Proba: ESA's Autonomy and Technology Demonstration Mission

SSC99-V-8

PROBA: ESA’s AUTONOMY AND TECHNOLOGY DEMONSTRATION MISSION

Frederic Teston, Richard Creasey - ESA/ESTEC Keplerlaan 1, 2201 AZ Noordwijk, the Netherlands

J. Bermyn, D. Bernaerts, K. Mellab – Verhaert Design and Development Hogenakkerhoekstraat 9, B-9150 Kruibeke, Belgium

ABSTRACT: Proba is an ESA mission conceived for the purpose of demonstrating the opportunities and benefits of on-board autonomy. To this end, it offers a flight opportunity centered around the validation of the associated technological capabilities. The Proba spacecraft is equipped with a selected set of technologies providing advanced on-board functions for performing a number of mission operations functions with minimum ground involvement. The autonomy is exercised in realistic scenarios through the accommodation of four payload instruments.

INTRODUCTION PROBA consortium and responsibilities : · Verhaert Design and Development In the framework of demonstrating the (Belgium) : prime contractor, system feasibility of small and low-cost missions by tasks, structure and launch increasing operational autonomy, ESA is · Space Innovations Ltd (UK) : Data undertaking two main activities: one is the handling, power and communication design, development, and operations of an system actual mission dedicated to autonomy · Spacebel Informatique (Belgium) : On- performed by Verhaert D&D as Prime board software contractor and called PROBA (PRoject for · SAS (Belgium) : Ground segment and On-Board Autonomy) and the other is a operations research activity performed by MMS (Matra · UdS (Ca) : AOCS system Marconi Space) which aims at the production · SSF (Finland) : Software Validation of flight software for supporting on-board Facility autonomy. · FIAR (Italy) : Solar cells Other European companies participate as Proba is an approved project under ESA’s unit suppliers (e.g. Technical University of General Study Program (which is part of the Denmark for the Star Tracker and SSTL in Technological Demonstration Program). The UK for the GPS). project has been kicked-off in February 1998, for a two years development. It is being Proba’s System Design Review has been held performed by an industrial consortium led by in June ‘98. The main scientific payload Verhaert Design and Development consists of an Earth observation instrument. (Belgium). The cost of the Proba mission is In addition, two instruments are taken on- targeted at below 10 MECU. board for the purpose of radiation and space The project will realized by following debris monitoring. A fourth instrument for consortium : high-resolution pictures (10m) will be flown as well.

J. Bermyn 13th AIAA/USU Conference on Small Satellites Proba intends to demonstrate the benefits of axis stabilized by means of attitude autonomy and to validate the supporting measurements provided by an autonomous star advanced technologies in orbit. In particular, tracker, a GPS-based attitude sensor and a the following autonomy functions are being three-axis magnetometer and by on-board implemented: control through a set of reaction wheels and magneto-torquers. 1. commanding for management of on-board resource and house-keeping functions; Payload Instruments 2. scheduling, preparation and execution of An Earth Observation Instrument, scientific observations (for instance: slew, CHRIS, has been chosen as the primary attitude pointing, instrument settings); payload instrument on the basis of its 3. scientific data collection, storage, strength of autonomy demonstration and its processing, and distribution; associated scientific merit. This instrument 4. data communications management will require a specific attitude (pointing and between Proba, the scientific users, and rates) of the spacecraft in order to be able to the ground station; perform its mission. It will be the prime driver 5. performance evaluation and estimation of for Proba operations since observations are drifts, trends; prepared on-ground by the scientist and the 6. failure detection, reconfigurations and amount of data generated is relatively high. The software exchanges. observation request itself should be ‘goal oriented’ (for instance, in terms of target This paper presents a description of the location and time of observation). This request Proba mission, including programmatic will then be translated into a schedule of aspects, system design, payload instruments, activities, resource management decisions and selected technology demonstrations, and the satellite pointing commands. baseline mission operations concept. The main instrument, CHRIS, was selected PROBA MISSION SUMMARY through an Announcement of Opportunity.

