blanking
techniques to remove pulse-type signals from the data stream
Just as optical astronomy faces serious problems with light pollution, radio astronomy is similarly plagued by its own brand of light pollution: radio frequency interference (RFI). Growing technological resources usurp more and more radio frequency spectrum in the VHF, UHF, and microwave bands; and all have potential to interfere with radio astronomy observations. Signals from some existing satellites leak into protected radio astronomy bands, and the problem will likely increase as more satellites are put into orbit. Low-earth-orbit satellites (LEOS) create an RFI problem no matter where on the earth a radio telescope is located, even in remote locations as Tasmania and the Antarctic! Increasing congestion of the radio spectrum is making astrophysical research in the designated radio and microwave bands ever more difficult, and the situation is quite a bit worse outside the bands protected for astronomy.
Every year, an increasing amount of radio frequency spectrum in the VHF, UHF, and microwave bands is being used to support new commercial and military ventures, and all have the potential to interfere with radio astronomy observations. Such services already cause problems for radio astronomy even in very remote observing sites, and the potential for this form of light pollution to grow is alarming. Preventive measures to eliminate interference through FCC legislation and ITU agreements can be effective, however, many times this approach is inadequate and interference excision at the receiver is necessary. When prevention is not possible, various methods to reduce or eliminate interference have been employed; conventional approaches include:
blanking
techniques to remove pulse-type signals from the data stream
filtering
techniques such as superconducting notch filters to remove
fixed-frequency interference
radio
frequency (RF) shielding to suppress spurious
digital signals and local oscillator signals from adjacent electronic
equipment or communication systems
post-processing
techniques on array systems such as sidelobe-beam nulling to remove
fixed-frequency signals
adaptive
beam forming techniques.
All these schemes are successful to some degree, yet each suffers from either insufficient interference cancellation, inability to adapt to changing statistics of the interference signal, partial removal of wanted data, or excessively large amounts of post-processing of the accumulated data. Clearly, we need to investigate new approaches to interference excision that have potential to improve upon the shortcomings of conventional techniques.
Adaptive interference cancellation is a real-time approach to interference excision that has not been used before in radio astronomy. For the first time, we have used adaptive interference cancellation inradio astronomy by building a prototype low-frequency adaptive filter receiver.
Up until now, not very much!! Removing RFI in a radiometer is a very difficult exercise. Astronomical signals are weak compared to ground (or satellite) signals, and so large attenuation capability is necessary. Often, RFI and astronomical signals occur at the same frequency, and conventional rejection schemes, such as notch filters, matched filters, and beam nulling, remove some or all of the astronomical signal along with the RFI. For example:
A
notch filter is a very sharp band-reject filter
that can be quite useful in removing strong interference which can
cause saturation to occur in the amplifier and correlator. However,
this type of filter is not easily adjustable in frequency and removes
some of the desired signal.
A
matched filter is a digital filter whose
characteristics "match" that of the interference and remove
it from the radiometer data. However, the spectrum of the
interference signal must be known a priori and in some cases
the filter characteristics are not physically realizable. The matched
filter can also cause distortion of the desired signal.
In
array systems beam-nulling has been tried through
extensive post-processing of the data; although somewhat successful,
this approach is very time consuming and does not attenuate enough
for astronomy observations at frequencies that are plagued by RFI.
An
adaptive beam-former is being designed for the
Square Kilometer Array Interferometer, but the problem here is that
the weighting coefficients that create the null can also cause small
distortions in the main telescope beam.
a
linear filter is used to minimize the least mean
square value of the difference between the desired response and the
actual output. The resulting solution is commonly known as the Wiener
solution, but the Wiener solution is inadequate because it
is not adaptive to changing environmental conditions.
If environmental conditions were always stationary, the Wiener solution would work well enough for astronomy. With a Wiener filter a reference channel that monitors only the RFI is compared with the corrupted signal coming in through the telescope or main channel, the RFI is identified and cancelled, leaving the astronomical signal. However, in the real world environmental conditions change as the telescope tracks or as propagation phenomena, such as reflection and multi-path, affect the RFI at the reference differently than at the main feed. In these non-stationary conditions, correcting coefficients must be able to update in real-time as the statistics change. To cancel RFI well enough to retrieve astronomical signals, the canceling scheme must be able to adapt to changing statistics.
Given the serious, ever-growing problems that RFI poses to radio astronomy, new and sometimes radical approaches for interference cancellation must be examined. If the adaptive interference canceling scheme is shown to be viable for use in radio astronomy, the potential would be significant. Initial bench tests with our prototype system are very encouraging; our adaptive filter achieved attenuation of strong frequency-modulated interference by more than 72 dB (a factor of ten million), at the limit of the system's measurement capabilities.