RapidEye Satellite Constellation

RapidEye is an Earth observation mission of RapidEye AG of Brandenburg (a city south-west of Berlin), Germany, that includes a constellation of five minisatellites. The mission will provide high-resolution (6.5 m) multispectral imagery along with an operational GIS (Geographic Information System) service on a commercial basis. The objectives are to provide a range of Earth-observation products and services to a global user community. Typical fields of application and services are: ^{1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)}

• Agricultural producers (farmers): Crop monitoring and mapping, yield prediction

• Agricultural insurance:Provision of regularly updated field maps to help insurers assess insurance contracts and claims by providing quick and reliable information about damaged areas

• Cartography - satellite based maps (scale 1:25,000), ortho photos, DEM (Digital Elevation Model) generation

• Other markets - disaster assessment, 3-D visualization

• Service spectrum at completion mission: Guaranteed daily revisit, global coverage, product delivery to the customer within 24 hours, possibility of dedicated programming, capability of direct transmission and imagery transfer within hours, global digital database of "orthomaps" of 1:25,000 scale and DEMs of 20 m x 20 m resolution. The service permits also the merging of multi-temporal imagery with information from other sources.

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 and MDA (MacDonald, Dettwiler and Associates Ltd) 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. ¹⁵⁾

Space segment:

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: 0.8 m x 0.9 m x 1.1 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. The electric power system consists of three GaAs solar panels located on the +x, -z, and -x faces of the S/C. About 100 W of power are provided (solar cell efficiency of 19% at BOL). A 15 Ah Li-ion battery is used. The S/C mass is about 175 kg. ¹⁶⁾

Figure 1: Illustration of the RapidEye spacecraft (image credit: RapidEye)

Figure 2: RapidEye FM2 spacecraft at SSTL (image credit: SSTL)

The attitude control system relies on four reaction wheels for three-axis control with redundant magnetic torques for momentum management. 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 ±25º FOR (Field of Regard) for camera observations into any direction (however, the feature is planned to be used only in the cross-track direction).

Figure 3: The Altair HB star tracker, camera head module and baffle (image credit: SSTL)

The propulsion subsystem is based on a cold 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 36 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. The propulsion unit may require up to 50 W from the platform power system. The onboard propulsion system is being used for constellation maintenance.

Figure 4: Illustration of the resistojet thruster (image credit: SSTL)

Figure 5: Cold gas tank for the resistojet thruster (image credit: SSTL)

Onboard data handling and monitoring functions are provided by redundant OBCs. A dual-redundant CAN (Control Area Network) bus provides communication between all subsystems and the OBC, including the payload.

Figure 6: Block diagram of RapidEye and spacecraft architecture (image credit: MDA, SSTL)

Launch: A single launch of the RapidEye minisatellite constellation on a Dnepr launch vehicle (launch provider: ISC Kosmotras) is planned for the spring 2008 from the Baikonur Cosmodrome, Kazakhstan (all spacecraft are being placed into the same orbital plane).

Orbit: Sun-synchronous orbit (all five satellites are evenly spaced in a single orbital plane), altitude = 630 km (±10 km), inclination = 98º, local equator crossing time at 11:00 hours (± 1 hour) on the descending node, 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 (±70º 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.

Figure 7: RapidEye constellation in one orbital plane (image credit: RapidEye)

Figure 8: Artist's view of the RapidEye constellation (image credit: RapidEye)

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 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 RapidEye headquarters, Brandenburg, Germany. - Each RapidEye spacecraft employs a new compact X-band antenna design of SES (Saab Ericsson Space) as shown in Figure 9.

Figure 9: Illustration of small-size quadrifilar helix antenna with radome attached (image credit: SES) ¹⁷⁾

Sensor complement:

REIS (RapidEye Earth Imaging System), is a multispectral imaging system designed and developed by 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). 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. ^{18) 19) 20)}

Table 2: Spectral parameters of REIS

Table 3: Overview of REIS instrument parameters

Figure 10: Illustration of the REIS (JSS-56) instrument (image credit: Jena-Optronik GmbH)

Figure 11: Schematic of the TMA telescope design accommodated on RapidEye (image credit: Jena-Optronik GmbH)

The instrument features a dedicated PEU (Payload Electronics Unit), in close proximity to the focal plane assembly, in 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 13).

Figure 12: Data handling electronics of the REIS instrument

Each CCD produces 4 outputs, which are synchronously clocked by the detector readout electronics at a rate of about 3.2 MHz, providing a 12 bit digitization with low readout noise. The raw pixel data, totalling nearly 765 Mbit/s, is transferred to the PEU via a set of high speed serial interfaces. The PEU provides a separate processing chain for each CCD, with a redundant chain crossstrapped to the others. The first step in the processing chain is to correct CCD data, in real time, for gain and offset non-uniformities using a programmable table of coefficients. This occurs prior to data compression to prevent image defects from biasing the compressed data. CCD correction coefficients, derived from imaging of ground calibration sites, are periodically uploaded from the ground station.

Figure 13: Payload block diagram (image credit: MDA, RapidEye)

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 1500 km at 2.5 bit/pixel (equivalent to 3 Gbit of storage capacity).

Ground segment

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 extract DEMs from stereo imagery using an optimal mix of automated processing and manual editing. Note: Near real-time stereo imagery results from observation of the same target area of two successive spacecraft.

• A calibration capability to ensure the sensor performance and processing system

• An interface to the value-added information product processing facility

• A product catalogue and multi-tiered data archive for raw data, ortho-products, DEMs and information products

• Support to other data providers to obtain weather forecasts, cloud cover predictions, DEMs and other information.

Figure 14: Ground segment of RapidEye

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 15: Overview of the RapidEye system (image credit: RapidEye)

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 4: Image processing levels of RapidEye products

2) 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

3) 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

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7) Information provided by Manfred Krischke of RapidEye

8) http://www.rapideye.de/

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10) http://www.mda.ca/news/pr/backgrounder/RapidEye.pdf

11) http://www.mda.ca/news/pr/backgrounder/RapidEye_datasheet.pdf

12) "Financing successfully concluded," Press Release of RapidEye, June 21, 2004

13) http://www.gotomda.com/news/pr/backgrounder/RapidEye.pdf

14) http://www.sstd.rl.ac.uk/envtest/Latest_news.htm

15) "RapidEye and MDA Enter Partnership Agreement," URL: http://www.spaceref.com/news/viewpr.html?pid=9397

16) http://sm.mdacorporation.com/pdf/9755-1R3_Rapideye.pdf

17) http://www.esa.int/techresources/ESTEC-Article-fullArticle_par-29_1129904713903.html

18) 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

19) http://www.jena-optronik.com/cps/rde/xbcr/SID-26EE34DB-F0613590/optronik/jss_jo.pdf

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