From an experimental idea to a satellite

Transcription

1 From an experimental idea to a satellite Hansjörg Dittus Institute of Space Systems, Bremen German Aerospace Center

2 Looking back in History Yukawa potential Gravity at large scales Weak gravity Nordtvedt effect Time dependence of G LLR, LAGEOS, ASTROD, LATOR, OPTIS, SEE, DSGE Einstein s theory of gravity Metric theory of gravity Einstein s Equivalence Principle Gravitational redshift Perihelion shift Light deflection Gravit. time delay Lense-Thirring effect Schiff effect Gravitational waves GP-A, Cassini, LAGEOS, GP-B, LISA, HYPER, DSGE, ASTROD, LATOR, OPTIS Universality of Free Fall MICROSCOPE, STEP, GG, HYPER, DSGE Universality of Gravitational Redshift ACES /PHARAO, SPACETIME, OPTIS, DSGE Local Lorentz Invariance ACES, OPTIS, PARCS, RACE

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4 Everything outside the Solar System refuses to follow the laws of General Relativity. Joao Magueijo, Imperial College

5 Why satellite experiments to test EP Centrifugal force Torsionsfaden Gravity Test masses made of different materials Torsion balance uses only 0.3 % (earth mode) and 0.07 % (solare mode) of the inertial force wrt to earth gravity. Gravity acts vertically Centrifugal force acts horizontally Direct measurement (full signal) Influence of torsion fibre Periodic long termexperiment Periodic free fall in space.. Free fall experiment Y (+) N (+) N (-) Torsion balance experiment N (-) Y (-) Y (+)

6 EP-Maesurement on satellite dr 1 rη 3 1 = E m r 1 Differential orbit differences cannot be maesured directly For weak coupling, both test masses and the time satellite form a springmass-system 1 ORBIT Relative test mass position

7 Development of precise differential accelerometers k 2 (1) Moveable test masses: a β δx m β= 0 and weak k: high resolution position sensor Problem: Complicated test mass movement (2) Closed loop control: a acontrol ω testmass = k m β 2 + k control m 2 β control Stiffness can be much higher than k: large bandwidth, but lower resolution Problem: back-action by noise

8 Drag-free AOCS for satellites Concepts: (1) Closed loop control large bandwidth two test masses: aligned e.g. MICROSCOPE 10-7 m / s 2 at Hz (2) Open loop for two moveable test masses (aligned) very small bandwidth (3) Virtual reference point for more than one test mass m / s 2 at 10-3 to 10-4 Hz (4) Free floating control for more than one test mass misaligned needs multiple parameter control

9 Drag forces and torques (for 1.5 m 2 cross area) Atmospheric drag Linear drag: ca. 1mN Torque: 10 µn m Radiation pressure by Earth albedo Linear drag: ca. 10 µn Torque: 1 µn m Magnetic torque (interaction with Earth magnetic field) Torque: 100 µn m After torque compensation: 10 µn m Solar radiation pressure Linear drag: ca. 10 µn Torque: 1 µn m

10 Two-test-mass problem external perturbaitons Drag free point -Air-drag -Radiation pressure -Magnetic fields -Solar wind, etc. internal perturbations -Patch effects -Radiometer effect -Non-perfect shielding etc. Huge complexity of signal!

11 Low thrust propulsion systems Balancing drag forces and torques Atmospheric drag Linear drag: ca. 1mN Torque: 10 µn m Radiation pressure by Earth albedo Linear drag: ca. 10 µn Torque: 1 µn m Magnetic torque (interaction with Earth magnetic field) Torque: 100 µn m After torque compensation: 10 µn m Solar radiation pressure Linear drag: ca. 10 µn Torque: 1 µn m Propulsion system requirements: Thrust control: S < ± 0.1 µn Residual acceleration: < m/(s 2 Hz 0.5 ) Permanent operation Sourc:e: Purdue School of Aeronautics and Astronautics Sourc:e: NASA Sourc:e: NASA

12 Simulation Example: differential acceleration of 2 test masses Satellite orbit: ω o Test mass coupling: ω k 2 = k/m k typ. = N/m S/C spin rate: ω spin 3.0x10-7 a x (k,gg,rot) 3.0x10-7 a x (k,gg,rot) 2.0x x x x10-7 a / ms x10-7 a / ms x x x x x x x Orbit 1 Orbit 0 1x10 5 2x10 5 Time / s 5.0x x x x10 4 Time / s

13 EGM 6th order ω spin = 4.6ω o ω k η= 0 = 2.9ω o Amplitude 1x x x x x s5c5gm6neta Frequenz (ω orbit ) ω spin = 4.6ω o ω k = 2.9ω o η= Amplitude 1x x x x x x ω η s5c5gm6eta15 = ω o + ω spin = 5.6ω o Frequenz (ω orbit )

