green bank laser rangefinders
DESCRIPTION
Green Bank Laser Rangefinders. Richard Prestage on behalf of PTCS Team. FAST Presentation, 22 July 2010. LRF Overview. Project conceived in early 1990s, as an integral part of GBT Development. Requirement was for ~ 100µm ranges over ~ 100m distance. No commercial technology available. - PowerPoint PPT PresentationTRANSCRIPT
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Atacama Large Millimeter/submillimeter Array
Expanded Very Large ArrayRobert C. Byrd Green Bank Telescope
Very Long Baseline Array
Green Bank Laser Rangefinders
Richard Prestage on behalf of PTCS TeamFAST Presentation, 22 July 2010
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LRF Overview
• Project conceived in early 1990s, as an integral part of GBT Development.
• Requirement was for ~ 100µm ranges over ~ 100m distance.
• No commercial technology available.• Made possible by the evolution of commercial off the shelf
hardware and software. – E.g. availability of laser diodes with built in GRIN lenses
that can be directly modulated at 1500 MHz for less than $150 as a result of consumer electronics CD players.
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Principle of Operation
• 780nm semiconductor lasers modulated at 1.5GHz• Light returned by target retroreflectors, detected
and mixed with transmitted signal• Phase difference converted to distance (so
distance must be known a priori to ± 50mm
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The Rangefinder (20 built)
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Block Diagram
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Exploded View of Optics
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Intended Operation
• LRFs to provide absolute measurements of pose (pointing) and surface figure (efficiency)– Goal was to provide ~ 2” absolute pointing, and ~ 250-
300µm surface accuracy– Compensate for thermal and (slowly varying) wind
effects– Closed loop control of Active Surface
• Requires ~ 100µm accuracy over 100m ranges (1ppm)• Requires a few (2-5) range measurements per second, with
potentially large angular motions (1 radian)• Index of refraction of air varies by ~ 1ppm / °C
– Must measure atmospheric conditions and adjust index of refraction
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LRF Configurations
• 6 LRFs on feedarm trilaterate to retroreflector prisms (one per panel)– Correct for wind variations in position of feedarm by trilaterating
to reference retroreflectors– Correct for structural vibrations of feedarm using accelerometer
data– Use corrected ranges to solve for Zernike polynomial expansion of
the wavefront error
• 12 LRFs mounted on ground monuments trilaterate to retroreflectors mounted on alidade and tipping structure (including “triplet” retroreflectors mounted on rim of primary)– Measure “traditional” terms in pointing model directly (e.g.
elevation axis collimation angle, azimuth zero point).– Measure departure of structure from ideal performance.
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LRF Configurations
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Status as of mid 2003
• 20 “First Generation” LRF units constructed.• Basic performance at the ~100µm level for individual
ranges demonstrated• LRF control software and basic range measurement
software in place• Initial “phase closure” experiments performed
– Making measurements in a 2-d horizontal plane between 12 ground monuments
– ~ 200 µm accuracies in position achieved.
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“Phase Closure” Experiments
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2003 “NCP trilateration Experiment”• Antenna pointed to a ~stationary, bright calibrator at the
North Celestial Pole• Astronomical “peak” and “focus” measurements performed• Extensive environmental information (air and structural
temperatures, wind speed, etc) gathered• Ranges measured to tip of feedarm• Traditional survey to same targets performed with Topcon
surveying instrument.– Discrepancies of the Topcon survey and LRF-calculated
target positions of the order of a few mm.• At this point, further work on use of this version of
the LRF for GBT surface setting / pointing improvements was put on hold.
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Concerns with LRF Performance
• How to measure Group Index of Refraction
• Geometry of system: “long skinny triangles”; relaying coordinate systems
• System Integration Concerns
• Difficulty of integrating LRF usage into GBT control software, and astronomical (incremental, differential) improvements to pointing and surface adjustments
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Geometry Concern
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-1.5 -1 -0.5 0 0.5 1 1.5
x 105
-2
-1
0
1
2
x 105
0
5
10
15
x 104
Y
ZY104
ZY103
ZY105
ZY102
ZEG41040R
X
Node: ZEG41040R Azimuth: 0 Elevation: 38
Z
-1.5 -1 -0.5 0 0.5 1 1.5
x 105
-2
-1
0
1
2
x 105
0
5
10
15
x 104
Y
ZY104
ZY103
ZY105
ZY102
ZEG41040R
X
Node: ZEG41040R Azimuth: 0 Elevation: 38
Z• Estimated position error ~
1.4mm for nominal GRI and range errors
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Subsequent Developments
• 2005: “Second Generation” metrology system– Fixed baseline system, range and angle measurements
made by instruments mounted on GBT.– Relay a fiducial coordinate created at the pintle bearing
by high performance inclinometers (like “ALMA reference telescope”)
– Use a two-tone system with incommensurate frequencies
– DDS synthesisers– MEMS fiber optic switches
• Performance improved, but still not adequate– Cross talk, phase nonlinearities
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Subsequent Developments• 2008: “Third Generation” rangefinders
– Addressed problems with DDS synthesiser programming, cross-coupling, phase instabilities and component nonlinearities.
– Accuracy is currently limited by phse detection nonlinearities.
• These are systematic and potentially correctable by providing a defined reference target.
– This generation LRF should meet spec of ~ 100µm range over ~ 100m
• BUT: Currently we are close to pointing and surface accuracy specifications using advanced and innovative applications of “traditional” astronomical correction techniques (e.g. with phase and phase retrieval holography; quadrant detector, etc).
• Currently no plans to continue rangefinder development. 16