Technologies for Precise Distance and Angular Measurements In Space

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Technologies for Precise Distance and Angular
MeasurementsIn Space

• M. Shao JPL

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Technology and Flight Hardware Development of
Optical Metrology

• Components and subsystems for precise distance
  measurements, applicable to SIM, and future
  relativity missions.
• Moderate power lasers with long lifetimes.
• Metrology source (freq shifters etc.)
• Beam launchers (metrology gauges)
• Types of errors at the picometer level
• Angular measurements at the microarcsec level,
  the SIM technology program.
• End to end demonstration of micro-arcsec
  astrometric precision
• Astrometry as a tool to study dark matter in our
  galaxy, and the local group.

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Metrology Source

• Two major components
• Laser
• Frequency shifters and fiber distribution systems
• Laser
• SIMs laser is a NPRO diode pumped YAG laser,
  designed with redundant pump laser diodes to
  achieve gt 99.7 probability of working for 5
  years in space. (SIM has a spare laser, )
• The acousto-optic frequency shifters provide the
  optical signals needed for heterodyne
  interferometry.
• The major activity here is not developing new
  technology but engineering components for flight.

Engineering model built and tested (Shake,
thermal vac) in 2007.
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Instrumental Errors in Long Distance Metrology

• Pointing /diffraction
• Beam walk (imperfect optics)
• Laser freq stability
• Transmissive optics (dN/dT)
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

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Pointing Errors
If the outgoing wavefront is not properly
pointed at the other spacecraft the optical phase
of the wavefront may not represent the distance
between the two fiducials. This is minized if
the outgoing wavefront is spherical, centered on
the fiducial.
Light hitting a retroreflector reverses the
direction of the laser. The optical path measured
is separation of the fiducialscos(q). A 10m
distance and a 1 urad pointing error yields a 5
picometer distance error. For very long
distances, a collimated laser beam through
diffraction will turn into a spherical wavefront.
As a rough estimate, the pointing error applies
to the path where the wavefront hasnt become
spherical. (D2/l)
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Defining a Retroreflectors Vertex
The vertex of a CC is where the three planes
intersect. The plane as defined by where
metrology beam samples the CC. One has to be
careful if we want a definition more precise
than the fabrication of the surfaces. (l/100
l/1000)
If the footprint of the interrogating laser beam
moves by 1 of the beam dia, and the surface is
perfect to l/1000, one would expect the vertex
position to be stable to l/100,000
A cats eye retro will interrogate a few micron
spot on the mirror at focus. The vertex
definition is only as good as the quality of the
surface.
Cats eye retroreflector
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Metrology Beam Launchers

• Beam launcher, designed with critical alignment
  components fixed on a zerodur optical bench.
• Launcher includes provision for pointing the beam
  with 1urad accuracy.

Engineering model built and tested (Shake,
thermal vac) in 2007.
Laser pointing should be parallel to a vector
joining the vertices of the two CCs
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Optical Fiducials in Optical Trusses

• Several missions make use of precise (sub
  nanometer) optical trusses.
• SIM (optical truss to connect several stellar
  interf)
• Beacon (test of relativity)
• LISA?
• Optical trusses requires that multiple lasers
  reference the same optical fiducial.
• Dual corner cube, optically contacted
  construction.
• l/20 p-v wavefront to 1mm/edge
• Common vertex to 1um
• Measure vertex offset to 1nm.

