Advanced Stellar Compass Research Articles

Europa's Influence on the Jovian Energetic Electron Environment as Observed by Juno's Micro Advanced Stellar Compass

AbstractThe micro Advanced Stellar Compass is an attitude reference for the MAG investigation onboard Juno. The μASC camera head unit images the star field with a CCD that is also sensitive to particles with enough energy to pass through the camera shielding: >15 MeV electrons and >80 MeV protons. This provides the capability to monitor fluxes of high‐energy particles in Jupiter’s magnetosphere. A survey of energetic electron fluxes sampled during the first 47 Juno orbits reveals instances of variations observed when Juno is traversing the M‐shell of the Galilean moons. Juno's traversal of the Europa M‐shell often results in distinctly particle signatures. We present the μASC observations of increased electron flux during the crossing of Europa’s plasma wake, and depletion of energetic electron flux on the upstream side. The upstream/downstream differences indicate that the wake environment of Europa drives strong pitch angle scattering on relativistic electrons.

Numerical simulation of the sensor output on Macau Science Satellite-1

Macau Science Satellite-1 (MSS-1) will be launched at the early of 2023 into a near-circular orbit. The mission is designed to measure the Earth’s geomagnetic field with unpreceded accuracy through a new perspective. The most important component installed on the satellite, to ensure high accuracy, is the deployable boom (Optical Bench). A Vector Field Magnetometer (VFM), an Advanced Stellar Compass (ASC), and a Couple Dark State Magnetometers (CDSM) are deployed on the deployable boom. In order to maximize the mission’s scientific output, a numerical simulator on MSS-1’s deployable boom was required to evaluate the adaptability of all devices on the deployable boom and assist the satellite’s data pre-processing. This paper first briefly describes the synthesis of the Earth’s total magnetic field and then describes the method simulating the output of scalar and vector magnetometers. Finally, the calibration method is applied to the synthetic magnetometer data to analyze the possible noise/error of the relevant instruments. Our results show that the simulator can imitate the disturbance of different noise sources or errors in the measuring system, and is especially useful for the satellite's data processing group.

Energetic electron lensing caused by Ganymede's magnetic field

The micro Advanced Stellar Compass (μASC) continuously monitors high energy particle fluxes in Jupiter's magnetosphere while providing attitude reference for the Juno Magnetic Field investigation. The μASC camera head unit (CHU) serves as a energetic particle sensor monitoring the radiation environment at Jupiter by sensing electrons greater than 15 ​MeV that pass through the CHU shielding. A comparison of the energetic electron population sampled during many orbits reveals events observed when Juno is traversing Ganymede's M-shell. We present observations of energetic electron depletions related to the interaction with Ganymede's magnetic field. The magnitude and extent of these depletions is associated with Juno's proximity to the magnetic field line passing through Ganymede. Analysis of the observations reveal that the charged particle depletion likely arises from electron scattering in Ganymede's magnetic field, observed within ∼13° phase angle from the moon, beyond which the flux tubes refill due to diffusion.

Observations of MeV electrons in Jupiter's innermost radiation belts and polar regions by the Juno radiation monitoring investigation: Perijoves 1 and 3

AbstractJuno's “Perijove 1” (27 August 2016) and “Perijove 3” (11 December 2016) flybys through the innermost region of Jupiter's magnetosphere (radial distances <2 Jovian radii, 1.06 RJ at closest approach) provided the first in situ look at this region's radiation environment. Juno's Radiation Monitoring Investigation collected particle counts and noise signatures from penetrating high‐energy particle impacts in images acquired by the Stellar Reference Unit and Advanced Stellar Compass star trackers, and the Jupiter Infrared Auroral Mapper infrared imager. This coordinated observation campaign sampled radiation at the inner edges of the high‐latitude lobes of the synchrotron emission region and more distant environments. Inferred omnidirectional >5 MeV and >10 MeV electron fluxes derived from these measurements provide valuable constraints for models of relativistic electron environments in the inner radiation belts. Several intense bursts of high‐energy particle counts were also observed by the Advanced Stellar Compass in polar regions outside the radiation belts.

Characterization and compensation of thermo-elastic instability of SWARM optical bench on micro Advanced Stellar Compass attitude observations

Launched into orbit on November 22, 2013, the Swarm constellation of three satellites precisely measures magnetic signal of the Earth. To ensure the high accuracy of magnetic observation by vector magnetometer (VFM), its inertial attitude is precisely determined by µASC (micro Advanced Stellar Compass). Each of the three Swarm satellites is equipped with three µASC Camera Head Units (CHU) mounted on a common optical bench (OB), which has a purpose of transference of the attitude from the star trackers to the magnetometer measurements. Although substantial pre-launch analyses were made to maximize thermal and mechanical stability of the OB, significant signal with thermal signature is discovered when comparing relative attitude between the three CHU's (Inter Boresight Angle, IBA). These misalignments between CHU's, and consequently geomagnetic reference frame, are found to be correlated with the period of angle between Swarm orbital plane and the Sun (ca. 267 days), which suggests sensitivity of optical bench system on temperature variation.In this paper, we investigate the propagation of thermal effects into the µASC attitude observations and demonstrate how thermally induced attitude variation can be predicted and corrected in the Swarm data processing. The results after applying thermal corrections show decrease in IBA RMS from 6.41 to 2.58″. The model significantly improves attitude determination which, after correction, meets the requirements of Swarm satellite mission. This study demonstrates the importance of the OB pre-launch analysis to ensure minimum thermal gradient on satellite optical system and therefore maximum attitude accuracy.