Launch and Orbit CHRIS (Compact High Resolution Imaging Proba is planned to be launched in mid 2000 on Spectrometer) is an imager proposed by PSLV. However, the spacecraft design is SIRA electro-optics (UK) which works in compatible with many of the current launch the spectral range 450 nm to 1050 nm, vehicles. It will be injected directly into its final targeted in particular towards directional polar, sun-synchronous orbit at an altitude of reflectance of land areas. It has a spatial 817 km, 98.7 degrees inclination. On-board resolution of 25m at nadir, and a spectral propulsion is not provided, so the orbit can not resolution of 5 nm to 12nm.. be corrected or maintained during the planned two years of operational lifetime of the In addition to its scientific interest, CHRIS spacecraft. The orbital drift (away from sun- puts severe requirements on the spacecraft synchronism) amounts to about 2 degrees per autonomy in terms of AOCS, data handling year and is acceptable. and resources management. For example, CHRIS uses the spacecraft slewing Navigation of the spacecraft is performed capabilities to perform multiple images of the autonomously on-board by a combination of same scene on Earth from different viewing GPS measurements and orbit propagation. angles. NORAD two-line elements are used for on- ground navigation and as a back-up in case of a In addition to the main Earth Observation GPS unit failure. The spacecraft is kept three- Instrument, Proba will have three secondary 2

J. Bermyn 13th AIAA/USU Conference on Small Satellites payloads: one is the Space Radiation Environment Monitoring (SREM), proposed by Contraves (CH), instrument providing measurements of electron and proton fluences and of the total radiation dose received [Ref. 1]. SREM is a standard off-the-shelf unit contained in a box of 9/12/22 cm, weighting 2.5 kg.

The second one is DEBIE (DEBris In orbit Evaluator) proposed by Finnavitec (Finland) which measures mass, impact speed and penetration power of the dust environment around the spacecraft using 2 impact detectors located on the panel looking in the flight direction and on the panel looking to deep space of the satellite. These instruments enhance Proba’s mission objectives because Figure 1 - PROBA Exploded View of their specific accommodation and (Courtesy of Verhaert Design and operations requirements and by their Development nv) generation of valuable scientific data. Spacecraft Characteristics The standard instrument units SREM and Proba has a weight of about 100 kg with DEBIE are beneficial to any satellite which dimensions 600 x 600 x 800 mm (ASAP5 / requires autonomous reactions to radiation PSLV compatible) and belongs to the class or debris flux levels. For instance, SREM is of mini-satellites. Its structure (Figure 1) is foreseen on ESA’s Integral spacecraft whose built in a classical manner using aluminum payload instruments must be switched off honeycomb panels. The carrying part of the when the level of radiation exceeds a certain structure is composed of 3 panels mounted in specified limit and then switched on again a H-structure carrying the different units. when the level drops below the limit again. The satellite’s outside panels have body- The third one is HRC (High Resolution mounted Gallium Arsenide solar cells with Camera) built by OIP (Belgium), which integrated diode, providing power to the allows the demonstration of taking high spacecraft (90 W peak) , a 7Ah NiCd battery resolution pictures (10m) with a small three- is used for energy storage. A centrally axis stabilized satellite. switched and protected 28 V regulated bus distributes the power to the units and instruments. The spacecraft has passive thermal control,

A high-performance redundant central computer performs all computing tasks and interfaces to every unit of the spacecraft with high-speed data links and house-keeping data acquisition links. The telecommunications subsystem provides quasi omni-directional CCSDS-compatible up-link and down-link 3

