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Title: Subspace Projection Methods for RFI Mitigation in Radio Astronomy


1
Subspace Projection Methods for RFI Mitigation in
Radio Astronomy
  • Brian Jeffs
  • July 25, 2002

2
References
  1. J. Raza, A-J Boonstra and A-J van der Veen,
    Spatial Filtering of RF Interference in Radio
    Astronomy, IEEE SP Letters, vol. 9, no. 2, Feb.
    2002.
  2. A. Leshem, A-J van der Veen, Radio-Astronomical
    Imaging in the Presence of Strong Radio
    Interference, IEEE Trans. On Information Theory,
    vol. 46, no. 5, Aug. 2000.

3
Correlation Across Array Elements
Primary Array
GLONASS satellite
Auxiliary Antenna
4
The Array Covariance Matrix is the Basis for
Synthesis Imaging
  • Elements of R are image frequency domain samples.
  • Earth rotation moves baselines for new R, more
    frequency samples.
  • Interference effects must be removed from R
    directly, beamforming to place nulls is not
    possible since correlations from all array pairs
    are needed.

(b) VLA frequency samples with Earth Rotation (d)
VLA frequency sample snapshot
5
Subspace Projection Approach
  • Interference component of R spans a subspace of
    rank P number of interferers.ap is array
    response to pth interferer with power .
  • Find a projection operator orthogonal to Ri

Use this in imaging. No interferer left!!
6
Methods of computing P
  • If array is calibrated and interference direction
    known
  • If ISNR gtgt 0 dB at feeds and direction
    unknown
  • If interference moves, use short-term integration
    for

7
Problems
  • Interference moves durning integration.
  • Solution use short-term integartions, short term
    projections
  • Projection biases signal subspace,
  • Can not invert because P is singular.
  • Solution use smoothing over short-term
    integrations to build rank
  • Now

8
Problems (cont.)
  • For high gain antennas, usually ISNR ltlt 0 dB at
    feed.
  • Poor interference subspace estimate leads to poor
    interference rejection from projection matrix P.
  • Sometimes the signal is identified as the
    interferer, and is projected out.

9
Solution to Bad Subspace EstimatesUse Auxiliary
Antennas
  • Array consists of high gain primaries and low
    gain auxiliaries, perhaps steered to
    interference,
  • Auxiliary antennas see high ISNR to guide
    subspace estimation for the primary array.
  • Four different approaches for computing P have
    been evaluated.

10
1. Conventional Full Array Subspace Projection
  • Use the full array, including auxiliaries, with
    no distinction as to antenna type.
  • Compute a truncated projection matrix
  • Significant performance improvement over using
    primaries only. Handles weaker interferers.

11
2. Array Multiple Sidelobe Canceller (MSC)
  • Form an MSC adaptive array processor separately
    on each primary antenna.
  • This is an oldie but a goodie.
  • Low probability of signal capturing the
    interference subspace.

12
3. Auxiliary Assisted Subspace Projection
  • Use only the primaries in final estimate
  • Projection uses only cross correlations between
    primaries and auxiliaries to strongly emphasize
    the interferer.
  • Best overall performance.

13
Examples
VLA, 1612 MHz with one 3m aux. dish, 200 Jy
source with one GLONASS interferer.
14
Examples (cont.)
VLA, 1612 MHz with two 3m aux. dishes, 20 Jy
source with two GLONASS interferers.
15
Examples (cont.)
VLA, 1612 MHz with one 0 dB omni aux., 200 Jy
source with one GLONASS interferer.
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