Noise features of the CHAMP vector magnetometer in the 1–25 Hz frequency range

CHAMP (CHAllenging Minisatellite Payload), a German small satellite mission to study the Earth's gravity field, magnetic field and upper atmosphere, ended in space on 19 September 2010. Thanks to the good quality of the satellite and to several altitude maneuvers, the satellite provided continuous and reliable observations including house-keeping data at different processing levels for more than 10 years. Among them the high-resolution vector magnetic field data, provided by the FGM (FluxGate vector Magnetometer) at 50Hz sample rate, are one of the most important data products. The 50Hz data allows us to investigate all signals from DC to 25Hz. Here we report on noise features found in the frequency range above 1Hz. There are predominantly three types of disturbances found regularly in the vector magnetic field data. One is at ∼2.1Hz and harmonic signals, an other is an 8Hz single frequency signal. The features of these two signals are characterized and analyzed, which helps us to trace the signals to the source of disturbance. The 2.1Hz harmonic signals mainly depend on the incident angle of sunlight, which points at the star tracker instrument ASC (Advanced Stellar Compass), as the signal depends on the level of blinding. The 8Hz sinusoidal signal is continuously present and its amplitude is controlled by the temperature inside of the satellite. For higher temperatures the signal gets smaller, and it practically disappears above 20°C. Only the FGM x-axis is affected by the 8Hz signal. The third artificial signal affects the FGM y-axis. A frequency sweep from the Nyquist frequency down to zero and up again occurs when the readings of the FGM y-axis go through zero. This is probably related to a ringing of the analog-to-digital converter at the sign change. The amplitudes of all these noise features are generally below 50pT.

Characterization of CHAMP magnetic data anomalies: magnetic contamination and measurement timing

The CHAMP (CHAllenging Minisatellite Payload) mission ended after more than ten years in space on 19 September 2010. For achieving a high measurement accuracy of the magnetometers on CHAMP, detailed analyses of spacecraft magnetic characteristics in orbit are required. A decade of continuous magnetometer and housekeeping data are a good basis for evaluating some of the effects of variable spacecraft magnetic fields on the ambient field determination. It was found that some perturbations of FGM (FluxGate vector Magnetometer) or OVM (OVerhauser scalar Magnetometer) measurements are caused by stray fields induced by the power system, the ASC (advanced stellar compass) instrument or magneto-torquer currents. The magnetic effect of solar currents on FGM measurements varies with the local time of the orbit and amounts to 0.2 nT. In cases when one head of the ASC instrument was blinded by the sun, sometimes transient drops in instrument current strength occur, which were accompanied by magnetic disturbance signals (∼0.3 nT) in FGM measurements. The magnetic residual contamination of OVM data by the torquer currents was of order 0.1 nT but still detectable. An improved torquer correction matrix is derived which eliminates this effect. In-flight scalar calibration parameters revealed some of the effects of timing anomalies. Time lags between FGM and OVM readings are misinterpreted by the scalar calibration as variations of the angles between some of the sensor axes. The resulting amplitudes of the anomalies presented here lie in the range of some 0.1 nT, but they are systematic in nature.

Advanced stellar compass deep space navigation, ground testing results

Deep space exploration is in the agenda of the major space agencies worldwide and at least the European Space Agency (SMART & Aurora Programs) and the American NASA (New Millennium Program) have set up programs to allow the development and the demonstration of technologies that can reduce the risks and the costs of the deep space missions. Navigation is the Achilles’ heel of deep space. Being performed on ground, it imposes considerable constraints on the system and the operations, it is very expensive to execute, especially when the mission lasts several years and, above all, it is not failure tolerant. Nevertheless, up to now, ground navigation has been the only possible solution. The technological breakthrough of advanced star trackers, like the micro-advanced stellar compass ( μ ASC ) might change this situation. Indeed, exploiting the capabilities of this instrument, the authors have devised a method to determine the orbit of a spacecraft autonomously, on-board and without any a priori knowledge of any kind. The solution is robust, elegant and fast. This paper presents the preliminary performances obtained during the ground tests. The results are very positive and encouraging.

