Thermal Focus and Pointing Corrections

1
Thermal Focus and Pointing Corrections

• K. Constantikes

2
Status

• New thermal model, new gravity model parameters
• Uses 19 structural temperature sensors
• Linear thermal, tradional gravity
• Focus peformance lt 3 mm (excludes midday) during
  30 mm thermal focus shift
• Elevation performance lt3 1s , lt1/hour
  (excludes midday) during 30 thermal pointing
  shift
• Azimuth performance lt3 1s , lt1/hour (excludes
  midday)
• Unanticipated dominance of horizontal feed arm
  influence
• PTCS/PN/25 Thermally-Neutral Traditional
  Pointing Models and Thermal Corrections to
  Pointing and Focus
• Test of real-time focus corrections (Balser and
  Prestage,11/20/03)

3
Structural Temperature

• 19 locations, 0.2C interchangeable accuracy,
  0.01C resolution, 1Hz, range 35 to 40C. (actual
  accuracy is 0.1C, temp control of conversion
  elex)
• Design documentation
• PTCS Wiki (AntennaInstrumentation)
• PTCS Project Note PTCS/PN12
• Accuracy tested in lab
• Solar/convective loading
• Selected unit-to-unit accuracy, repeatability
• Electronics temperature range
• RFI mitigated, ESD protected
• Two thermistor failures, forensics with YSI
• Integrated into MC
• First cut pointing, focus predictive algorithms
  tested

4
Structural Temperature
5
Structural Temperature
6
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 coefs
• Estimate coefs using 9/11, 10/2, 11/10 data
• Test models using 9/5, 11/20 data

7
Focus Model
 
 
8
Focus Model Estimation
9
Focus Model Tests

• Wind lt 2.5 m/s
• 15 lt elevation lt 85
• 9/5 is NCP
• 11/20 is all-sky
• Excludes 1000-1800
• Graphs show thermal contributions only

10
Focus Model Tests
11
Elevation Model
12
Elevation Model Estimation
s 3.6
13
Elevation Model Test
14
Elevation Model Test
15
Azimuth Model
16
Azimuth Model Estimation
s 3.9
17
Azimuth Model Test
18
Azimuth Model Test
19
Why does it work?

• Didnt for 140 (von Hoerner), why should GBT?
• Thermal diffusivity?
• Time constants?
• Characteristic length of perturbations?
• Surface area of structural supports?
• Better temperature measurement technology !
• Better homology?

20
Conclusions

• Focus and elevation greatly improved with
  thermally-neutral traditional model and
  temperature corrections
• Azimuth performance improvement marginal (but
  its already pretty good)
• Use of thermal imaging to improve locations
• Add sensors to HFA, BUS
• Work on graceful degradations
• Production implementation
• Further tests for confidence
• Thermal stability model

21
The Details.

22
Previous Focus Tracking Curves
23
Previous Focus Tracking Curves
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Temperature Sensor Locations
TF1
TF5
TSR
TF3
TF4
TF2
TH3
TB2
TH2
TE2
TB1
TB3
TB4
TB5
TE1
TA4
TA2
TA3
TA1
25
Focus Model

• Gravity
• SR-Primary
• VFA-Primary
• HFA
• BUS

26
Elevation Model

• Gravity
• BUS
• HFA
• VFA
• Alidade

27
Azimuth Model

• Gravity
• Alidade
• HFA
• BUS
• VFA

28
Optimization

• Focus optimization using pseudo-inverse for LSE
  solution
• Coupled Az and El gravity models (AN, AW
  constraint)
• Gradient descent