THE MICRO ADVANCED STELLAR COMPASS FOR ESA S PROBA 2 MISSION

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1 THE MICRO ADVANCED STELLAR COMPASS FOR ESA S PROBA 2 MISSION P.S. Jørgensen (1), J.L. Jørgensen (1), T. Denver (1), Pieter van den Braembuche (2) (1) Technical University of Denmark, Ørsted*DTU, Measurement and Instrumentation Systems, 2800 Lyngby, Denmark. Phone: , psj@oersted.dtu.dk. (2) Verhaert Design & Development, Hogenakkerhoekstraat 9, 9150 Kruibeke, Belgium. ABSTRACT In the wake of the successfullproba-1 ESA technology demonstrator its successor, PROBA-2, is under preparation. While maintaining focus on technology demonstration the PROBA-2 satellite will also serve scientific purposes, mainly observing the Sun. Part of the technology demonstration on PROBA-2 is the first flight of the next generation star tracker, the Micro Advanced Stellar Compass (µasc). This miniature fully autonomous star tracker is based on the well-proven design of the Advanced Stellar Compass (ASC), flown on numerous missions. The basic design drivers taken into account in the µasc were: reliability, mass, power, size, and also maintaining full heritage from the flight-proven ASC SW and latch-up protection philosophy. The µasc features pure 3V SMD technology, low power consumption, full EDAC on all data transfer and up to 32 Hz true sampling on a single electronics box (micro Data Processing Unit or µdpu). This new technology allows for a fully hot- or cold redundant star tracker (two separate µdpus) in just (10 by 10 by 4.5cm) operating with four fully cross-strapped camera head units (CHU s, 5 by 5 by 5cm each). 1. INTRODUCTION The first of ESA s PROBA technology demonstration satellites was launched in October 2001 and has since then operated successfully [1]. PROBA-1 demonstrated, along with a number of new enabling technologies, an unprecedented degree of autonomy in observation planning and operations. In continuation of this success the next ESA technology demonstration satellite, PROBA-2, is under preparation with a launch scheduled in The PROBA-2 satellite will carry a number of new technologies and instrumentation. One of these is the micro Advanced Stellar Compass (µasc), which is the subject of this paper. The main purpose of the PROBA-2 satellite is technology demonstration. However, instrumentation for Sun observation is also added, ensuring a scientific return as well. The PROBA-2 Sun observations will require arcsecond level accuracy, reliable and timely knowledge of the SC attitude, which will be provided by the µasc. In addition to being AOCS sensor and attitude reference for the scientific observations on PROBA- 2 the µasc is also the next generation in star tracker technology, thus fulfilling both mission objectives new technology and science. The µasc has been designed using the basic principles of the Advanced Stellar Com-

2 pass, which successfully has flown on over 15 missions with all the major space agencies including ESA (PROBA-1 and SMART-1). The new µasc design effort focused on developing a more compact, higher performance, less power consuming and more flexible instrument with even higher reliability numbers. The design drivers should be realized, while maintaining the important key features from the well-proven ASC design (accuracy, autonomy and flexibility). This paper describes the main design drivers applied for the µasc development and the resulting instrument developed for the PROBA-2 mission. 2. STAR TRACKER DESIGN DRIVERS The need for an increased level in the autonomy of advanced space instruments has been ever growing for the last two decades, i.e. as long as powerful onboard computers have existed. The demand has mainly been focused on the bus instrumentation (AOCS, TM/TC and on-board data handling) and amongst these, a significant interest has been in highly autonomous and accurate star sensors. However, also science measurements can gain significantly from autonomous care-free instruments. The autonomous star sensors (or star trackers) offer considerable advantages in the SC design, especially for the AOCS. Autonomous star sensors measures and reports directly and accurately the orientation of the spacecraft relative to an inertial reference frame. Therefore, attitude measurements are directly available for the AOCS without any post-processing, thus simplifying the AOCS algorithms. At the same time a sufficient number of star trackers with sufficient operational envelope and robustness will reduce the types and number of different attitude sensors on the SC, hence lowering both design and operational complexity. Most of the last generation of star sensors are quite robust against environmental disturbances and are autonomous, e.g. they can solve the lost in space problem and thus do not need to be seeded to begin operations. The autonomy also provides for automatic rejection of inaccurate or invalid attitude updates, online accuracy assessment, anomaly detection and isolation and failure trapping and recovery. However, several nonnominal conditions adversely impact their performance, e.g. high radiation dose rates, non-stellar luminous objects and blinding by Sun etc. Further enhancements in the autonomy are consequently called for. Another system-level driving requirement is to have large payload-to-bus mass, power and operational cost ratios, i.e. to have "more value for money" as, ultimately, this advances the scientific or commercial return of the mission. Typically, this is achieved by using small but still reliable instruments. Especially for star trackers, sharing of attitude data by both bus and payload has been employed to reduce cost, mass and allocation space. Autonomous, small and reliable implies that redundant units can be implemented with minor penalties only. At the same time considerably increasing the instrument's performance especially when they can be operated simultaneously, e.g. in a hot redundant, cross-strapped configuration.