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Precise Measurement of Angles Between Stars
Internal Path Delay
D? _at_ Dx/B
An interferometer measures (Bs) ? the dot
product of the baseline vector a unit vector to
the star,
or, the projection of the star vector in the
direction of the baseline
The peak of the interference pattern occurs when
Internal delay External delay
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SIM Technology Flow
Component Technology
Subsystem-Level Testbeds
System-Level
1999
4Oct2002
8Jul2005
2001
Metrology Source
Absolute Metrology
4 Kite Testbed (Metrology Truss)
3Sep2002 5Mar2003 6Sep2003 7Jun2004
8 Overall system Performance via Modeling/Testbe
d Integration
Picometer Knowledge Technology
1999
Multi-Facet Fiducials
Numbers before box labels indicate HQ Tech Gate
s (1 through 8)
1Aug2001
1 Beam Launchers
1998
2000
3, 5, 6, 7 MAM Testbed (single baseline
picometer testbed) Narrow Wide Angle Tests
TOM Testbed (distortion of front end optics)
High Speed CCD
Fringe Tracking Camera
All 8 Completed
2Nov2001
Optical Delay Line
Nanometer Control Technology
1998
Hexapod Reaction Wheel Isolator
1999
STB-1 (single baseline nanometer testbed)
2 STB-3 (three baseline nanometer testbed)
1998
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STB-3 on 9-meter Flexible Structure
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The Micro Arcsec Metrology Testbed
Laser metrology measures the position of the
IIPS. Test is to compare metrology to whitelight
(starlight) fringe position.
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Wide Angle Astrometry
SIM goal is 4uas global astrometry (end of
mission) Single epoch accuracy 10uas.
Instrumental error vs position in the field of
regard. Met milestone 4 uas error (end of
mission) 10uas single epoch error. Dominated
by field dependent biases and thermal drift over
1 hr (versus 90sec for NA)
Wide angle test sequence looks at 60 stars over
a 15 deg field of regard. (1hr test)
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Narrow Angle Astrometry
MAM test 4 ref stars, 1 target star, (T, R1,
T, R2, T, R3, T, R4 . Repeat) 20 runs
conducted over 1 week.
1 uas total error 0.7 to photon noise 0.7 to
instrument 0.5 to science interf 0.5uas 25
pm Meet 25pm in 8 chops Each dot is an 8 chop
average
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Thermal Drift, 1/f type noise

• Thermal drift will change optical pathlengths.
  But most thermal drift on SIM is benign, because
  its accurately monitored by laser metrology.
  (accurate means accurate at the few picometer
  level)
• Astrometric errors occur when the alignment of
  the starlight and metrology light diverge. Since
  both starlight and metrology light are actively
  control, this happens when the alignment sensors
  in the ABC (astrometric beam combiner) move wrt
  each other.

• Dimensional instability (from thermal
  instability) of the ABC bench can cause
  star-light and metrology to diverge.
• ABC bench is a box within a box. The ABC
  enclosure is controlled to 10mK. The ABC optical
  bench inside the enclosure is stable to better
  than 1 mK.

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Thermal Stability of the Lab Testbed vs Model of
SIM on Orbit
Multi-100 node thermal model of SIM-(lite) in
solar orbit executing an orange peel. Plot is
temperature on the ABC bench.
Inside Testbed Vac Tank temperature measurement
The MAM optics in the MAM vacuum chamber was
reconfigured and the testbed called SCDU. But the
thermal properties of the chamber were overall
unchanged. (Shorter 6hr allan variance data
taken showed that the new setup is slightly
better than before. The plot on prior page 2 over
estimates the thermal error.
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• We have two squiggly lines for thermal drift. How
  do we compare them? We compare their power
  spectra.
• SIM in solar orbit is expected to be more stable
  than the inside of the MAM vacuum tank. (Thermal
  instability even in the MAM tank is not the
  dominant error/noise source.)
• The reason chopped astrometry error goes as
  sqrt(T) is because were sensitive to the noise
  at 0.01 hz, (90sec chop period). The rms error
  of a 1000sec integration of a chopped signal is
  roughly a 0.001hz bandwidth around 0.01hz.

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Effect of Chopping on Thermal Drift

• While the drift of the starlight-metrology
  optical path can be quite large over long periods
  of time, the chopped signal only sees changes on
  a time scale of 90 sec.

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Instrumental Systematic Error

• Instrumental errors in the SIM testbed (chopped)
  does integrate down as sqrt(T)
• At least down to 1 picometer after 12x105 sec

Terrestrial Planet search Single epoch precision
1mas
Systematic error floor 40 nanoarcsec
MAM testbed March 2006
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Summary

• The SIM technology program has demonstrated the
  ability to make precise angular measurements in
  space.
• The activities have changed from (demonstrating
  it can be done) to building engineering units
  that can survive launch loads and operate in
  space, with high reliability over many years. (A
  series of engineering milestones have replaced
  the technology milestones).
• In subsequent talks at this conference S.
  Majewswski ,and E. Shaya will talk about how
  they would use SIM to study Dark Matter in our
  galaxy and the local group.
• The components that have been flight qualified
  have uses in other space missions that test
  relativity. (Beacon will be discussed by B. Lane
  later today.)