Removal of Radio-frequency Interference (RFI) from Terrestrial Broadcast Stations in the Murchison Widefield Array. A/Prof.

Transcription

1 Removal of Radio-frequency Interference (RFI) from Terrestrial Broadcast Stations in the Murchison Widefield Array Present by Supervisors: Chairperson: Bach Nguyen Dr. Adrian Sutinjo A/Prof. Randall Wayth Prof. Yue Rong

2 Outline Motivations & Significance Literature Review Research methods Expected Outputs Timeline Radio-over-Fiber Calibration for Radio Astronomy 2

3 Introduction Radio-frequency Interference: any source of transmission that is within the observed frequency band other than the celestial sources themselves. Impact: contaminates the data collected by radio telescopes, potentially leading astronomers to erroneous interpretations. Major sources of RFI at the MWA: FM stations Digital TV stations Satellite Aircraft 3

4 Introduction 4

5 Motivation & Significance Objectives: To explore the application of an RFI subtraction technique using reference antennas introduced in (Briggs, 2000) to broadcast FM and DTV RFI at the MWA. To design and implement an RFI mitigation system with an optimum number of reference antenna to subtract terrestrial FM and DTV RFI from the cross-correlation product. Significance: Allows radio astronomers to perform observations in the the low frequency bands of FM and DTV within an error of less than 1% of the true sky visibility. 5

6 Literature review RFI Mitigation technique: Blanking/Flagging: involves the identification of the contaminated frequencies and removal of these channels from the spectrum of the correlator outputs. Null Steering: steering the nulls in the radiation pattern of the telescope towards the source of RFI. Adaptive filtering: involves the implementation of one or more reference antennas together with a filter to subtract RFI from the output spectrum Spatial filtering: based on the estimation of the spatial signature vector of the interferer from the spatial covariance matrices, followed by the subspace projection and further averaging. Current RFI removal method at MWA: Blanking/Flagging. 6

7 Literature review (cont.) An RFI subtraction technique using reference antennas by (Briggs, 2000): Figure 1: RFI subtraction model configuration (Briggs, 2000) 7

8 Literature Review (cont.) Correlation products: = g A g B A A A B + g 1 g 2 I 2 8

9 Literature Review (cont.) Correction terms CX ij : Filter design: Corrected spectrum: 9

10 Literature Review (cont.) Figure 2: The Power Spectra for the 4 channels at the Parkes telescope. These spectra are the averages of 25 seconds of data. The upper panels show the spectra both before and after cancellation. (Briggs, 2000). 10

11 Literature Review (cont.) Application on MWA This method has been analysed and demonstrated on a dishbased telescope only. There are a few challenges that require further rethinking of this technique: The MWA is wideband ( MHz) and has a wide field-of-view (FoV). The MWA is sky-noise dominated in the FM and VHF DTV bands. The reference antennas will receive appreciable sky noise. Terrestrial broadcast signals may arrive from a few directions. It is not practicable to dedicate one (or two) fixed reference antenna(s) to each station. 11

12 Literature Review (cont.) Application on MWA (cont.) 12

13 Literature Review (cont.) Figure 3: A simulation of the RFI subtraction method with sky leakage in reference antennas 13

14 Literature Review (cont.) Application on MWA (cont.) 14

15 Literature Review (cont.) Antenna Design The specifications for this reference antenna design are suggested as follow: High-gain, cost-effective omni-direction antenna. Zero (or as small as possible) response towards zenith, with maximum gain towards horizon Feasible model: Hertzian dipole array Half-wave dipole array 15

16 Literature Review (cont.) Antenna Design (cont.) 3D Radiation pattern (Stutzman & Thiele, Antenna Theory and Design 3 rd edition, 1981) Radiation pattern of Hertzian dipole and half-wave dipole 16

17 Literature Review (cont.) Antenna Design (cont.) Hertzian dipole 2D prototype Radiation pattern of Hertzian dipole array 17

18 Research milestones Reference antenna design: First feasible design to start with is Hertzian dipole array Identify the optimal number of element in the array as well as separation distance between elements Find out the actual gain of the MWA towards RFI at horizon Practical deployment: Expansion to DTV RFI removal: 18

19 Research milestones (cont.) Performance comparison between Hertzian and half-wave array with the same number of elements. Baseline = 60m 19

20 Research milestones (cont.) Performance comparison between Hertzian array with 1, 4 and 7 elements. Baseline = 200m (left) and 1000m(right) 20

21 Research milestones (cont.) Reference antenna design: Practical deployment: Find out the proper position for the 2 reference arrays in the MWA telescope Deploy the RFI subtraction system to the MWA and verify its performance Expansion to DTV RFI removal: 21

22 Research milestones (cont.) Specifications for the second simulation model: All the antenna aligned East-West : Rx1 - Rx3 - Rx4 - Rx2 The reference antenna : four-hertzian-dipole array. The real radiation patterns of the MWA tile and Hertzian dipole array The sky pattern for the FM band : constructed from the Haslam map at 408 MHz. Power of incident RFI signal was approximated as -125 dbm for 10 khz bandwidth from a user-defined direction (5 degree above horizon from the East) and remained constant over the FM band (80-120MHz). Constant distance of 10m between the two reference antennas Separation between elements within the reference array is 1.5m. 22

23 Research milestones (cont.) Configuration models: Close: The arrays is very close to Rx1 (only 10m away from Rx1) Centre: The reference antennas is exactly in the middle of Rx1 and Rx2 Far: The reference arrays is between close and centre position, but closer to centre position 23

24 Research milestones (cont.) Simulation results: Baseline = 60m 24

25 Research milestones (cont.) Baseline = 200m 25

26 Research milestones (cont.) Baseline = 700m 26

27 Research milestones (cont.) Baseline = 1000m 27

28 Research milestones (cont.) Reference antenna design: Practical deployment: Find out the proper position for the 2 reference arrays in the MWA telescope Deploy the RFI subtraction system to the MWA and verify its performance Expansion to DTV RFI removal: 28

29 Research milestones Reference antenna design: Practical deployment: Expansion to DTV RFI removal: The reference antenna design needs to be reconsidered for DTV RFI case Analyse possibility that a common design of RFI removal system could be used for both FM and DTV RFI at the MWA telescope 29

30 Research outputs Demonstrable evidence of the efficiency of this RFI removal technique for the MWA telescope including: Results of simulations where all practical circumstances expected at the MWA are accounted for. Prototype antenna array design for FM and DTV RFI removal successfully deployed at the MWA. Successfully verifying the performance of this system for AAVS1 and MWA telescope with demonstrable results. 30

31 Schedule 31

32 Questions & Answers Thank you for your listening! Any questions? 32