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2 Satellite gravimetry Mapping the global gravity field Static and dynamic components Many applications in geosciences Techniques Orbit determination and tracking Satellite-to-satellite tracking (SST) Gravity gradiometry Recent gravity field satellites CHAMP GRACE Earth s gravity GOCE GRAIL Lunar gravity Image credit: NASA Image credit: GGOS Image credit: CSR GFZ Potsdam Image credit: GGOS Daniel.Schuetze@aei.mpg.de 2

3 What can we learn from observing Earth s gravity? Gravity field is determined by mass distribution Mass redistribution leads to changes in the gravity field Mass transport is associated with many interesting geophysical phenomena, e.g.: Glacial isostatic adjustment Glacial / polar ice changes Solid Earth deformation Hydrology Image credit: GFZ Daniel.Schuetze@aei.mpg.de 3

4 Mass transport example: Hydrology CSR Image credit: NASA 4

5 GRACE observing seasonal water storage changes CSR B. D. Tapley et al., GRACE Measurements of Mass Variability in the Earth System, Science 305, Image credit: NASA (2004). 5

6 Long term trends observed with GRACE Velicogna, Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophys. Research Lett. 36, L19503 (2009). CSR Tiwari et al., Dwindling groundwater resources in northern India, from satellite gravity observations, Geophys. Research Lett. 36, L18401 (2009). J. S. Famiglietti et al., Satellites measure recent rates of groundwater depletion in California s Central Valley, Geophys. Research Lett. 38, L03403 (2010). Image credit: NASA Daniel.Schuetze@aei.mpg.de 6

7 Overview of observable geophysical processes CSR Image credit: NASA Ilk et al.,

8 GRACE: Low-orbit satellite-to-satellite tracking V7.8 km/s CSR Mass Anomaly Image credit: NASA 8

9 GRACE satellites Two identical spacecraft launched March 2002 Polar low-earth orbit 450 km altitude, 220 km spacecraft separation Freely decaying (no drag compensation) Two-way K-band microwave ranging Dual-frequencies for ionosphere correction Micrometre performance 3-axis SuperSTAR accelerometer Measurement of non-gravitational forces down to m s -2 / Hz Mass-trim to position S/C COM at accelerometer GPS receivers and laser retroreflector Orbit determination and absolute spacecraft position/velocity Attitude and orbit control system 2 star trackers Magnetic torquers Cold gas thrusters for orbit maintenance Schmidt et al. Surv Geophys 28, 319 (2008) Image credit: CSR Daniel.Schuetze@aei.mpg.de 9

10 GRACE follow-on mission GRACE mission extended to the end of its on-orbit life (expected around 2016) Degraded battery, freely decaying orbit Continuation of the data series requires a new mission: GRACE follow-on Near rebuild of GRACE using microwave ranging to minimize data gap Launch in 2017 Will include Laser Ranging Interferometer (LRI) as technology demonstrator Daniel.Schuetze@aei.mpg.de 10

11 GRACE follow-on & LISA LISA Laser interferometer SST Inter-satellite distance 5 million km km Orbit Heliocentric (1 a.u.) Low Earth Orbit ( km) Orbit environment Attitude and Orbit Control System No atmospheric drag, stable thermal environment Drag-free using µn thrusters Atmospheric drag, large thermal CSR disturbances Attitude control with magnetotorquers & cold gas thrusters Measurement band 100 µhz 1 Hz 200 µhz 100 mhz Measurement noise 40 pm/ Hz ( freq. dep.) 80 nm/ Hz ( freq. dep.) Telescope aperture diameter 38 cm 1 cm Transmit beam waist radius 17 cm 2 mm Transmit power 1 W 30 mw Effective received power (at photodetector) 100 pw 100 pw Maximum relative L.O.S. velocity ±15 m/s ±3 m/s (depending on orbits) Daniel.Schuetze@aei.mpg.de 11

12 LRI performance requirements CSR 12

13 Laser frequency stabilization Frequency noise coupling proportional to arm-length mismatch: x L Stabilization is required (30 Hz/ Hz) LISA tricks (armlocking, TDI) not applicable Space-qualified reference cavity developed by Ball Aerospace and tested at JPL Even with stabilization laser frequency noise will be a significant component of error budget W. M. Folkner et al., Laser Frequency Stabilization for GRACE-2, Proc. ESTF Daniel.Schuetze@aei.mpg.de 13 CSR

