Interference Mitigation Using a Multiple Feed Array for Radio Astronomy

1
Interference Mitigation Using a MultipleFeed
Array for Radio Astronomy

• Chad Hansen, Karl F. Warnick, and Brian D. Jeffs
• Department of Electrical and Computer Engineering
• Brigham Young University
• Provo, UT
• J. Richard Fisher and Richard Bradley
• National Radio Astronomy Observatory
• Green Bank, West Virginia
• June 11, 2004

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RFI Mitigation

• Techniques
• Spatial filtering
• Requires multiple spatially separated looks at
  interferer
• Adaptive cancellation
• Time blanking

3
Array Feed - Design Goals

• High Sensitivity
• Senstivitity SNR
• TsysTreceiver Tspillover TInterference
  Tatmosphere Tcmb
• Beam steering
• Beam shape control
• Gain stability
• RFI Mitigation

Gain
System Temperature
4
Previous Work Array Feeds

• Most implementations 1 feed 1 beam

5

19-element Array at NRAO

• Electrically small elements
• Hexagonal array
• Beamforming

6
Approach

• 25 meter Very Large Array (VLA) type reflector
• GRASP8 (TICRA) PTD reflector analysis software
• Array weights three methods
• Conjugate field match (CFM)
• Brute force sensitivity optimization
• Max SNR/LCMV (beamforming RFI nulling)
• Compare to single waveguide feed

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Assumptions

• Operating frequency 1612 MHz
• 7 and 19-element hexagonal arrays with 0.6?
  spacing
• Hertzian dipoles
• No mutual coupling between array elements
• Individual LNA noise temperature 15 K
• Low noise temperature emphasizes importance of
  spillover efficiency
• Spillover noise 300K warm ground below
  reflector
• Atmospheric and cosmic background noise is
  neglected

8
Sensitivity

• 25 meter reflector
• Boresight beam

9
Gain and Spillover Efficiency
10
Reflector Illumination Pattern
11
Steered Beams/Offset Feed
12
Focal Field Distribution
Boresight
Beam steered to .3?
dB
dB
?
?
13
Interference Mitigation
max-SNR/LCMV
x1n
w1
x2n
yn
w2
xNn
wN
Spillover noise
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Results (7 Element Array)
Interferer at 30 degrees, INR0 dB
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Main Beam Distortion
16
Interferer at 30 deg, INRIn changing
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Moving Interferer
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Interference Rejection
- Low sensitivity corresponds to poor spillover
efficiency and gain loss
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Signal/Interferer Array Responses

• Angle cosine between
• interferer and signal response
• vectors
• Sensitivity decreases when
• responses are similar
• Sensitivity loss is a
• grating lobe-like effect

20
Conclusions

• High sensitivity can be achieved using an array
  feed
• High sensitivity can be maintained in presence of
  interferer
• Small beam distortions occur due to beam
  steering/RFI mitigation
• Some angles of arrival lead to decreased
  sensitivity
• Future work
• Algorithms beam shape control, defocusing
  (larger arrays)?
• Broadband elements
• Mutual coupling
• Prototype

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Gain and Spillover Efficiency
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Multiple Beams
?.3?
?-cut
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Sensitivity
24
19-element array, moving interferer
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Sum of outer weights
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Center element, INRIN
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Assumptions
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