3 2.1 General Requirements for Autonomous Star trackers The typical main requirements for an autonomous star tracker are given in table 1. However, the need to improve the performance and lifetime while decreasing the resources allocated to the star tracker is strongly felt for the deep space missions as well as for the commercial satellites. Class Requirement ASC µasc (*) Dual redundant Initial acquisition (solve lostin < 1 min 80msec 30msec space) Accuracy (EOL) [arcsec] 30(3 σ) 3(3 σ) 2(3 σ) Attitude rate [ /s] Up to 1 Up to 7 Up to 20 Update rate [Hz] Up to 4 Up to 4 Up to 32 Availability [%] Power [W] < n*0.3 Mass [Kg] < * Size [cm] Proc. Unit: ~10x10x10 Camera: ~5x5x5 DPU: 10x10x10 CHU: 5x5x5 DPU: 10x10x4.5* CHU: 5x5x5 Lifetime [years] Reliability [%] Table1: Typical requirements and example specifications for autonomous star trackers. 3. THE NEXT GENERATION STAR TRACKER: THE µasc To further improve and miniaturize the star tracker technology, the micro-advanced Stellar Compass (µasc) development was initiated. Specific goals during the design phase were, in addition to smaller size, mass and power consumption, to improve the update rate, accuracy and robustness as well as to increase the reliability to allow for 30+ years applications. The new µasc expands the multi-chu concept of the ASC by being able to operate up to four CHU s at once, thus providing full immunity to simultaneous blinding. In addition the standard µasc is built with two fully hot/cold redundant micro Data Processing Units (µdpu). The CHU s are fully cross-strapped; enabling in-flight reconfiguration for each of the µdpu s to operate the desired number of CHU s (zero to four). The CHU s can be up to 20m away from the µdpu. The µasc features a lower size, mass and Figure 1: The next generation star tracker the µasc. Here the double DPU is shown with one of the four camera head units. volume and an improved radiation tolerance that results in an increased Residual Design Margin (RDM) for all mission profiles studied so far. Like the ASC, the µasc is entirely tolerant to known increases in the radiation environment, such as those

4 encountered in the South Atlantic Anomaly (SAA) and the Van Allen belts. The instrument is designed to perform nominally even under extreme conditions, e.g. during extreme solar storms. This performance has been achieved through a coupled HW/SW radiation impact rejection strategy. The µasc specifications are given in table 1, where they can be directly compared with the star trackers main requirements. Throughout the design of the HW, the emphasis has been put on accuracy, functionality, low power, low thermal stressing, mass and size and on making system performance independent of the degradation of various parameters with age or radiation doses. By using 3V SMD highly integrated technology, the board population density has been increased and the mechanical stability has been maximized, while simultaneously lowering the power consumption. This has led to a compact design with a low IC-count, full inline EDAC and a very high data handling capacity. Due to the adopted design procedure, originally developed for the ASC, all IEEE parts applied in the design have been thoroughly screened for radiation tolerance with respect to total-dose, dose-rate for Single Event Upset (SEU) and Single Event Latch-up (SEL) [2]. This selection procedure ensures correct operations during exposure to solar wind and trapped protons in the Van Allen belts. To cope with SEL's generated by the (rare) cosmic particles, all circuit blocks are protected by individual latch-up protection circuitry. The projected SEL rate in GEO is less than one per year. Every SEL will result in the loss of attitude updates for 4.5 seconds. However, in case of a critical maneuver, the µasc may be operated in hot redundant configuration efficiently removing the risk of attitude update loss. Thermally, the µasc design is such that no component has a temperature that deviates more than 8K from the box surface reference point, this is verified in vacuum. The low thermal gradient associated with the low power dissipation and strong thermal coupling result in an extraordinary thermal cycling resilience and do provide for a very high reliability figure, as proven by the accelerated lifetime tests performed. The compact design and the miniaturization provide a rugged design with low amplification factors and high resonant frequencies, resulting in high shock and vibration level tolerances. The µasc interface will support the 1553B and the RS422 interface standards over which either PUS or CCSDS packets are transmitted. 4. QUALIFICATION STATUS The µasc has undergone a full set of qualification tests including radiation screening TID, EMC, accelerated lifetime test (200 30C) and shake. Also, the µasc has been tested on the real sky and results demonstrate improved performance as expected. With the PROBA-2 flight the micro Advanced Stellar Compass will earn its flight heritage enabling this new technology to be used on a suite of future satellite missions.

5 5. REFERENCES [1] Jørgensen, J. L et al., The Proba Satillite Star Tracker Performance, Abstract in Small Satellites for Earth Observation, 4th Int. Symp. of the IAA, pp , 2003 [2] Thuesen, G. G et al., Application Specific Radiation Tests for Cots EEE Components, Abstract in Small Satellites for Earth Observation, 4th Int. Symp. of the IAA, pp , 2003