J. Bermyn 13th AIAA/USU Conference on Small Satellites for S-band communications with the ground. Software experiments may be accommodated The link capacities are 4 Kbit/s for the packet as well since the on-board software will be telecommanding and a maximum of 1 Mb/s entirely re-programmable. However, for the packet telemetry. software updates for in-flight testing of new functions or algorithms will be introduced The spacecraft provides nadir and inertial only in the later phase of the mission. off-nadir pointing capabilities. Three-axis attitude control and fine pointing are On-Board Computer provided by a high-accuracy autonomous Proba’s high-performance on-board double-head star tracker, a GPS receiver and computer (built around components a set of reaction wheels. Two three-axis sponsored by ESA) provides sufficient magnetometers complements the attitude performances to support spacecraft sensing, and four magneto-torquers are autonomy. It is able to support the available for momentum management. A set processing normally performed on-ground of gyroscopes may be accommodated for (and also migrated on-board in Proba’s case) improving the short-term pointing stability. A as well as the on-board scientific data propulsion capability is not foreseen. processing. The latter capability enhances the spacecraft autonomy with respect to data Whereas commonly available units and well- distribution through the selective use of the proven concepts are used for the power and downlink and the on-board mass memory. the communication subsystems, the system design of Proba is innovative in many A high-performance RISC processor, the respects, especially in the areas of attitude ERC 32, performs the spacecraft control and avionics. management including guidance, navigation, control, housekeeping and monitoring, on- TECHNOLOGIES for AUTONOMY board scheduling and resource management. The development of this processor which is a A core of technologies aiming at the space version of standard commercial demonstration of spacecraft autonomy is processors have been initiated by ESA to accommodated in the attitude control and the establish and validate a computing core for avionics subsystems and forms an integral future spacecraft. part of the Proba system design. Others will be implemented as ‘technological The ERC 32 [Ref. 2] is a radiation tolerant experiments’ since their use for autonomy is (> 80 Krad) SPARC V7 processor providing optional or their novelty would present a 10 MIPS and 2 MFLOPS with a floating- relatively high risk. point unit. A memory controller includes all the peripheral functions needed by the The technologies flown on Proba as part of processor, such as the address decoders, the the system are a GPS receiver for navigation bus arbiter, the EDAC, 2 UARTS, 3 timers and attitude determination, an autonomous and a watchdog. The chip set is star tracker for attitude determination and a manufactured with the MHS 0.8 micron high-performance computer. CMOS/EPI radiation tolerant technology.

Further hardware technological experiments Autonomous Star Tracker will be accommodated in a ‘PPU’ box The main attitude determination sensor is an (Payload Processing Unit) or as stand-alone autonomous star tracker which provides the units. The PPU also contains a Digital Signal full-sky coverage and achieves the high- Processor for on-board scientific data pointing accuracy required in Earth processing and analysis, and a mass memory. observation and in astronomy. The sensor 4

