Transcript Высокочувствительный комплекс радиомет

Techniques for interference
mitigation on RATAN-600 radio
telescope in dm ranges
A. B. Berlin, N. A. Nizhel'skij, M. G. Mingaliev,
P. G. Tsybulev , D. V. Kratov, R.Y.Udovitskiy
RATAN-600 is one of the main the radio telescope of the Russian Academy
of Sciences with a ring multi-elements 600-m antenna.
Maximum angular resolution
Frequency range
Accuracy of coordinate determination
2 arcsec
610 - 30000 MHz
1-10 arcsec
The principals of Time – Frequency RFI suppression :analysis and synthesis
The technique time-frequency method of
interference mitigation is based on the following
idea:
Analysis - the input band of the radiometer is
subdivided into several "narrow" frequency
channels, with an independent detection section
and rapid signal processing in each.
Synthesis – the wide radiometric band is
synthesized by summing of the narrow channels
each of them cleaned from the short and large
pulses in the real-time digital signal processing.
The typical appearance of pulsed
interference in all "narrow"
channels of the 31-cm radiometer.
Shown the individual half-periods of
modulation in all four channels of
the radiometer in the "antenna" —
“noise source" sequence for each
channel. As can be seeing the
interference appears in the
“antenna” half-period only. This
behavior is indicative of
interference that is external with
respect to the radiometer.
Block diagram of the anti-interference adapter to the 13-cm radiometer.
1 - 2.12 - 2.52 GHz band microwave; 2 - gates; 3 – signal divider by 2; 4 - amplifier(about 40 дB) for
compensating subsequent division of signal by 8; 5 – signal divider by 8; 6 – a bank of 50 MHz wide microwave
filters; 7 – preliminary low-frequency amplifier with quadratic detectors at the input; 8 – low-frequency filter to
match the operating band of the analog-digital converter;9 - ADC(10-12 bits) wiyh a multiplexer at the input;
10 – parallel-to-serial code converter; 11 – link to a digital signal processor on an ISA PC bus MOD – signal of
radiometer modulation; INPUT – input of the microwave signal.
The method of time-frequency mitigation at the level of signal processing consists of two stages. The first stage
involves on-line removal in the digital signal processor of pulses whose duration does not exceed 80% of the
half-period of modulation of the radiometer and whose amplitude exceeds the given threshold. Signal counts in
these intervals are substituted by the preceding "normal" count that met the 3сигма criterion. However, if the
duration of interference exceeds the specified limit, this interference is considered to be "long" so that its
removal would break significantly the statistics of the channel and the corresponding interference is not
removed. This determines the boundary of applicability of the method. The second stage consists in off-line
frequency interference mitigation. The central idea of the method consists in synthesizing the wide frequency
band of the radiometer from "narrow" channels in the process of subsequent data reduction
The demonstrates the efficiency of on-line filtration of pulsed interference.
Observations with the 13-cm radiometer were performed simultaneously with
and without on-line filtration in the full band of the radiometer (400 MHz). The
figure demonstrates complete suppression of radar-station pulses in record
(b). Residual "artefacts" indicate that the filtration algorithm operated at the
limits of its capability when the duration of the pulse exceeded the permissible
duration and the algorithm switched off for a short
Illustration of the method of time-and-frequency mitigation of interference
as applied to a 31 cm radio source (four “narrow” channels)
The application of the time-frequency method of interference mitigation to the
31-cm radiometer. It should be pointed out that 13-and 31-cm radiometers allowed
recording data in the full radiometric band with the possible switching off of online
filtration of pulsed interference (record f). It is evident from this record that during the
observation the radio source was recorded at the presents interferences that far
exceeded the amplitude of the source. The nature of this interference can be explained
as following: it is pulsed interference, because it has been successfully suppressed by
filtration in the digital signal processor. As a result, all "narrow" channels proved to be
suitable for synthesizing the full band of the radiometer (120 MHz), see record (e)
Record of signal from radio source 1936+046 at the input of
the 13 cm radiometer
A case of observation of a radio
source with the 13-cm radiometer in "narrow"
channels (b, c, d, e, f, g, h, i). The (off-line)
reduction of observations involved the
synthesis of the full band of radiometer
(record (a)) and the synthesis of the band
with rejection of channel (e), so we get
record (j).
