Interference Mitigation Using a Multiple Feed 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 July 13, 2004
RFI Mitigation Techniques: Spatial filtering Requires multiple spatially separated looks at interferer Adaptive cancellation Time blanking
Array Feed - Design Goals High Sensitivity Sensitivity = Gain System Temperature ~ SNR T sys =T receiver + T spillover + T Interference + T atmosphere +T cmb Beam steering Beam shape control Gain stability RFI Mitigation
Previous Work: Array Feeds Most implementations: 1 feed = 1 beam eg: Parkes HIPASS Array Multibeam feed
19-element Array at NRAO Electrically small elements Hexagonal array Beamforming
Approach 25 meter paraboloid 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
Assumptions Array: Operating frequency: 1612 MHz 7 and 19-element hexagonal arrays with 06λ spacing Hertzian dipoles No mutual coupling between array elements Hemispherical element patterns Noise model: Individual LNA noise temperature: 15 K Spillover noise: 300K warm ground below reflector Atmospheric and cosmic background noise is neglected
Interference Mitigation s[n] max-snr/lcmv x 1 [n] x 2 [n] x N [n] w 1 w 2 y[n] w N Spillover noise
Sensitivity 25 meter reflector Boresight beam 10 x 10 3 9 8 Array feed (optimum) Array feed (CFM) Waveguide feed Sensitivity (Jy 1 ) 7 6 5 4 3 2 1 1 05 0 05 1 Feed Displacement (wavelengths) closer to reflector further from reflector
Gain and Spillover Efficiency 51 50 Array feed (optimum) Array feed (CFM) Waveguide feed 1 095 Array feed (optimum) Array feed (CFM) Waveguide feed Gain (dbi) 49 48 47 46 45 44 1 05 0 05 1 Feed Displacement (wavelengths) Spillover Efficiency (%) 09 085 08 075 07 065 06 1 05 0 05 1 Feed Displacement (wavelengths)
Reflector Illumination Pattern 10 Array feed (optimum) Waveguide feed 0 Gain (dbi) 10 20 30 40 50 50 0 50 Degrees from Boresight
Steered Beams/Offset Feed Sensitivity (Jy 1 ) 12 x 10 3 10 8 6 4 Array feed, focal plane Array feed, 25 wavelengths Array feed, +15 wavelengths Array feed, +25 wavelengths Array feed, CFM Waveguide feed 2 0 02 04 06 08 1 Degrees from Boresight
Focal Field Distribution Boresight Beam steered to 3 1 1 0 08 5 08 5 06 10 06 10 04 15 04 15 02 20 02 20 0 25 db 0 25 db 02 30 02 30 04 35 04 35 06 40 06 40 08 45 08 45 1 1 08 06 04 02 0 02 04 06 08 1 50 1 1 08 06 04 02 0 02 04 06 08 1 50 λ λ
Results (7 Element Array) Interferer at 30 degrees, INR=0 db Sens eff =00073 Jy 1 Spillover eficiency=971 % Gain=493 dbi Sens eff =00073 Jy 1 Spillover eficiency=971 % Gain=493 dbi 40 20 20 30 Gain (dbi) 0 20 Gain (dbi) 40 50 40 60 60 50 0 50 Degrees from Boresight 70 25 30 35 Degrees from Boresight
Main Beam Distortion Sens =00073 Jy 1 eff 50 Spillover eficiency=971 % Gain=493 dbi 40 30 Gain (dbi) 20 10 0 10 20 30 3 2 1 0 1 2 3 Degrees from Boresight Distored Beampattern with Interference Normal Beampattern with no Interference
Interferer at 30 deg, INR In changing 8 x 10 3 Array feed (max SNR) Waveguide feed Effective Sensitivity (Jy 1 ) 7 6 5 4 3 2 1 100 50 0 50 100 INR In (db)
Moving Interferer 9 x 10 3 8 Effective Sensitivity (Jy 1 ) 7 6 5 4 3 2 1 0 0 10 20 30 40 50 Degrees from Boresight
Interference Rejection 180 160 Interference Rejection (db) 140 120 100 80 60 40 20 0 0 10 20 30 40 50 Degrees from Boresight - Low sensitivity corresponds to poor spillover efficiency and gain loss
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 cosψ 06 05 04 03 02 01 0 20 25 30 35 40 45 50 Degrees from Boresight
19-element array, moving interferer Effective Sensitivity (Jy 1 ) 0025 002 0015 001 0005 7 element array feed 19 element array feed Theoretical maximum 0 10 20 30 40 50 Degrees from Boresight
Conclusions Good sensitivity can be achieved using an array feed In the presence of an interferer Interference at all INR levels and all angles was effectively rejected Main beam distortion occurs due to beam steering/rfi mitigation Sensitivity fluctuates by a few db with moving angle of arrival Future work: Algorithms: beam shape control, defocusing (larger arrays)? Broadband elements Mutual coupling Prototype
Gain and Spillover Efficiency Gain (dbi) 52 50 48 46 44 Array feed, focal plane 42 Array feed, 25 wavelengths Array feed, +15 wavelengths 40 Array feed, +25 wavelengths Array feed, CFM Waveguide feed 38 0 02 04 06 Degrees from Boresight 08 1 Spillover Efficiency (%) 100 95 90 85 80 75 70 Array feed, focal plane Array feed, 25 wavelengths Array feed, +15 wavelengths Array feed, +25 wavelengths Array feed, CFM Waveguide feed 65 0 02 04 06 08 1 Degrees from Boresight
Multiple Beams θ=3 θ-cut Gain(dBi) 50 48 46 44 42 Gain(dBi) 51 50 49 48 47 46 45 44 40 0 50 100 150 200 250 300 350 Phi 43 0 02 04 06 08 1 Degrees from Boresight
Sensitivity x 10 3 Sensitivity (Jy 1 ) 10 9 8 7 6 5 4 3 2 1 1 2 3 4 Beam 5 6 7
Sum of outer weights 0014 0012 Effective Sensitivity (Jy 1 ) 001 0008 0006 0004 0002 0 01 015 02 025 03 035 04 Sum of amplitudes of outer six element weights
Center element, INR IN 20 15 10 INR IN (db) 5 0 5 10 15 20 20 25 30 35 40 45 50 Degrees from Boresight
Assumptions 15 K W 1 LNA 15 K W 1 W 2 15 K W 2 15 K W N G S = T rec + T spill T = (W ) rec W N N i= 1 i 2 T i