J. Bermyn 13th AIAA/USU Conference on Small Satellites can autonomously reconstruct the Autonomy requires larger and more reactive spacecraft’s inertial attitude starting from a software than is the case on conventional ‘lost in space’ attitude. This is done with a spacecraft. The software development time typical performance of a few arcseconds to and its validation are therefore key aspects to an arcminute, depending on the sensor’s be taken into consideration in the system characteristics and the measurement design. Open standards have been considered conditions. The attitude can be reconstructed in order to ease the development effort. A even at relatively high inertial rates. Several standard commercial off-the-shelf kernel will star trackers with adequate characteristics be used: this will allow the support of and performances for the Proba mission have custom, re-usable, and automatically- been evaluated during the feasibility study. generated software. The first category of Finally,: the Advanced Star Tracker (double- software should normally be in Ada while the head) from DTU was selected because of its second type is often in C language. superior performances and its novelty. Furthermore, when Proba will be subjected to the in-orbit testing of additional GPS (Global Positioning System) autonomous functions, openness will Proba will accommodate a GPS L1, a C/A facilitate the development and the uplink of receiver, and 4 antennae for position and the supporting software. medium-accuracy attitude determination. This is considered a crucial technology for For verification and validation of the autonomy demonstration because this single software a set of tools will be implemented, unit is able to provide all essential AOCS the Software Validation Facility [Ref. 3] measurements without any ground allows to exercise and validate the software intervention for a low-altitude orbiting in a realistic environment. spacecraft. Autonomy in low-Earth orbit depends on the accurate on-board knowledge Additional Opportunities for of the satellite position in ground Demonstration coordinates. This knowledge is required for During the system feasibility study, several Earth observation purposes, for data companies have indicated their interest in in- downlink transmissions, as well as for flight testing of some of their new technology spacecraft control functions. Furthermore, it developments. Technologies supported by is required for high-precision attitude hardware are included in a ‘Payload determination in the Earth-orbital frame Processing Unit (PPU)’ or accommodated as using star tracker measurements. stand-alone units. The PPU box provides the Autonomous operations also require mechanical, electrical and functional adequate time accuracy on-board: the GPS interfaces with the spacecraft. The envisaged time determination is used for on-board time technological demonstrations are, a new keeping and correlation with UTC. advanced autonomous star tracker, a set of Most of these GPS functions are actually Smart Sensors (called DIP) measuring somewhat redundant: the attitude estimation temperature and radiation, and solid state may also be provided by the star tracker and gyroscopes. Another candidate is a high the spacecraft position may be obtained by resolution camera (approx. 10m), in order to orbit propagation using the Norad two-line evaluate the platform stability and pointing elements. This ‘redundancy’ may in fact accuracy. Final accommodation depends on allow interesting accuracy comparisons and the available mass left for additional calibrations. payloads. The selected GPS receiver is from SSTL. On-Board Software Technological experiments supported by software alone will be tested on the 5

J. Bermyn 13th AIAA/USU Conference on Small Satellites spacecraft in the later phase of the mission It is generally expected that, in the near future, after the first year in orbit. These mission operations activities will evolve from experiments will be validated on-ground ground-based control towards on-board using the software validation facility before monitoring and control. This will clearly be the being uplinked to the spacecraft. It is planned case for the so-called ‘routine’ functions that the spacecraft software will use a requiring relatively straightforward and low- standard programming language and a real- risk decision-making. time kernel so that the uplink of new software is facilitated. In the future, more complex functions may gradually be migrated to the on-board IMAGER DATA PROCESSING computer. On-ground operations spacecraft activities by humans will essentially be limited The Chris imager data (main payload) is to the initialization phase (establishing the processed by a DSP and stored in a MMU, routine operations) and to emergency support. both located in the Payload Processing Unit. In order to be able to arrive at an effective overall system design (both from performance DSP and cost points of view!), it is essential that the The DSP is a space qualified ASDP21020 design of spacecraft and the associated device which is used to perform image operations are performed together in an overall processing on the acquired image both, post system-level concept rather than in isolation acquisition and during acquisition. Typical (Ref. 4). image processing tasks include image compression. Proba provides considerable flexibility in the allocation of on-board resources and in Memory Management Unit scheduling of operations when compared to the Autonomy requires the accommodation of a relatively rigid concepts used in conventional large telemetry buffer in order to be able to missions. By its nature, Proba offers an decouple the data generation during periods excellent opportunity for validating and of observation from the data transmission demonstrating novel operations concepts which during ground contacts. Fast random access may then be applied in future missions. In will be used for supporting autonomous data particular, the following operations functions transmissions directly to a user's station. are being planned for implementation on Proba: Appropriate memory management will allow the efficient usage of the storage area by the 1. On-board housekeeping: Proba will on-board scientific data analysis and autonomously take care of all routine compression software. Although relatively housekeeping and resource management tasks. small at 1 Gbit, the mass memory is adequate This includes also the decision-making process for fulfilling Proba’s mission requirements. in the case of (foreseeable only!) anomalies, i.e. The Mass memory is located into the PPU failure detection, failure identification and first- and is part of the Main instrument. level recovery actions. A summary of the Data stored in the MMU can be accessed information available on-board is downlinked to from both the DSP and the DHS. the control centre at regular intervals. More detailed information may be downlinked on specific request by the control centre.

