ONBOARD AUTONOMOUS CORRECTIONS FOR ACCURATE IRF POINTING

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1 ONBOARD AUTONOMOUS CORRECTIONS FOR ACCURATE IRF POINTING John L. Jørgensen, Maurizio Betto, Peter S. Jørgensen, Troelz Denver Technical University of Denmark, Ørsted.DTU, Dept. of Measurement and Instrumentation Building 327 DK-2800 Lyngby, Denmark. Phone: , Fax: , ABSTRACT The dramatic improvement in the performance of star trackers both in accuracy, reliability and autonomy has led to a significant broadening of the tasks handled by star trackers. Current measurement accuracies are in the range of 1 arc second. Measurement noise, accuracy and biases can be described by three measures: Noise Equivalent Angle, Relative Instrument Accuracy and Absolute Instrument Accuracy, describing noise during constant external conditions, noise during varying external conditions including NEA but excluding biases and the total measurement noise, including NEA, RIA and biases respectively. Sensor improvements and increased instrument autonomy are the main reasons for the improved star tracker performance. The Advanced Stellar Compass (ASC) is a state of the art, fully autonomous star tracker. Its full potential was first fully exploited onboard the German CHAMP satellite. CHAMP includes two ASC s each with two camera heads fixed together in optical benches. The ASC configuration onboard CHAMP provides an elegant platform for testing measurement accuracy and for confirming onboard corrections, which can be performed online by the ASC e.g. the correction for astronomical aberration correction. 1. INTRODUCTION Over the past decade a dramatic improvement in the performance of star tracker technology has taken place. Not only has the accuracy been improved, also other important performance parameters such as the acquisition time, i.e. the time from no knowledge of the attitude till the first accurate absolute update is output, and the timeliness of the update, i.e. the time from measurement till the attitude has been calculated and outputted. But also for less obvious performance characteristics the improvements has been substantial, e.g. autonomous rejection of false-positive attitudes, automatic radiation impact suppression and proper handling of non-stellar objects. The result of these improvements is, that star sensors on spacecrafts have changed from their secondary role as attitude calibration units a decade ago, to the prime real-time attitude sensor for precision navigated spacecrafts today. 2. NOISE AND BIAS TERMINOLOGY The most dramatic improvement for the science users has been the increase in precision and accuracy. The noises and biases influencing the accuracy of the attitude measurement, may for star trackers conveniently be divided into three groups: 1) The noise sources that are present in the case where all external variables are kept constant leads to a deviation from the actual attitude termed the Noise Equivalent Angle, NEA. Typical effects adding to the NEA are photon count noise, photo sensor read-out noise, A/D noise etc. The NEA is conveniently measured over short times with the star tracker pointed to the same point on the night sky. 2) The Relative Instrument Accuracy, RIA, is the accuracy the instrument exhibits over time spans where biases are negligible but including NEA, varying star patterns, moving non-

