january 6, 2005 ursi nation radio science meeting, boulder, co. an update on the gbt metrology...

January 6, 2005URSI Nation Radio Science Meeting, Boulder , CO.

An Update on the GBT Metrology SystemK. T. ConstantikesNRAOGreen Bank

2January 6, 2005

URSI National Radio Science Meeting, Boulder, CO.

GBT Metrology System

• New Approach– Parametric corrections using astronomy, e.g., Thermal Model– Parametric corrections using direct measurements, e.g., elevation axle

pose– Combinations of angle, distance, and dynamical data, i.e. Quadrant

Detector (QD), Laser Range Finder (LRF), Accelerometers for direct measurements of e.g. primary tilt

– Combination of holography (surface figure at one elevation, no temp gradient) and few (> 6) direct measurements of primary to correct for smoothly varying FEA error (vs grav) and thermal errors

– Fiducial and relayable tipping structure coordinate system (including orientation) tied to primary rim

– Inclinometers on el axles tie all to topocentic frame, perhaps using a track map, perhaps direct alidade measurements

– No rangefinders on ground– Fixed baseline range and angle measurements

• System study underway, i.e., can pointing/focus/collimation/efficiency be met with new instruments and notional geometry of metrology components and telescope?

3January 6, 2005


Current Instrument Set

Quadrant Detector IlluminatorStructure Temperature

Air Temperature

Quadrant Detector IlluminatorStructure Temperature

Air Temperature

Structure Temperatures (4)Air Temperature


Structure Temperatures (2)Structure Temperatures (2)

Structure Temperatures (2)Structure Temperatures (2)Structure Temperatures (2)Structure Temperatures (2)

Air TemperatureAir Temperature


Quadrant Detector2-Axis Inclinometers (2)

3-Axis Accelerometers (2)Elevation Encoder


Quadrant Detector2-Axis Inclinometers (2)

3-Axis Accelerometers (2)Elevation Encoder

Air TemperatureAir Temperature

3-Axis Accelerometer3-Axis AccelerometerStructure Temperatures (4)Structure Temperatures (4)



Structure TemperatureStructure Temperature

Azimuth EncoderAzimuth Encoder

4January 6, 2005


Communications Infrastructure

RS232 to 802.x Concentrator (8 ports)

Alidade Level


Alidade Level

2 RS232 to 802.x Concentrators (16 ports)Active Surface Actuator

Control Room

2 RS232 to 802.x Concentrators (16 ports)Active Surface Actuator

Control Room

2 RS232 to 802.x Concentrator (16 ports)

Vertex

2 RS232 to 802.x Concentrator (16 ports)

Vertex


Receiver Room


Receiver Room

2 RS485 to RS232 Transceivers (4 drops)

Active Surface Actuator Control Room, Elevation

Bearings

2 RS485 to RS232 Transceivers (4 drops)

Active Surface Actuator Control Room, Elevation

Bearings

5January 6, 2005


Additional Design Constraints have been Addressed

• RFI Mitigation and Testing• Thermal, Thermal/Mechanical Design• Maintainability• Availability (System MTBF)• Alignment/Calibration• Design for Installation

6January 6, 2005


Structural/Air Temperature Sensors

• 0.15 C accuracy, -35 to 40 C• 0.10 C interchangable thermistors• 0.01 C resolution, 1 sec sampling• 23 structure sensors • 5 air sensors , forced convection cells, ~ 5 sec

time constant • RS232 communications• Automated testing (daily)• Structure thermal distortions• Vertical air lapse• Laser rangefinder group index calculations

Thermistor in mounting slug

Environment Enclosure

RFI Enclosure ESD Protection

Forced Convection Cell

Mounted with VHB Tape and

Delrin plate

7January 6, 2005


Inclinometers

• 2-axis (horizontal plane), both elevation bearings

• 0.1” short-term accuracy, 0.01” resolution

• ~1 sec damping, 17 Hz resonance

• 5 Hz sampling rate, 0.3” noise at 5 Hz

• Azimuth track maps• Real time measure/correct

Az/El • Verify thermal effects• Wind force spring balance• Structural resonances

8January 6, 2005


Inclinometers, Cont.

Accelerometer Cube

X InclinometerY Inclinometer

Elevation Bearing Casting

Three Point, Spherical

Washer and Shim Leveled

Mount

9January 6, 2005


Accelerometers

• 3-axis, elevation bearings and receiver cabin

• MEMS torsion, capacitive readout, nickel

• 2 micro-G/root Hz• 10 Hz sampling• 1 x 1 x 0.1 G dynamic range• 24 x 24 x 16 bit mixed signal

ADC/microprocessor

• Structural resonances• Receiver room vibration• Feed Arm motions• Vertical dynamics at El

Bearings

10January 6, 2005


Quadrant Detector (Version 3)

• Sub-arcsec angle-angle measurements, ~1000” FOV

• ~5 Hz bandwidth, sampling at 10 Hz• Instrument noise ~ 0.2 arcsec

(in lab)• Good relative measurements on ½

hour time scales• Degraded by turbulence, index

gradients (19”/100m/1°K/m)

• Feed arm position/motion WRT ~elevation shaft (tipping structure coordinates)

• Structural resonances

High Intensity Green LED Illuminator

5 Accuracy, 1.5 Repeatability,0.1 ResolutionX-Z Translation Stage, Computer Control

Optical Tube, 800 mm fl lens, detector and TIA preamp assy.