2. On-board data management: Proba will MISSION OPERATIONS CONCEPT also autonomously take care of all management tasks related to the on-board data handling, storage, and downlinks. The spacecraft 6

J. Bermyn 13th AIAA/USU Conference on Small Satellites accommodates in the PPU a 1 Gbit mass 4. Instrument commanding: all preparatory, memory for data recording and a tuneable 2 commanding, and data processing activities Kbit/s to 1 Mbit/s down-link. As a worst case, related to the instrument operations are planned two passes of about 10 min every 12 hours are to be performed on-board (after an appropriate available for downlink of telemetry data (as initialization period). This includes the planning, well as for command uplinks). Therefore, the scheduling, resource management, complete mass memory contents can be navigation, and instrument pointing as well dumped to ground every 12 hours if desired. It as the downlinks of the processed data. In the is intended to downlink only a synthetic house- case of the main Earth Observation Instrument, keeping report (unless specific data are a specific attitude (pointing and rates) of the explicitly requested) so the full down-link will spacecraft will be required to perform its Earth normally be available for science data. observation function. The calculations of the relevant slew characteristics will be based on a 3. On-board resources usage: Proba will request file (containing the coordinates of the autonomously take care of all management target area and the observation duration) which tasks related to the on-board power usage. Any is uplinked from ground. excess power and energy (above the basic spacecraft control requirements during daylight The planning and scheduling of the instrument and eclipse phases) will be allocated to the requests together with other spacecraft instruments and to the spacecraft subsystems activities will be resolved on-board using a supporting the specific operations of the combination of constraints solver and optimiser instruments, for instance to the wheels for to achieve the best possible mission data return. attitude manoeuvres. The allocation will be performed on a dynamic basis, resolving task 5. Science data distribution: the collected constraints and priorities. Constraints include science data are normally downlinked to the for each activity the power and data storage (nominal) ground station from where they area needed, the pointing requested, etc. may be routed automatically or on request to a user’s site using Internet links. Because of the criticality of this function Furthermore, it is planned to demonstrate an precautions will be introduced as follows: automatic direct data distribution capability to different user ground antennae upon their à the scheduling of on-board activities requests without human involvement and obtained after solving the constraints and with minimum possible delay. The optimal priorities may either be done on-ground or downlink times may be uplinked in the on-board with ground acknowledgement; request file or calculated on-board. since this scheduling may also be programmed in on-board software, fully autonomous scheduling is feasible in principle; however, this step should be taken CONDUCT of MISSION OPERATIONS very carefully and only after a build-up of operational experience has been established; Ground Segment à a safety function will continuously monitor With the ground station located in a mid- the spacecraft ensuring that sufficient energy latitude region (Belgium) about 4 time 10 will be available during the following eclipse minutes of visibility per day will be available in phase based on a specified depth-of- average. The station consists of a portable 2.4 discharge of the battery, while guaranteeing m dish with RF front-end and a control centre that also enough energy is available for the with relatively limited facilities. The centre will down-link of house-keeping data. be directly connected to the ground antenna; it will also be connected to a communications 7

