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STECCO, a Laser Ranged Nanosatellite
Abstract
The Space Traveling Egg-Controlled Catadioptric Object (STECCO) is a 30 cm x 5 cm x 5 cm PocketQube nanosatellite to be launched in 2020. Its shape and the presence of a passive viscous damper shall allow gravity gradient stabiliza-tion of the satellite, with one of the two smaller faces pointing toward nadir. Each one of the two smaller faces will be equipped with one Corner Cube Re-flector (CCRs), so that it will be possible to track the satellite and reconstruct its orbit by means of laser ranging. One of the two CCRs will have the back faces coated while the other one will be uncoated: this will allow distinguishing which face is pointing toward the ground, because of the different sensitivity to the po-larization of the laser impulses used for ranging. CCRs for STECCO will not be custom built units but will be Commercial-Off-The-Shelf (COTS) ones. Optical tests on the reflectors and on the mounting system will be performed in the thermovacuum and optical test lab of the School of Aerospace Engineering of Sapienza University of Rome.
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The subject of this work is the implementation and experimental testing of a purely magnetic attitude control strategy, which can provide stabilization after the deployment and pointing of the spacecraft without any attitude information. In particular, the control produces the detumbling of the satellite and leads it to a desired attitude with respect to the direction of the Earth magnetic field, based on the only information provided by a three-axis magnetometer. The system is meant to be used as a backup solution, in case of failure of the primary strategy and is designed considering the constraints set on time of operations, power consumption, and peak electric current for a typical CubeSat mission. The detumbling and pointing algorithms are implemented on the FPGA core of a CubeSat on-board computer and tested by Hardware-in-the-loop simulations. The simulation setup includes a Helmholtz cage, recreating the magnetic environment along the orbit, the on-board computer, a MEMS three-axis magnetometer and Simulink software, on which the attitude dynamics is propagated. Test on the real system can provide useful information to select the parameters of the control, such as the gains, to estimate the limits of the system, the time of operations and prevent failures.
Lunar laser ranging has made significant contributions to the study of gravitational physics and the Earth-Moon system. The best results for fundamental gravitational experiments have been achieved using lunar laser ranging data accumulated so far. However, corner cube retroreflector arrays placed on the Moon currently set a limit on the laser-ranging precision, which is approximately several centimeters for a single photon received. To achieve millimeter precision, next generation of lunar laser ranging using a single hollow retroreflector with a large aperture has been proposed. We developed a prototype hollow retroreflector with a 100-mm aperture using silicate bonding together with a new fabrication method. Dihedral angle offsets of 0.5 ′′ , 0.8 ′′ and 1.9 ′′ were realized, which partly come close to meeting the requirements (offset of 0.6 ′′ for each dihedral angle) for lunar laser ranging. Fluctuation of the wavefront is approximately 1.038λ at 633 nm. A thermal cycle test ranging from −40 • C to +75 • C was carried out for 18.5 periods (approximately 5 d). After this test, the dihedral angle offsets were measured to be 0.39 ′′ , 1.00 ′′ and 2.06 ′′. The results indicate the potential application of our method for manufacturing a hollow retroreflector with a large aperture to realize lunar laser ranging.
A single aperture and hollow retroreflector [corner-cube mirror (CCM)] that in principle has no internal optical path difference is a key instrument for achieving lunar laser ranging one order or more accurate than the current level (∼2 cm). We are developing CCM whose aperture is 20 cm with optimized dihedral angles. The 20-cm CCM yields two times peak height for returned laser pulse compared with Apollo 15's retroreflector. Two investigations were conducted to confirm the feasibility of the 20-cm aperture CCM. The first is thermo-optical simulation and evaluation of the 20-cm CCM in the lunar thermal environment. Through this simulation, it has turned out for the first time that 20-cm aperture CCM made of single-crystal Si or "ultra-low expansion glass-ceramics" such as CCZ-EX® (OHARA Inc.) can be used for CCM with no thermal control, if the perfectly fixed point of CCM is limited to one. The second is annealing and shear loading experiments of single-crystal silicon (Si) samples. Through these experiments, high-temperature annealing from 100 to 1000 °C is confirmed to be effective for the enhancement of the adhesive strength between optically contacted surfaces with no optical damage in roughness and accuracy, indicating that this annealing process would enhance the rigidity of CCM fabricated by the optically contacted plates.
