Beam Simulator

Description

This package contains tools for simulating the beam pattern of an arbitray phased array. It consists of 3 modules:

• nec.py - Tools for reading in an output file from a NEC4 simulation and representing the gain pattern of an individual antenna.
• station.py - Collection of classes used to build objects representing the different levels of an array.
• beamformer.py - Beamforming related functions for a given <Station> object.
• skymap.py - Collection of functions which can simulate the observed sky for a <Station> object and simulate the measured spectrum or driftcurve.

Information about a station can be supplied in a text file which can be loaded in to generate a fully populated <Station> object. A template for such text files is provided. LWA SSMIF's are also supported to quickly load in a full LWA station.

Requirements

• python >= 3.8
• numpy >= 1.19.2
• scipy >= 1.5.4
• astropy >= 4.1
• matplotlib >= 3.3.2
• numba >= 0.51.2
• pygdsm >= 1.3.0
• tqdm >= 4.62.2
• ephem >= 4.1
• healpy >= 1.15.0
• lsl >= 2.0.2 (for LWA SSMIF compatability)

Example

Below is an example showing how to:

1. Populate a <Station> object describing LWA-SV using its Station Static MIB Initialization File (SSMIF).
2. Represent the gain pattern of a LWA dipole.
3. Simulate the beam pattern for LWA-SV for a given pointing center and frequency.

NOTES:

• "lwasv-ssmif.txt" can be found within the LWA Software Library.
• The NEC4 output files for the LWA dipole can be found here

Setting up the Station.

from beam_simulator import station

lwasv = station.load_LWA("lwasv-ssmif.txt")

Representing the LWA dipole gain pattern and fitting spherical harmonics to it as a function of frequency.

The LWA dipole gain pattern has been modeled using NEC4 at multiple frequencies. These can be loaded into Beam Simulator in order to build a model of the dipole gain pattern.

from beam_simulator import nec

#We need to read in all NEC4 output files for a series of frequencies.
freqs = [10, 20, 30, 40, 50, 60, 70, 80, 88] #MHz

p1 = [f'lwa1_xep_{freq}.out' for freq in freqs]
t1 = [f'lwa1_xet_{freq}.out' for freq in freqs]
p2 = [f'lwa1_yep_{freq}.out' for freq in freqs]
t2 = [f'lwa1_yet_{freq}.out' for freq in freqs]

#Fit the spherical harmonic decomposition as a polynomial in frequency.
nec.fit_antenna_response(freqs, p1, t1, p2, t2, lmax=12)  

This will create a file named "beam_cofficients.npz" which can be used by the beamformer module.

Simulating the beam pattern of LWA-SV.

from beam_simulator import beamformer

#Generate the weighting vector for the station.
w = beamformer.generate_uniform_weights(lwasv) #All antennas have the same weighting of 1.0

#Simulate the beam for a pointing center of az = 180 deg, el = 75 deg at 74 MHz with 1 degree resolution.
pwr = beamformer.beamform(lwasv, w, freq=74e6, azimuth=180.0, elevation=75.0, resolution=1.0,
                          ant_gain_file='beam_coefficients.npz', dB=False)