J. Bermyn 13th AIAA/USU Conference on Small Satellites network (using the Internet) for easy remote a check-out of the spacecraft status is access by the users. performed. After the spacecraft health has been confirmed and a first fix of navigation data is The ground station will provide the following achieved, the GPS receiver is switched on and functions: the AOCS is commanded into a nominal (Earth-pointing) mode. 1. automatic link acquisition based on Norad elements and spacecraft navigation data; Commissioning Phase 2. communications set-up protocol for the The spacecraft is kept pointing towards nadir types of data (and their bit rates) to be (or nadir plus an offset in case of a specific received; target) by means of a star tracker, on-board 3. automatic uplink of previously screened navigation data and the set of wheels. During observation requests and spacecraft the spacecraft commissioning the performances planning commands; of the spacecraft subsystems and the autonomy 4. automated call of ground staff in case of support will be checked , for instance: detection of on board anomaly; 5. science data filing, notification of scientists, · Star tracker accuracy in inertial and nadir- and data distribution. pointing mode; · GPS attitude measurement accuracy using The spacecraft may also be accessed by other the star tracker as a reference; standard S-band ground stations (for instance: · Navigation accuracy; ESA, CNES). For what concerns the telemetry, · Wheels performances (perturbations); it is planned that science data can be · Magneto-meter accuracy, using magnetic transmitted directly to a ‘user station’ as field model; specified in the observation request. · Power generation performances; · Thermal and thermo-elastic performances, The user request shall also specify the data using the double head star tracker; transmission type: if it is a ‘send and forget’ · RF link performance; type, the data will be removed from the mass · Avionics performance (time accuracy, memory after transmission; if it is a ‘confirmed SEU); delivery’ type, the data will be removed from · Instruments performances; the mass memory only after a positive · Operations functions. acknowledgement of receipt has been received from ground. The handling of transmission anomalies, i.e. re-transmissions and time-outs, will follow a pre-programmed algorithm. Nominal Phase Nominal spacecraft-to-ground interfaces are LEOP Operations limited to the downlink of the spacecraft health The spacecraft is launched in a powered-off status and house-keeping data. Specific configuration. Five seconds after the spacecraft operations activities, however, are required to detects the separation, the avionics and attitude support the demonstration objectives of the acquisition units are powered on, as well as the spacecraft (e.g., switch to GPS attitude, spacecraft receivers. The spacecraft separation software uploads). Emergency and specific rates are damped first by magneto- operations are the only periods when human meters/magneto-torquers and then by star interaction is required; during the remainder of tracker/wheels. Subsequently, the spacecraft the time, operations will be performed acquires a rough nadir pointing attitude until automatically by the control centre and ground the first ground contact. When ground contact is established the transmitter is switched on and 8

J. Bermyn 13th AIAA/USU Conference on Small Satellites staff can have remote access to downlinked For cases (1), (2), (3) the recovery is data whenever they wish. performed autonomously on-board, with priority given first to the safety of the The nominal instrument operations are limited spacecraft and the downlink capability to to observation requests remotely transmitted ground, and secondly to the recovery of the from the scientists to the control centre. The complete mission. For case (4), the requests are automatically checked, a spacecraft is designed to be recoverable even preliminary schedule of the requested if left uncontrolled by the provision of operations is prepared, and the scientists are passive thermal control, and a quasi-omni- informed about the results. The requests are directional link coverage from ground. In then uplinked to the spacecraft which may re- addition, built-in protections are provided to schedule them in accordance with its more ensure that the computer and receivers are precise on-board knowledge. After the always powered on. The handling of on-board scientific data are collected they are downlinked failures and recoveries relies either on to ground at the first possible opportunity and redundant identical units or on functional the scientist is notified of the data availability. redundancies.

Spacecraft Contingency Phase Mission Planning and Scheduling Whenever a failure is detected on-board, the Three options will be provided for mission ground staff is automatically warned (by means planning and scheduling featuring different of a pager, telephone, or e-mail). The station allocations between on-ground and on-board triggers the warning procedure through activities. In this manner, a gradual progression monitoring of the telemetry data or when it fails towards an increased on-board autonomy may to receive the spacecraft’s periodic status be implemented during the course of the report. mission:

The detection of on-board anomalies and 1) conventional: mission planning and failures is performed autonomously using the scheduling are done manually on-ground; following parameters: checks of the proposed planning are performed by both the on-ground and the 1. Unit technological parameters are monitored On-Board Mission Manager (OBMM) by the central computer for hard errors (as mission planning tools; usual); 2) semi-autonomous: the on-ground and the 2. Drifts, biases, and systematic errors may be OBMM mission planning tools are used to detected by simple coherency checks and by schedule the activities, but the resulting analytical redundancy techniques; schedule must be confirmed on-ground 3. Failure of the central software performing prior to its execution on board; these functions is detected by conventional 3) completely autonomous: the operations mechanisms (such as watch-dogs and requests are evaluated on-ground by the processor alarms) or perhaps by a mission planning tool and then uplinked to monitoring function in the redundant the spacecraft which schedules the activities processor; in the most efficient way while accounting 4. In case of a failure which can not be for the relevant constraints. recovered autonomously, ground is informed by the absence of the ‘alive’ signal The commanding of the spacecraft is performed from the spacecraft and will be able to using synthetic and goal-oriented commands. In access the spacecraft using direct order to accomplish autonomous mission commands. management, goal-oriented commands are first expanded into elementary tasks, then these 9

