Title: Subspace Projection Methods for RFI Mitigation in Radio Astronomy
1Subspace Projection Methods for RFI Mitigation in
Radio Astronomy
- Brian Jeffs
- July 25, 2002
2References
- 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. - 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.
3Correlation Across Array Elements
Primary Array
GLONASS satellite
Auxiliary Antenna
4The 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
5Subspace 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!!
6Methods 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
7Problems
- 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
8Problems (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.
9Solution 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.
101. 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.
112. 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.
123. 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.
13Examples
VLA, 1612 MHz with one 3m aux. dish, 200 Jy
source with one GLONASS interferer.
14Examples (cont.)
VLA, 1612 MHz with two 3m aux. dishes, 20 Jy
source with two GLONASS interferers.
15Examples (cont.)
VLA, 1612 MHz with one 0 dB omni aux., 200 Jy
source with one GLONASS interferer.