2 stellar objects etc. 3) The Absolute Instrument Accuracy, AIA, is the overall accuracy of the instrument, including all biases. The NEA has consequently improved from 2-5 arcseconds 1σ to a sub arcsecond level, the RIA has dropped from a few tens to 2-3 arcsecond, and the AIA, which for precision mounts typically is dominated by the RIA may be as low as 3-5 arcseconds over years. 3. ONBOARD AUTONOMY This improved performance is partly due to improved sensor-technology with enhanced signal to noise ratios, partly due to improved processing electronics, which allows for more sophisticated and faster signal processing. However, the main reason for the increased precision, is the application of onboard autonomy, which apart from simple outlier rejection also allows for a reliable removal of "false positive" answers, and other "unexpected" noise sources, that otherwise would degrade the quality of the measurements, e.g. discrimination between signals caused by starlight and ionising radiation. The application of autonomy in the signal processing has also provided the means for important onboard processing steps, such as the autonomous recovery from lost in space, where the attitude instrument without a priori knowledge derive the absolute attitude, i.e. in IRF coordinates, within fractions of a second. Combined with precise orbital state or position data, the absolute attitude information opens for multiple ways to improve the mission performance, either by reducing operations costs, by increasing pointing accuracy, by reducing mission expendables, or by providing backup decision information in case of anomalies. It is important to note, however, that despite the intrinsic high accuracy of the instrument, a given mission may not be able to utilize it to the full, unless attention is paid to the following problem: An attitude measurement noise in the arcsecond range cannot directly be translated to attitude knowledge or pointing accuracy of the spacecraft, because some residual systematic noise or bias effects exists. These residuals encompass relativistic effects such as orbital and annual aberration, light-time effects, perceived noise terms such as proper motion, or bias terms such as precession and nutation, and of course uncompensated thermal generated offsets. Even though the aforementioned residuals are all of moderate amplitude or size, i.e. tens of arcseconds, and regardless most of them can be accounted for, so that removal via on ground post-processing is viable, the process of doing so is rather complex, time consuming and relatively imprecise as compared to what can be achieved onboard. Furthermore, for mission profiles where a demand for real-time accurate attitude exists, such as highresolution telescopes, onboard removal of the aforementioned bias terms is the only way to meet stringent pointing requirements. 4. THE ADVANCED STELLAR COMPASS The Advanced Stellar Compass, ASC, developed and manufactured by a consortium led by the Technical University of Denmark, is a miniature fully autonomous star tracker that has realized all of the above-mentioned improvements. It recovers from the lost in space conditions in only 30ms, has proven capable to operate under severe radiation conditions such as the massive mass ejections encountered in the past solar maximum. It exhibits an NEA level less than half an arcsecond, and mounted on an optical bench thermo-elastic biases less that 2 arcseconds may be achieved. The ASC is designed to use all available stars in its field of view, wherefore proper motion, non-stellar object and radiation damage effects

3 are efficiently averaged out. Furthermore, the ASC is capable of, when requested by the user, to removing both astronomical aberration, precession and nutation effects. This function is enabled by the implementation of a full orbital state vector model with automatic cloud filtered GPS updates, a world time clock, astrometric correction tables, and a attitude output transform system, that allow the ASC to deliver the spacecraft attitude relative to the Inertial Reference Frame (IRF) in real-time. 5. THE CHAMP SATELLITE The combination of the improved performance and the onboard correction for the astrometric terms has led to an instrument that enables onboard pointing accuracies in the arcsecond range. The first spacecraft to use the ASC performance level in full is the German mini-satellite, CHAMP, a geo-potential mapping mission led by GeoForschungsZentrum Potsdam. The Champ spacecraft has two scientific instruments requiring high accuracy attitude knowledge: A gravitometer and a vector magnetometer. Since the gravitometer must be located very close to the centre of gravity, and since the extremely sensitive magnetometer has to be mounted as far as possible from magnetic disturbances, and hence electric currents, a star tracker for each was necessary. Furthermore, since the CHAMP mission requires a slow scanning of all of the Earth and all local time, a slow drift polar orbit was chosen. This orbit implies that the Sun slowly will scan all parts of the spacecraft, and consequently blind a star sensor with regular intervals. The ASC consist of a Data Processing Unit, DPU, and one or two Camera Head Unit, CHU. There solution to the blinding is therefore a configuration with two CHU pointed such that the Sun cannot blind both simultaneous. Consequently CHAMP is configured with two ASC, each with two CHU s. The actual configuration is shown in figure 1. Figure 1 The attitude information of the CHAMP spacecraft is used both by the AOCS system and by the two main vector instruments. The AOCS is based on a cold gas thrusters limited dead band controller, assisted by a magnetorquer proportional regulator. Since the orbit and attitude chosen will lead to expose of the CHU to direct sunlight exposure for 14 day period regularly, and since intrusion of the full Moon also will happen at a regular basis, a strategy has been chosen where the AOCS relies the output from CHU1 when available. Should valid attitude from CHU1 be unavailable, e.g. due to a solar blinding, the attitude from CHU2 is used instead. Should also CHU2 data be missing, CHU3 is used, etc. This strategy may not be the optimal for a pointing mission, but for the implemented dead band regulator, it is a robust and adequate solution.