Enhanced mission performance from autonomous instrument guidance

During the last decade improvements in electronics, on-board processing power and software design has lead to significant advances in the development of autonomous instrumentation for spacecraft use. The Advanced Stellar Compass (ASC) and the newly developed micro-ASC ( μ ASC ) are excellent examples of such autonomous space instrumentation. With its full autonomy, this star tracker is capable of providing, in real-time, the absolute orientation with respect to the celestial reference frame with an accuracy of a few arc seconds. This high accuracy along with the robust operations, low weight and power consumption makes the μ ASC an ideal instrument for small, high yielding satellite missions. The ASC has hitherto been used by the satellite AOCS and the high accuracy scientific instrument for attitude recovery (among others onboard ØRSTED, CHAMP, and GRACE), and satellite high accuracy target acquisition and pointing (PROBA). Here three applications of the μ ASC as an autonomous onboard precision guide for precision vector instrumentation are presented. These are autonomous onboard antenna guidance, telescope guidance and tracking and high accuracy and wide range laser rangers.

The PROBA satellite star tracker performance

ESA's PROBA satellite features a high degree of autonomy, both technologically and scientifically. It is build around a powerful command, data and AOCS controller and with its less than 100 kg it is a true microsatellite. The scientific mission of PROBA includes a scanning telescope, which calls for high pointing accuracy and agility. The Advanced Stellar Compass (ASC) provides the pointing knowledge with arc-second accuracy. A description of the PROBA ASC is given and various aspects of the ASC are treated: Handling of solar and lunar blinding and dark and lit Earth horizons in the field of view. The accuracy of the PROBA ASC is analyzed comparing the measurements from the ASC's two camera-heads. Investigation of this Inter-Boresight Angle demonstrates the spacecraft thermal flexures, and models this to arc-second level. The astronomical aberration corrector of the ASC is investigated and confirmed. Finally timing and radiation handling of the ASC is treated.

In-flight quality and accuracy of attitude measurements from the CHAMP advanced stellar compass

The German geo-observations satellite CHAMP carries highly accurate vector instruments. The orientation of these relative to the inertial reference frame is obtained using star trackers. These advanced stellar compasses (ASC) are fully autonomous units, which provide, in real time, the absolute attitude with accuracy in the arc second range. In order to investigate the in-flight accuracy of the ASC, the terminology to characterize noise and biases is introduced. Relative instrument accuracy (RIA) and absolute instrument accuracy (AIA) can in principle be determined in-flight. However problems with modeling external noise sources often arise. The special CHAMP configuration with two star tracker cameras mounted fixed together provides an excellent opportunity to determine the AIA in-flight using the inter boresight angle.

Advanced stellar compass onboard autonomous orbit determination, preliminary performance.

Deep space exploration is in the agenda of the major space agencies worldwide; certainly the European Space Agency (SMART Program) and the American NASA (New Millennium Program) have set up programs to allow the development and the demonstration of technologies that can reduce the risks and the cost of deep space missions. From past experience, it appears that navigation is the Achilles heel of deep space missions. Performed on ground, this imposes considerable constraints on the entire system and limits operations. This makes it is very expensive to execute, especially when the mission lasts several years and, furthermore, it is not failure tolerant. Nevertheless, to date, ground navigation has been the only viable solution. The technology breakthrough of advanced star trackers, like the advanced stellar compass (ASC), might change this situation. Indeed, exploiting the capabilities of this instrument, the authors have devised a method to determine the orbit of a spacecraft autonomously, onboard, and without a priori knowledge of any kind. The solution is robust and fast. This paper presents the preliminary performance obtained during the ground testing in August 2002 at the Mauna Kea Observatories. The main goals were: (1) to assess the robustness of the method in solving autonomously, onboard, the position lost-in-space problem; (2) to assess the preliminary accuracy achievable with a single planet and a single observation; (3) to verify the autonomous navigation (AutoNav) module could be implemented into an ASC without degrading the attitude measurements; and (4) to identify the areas of development and consolidation. The results obtained are very encouraging.

Operation and performance of a second generation, solid state, star tracker, the ASC

The Advanced Stellar Compass (ASC) is a second generation star tracker, consisting of a CCD camera and its associated microcomputer. The ASC operates by matching the star images acquired by the camera with its internal star catalogs. An initial attitude acquisition (solving the lost in space problem) is performed, and successively, the attitude of the camera is calculated in celestial coordinates by averaging the position of a large number of star observations for each image. Key parameters of the ASC for the Ørsted satellite and Astrid II satellite versions are: mass as low as 900 g, power consumption as low as 5.5W, relative attitude angle errors less than 1.4 arcseconds in declination, and 13 arcseconds in roll, RMS, as measured at the Mauna Kea, HI observatories of the University of Hawaii in June 1996.

The advanced stellar compass, development and operations

The science objective of the Danish Geomagnetic Research Satellite “Ørsted” is to map the magnetic field of the Earth, with a vector precision of a fraction of a nanotesla. This necessitates an attitude reference instrument with a precision of a few arcseconds onboard the satellite. To meet this demand the Advanced Stellar Compass (ASC), a fully autonomous miniature star tracker, was developed. This ASC is capable of both solving the “lost in space” problem and determine the attitude with arcseconds precision. The development, principles of operation and instrument autonomy of the ASC is described. This is followed by a description of test and performance verification methods, and finally key physical and performance data are given.