tulip, D. Schulten, M. Krischke, M. Oxfort, ”The RapidEye Mission Design,” 4th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 7-11, 2003; also available in: George Tyc, John Tulip, Daniel Schulten, M. Krischke, M. Oxfort, “The RapidEye mission design,” Acta Astronautica Volume 56, Issues 1-2, January 2005, pp. 213-219
7) G. Tyc, G. Buttner, M. Krischke, M. Oxfort, ”The RapidEye Spacecraft,” 4th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 7-11, 2003, URL: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/-archiv4/IAA-B4-1105P.pdf
8) M. Krischke, W. Niemeyer, et al., ”RapidEye - Satellite Based Geo-Information System,” IAA 2nd International Symposium on Small Satellites for Earth Observation, Berlin, April 12-16, 1999, pp. 249-252
9) Information provided by Manfred Krischke of RapidEye
11) M. Krischke, F. Jung-Rothenhäusler, D. Schulten, G. Tyc, ”The New Approach Towards Commercial Earth Observation - RapidEye,” 4th IAA Symposium on Small Satellites for Earth Observation, Berlin, April 7-11, 2003
13) “RapidEye Mission,” MacDonald Dettwiler, URL: http://www.gotomda.com/corporate/news/pr/backgrounder/RapidEye.pdf
14) “Iunctus Group Acquires Assets of RapidEye AG, New Corporate Vision More Customer Focused,“ Press Release of Iunctus Geomatics Corp., Sept. 1, 2011, URL: http://blackbridge.com/geomatics/uploads/2011%2009%2001_Press-Release_Iunctus%20Acquires%20RapidEye.pdf
15) “Inctus Group Acquires Assets of RapidEye AG,” Sept. 1, 2011, URL: http://www.rapideye.de/news/pr/2011-iunctuscorp.htm
16) “RapidEye is now BlackBridge,” BlackBridge Press Release, Nov. 6, 2013, URL: http://www.blackbridge.com/rapideye/news/pr/2013-blackbridge.htm
17) Mark Pollard, “The Design, Build, Test and In-orbit Performance of Low cost Xenon Warm Gas Propulsion Systems,” Proceedings of Space Propulsion 2010, San Sebastian, Spain, May 3-6, 2010
18) RapidEye Newsletter, September 2008, URL: http://www.docstoc.com/docs/4732039/Newsletter-Launch-Edition-September-RapidEye-Launch-Successful-On-August
19) Information provided by Michael Oxfort of BlackBridge, Berlin, Germany.
20) Enrico Stoll, Harald Konstanski, Cody Anderson, Kim Douglass, Michael Oxfort, “The RapidEye Constellation and its Data Products,” Proceedings of the 2012 IEEE Aerospace Conference, Big Sky, Montana, USA, March 3-10, 2012
21) “BlackBridge Supports the European Space Agency’s Sentinel-2 Mission,” BlackBridge, Nov. 20, 2013, URL: http://www.blackbridge.com/rapideye/news/pr/2013-sentinel.html
22) “From the Corn Belt to Canada, RapidEye Completes Its North American Agricultural Imaging Campaign,” RapidEye, Sept. 30 ,2013, URL: http://www.blackbridge.com/rapideye/news/pr/2013ag_campaign.html
23) “RapidEye kicks off 2013 North American Agricultural Imaging Campaign,” RapidEye Press Release, May 13, 2013, URL: http://www.rapideye.com/news/pr/2013-agcampaign.html
24) Information provided by Claudia Hahm of RapidEye.