Several small satellites in the class of pico- and nano-satellites will be equipped with multiple small corner cubes: OPS-SAT (ESA), S-Net and TechnoSat (8 kg resp. 15 kg; Technical University Berlin), and CubETH (ETH Zuerich). The size of these satellites is in the range from 10x10x30 cm up to about 40x40x30 cm; the planned circular orbits are in the 450 – 620 km range. Commercially available 10 mm and 0.5” corner cubes will be used for SLR; a single corner cube of this size will be sufficient for SLR to the planned LEO orbits. Placing several of these corner cubes on each side of the satellites will not only allow for standard SLR and POD, but also for an independent attitude determination with < 1° accuracy, even after the end of the satellites lifetime, or in case of problems or satellite failure. For multiple satellites flying in close formation, it will be possible to distinguish the sequence of single satellites within the formation.
The laser-ranging technique and an extremely precise knowledge of the Earth gravitational field paved the way to perform very accurate measurements in General Relativity and Earth science using passive satellites. LARES 2 belongs to this category of satellites and is approved by the Italian Space Agency with a possible launch date at the end of 2019. One of the most interesting effects predicted by General Relativity is frame-dragging according to which the inertial reference frames are dragged by currents of mass-energy, such as a rotating mass. Indeed, the Earth rotation produces this effect that has already been measured by a very accurate orbit determination of the LAGEOS and LARES satellites. With this new satellite it will be possible to improve the accuracy of the measurement by one order of magnitude. This very demanding objective can be reached thanks to the unique orbit and the special design of the LARES 2 satellite. The paper will outline the physics behind the experiment and will describe the mission details.
LARES is a satellite of the Italian Space Agency, successfully launched with the new VEGA launcher in the occasion of its inaugural flight, VV01. It was put in a circular orbit at 1450 km altitude. This altitude was required to reduce atmospheric drag, whereas the satellite was designed to minimize the non-gravitational perturbations acting on the surface of the satellite. This was of paramount importance because the mission objective is to test Einstein general relativity, and any unmodeled effect could spoil the accuracy of the relativistic measurement. With the optimal design achieved, this non-gravitational unmodeled effects are maintained below 1% of frame-dragging or Lense-Thirring effect. This effect is the orbital node shift induced by the Earth rotation as predicted by general relativity. To achieve the accuracy required for the test, it was conceived a constellation of three laser ranged satellites (LAGEOS 1, LAGEOS 2 and LARES) along with the latest determination of the Earth gravitational field by GRACE satellite. The satellite is a passive system and embedded with 92 Cube Corner Reflectors that have the properties of reflecting back to the emitting ground station the laser pulses, thus allowing its precise orbital determination. In this paper engineering aspects of the mission will be addressed.
To date, lunar laser ranging to the Apollo retroreflector arrays, which are still operational after four decades, has produced some of the best tests of General Relativity. Since the ground Observatories have improved their accuracy by a factor of 200, the lunar hardware, due to the lunar librations, now limits the ranging accuracy. The Lunar Laser Ranging Retroreflector Array for the 21st Century program plans to deploy new packages that will improve the ranging accuracy by a factor of ten to one hundred in the next few years.
This classic book is the first comprehensive presentation of data, theory, and practice in attitude analysis. It was written by 33 senior technical staff members in the Spacecraft Attitude Department of the Computer Sciences Corporation and incorporates their experience in supporting more than 30 space missions. Because of the extensive cross-references, complete index, and 13 technical appendices, this book can be either a self teaching text, or a reference handbook. Among its unique features are orthographic globe projections to eliminate confusion in vector drawings; discussions of common data anomalies, data validation, attitude hardware, and associated mathematical models; and a presentation of new geometrical procedures for mission analysis and attitude accuracy studies which can eliminate many complex simulations.
Progress in laser systems for precision ranging, angle measurements, photometry, and data transfer
- V V Vasiliev
- S Shargorodsky
- Novikov
V. Vasiliev V. Shargorodsky S. Novikov, et al. "Progress in laser systems for precision ranging, angle measurements,
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Analysis of ILRS Data from STPSat-2 Retro-reflector
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A tutorial on retroreflectors and arrays for SLR
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