4 The boom carrying CHU1 and 2 is hinged such that it folds in over the body in the launch configuration. Both the body and the boom couple of CHU are mounted an optical bench, wherefore the angle between these two could be accurately measured prior to launch. However, since the accurate zero gravity deployment angle of the boom was unknown prior to launch, this angle had to be measured in flight. During LEOP, before this angle could be measured, the offset angle led in combination with the chosen CHU1 through 4 switching was clearly visible: Every time the AOCS switched from the boom mounted CHU to the body mounted, an apparent jump in the attitude was registered and often causing the cold gas system to react. The actual boom deployment value was easily found from the deviation between the two sets of CHU, the offset uploaded as a correction to the body mounted CHU. 6. THE CHAMP ASC OPERATIONS After the initial adjustments, the ASC onboard CHAMP has been operating impeccable. Solar intrusion for up to 14 consecutive days are handled as designed, and full operation commences as soon as the blinding state seizes. Lunar intrusions are as designed, with no degradation with a Moon up to 50% phase, intermittent drops in attitude fixes from 50-75% Moon, and proper operations with the full Moon more than 7 from the boresight. The autonomous functions have shown a remarkable robustness during the massive solar mass ejections encountered during the last solar max. Even during the worst of the storms, with particle fluxes more than times above the level of quiet times, the ASC kept outputting valid attitudes enabling the AOCS to maintain proper control of the spacecraft. 7. MEASUREMENT ACCURACY AND ONBOARD CORRECTIONS Normally, the assessment of the measurement accuracy of a star tracker in space is extremely difficult because no other instrument may act as the reference source. Therefore secondary reference sources, such as short interval noise assessment, or noise relative to a science model, e.g. the magnetic field is used. Both methods are only indicative at best. The first, because it on the one hand inevitably will include spacecraft motion and actuator noise, on the other only will include tracker noise terms up to the minute level. The second, because it unavoidably will include noise from the science model itself. Since multiple CHU s are flown on CHAMP, and since they do not point to the same portion of the sky, i.e. they are viewing different star patterns, an excellent measure of the accuracy of the ASC may be found by comparing the attitude measured by the CHU at a given time. Obviously this accuracy includes all noise and bias terms, including thermoelastic deflections of the optical bench, but assuming a reasonable stability of the latter, a very good assessment of the AIA may be arrived at, by using the attitude measured over several orbits. In the following we are using a very simple measure, to Aberration direction and size Aberration induced change of apparent angle Earth heliocentric velocity Figure 2 camera 1 camera 2 Orbital velocity camera 1 camera 2

5 assess the pointing accuracy of the ASC, namely the so-called Inter Boresight Angle, IBA. The IBA is found from the dot-product of the measured pointing direction from simultaneous measurements from two CHU s. The IBA has amongst other been used onboard CHAMP, to verify proper operations of the astronomical aberration corrector, and subsequently correct operations of the orbit-state updates and propagator. Figure 2 show how the astronomical aberration will influence the measurements of the CHU at various stages of the orbit. The correction is always towards the heliocentric direction of motion and is largest when the pointing direction is perpendicular to the boresight. Since the orbit shown causes first the one, then the other CHU to be pointed perpendicular to the heliocentric velocity, the measured IBA will vary accordingly over the orbit. The direction of the change in the IBA is also indicated in figure 2. Figure 3 Figure 3 show the measured IBA as a function of the spacecraft latitude. The data shown is four consecutive orbits, with the aberration correction function turned off. The expected shape of the variation in the IBA is an ellipse, however, since the one CHU is blinded during part of the orbit, no IBA has been measured. Since the measured IBA include parts of the orbit where one CHU has just left solar blinding and the subsequent thermal loading of the optical bench, the measured angle also include a thermal bias term close to its maximum value.

6 Figure 4 Figure 4 show the consecutive four orbits, where the aberration correction was enabled. Clearly the ellipse shape from the aberration effect has disappeared and the attitude output is now in the true orbital frame of the spacecraft. Note the remarkable repeatability of the measured IBA over multiple orbits. From analysing short sections it is found, that the NEA is in the range of 0.4 arcseonds. From similarly measuring the deviation in the IBA over several days, it is found, that the bias from thermo-elastic deformation is in the range of one arcsecond. Finally it shall be noted, that since the IBA essentially is a comparison between two uncorrelated measurements, the RIA of a single CHU is about 22 arcseconds 6σ, or approximately 3.5 arcseconds 1σ.