14 LRI accomodation challenge

15 GRACE follow-on Laser Ranging Interferometer 15

16 Aquisition strategy (AEI/ANU/JPL) Two modes: Initial acquisition with large unknown biases ( mrad) and frequency offset Re-acquisition after loss of lock should be much faster Daniel.Schuetze@aei.mpg.de 16

17 Aquisition strategy (AEI/ANU/JPL) Challenging due to: 5 degrees of freedom (two angles on each S/C plus freq. offset) No real-time communication between the S/C No guarantee that the heterodyne signals will be detected at both ends for near good alignment Must demonstrate that software will work autonomously Daniel.Schuetze@aei.mpg.de 17

18 GRACE follow-on Laser Ranging Interferometer 18

19 Triple Mirror Assembly - basic properties PP, V Place virtual intersection point inside accelerometer Rotation around virtual intersection point preserves: round-trip pathlength d beams antiparallel impact parameter p Daniel.Schuetze@aei.mpg.de 19

20 Triple Mirror Assembly prototyping at ANU/CSIRO Main requirements Beam coalignment error: <40 µrad Vertex (actually the point of minimal coupling ) must be placed within about 100 µm of the ACC in two axes lateral to line of sight Pathlength stability < 25 nm/ Hz NSF(f) in measurement band Robust enough to survive launch CFRP prototype constructed and currently being tested, All glass ULE prototype also being developed as risk reduction Daniel.Schuetze@aei.mpg.de 20

21 Triple Mirror Assembly prototyping at ANU/CSIRO 21

22 Triple Mirror Assembly - coupling of displacement into length measurement Offsets V PP Coupling factors Rotation angles Daniel.Schuetze@aei.mpg.de 22

23 Triple Mirror Assembly testing at AEI Determine Point of minimal coupling TMA is rotated by Hexapod around a grid of points Interferometric monitoring of round-trip pathlength Daniel.Schuetze@aei.mpg.de 23

24 GRACE follow-on Laser Ranging Interferometer 24

25 Optical Bench Measure the average phase difference of the local and received beams Measure the tilt of the received beam (DWS) Send out the transmit beam in the same direction as the input beam (using steering mirror) 25

26 Optical Bench - Differential Wavefront Sensing Daniel.Schuetze@aei.mpg.de 26 Phase difference proportional to wavefront tilt D B C A R L D C B A B T 3 16r R L Approximation for flat-top beams and small tilts Simultaneous readout of two independent axes

27 Optical Bench - Differential Wavefront Sensing Daniel.Schuetze@aei.mpg.de 27

28 Optical Bench - Differential Wavefront Sensing When spacecraft rotates the incoming beam is tilted relative to the local beam leading to a non-zero DWS signals, reduction of contrast and misaligned outgoing beam Daniel.Schuetze@aei.mpg.de 28

29 Optical Bench - Differential Wavefront Sensing Appropriately feeding back the DWS signals to the steering mirror zeros the DWS signals and corrects the outgoing beam direction Daniel.Schuetze@aei.mpg.de 29

30 Optical Bench compensation plate Due to 45 degree angle of incidence, the pathlength through the beamsplitter changes almost linearly with changes of the angle of incidence for yaw Angular coupling for pitch is only quadratic 30

31 Optical Bench compensation plate Simulation 31

32 Optical Bench compensation plate Experiment on Optical Bench prototype (AEI) 32

33 Optical Bench prototyping at AEI QPD on translation stage Local beam from fiber collimator, w 2.5 mm Daniel.Schuetze@aei.mpg.de 33

34 Optical Bench prototyping at AEI QPD on translation stage Received beam, expanded by commercial 20x beam expander, w 20 mm Daniel.Schuetze@aei.mpg.de 34

35 Optical Bench prototyping at AEI 35

36 Optical Bench prototyping at AEI 36

37 Optical Bench prototyping at AEI 37

38 Summary GRACE follow-on will be a milestone as the first inter-satellite laser interferometer Good progress in testing key concepts and components For detailed information on the LRI concept see: Sheard et al., Intersatellite laser ranging instrument for the GRACE follow-on mission, Journal of Geodesy, 2012 (DOI: /s ). Thanks for your attention! Daniel.Schuetze@aei.mpg.de 38