Interference is apparent only in
channel (e), thereby preventing observation
of the source in the full band of the
radiometer. The source can be seen clearly
in record (j) and its parameters can be
measured with high accuracy.
а – the signal of the “wide-band”(400 MHz) channel
without noise protection;
b,c,d,f,g,h, i - signals in “narrow” channels(each with a
50 MHz bandwidth) of the anti-interference system;
j - sum of the signals of the seven “narrow” channels
uncontaminated by interference.
Block diagram one of the possible interference-measurement setup at RATAN-600.
To study the electro magnetic environment, the wide band (0.5—3000 MHz)
measurement system has been developed on RATAN-600. The detector fully covers the
operating decimeter - waves on RATAN-600. However, the sensitivity of this detector combined
with a two-meter parabolic antenna, is much lower than that of the modern radiometer of
RATAN-600, and that is why for precision measurements we acquired and installed an
additional low-noise amplifier at the detector input (+17 dB). Such a measurement-instrument
kit can be used for visual recording of interference.
To allow recording the detected interference to a personal computer (PC) and to
receive measurement reports, we developed additional software, which allows controlling
measurement modes and recording data to a personal computer (PC).
The second measuring setup for the electro magnetic environment investigation
at RATAN-600.
The another way to measure electromagnetic environment at the radio telescope
site: the spectrum analyzer FSU-46 (0-46GHz) used as a radiometer baсk-end (after the
band pass filter or before band pass filter ). That lets to carry on an absolute measurements
in an absolute power scale (dBm, e.t.c.).
Radio-frequency emission detected at RATAN-600 using the electromagnetic
environment measuring setup
RATAN-600 frequency ranges (green zones):
546-677MHz (49 cm), 900-1020MHz (31 cm),
and 2120-2520MHz (13 cm).
Red lines are subdivided narrow channels
Radio-frequency emission detected at RATAN-600 using the spectrum analyzer
FSU-46 (0-46GHz) as a radiometer baсk-end
0-3000MHZ
850-1020MHZ
100-800MHZ
800-1500MHZ
CONCLUSIONS
Complex electromagnetic environment requires
the use of modern hardware and software tools to success
interference mitigation in radio astronomy. The timefrequency approach described in this talk proved to be an
efficient method for the worst frequency ranges in radio
astronomy. Further development of hardware and algorithms
for quality radio-astronomical observations required.
Also, we need to develop other approaches for
interference mitigation in radio astronomical ranges. Ones
must be as a technical (for example adaptive filtering) so an
organizing ways.
Thank you !
n
Loss of Information in the Method of Time and Frequency Interference
Mitigation
Let the total band of a microwave radiometer be equal to ΔF. This band is subdivided into N
adjacent "narrow" bands or frequency channels of different width δfi. In this case, if the root
mean squared (RMS) noise of a radiometer with a transmission band ΔF is equal to up then,
given that the RMS noise at the radiometer output is proportional to the square root of the
microwave band width, the RMS noise in a δfi -wide channel is equal to:
This means that in the case of "long" interference in N-1 channels where the algorithm of
cutting pulsed interference in these channels ceases to work, the resulting synthesis can be
performed only using one "narrow" channel with an root of N times lower sensitivity compared
to the entire band of the radiometer.
- The noise of the NG half-period
- The dispersion for the "sky" noise
In this case, as a result of subtraction of varying-accuracy data of half-periods, their dispersions
would add up:
Thus in the most unfavorable case where interference still does not prevent observations with
3N
the given radiometer the radiometer sensitivity should be about
times lower than in
the case of interference-free observations