11January 6, 2005


QD V3 Performance

65 Hour calibration, 225 points (dithered) per affine transform estimate, 161 iterations70mm dynamic range at 18m (~ 800”), air strongly mixed with fans

Horizontal: zero point 1 = 0”.18, scale 1 = 0.06% Vertical: zero point 1 = 0”.85 (dominated by vertical index gradient), scale 1 = 0.1%Compare to PSD nonlinearity spec of 0.05%

1 measurement noise = 1”.1 at 10 Hz (Telescope configuration SNR, fan turbulence)Equivalent to 1.4” on sky @ 10 HzStationary telescope performance ~ 0”.2 to 0”.4 on sky @ 10 Hz (100 s linear detrend, clear day)1700’/’ azimuth slew introduces ~ 2” of structural resonance modes.

12January 6, 2005


Quadrant Detector (Version 4)

QD V4 with 500 mm fl catadioptric lens, temp control, TIA, 4x4mm PSD

Analog Processor and Data Acquisition

Interim Configuration4 channel BP filter and True RMS to DC Conversion1x 24 bit sigma-delta, 3x 16 bit sigma-delta512 Hz carrier modulation, 10 Hz sampling7ppm local clockRS232 status, control, and dataTime transfer over RS232/Ethernet

Final ConfigurationTunable filters (SCF) for three channelsThree illuminator modulation frequenciesAutocollimation and angle-angle measurements

13January 6, 2005


IR Thermography

~ 2C cooler in Rcvr Room Shadow

Conduction into BUS ribs and hoops clearly visible, hot band in hoop direction ?

Ripple due to conduction into BUS?

14January 6, 2005


Optical Telescope

USB 2.0 to Ethernet

Fiber Media Converter

TE cooledmicro-lens arrayfull frame CCDmechanical shutter765x510, 9 pitch500 mm F/43”.8 IFOV 0.8° x 0.5° FOV0.1 s min exposure

Power Supply

Primary use: Star tracking, i.e. orientation in inertial frame.Interpolate to ~ 1”, limiting magnitude better than 14 with 1 s exposureImplementation will include 5° tip-tilt stage, focus servo. Alternative to Inclinometers when dynamic range > 1° WRT gravity. Expect (95%) 2 stars > Mag 11 in FOV. NOT for use as “Pointing/Tracking Telescope”

15January 6, 2005


Next Generation Laser Rangefinder

Synthesizers Clock

IF Subsystem

DSP

PA

VGA

TE Cooler,Bias-T,

Pigtailed Visible LD

TEC Controller,LD Power

Supply

Detector Bias Supply

Fiber Optic Switches

Detectors, Xmit and

Rcvr Optics

16January 6, 2005


Next Generation Laser Rangefinder, Cont.

Xmit Aperture,

~ 1cm

RcvrAperture,

~ 1 cm

Fiber Coupled JFET TIA Detector

1 kHz Offset @ 150 MHz,~ 16 kHz BW

24 Bit Digitized I/Q

17January 6, 2005


Rangefinder Improvements

• Frequency diverse (100-300 MHz), absolute range (incommensurate wavelengths)• Fiber optically coupled optics:

– MEMS chopping, zero points, xmit to rcvr coupling @ 100 Hz– Fiber reference loop– Multiplexing multiple remote heads from one EO package– No longer share a single aperture- no need for polarization decoupling – Xmit and rcvr optics are small– No phase uncertainty associated with photon centroid on detector (group

delay uncertainty in detector)– RFI Mitigation: No “bare” detectors or radiators, can use optical cutoff tube

and fiber• Visible LD:

– TE cooled to mitigate lasing wavelength changes (secondary group index error)

– Mitigate eye hazard– Easy alignment– Looks cool….– Might be marginally worse (?) for path loss, Mie scattering?– Easy to convert to NIR if needed

18January 6, 2005


Rangefinder Improvements, Cont.

• Diverged beam (~ 5 mrad) mitigates pointing problems, small scale turbulence

• Fiber coupled optics could be mounted on existing pointing heads with fiber wrap

• Much lower cost and volume: Telecom/OEM subsystems• Design Goals: Instrument noise less than group index

fluctuation/uncertainty (0.3-1 ppm)• Current Measured Performance (single phase measurement):

23 on 20m path, 10Hz, 5 mrad beam, baseband13µ @ 1kHz offset

• Instrument noise dominated by AM, shot noise limited (at detector diode) @ 1 kHz offset

• Measurement noise dominated by index fluctuations @ 10 Hz• Disciplined measurement (e.g., structural modes) can have much

lower measurement noise if needed

19January 6, 2005


Acknowledgements, Etc.

PTCS Instrumentation Team

K. Constantikes, J. Ray, JD Nelson, J. Cromer,

J. Shelton, R. McCullough, M. Stennes

january 6, 2005 ursi nation radio science meeting, boulder, co. an update on the gbt metrology...

Documents

structure sensors

ports vertex rs232

ports receiver room

air sensors

rs232 transceivers

ports alidade level

x concentrators

elevation bearings