Diapositive 1

1
DSP enabled Radio Astronomy in Highly Corruptive
Environment R. Weber1, L. Denis2, G. Kenfack2 ,
C.Viou2, A.Coffre2, P.Picard2 1LESI-Laboratoire
d'Electronique, Signaux, ImagesUniversité
d'Orléans, 12 Rue de Blois, BP 6744,
F-45067Orléans Cedex 2 2Station de
radioastronomie, F-18330 Nançay, France
As a result of the development of
telecommunications, radio astronomers have to
deal with an increasing number of observations
polluted by man-made radio frequency
interferences (RFI). Current receiver designs,
however, which are based on the hypothesis of a
non-corruptive environment, are no longer
suitable as their performances in terms of
linearity and frequency rejection are
insufficient to cope with high dynamic RFI.
Moreover, their hardware architecture is too
specific to allow additional functions to be
implemented. In this paper, a new generation of
radio astronomic receiver is presented. It has
been specifically designed for the single dish
telescopes of the Nançay observatory (France).
Eight independent channels (14 MHz bandwidth
each) are processed by a fully reconfigurable
digital unit. This system is based on a HUNT
Engineering system (Heron modules and HEPC 9
boards). Its primary function is to provide high
resolution dynamic spectra. Depending on the
interference context, part of the calculation
power can also be used to carry out real time RFI
detection in order to clean up the spectra.
Hardware upgrade is under progress with a view to
implementing a 4096-bin polyphase filter bank and
to developing more complex RFI mitigation
algorithms.
Architecture of the tolerant receiver
Architecture of the decimation filters

• Basic configuration for each bank
• Undersampling at 56 MHz (14 bits ADC),
• Digital Down Conversion (FPGA),
• Final bandwidth from 14 MHz to 0.875 MHz,
• High dynamic range (gt70 dB),
• Channelization with a 256 to 4096 bin FFT (FPGA),
  and 8192 bins polyphase filter bank (49152
  coefficients),
• Post-detection RFI mitigation (2 DSP available).
• Cross-Correlation between 2 input signals. From
  256 to 4096 channels.

Example of data acquisitions

• Science and RFI mitigation

(a)
(b)

• Total computing power
• 8 FPGA virtex II 1000
• 8 FPGA virtex II 3000
• 16 DSP TMS6203
• Data flow Rate 400Mo/s

8 analog inputs with 14 MHz bandwidth each
configurable from 2 channels with 56 MHz
bandwidth (by merging 4 banks) to 8 channels with
14 MHz bandwidth
Measured spectrum Expected profile
(c)
(d)
Overview of the tolerant receiver. The basic flow
for each bank is first digital down conversion
(DDC) with a FPGA, then channelisation (FFT) with
another FPGA and finally RFI mitigation with 2
DSPs. The flow can be reconfigured to share the
calculation power between all the banks.
Overview of the performances - Total bandwidth
(BW) from 875 kHz to 56 MHz -
Frequency resolution (depending on BW) from 427
Hz to 6.8 kHz - Temporal resolution
(depending on the BW) from 73 µs to 1.17 ms
RFI impact on the observation of the mega maser
IIIZW35. RFI bursts are due to downlink satellite
emissions.(a) Example of time-frequency
representation. The signal has been acquired with
our digital robust receiver, but no blanking has
been applied. The RFI bursts are clearly visible.
The SOI being buried in the noise, a spectrum
averaging must be done to make the SOI profile
visible. (b) Averaged spectrum. The averaging
time is 13 minutes. The RFI bursts dominate the
spectrum. (c) Zoom on averaged spectrum.
Compared with (b), the filter spectral shape has
been removed. The spectral profile of IIIZW35
(dash red line) is completely scrambled by the
RFI bursts. (d) Spectra of IIIZW35 after a
integration time of 13 minutes. The spectral
shape of the receiver filters has been removed.
Compared to Figure 1.c, the IIIZW35 source is
clearly visible. The result has been obtained in
real time with our digital robust receiver. (
Thanks to P.Zarka, P.Colom and JM.Martin for this
result on IIIZW35 ).
Future developments Digital receiver for FASR
interferometer
The Frequency Agile Solar Radiotelescope is a
solar-dedicated instrument designed to perform
broadband imaging spectroscopy.
FASR will be designed to support temporal,
spatial, and frequency resolutions well-matched
to problems in solar physics.
Specification band