The main challenges for the RSI development were: - to introduce innovative technologies in order to meet the high performance requirements while achieving the design simplicity necessary for the mission (low mass, low power) - to have a development approach and verification compatible with the very tight development schedule This paper describes the instrument design together with the development and verification logic that were implemented to successfully meet these objectives. |
1.INTRODUCTIONEADS-Astrium has recently completed the development of a 2m-resolution camera, so-called RSI (Remote Sensing Instrument), for the small-satellite ROCSAT-2, which is the second component of the long-term space program of the Republic of China. The National Space Program Office of Taïwan selected EADS-Astrium as the Prime Contractor for the development of the spacecraft, including the bus and the main instrument RSI. The main challenges for the RSI development were:
This paper describes the instrument design together with the development and verification logic that were implemented to successfully meet these objectives. 2.ROCSAT-2 MISSION AND SATELLITEThe ROCSAT-2 mission mainly aims at monitoring the terrestrial and marine environment and resources throughout Taïwan with:
The solution proposed by EADS-Astrium was based on the LEOSTAR generic platform pre-developed by EADS-Astrium and addressing a wide range of missions, from earth observation to scientific missions (Mars Express, Rosetta). For the purpose of the mission, a helio-synchronous orbit of 891km was selected to satisfy the 24-hours revisit time. Thanks to the platform pitch/roll performance capability (+/-45°), a multi-strip approach was offered in response to the large coverage requirements The Remote Sensing Instrument (RSI) of ROCSAT-2 satellite provides a 2m Ground Sampling Distance (GSD) in Panchromatic (PAN) band, and a 8m GSD in multi-spectral (MS) bands, over 24 km swath width, in the Nadir direction. 3.RSI DESIGN3.1RSI overall configurationThe Instrument features two main parts:
3.2RSI Camera♦Telescope optical conceptThe camera is based on a compact Cassegrain-type telescope and a four-lenses field corrector ♦Silicon carbide for mirrors and structureThe RSI design is based on an all-SiC opto-mechanical architecture (telescope structure, mirrors, and focal plane structural elements). This monolithic design approach, combined with the intrinsic SiC100 properties (high stiffness, low density, low thermal expansion, high thermal conductivity) allows to combine a high level of stability together with a low mass. ♦Telescope structureThe telescope structure is only featuring three main parts: the main plate (supporting the primary mirror), the secondary mirror support, and the rod connecting those two parts. ♦Telescope mirrorsSiC mirrors can be light-weighted and polished with a high accuracy. Both mirrors were SiC CVD1 coated before polishing in order to minimize the roughness The Wave-front Error (WFE) was measured below 20 nm rms for each mirror, with a roughness lower than 1.0 nm rms. ♦Refocusing capabilityThe secondary mirror is fixed on the structure by its interface flange. The primary mirror is fixed on to the structure through three iso-static invar mounts and thus thermally decoupled from the structure. Its temperature is controlled by a heater plate located between the mirror and the mounting plate. Setting different thermal control set points between the telescope structure and the primary mirror leads to a variation of the focal plane position, thanks to the low - but nonnull – thermal expansion coefficient of silicon carbide. The refocusing capability is +/- 200μm for a +/- 5°C thermal set point variation. ♦Focal Plane Assembly (FPA)The focal plane assembly features only two CCD for the 5 required spectral bands. One CCD is dealing with the Panchromatic band and the other one is dealing with the multi-spectral bands. The separation of the entrance optical bean is ensured by an optical field separator. 3.2A4-line CCD for Multi-spectral bandsROCSAT2 took benefit of the pre-development performed by Atmel, under a CNES R&D contract. The TH31547 multi-spectral CCD consists of 4 photodetector lines, each line being made of 6000 photodiodes with 13μm step. The detector is operated at 5 Mpixel/s per video output. Each CCD line is coupled with a spectral band filter. The four slit filters are coated on the same glass substrate glued on the CCD. High speed video processing for the panchro- matic channelThe panchromatic detection chain is based on the wellknown TH7834B detector (12000 useful 6.5 x 6.5 μm2 pixels). The challenge was to operate the four serial read-out registers at a 10 MHz pixel rate for satisfying the 308μs integration time required to achieve the 2- meter resolution. 3.3Integrated Video Processing FunctionThe Instrument Processing Unit (IPU) is gathering the instrument electronics functions in a modular and highly integrated assembly. The IPU is coupled with the Focal Plane Assembly front-end electronics - Panchromatic Electronics Board (PEB) & Multi-spectral Electronics Boards (MEB) – and also with three Spacecraft main units: the On Board Management Unit (OBMU), the Solid State Recorder (SSR), and the Distribution & regulation Unit (DRU). Each IPU includes the necessary functions:
These functions are split on seven electronics boards racked in the same unit. 4.DEVELOPMENT AND VERIFICATION APPROACHTo cope with the required tight development schedule (2.5 years), a straightforward approach was necessary. This approach concerned two main areas: 4.1Industrial organizationThe key issue was to favour the flexibility and reactivity in terms of design, development and verification activities. This led to a two-step approach consisting in:
The main partners were:
Such an organization and partnership allowed to have an efficient and fruitful concurrent engineering phase at the very beginning of the program in order to consolidate at an early stage (T0 + 3 months) the equipment specifications of the schedule critical items. 4.2Model and verification philosophyThe selected approach was to develop a single model of the Instrument (Proto-Flight Model), but to mitigate the risks by conducting pre-validation activities where deemed necessary. ♦Supporting programsTwo main areas of concern were identified: Detection:
Opto-mechanical aspects: Therefore, two support programs were conducted : Detection support campaignThis support program consisted in testing and characterizing the detection chains operating conditions and performances (PAN & MS) using a Focal Plane Assembly and Instrument Processing Unit Development Models (DM). Mechanical support campaignThis support program consisted in verifying the telescope stability by testing a Structural Model (SM) of the Instrument. This SM was built of the telescope structure flight model equipped with mirror blanks and the FPA DM The stability of the structure was verified using a 3-D machine. 4.3Proto-flight assembly and qualification tests♦RSI Assembly and integrationThe RSI FM AIT started at the completion of the support programs and delivery of the telescope mirrors. ♦Proto-qualification testsAll the environmental tests were performed at the same place (Centre Spatial de Liège). The qualification sequence was the following: Vibration testsThe Instrument was submitted to low level sine vibration and quasi-static load tests (15g axial, 9g lateral). The compatibility with the specified acoustic levels was in a first step demonstrated by analysis and tested at spacecraft level. 4.4Performance test resultsThe measurements performed under thermal vacuum conditions have demonstrated the full compliance to the performance requirements. The following sections gives some test results concerning CTF and SNR test results. 5.OVERVIEW OF THE ACHIEVED PLANNINGThe main program phases can be summarized as follow:
6.CONCLUSIONWith this program, EADS-Astrium entered the export market with the first worldwide contract for a high-resolution civilian observation mission. The main challenge, successfully achieved, was to combine an innovative design to a tight development schedule. ROCSAT is to be launched in April 2004 on a OSC TAURUS launcher. |