14 High Performance Simulation Objectives Provide comprehensive simulation of the real system including science signal and error sources Provide simulation environment for control system performance validation Generate data needed to test data reduction methods Provide capability for identification of the satellite and instrument Core features Simulation of full satellite and test mass/experiment dynamics in six degrees of freedom by numerical integration of the equations of motion Multi-body system Consideration of linear and nonlinear coupling forces and torques between S/C and TMes Modelling of cross-coupling interaction Earth gravity model up to 360th degree and order + solar system effects Gravity-gradient forces and torques 5th order Runge-Kutta numerical integration, Bulirsch-Stoehr, Euler-Cauchy Misalignment, displacements, attitude errors, coupling biases

15 Satellite Dynamics and Interface Experiment / Spacecraft Important problem in order to keep accuracy Needs modelling of all experimental components and S/C subsystems Combining thermal and mechanical requirements Out1 Orbit Disturbances Simulation software High precision models 1 Guidance In1 Out1 Controller In1 Out1 In1 Out1 Thruster Actuation Actuators & System Configuration Out1 In1 Sensors In1 Out1 Spacecraft Dynamic In1 Analysis Mission Req Validation with flight data Hardware-in-the-Loop-Testbed Modular design Applicable to different missions Test facilities for AOCS components GPS-Simulator 3-Axis Rotation Table

16 Precise thermal models Albedo and Planetshine Earth Model Improved S/C thermal models: based on FEM 1,0 0,8 Albedo per Latitude 0,6 Planetshine Map 0,4 0,2 0, Planetshine per Latitude 300,0 200,0 100,0 0, Südpol Äquator Nordpol

17 Mission Scenarios Science Satellites Low earth orbit S/C Gravity Probe B Launched in a 642 km circular, polar orbit S/Cmass: 3,145 kg Pointing accuracy: 0.2 arcs achieved by means of a Cassegrain telescope Residual acceleration requirement: < 10-9 m/(s 2 Hz 0.5 ), achieved: < m/(s 2 Hz 0.5 ) Use of gas proportional thrusters with He-boil-off) Use of gyroscopes (niobium coated exact silica spheres) with SQUID-based rerad-out Measurement of the precession of a gyroscope due to space-time curvature r ds dt r r = Ω S = 6,6 arcsec per year by geodetic precession; confirmed within 0.5 % 0,041 arcsec per year due to frame dragging (Lense-Thirring precession), confirmed within r r v a r r r v U + h S

18 Mission Scenarios Science Satellites Drag-free performance of GP-B Proof mass acceleation (m-sec -2 ) Acceleration (m/sec 2 ) x10 x -8 Engagement of DF control on Gyro 1 (Z axis) Drag-free off Drag-free on Day of Year, 2004 Acceleration (m/sec 2 ) Control effort, m/sec 2 Gyro 1 - Space Vehicle and Gyro Ctrl Effort - Inertial space (2005/200, 14 days) Gravity gradient Cross-axis avg. 1.1 x g Roll rate X Gyro CE X SV CE 2 Orbit Frequency (Hz) F. Everitt et al.

19 Machining Cylindrical test masse Identical moments of inertia in all axes in order to minimize influences of the gravity gradient High precision alignment Gold coating of the zerodurstructures and electrodes Blocking system for test masses during launch Ultra high vacuum Fixed Stops Electrical Connectors Housing Mobile Stops Vacuum System External Acc Electrode Cylinders External PM (PtRh10 or TA6V) Internal PM (PtRh10) Internal Acc Electrode Cylinders Magentic shielding Sole Plate Blocking System

20 Test mass selection and machining characteristics η = N + Z N Z 5 ( γ 1) c + + baryon clepton µ Atom µ Atom µ Atom E 1 Pt Au Analyis for η E = f (N+Z), (N-Z), (Z-(Z-1)(N+Z) 1/3 Ti V TiC Cu Zr Nb 0.2 Be C CH Mg 2 Al Si 0-0.2

21 Test mass machining and tolerances Ti-Testmasse, PTB Macining precision in all axes: 1.5 µm Misalignment along symmetry axes dependent on: Maching accuracy (12µm worst case) Capacitive metrology due to test mass conicity (worst case 17µm, 8.5µm due to improved metrology x

22 Test and verification environment with free flyer

23 Conclusion Extremely challenging space experiment Various innovative space technology developments: S/C AOCS of highest precision ( no moving parts ) S/C dynamic simulation Low thrust propulsion Thermal modelling Differential accelerometers Machining on sub mµ-level Coating of non flat surfaces Stimulus for many new mission concepts: LISA STAR / OPTIS STE-QUEST X-ray telescope missions GRACE-Follow-on Earth observation on high precision level