J. Bermyn 13th AIAA/USU Conference on Small Satellites tasks are merged with all other operations to be which has access to a library of on-board performed, and finally, the constraint problem is services; resolved. Each task has its own associated · software updates are seen as complex constraints, resource requirements, and priority operational procedures, and any ground parameters. In addition, cost functions are interaction is executed through a dedicated introduced in order to allow for optimisation of spacecraft command language and an on- the long-term planning, in contrast to the board interpreter. scheduling task which has a shorter-term view and does accommodate (but not optimise) a For what concerns the lower-level commanding possible sequence of tasks. design, packet telecommanding is supported using the COP-1 protocol. Commands are Operational Language and Software interpreted at spacecraft computer level or at For executing a science request on-board, the instrument level. Direct priority commands are following information should be provided: supported for emergency recovery situations.

1. which area shall be observed; CONCLUSIONS 2. which instrument configuration shall be selected; Proba is an ESA mission aiming to 3. where shall the data be sent. demonstrate the opportunities and benefits of on-board autonomy. It offers a flight These are the only items of information that will opportunity for the validation of the be contained in a spacecraft command. The on- associated technological capabilities. board computer will translate item (1) into a time sequence of activities with the aid of (in The on-board autonomy is exercised in particular) the on-board navigation function; realistic scenarios through the item (2) will be expanded into a configuration accommodation of three payload sequence and item (3) will be translated into a instruments: an Earth Observation downlink sequence. instrument, a space radiation monitoring instrument, and a debris detector. The Specific pre-defined operations procedures are autonomy capabilities focus on the following called up as required by specifying items (2) operations functions: and (3). These procedures will be resident on- board, or may be uplinked from ground. · scheduling, preparation and execution of Therefore, a spacecraft command language will scientific observations; need to be implemented on-board on top of the · scientific data collection, storage, and basic telecommand execution module. processing; · data communications management One of two radically different options (which between Proba, the scientific users and the are still being evaluated) may be selected for ground station; implementing operations procedures, while · management of on-board resources and taking account of the fact that complex house-keeping functions; operations procedures as well as on-board · state estimation and trend monitoring. software updates need to be executed: · failure detection, reconfigurations and software exchanges. · complex operations are considered as on- board software updates so that any complex The Proba spacecraft provides a unique ground interaction is executed by the uplink opportunity for the in-orbit demonstration of of an on-board software task which is a number of new technologies, such as for scheduled by the on-board scheduler and instance: 10

J. Bermyn 13th AIAA/USU Conference on Small Satellites · a double-head autonomous star tracker 1. www.estec.esa.nl/wscwww/srem/srem.ht for high-accuracy attitude estimation; ml. · GPS receiver for position and attitude 2. www.estec.esa.nl/wsmwww/erc32/erc32. estimation; html. · novel, high-performance, and space- 3. S. Ekholm : “Software Validation hardened on-board computer; Facilities”, Data Systems in Aerospace · high-integration solid state recorder; Symposium, Sevilla, Spain, May 1997. · GaAs solar panels with integrated diode. 4. Edited by J. R.Wertz & W. J. Larson : “Reducing Space Mission Costs”, REFERENCES Microcosm & Kluwer, 1996.

PROBA Structural / Thermal Model (Verhaert)

11

J. Bermyn 13th AIAA/USU Conference on Small Satellites