25) Michael Oxfort, “RapidEye Large Area Imaging Capabilities and New Countrywide Mosaic Product,” 12th Annual JACIE (Joint Agency Commercial Imagery Evaluation) Workshop , St. Louis, MO, USA, April 16-18, 2013, URL: https://calval.cr.usgs.gov/wordpress/wp-content/uploads/JACIE_2013_LargeAreas_Mosaics.pdf
26) Harald Konstanski, Enrico Stoll, Brian D'Souza, Michael Oxfort, “Four years of operating RapidEye satellites - Continuous performance improvement and adaptation to customer needs and markets,” Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013
27) RapidEye secures enhanced data downlink support for Australia and Far-East regions,” RapidEye Press Release, June 24, 2013, URL: http://www.rapideye.com/news/pr/2013-datadownlink.html
28) “RapidEye Commits to Data Continuity; discusses system health and life span,” RapidEye, Jan. 17, 2013, URL: http://www.rapideye.com/news/pr/2013-1-continuity.html
29) “RapidEye joins ESA Third Party Missions Programme,” ESA, Nov. 8, 2012, URL: /web/guest/news/-/asset_publisher/G2mU/content/rapideye-joins-esa-third-party-missions-programme
30) M. Vitale, “RapidEye Mission and Ground Segment,” 3rd GSCB (Ground Segment Coordination Body) Workshop, 2012, ESA/ESRIN, Frascati, Italy, June 6-7, 2012, URL: http://earth.esa.int/gscb/papers/2012/27-RapidEye_Mission_and_Ground_Segment.pdf
31) RapidEye Contracted to cover China Third Year Running,” RapidEye News Release, Dec. 7, 2011, URL: http://www.rapideye.net/news/pr/2012-china-mlr.htm
33) Enrico Stoll, Brian D'Souza, Harald Konstanski, Michael Oxfort, “2 Years of Operating the RapidEye Constellation – an Evolving Mission Concept and Lessons Learned,” 8th IAA (International Academy of Astronautics) Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2011; URL of presentation, IAA-B8-1504, http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv8/Presentations/IAA-B8-1504.pdf
34) Horst Weichelt, Scott Douglas, Massimiliano Vitale, Frederik Jung-Rothenhäusler, “New RapidEye Image Products,” 10th Annual JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29-31, 2011, Boulder CO, USA URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/Wed-AM/1020_Weichelt_JACIE_11.160.pdf
35) Andreas Brunn, Denis Naughton, Horst Weichelt, Michael Thiele, Scott Douglass, Michael Oxfort, “Relative Comparison of the Spectral Resolution of RapidEye Products,” JACIE (Joint Agency Commercial Imagery Evaluation) Workshop, March 29 - 31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedAM/950_Brunn_JACIE_11.010.pdf
36) Denis Naughton, Andreas Brunn, Michael Thiele, “Radiometric Performance Assessment of the RapidEye Constellation_2010,” JACIE ( Joint Agency Commercial Imagery Evaluation) Workshop, March 29 - 31, 2011, Boulder CO, USA URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Presentations/WedAM-/1005_Naughton_JACIE_11.100.pdf
37) Aparajithan Sampath, Md. Obaidul Haque, Gyanesh Chander, “Radiometric & Geometric Assessment of Data from RapidEye Constellation of Satellites,” JACIE Workshop, March 29 - 31, 2011, Boulder CO, USA, URL: http://calval.cr.usgs.gov/JACIE_files/JACIE11/Posters/11.015_Chander-Obaidul_Sampath_JACIE2011_Poster.pdf
38) “2 Years in Space: RapidEye Establishes its Place in the Geospatial Industry,” RapidEye AG Press Release, Aug. 30, 2010
39) “Change Detection Services Show Damages in Areas Affected by the 2010 Chilean Quake,” March 5, 2010, URL: http://www.directionsmag.com/pressreleases/rapideye146s-change-detection-services-show-damages-in-areas-affected-by-th/120492
41) “Time Flies: RapidEye Celebrates One Year In Space,” RapidEye AG Press Release, Aug. 28, 2009, URL: http://www.rapideye.de/upload/Press_Releases/2009/RapidEye_Press_Release_One_Year_Aug_28_2009.pdf
42) “With Testing Completed, RapidEye :is Open For Business,” RapidEye AG Press Release, Feb. 4, 2009, URL: http://www.rapideye.de/upload/Press_Releases/2009/RapidEye_Press_Release_Feb_04_2009.pdf
43) D. Schulten, G. Tyc, J. Steyn, N. Hannaford, M. Oxfort, P. Widmer, “RapidEye - The first six months in orbit,” IAA-B7-1501, Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, May 4-8, 2009, Berlin, Germany, URL of presentation: http://media.dlr.de:8080/erez4/erez?cmd=get&src=os/IAA/archiv7/Presentations/1501%5FMDA-%20RapidEye%20Presentation%-20May%202009%20IAA%20Conference-%20Berlin.pdf
45) “RapidEye satellite constellation transmits first images,” DLR, October 22, 2008, URL: http://www.dlr.de/en/desktopdefault.aspx/tabid-3432/7418_read-13841/
46) Karin Schröter, “Multi-Spectral Optical Scanners for Commercial Earth Observation Missions,” Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-7, 2009
47) F. Doengi, W. Engel, A. Pillukat, S. Kirschstein, “JSS Multispectral Imagers for Earth Observation Missions,” Proceedings of the 5th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4-8, 2005
48) S. Kirschstein, A. Koch, J. Schöneich, F. Döngi, “Metal mirror TMA, telescopes of the JSS product line: design and analysis,” Proceedings of the SPIE, Building European OLED Infrastructure, edited by T. P. Pearsall, J. Halls, Vol. 5962, 2005, pp. 484-493, Sept. 12-16, 2005, Jena, Germany
49) Frank Doengi, Wolfgang Engel, Alexander Pillukat, Omar Kirschstein, “JSS Multispectral Imager s for Earth Observation Missions,” URL: http://www.dlr.de/iaa.symp/Portaldata/-49/Resources/dokumente/archiv5/0704_Doengi.pdf
50) Godard, Enrico Stoll, Cody Anderson, Roland Schulze, Brian D’Souza, “Integrating Advanced Calibration Techniques into Routine Spacecraft Operations,” Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL: http://www.spaceops2012.org/proceedings/documents/id1275279-Paper-001.pdf
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.