Simulation of visibility with RASCIL
The final stage of the three-stage RFI simulation, simulate_low_rfi_visibility_propagation.py uses RASCIL to
calculate the visibility measured by SKA-Low (LOW) for a number of emitters, and generate output images or measurement sets.
We are interested in the effects of RFI signals that cannot be detected in the visibility data. Therefore, in our
simulations we add transmitter apparent power and beam-gain information calculated in the previous stages.
As before, we study the effects of a TV station located in Perth, AU, emitting a broadband signal of a known power (information stored in CSV files in rfi/data/transmitters). We presume the following scenario:
The emission from the TV station arrives at LOW stations with phase delay and attenuation. We calculate Propagation attenuation with Pycraf. The RFI enters LOW stations in a side-lobe of the station beam. We perform Beam-gain calculation with OSKAR, which, together with the pre-calculated apparent power values, is used as an input for the RASCIL script. The RFI enters each LOW station with fixed delay and zero fringe rate (assuming no e.g. ionospheric ducting or reflection from a plane). When tracking a source on the sky, the signal from one station is delayed and fringe-rotated. Fringe rotation stops the fringe from a source at the phase tracking centre but phase-rotates the RFI, which now becomes time-variable. To de-correlate the RFI signal, the correlation data are time- and frequency-averaged over a timescale appropriate for the station field of view.
We want to study the effects of this RFI on statistics of the visibilities, and on images made on source and
at the pole. The simulate_low_rfi_visibility_propagation.py script averages the data producing baseline-dependent
de-correlation and uses
RASCIL functions
and input data from the previous stages to produce FITS images,
and un-averaged MeasurementSets (one per time chunk). The images are on signal channels and on pure noise channels,
and for the source of interest. Distributed processing is implemented via Dask.
Simulation inputs
SKA Low configuration
An input configuration file (txt or equivalent, called as the “antenna_file”) containing position information in longitude and latitude.
The default configuration file
(rfi/data/telescope_files/SKA1-LOW_SKO-0000422_Rev3_38m_SKALA4_spot_frequencies.tm/layout_wgs84.txt) is
used by the code if the --use_antfile argument is set to True, else it uses the RASCIL equivalent.
If --use_antfile == True, you can specify an alternative configuration file by setting the --antenna_file
CLI argument (see RASCIL Python script).
Transmitter apparent power
The apparent power of the transmitter is calculated by Pycraf Python script and stored in an HDF5 file, together with other relevant information, such as time and station-dependent azimuth and elevation data. For more information, follow Propagation attenuation with Pycraf.
Beam gain data
Beam gain values as a function of frequency, calculated by OSKAR Python script and stored in an HDF5 file, together with pointing information (i.e. right ascension and declination). For more information, follow Beam-gain calculation with OSKAR.
Usage and command line arguments
Power spectrum
The power_spectrum.py script can be used following the production of output FITS images from the simulation to
produce power spectrum plots.
Usage and command line arguments: Power Spectrum Python script