April 8/9, 2003 Green BankGBT PTCS Conceptual Design Review

Richard Prestage, Kim Constantikes,

Dana Balser, Jim Condon

The GBT Precision Telescope Control System

2URSI – Jan 5-8, 2004

How to make a 100m telescope work at 50GHz

(…with plans for 115GHz)

• Overview of GBT and the PTCS project

• Thermal Effects and their compensation

• Measurement of wind and servo effects

3URSI – Jan 5-8, 2004

The GBT is large….

4URSI – Jan 5-8, 2004

Telescope Structure and Optics

5URSI – Jan 5-8, 2004

Telescope Structure and Optics

6URSI – Jan 5-8, 2004

Telescope Structure and Optics

7URSI – Jan 5-8, 2004

Telescope Structure and Optics

8URSI – Jan 5-8, 2004

Scientific Requirements

9URSI – Jan 5-8, 2004

PTCS Project

• Aim of the project is to deliver 3mm operation.• Includes instrumentation, servos (existing), algorithm

and control system design, implementation.

• As delivered antenna => 15GHz operation (Fall 2001)• Active surface and initial pointing/focus tracking

model => 26GHz operation (Spring 2003)

• PTCS project initiated November 2002:– 50GHz operation: Fall 2003 (November)– 90GHz operation: Winter 2004/05– Full 115GHz: Winter 2005/06

10URSI – Jan 5-8, 2004

Gravity/Temperature Effects - Focus

Temperature EffectGravitational Effect

Measure focus over short time period NCP source 0117+8928

11URSI – Jan 5-8, 2004

Gravity/Temperature Effects - Pointing

12URSI – Jan 5-8, 2004

Structural Temperatures

13URSI – Jan 5-8, 2004

Structural Temperatures

14URSI – Jan 5-8, 2004

Algorithms

• Use existing GBT gravity pointing and focus models• Structure is linear: Thermal effects superpose• Temperature effect on focus, pointing assumed linear

in temperatures• No dependence on air or bulk temps, just differences• Simultaneously estimate gravity and temperature

model coefficients• Estimate coefficients using 9/11, 10/2, 11/10 data• Test models using 9/5, 11/20 data

15URSI – Jan 5-8, 2004

Focus Model

Term Coefficient Min-Max Significance Parameter

M1 1.086 13.1 14.3 SR-PriM2 -0.697 6.2 -4.3 VFA-PriM3 3.981 15.6 62.0 HFAM4 -7.326 0.9 -6.8 BUS V1M5 -0.688 12.1 -8.3 BUS V2M6 -2.576 12.1 -31.2 BUS FM7 -180.630 0.0 0.0 Offset

M8 66.189 .7 43.1 sin term

M9 196.949 0.6 110.8 cos term

16URSI – Jan 5-8, 2004

Focus Model Estimation

17URSI – Jan 5-8, 2004

Focus Model Test

18URSI – Jan 5-8, 2004

Elevation Model

Term Coefficient Min-Max Significance Parameter

M1 -4.6455 1.2 -5.3 BUSM2 1.7830 15.6 -27.8 HFAM3 4.4488 5.9 26.4 VFAM4 -8.4477 1.6 -14.0 AlidadeM5 62.2218 0.0 +0.000 -IE,d(0,0)M6 -55.8624 0.7 -62.792 HZCZ,b(0,1)M7 -22.8268 0.9 -38.216 HZSZ,d(0,1)M8 2.4960 2.0 +2.169 -AW,c(1,0)M9 -1.3360 2.0 -1.750 AN,d(1,0)

19URSI – Jan 5-8, 2004

Elevation Model Estimation

20URSI – Jan 5-8, 2004

Elevation Model Test

21URSI – Jan 5-8, 2004

Thermal Compensation Results

• Significantly improved “static” gravity models.

• Focus peformance ~< 3 mm (excludes midday) during ~30 mm thermal focus shift.

• Elevation performance ~<3” 1 , <1”/hour (excludes midday) during ~ 30” thermal pointing shift.

• Azimuth performance ~<3” 1 , <1”/hour (excludes midday).

• Unanticipated dominance of horizontal feed arm influence.

22URSI – Jan 5-8, 2004

Tracking Stability: Servo and Wind

• Thermal effects important on timescales ~ 0.5 hours

• Short term tracking stability dominated by:– Wind– Servo disturbances

• We are starting to characterize the effects

• Possibility of compensation looks promising

23URSI – Jan 5-8, 2004

14GHz half-power track

sec12 arc

)/'10,/'2(,

)5,1(),(

)58,290(),(

mmt

El

t

Az

ElAz

ElAz

24URSI – Jan 5-8, 2004

14GHz half-power track

25URSI – Jan 5-8, 2004

14GHz half-power track

26URSI – Jan 5-8, 2004

Servo effects

27URSI – Jan 5-8, 2004

Effects of wind

1''

12

2

11

68

2)(

sec16.0)(

smsat

windwind

arcsm

swind

28URSI – Jan 5-8, 2004

Effects of Wind

29URSI – Jan 5-8, 2004

Future DevelopmentsNew Instrumentation Existing Instrumentation/Techniques

Instrument/

Technique

Inclinometers Star Tracker Penn Array Receiver

Temperature Sensors

(struct. / air)

Quadrant Detector

Holography

(trad. / oof)

Laser Rangefinders

Algorithms/

Development

• Track irregularities• Wind lift/drag• Vibration

• Test 1” differential capability• Substructure rotations, e.g., primary

• Rapid beam maps•Surface improvements

• Quadrant Detector path• LRF GRI• Thermal imaging• Add sensors

• Residuals, stability • Wind• Vibration

• Extend over grav, temp, wind• Improve FEM• Surface Peak-up

• Beam expand• Pointing• Diode replacement• Surface / collimation improvements

Enable W-Band Performance Under Benign Conditionsand Q-Band Performance Under Normal Conditions

Prototyping, Commissioning Experiments and Transition to Production Capabilities

30URSI – Jan 5-8, 2004

Conclusions

mmfocus

arc

5.2)(

sec52

mmfocus

arc

5.1)(

sec7.22

sec12 arc

Blind Pointing: (1 point/focus)

Offset Pointing: (90 min)

Continuous Tracking: (30 min)

•GBT is capable of 50GHz operation under benign conditions:

31URSI – Jan 5-8, 2004

Conclusions

• Largest “non-repeatable” effects are thermal and wind.

• Thermal compensation works well apart from around mid-day; may be extended to all conditions.

• Next development: inclinometers:– Az-track irregularities– Confirmation of alidade thermal pointing effects– Wind compensation on ~10s timescales

• Servo disturbances are clearly visible - good chance that we will be able to compensate for these.

32URSI – Jan 5-8, 2004

Acknowledgements

• Joe Brandt, Ray Creager, Jeff Cromer, Paul Marganian, J.D. Nelson, Jason Ray.

• PTCS Project Team.