Ubuntu Manpage: cassbeam - Cassbeam is a Cassegrain antenna ray tracer

NAME

       cassbeam - Cassbeam is a Cassegrain antenna ray tracer

SYNOPSIS

       cassbeam input_file [key=value ...]

DESCRIPTION

       cassbeam is a program for Cassegrain antenna modelling.  It computes several properties of
       the antenna including gain, zenith system temperature, and the beam, in full polarization.
       All  calculations  are  done  in  the  transmit sense and use reciprocity to relate to the
       equivalent receiving system.

       Cassbeam is a non-interactive command line program that takes all of its  input  from  the
       command  line.   Note  that  this  does not preclude someone at a later date from making a
       graphical or web front end.  There is one required argument when running  cassbeam  -  the
       input  filename.   Additional  arguments  can supplement the parameters of the input file.
       These arguments are passed in the same key=value as required  in  the  input  file  except
       whitespace is not allowed around the equal sign.  If a parameter appears both in the input
       file and the command line, then the value on the command line supercedes the value on  the
       input file.

INPUT FILE

The main input file consists of a series of lines of the form key=value. Only one such
entry is allowed per line. The equal sign is optional. The input files allow comments to
be placed within the file. All comments begin with %. This character and any that follow
it on a given line are ignored by cassbeam. Depending on key, the value may be one of
five types: string, integer, double, vector, none. A string is a sequence of non-
whitespace characters not surrounded by quotes of any kind. A double value is a number
that can have a fractional part. A vector is a comma-separated list of doubles. The
`none' type expects no value. Below is a list of the allowed keys and the type of value
expected. If the range of legal values is restricted, the legal range will be contained
within brackets. Note that legal values do not imply a physical system that will generate
meaningful results! For the vector type, if a certain number of values are needed, they
will be indicated in parentheses. A required parameter will be indicated with a `*'. It
is important to realize that the secondary optical surface (i.e., the subreflector) is
defined based on the input geometry. Thus changing the feed placement will change the
geometry of the subreflector! To change parameters of the telescope without affecting the
shape of the subreflector, set the pathology parameters. Note that the order of the
parameters does not matter.

Antenna geometry parameters
feed_x The x value of the phase center of the feed. If no value is provided, 0 is
assumed. [double]

feed_y The y value of the phase center of the feed. If no value is provided, 0 is
assumed. [double]

feed_z The z value of the phase center of the feed. If no value is provided, 0 is
assumed. [double]

geom This string points to a disk file containing the primary optical surface geometry.
This file is a three column ascii text file, each containing space separated values
for r, z, and dz/dr for the antenna. There is no limit (other than your computer's
memory) to the number of lines in this file. It is assumed (but not checked!) that
the values of r start at 0 and are equally spaced. The radius, R, of the primary
is given by the value of r in the last row. Columns 1 and 2 are in meters, and
column 3 is dimensionless. [string]

hole_radius
The radius (in meters) of an unpanelled area at the center of the primary. If
omitted, no hole will be made. [double, > 0]

legapex
The z value where the legs (struts) intersect each other. Note that the legs might
terminate before reaching this point. The default value is 1.2*sub_h. [double, >
0]

legfoot
The r value where the legs (struts) intersect the primary surface. The default
value is half the antenna radius. [double, > 0]

legwidth
The effective width of the legs, used to compute blockage. Note that currently a
positive value indicates four equally spaced legs with one leg along the x axis.
If the value is negative, its absolute value is used in the blockage calculations,
but the legs are rotated 45°. If this parameter is not set, or if it is set to 0,
then no legs will be generated. [double]

name An optional name given to the antenna. If the name is ``VLBA'', then the true
strut geometry for the VLBA antennas is used rather than equispaced struts.
[string]

roughness
The RSS surface roughness in meters. This number represents the combined surface
error for the primary and secondary. If no roughness is provided, the default
value of 0 is used. [double, >= 0]

sub_h This value is the z value of the intersection of the subreflector with the z axis.
[double, > 0]

Feed pattern parameters
Note that either both feedtaper and feedangle or feedpattern must be provided.

feedangle
Sets the reference angle for the feed taper. [double, > 0]

feedpattern
The name of the file containing the pattern of the feed. This file contains two
space-separated columns of numbers: the angle in degrees and the taper in dB. The
first angle must equal 0, and the angles must be uniformly spaced. [string]

feedpatternscale
The factor by which to scale the pattern defined in feedpattern. [double, > 0]

feedtaper
This parameter sets the taper (in dB) of the feed at an angle feedangle from the
feed axis to 10^-feedtaper/10. [double, > 0]

Pathology parameters
None of the following operations change the shape of the subreflector - its geometry is
calculated before their application. Note that displacements of either the feed or the
subreflector result in a rotation of the feed that corrects for the mispointing caused by
the translations. Rotations of the feed act in addition to this correction. Composited
rotations (i.e., setting rsub_x and rsub_y are both provided), the operations on the
object being rotated proceed in reverse alphabetical order (z rotation before y rotation;
y rotation before x rotation) regardless of the order that the parameters are received.

dfeed_x
Displacement of the feed along the x axis. [double]

dfeed_y
Displacement of the feed along the y axis. [double]

dfeed_z
Displacement of the feed along the z axis. [double]

dsub_x Displacement of the subreflector along the x axis. [double]

dsub_y Displacement of the subreflector along the y axis. [double]

dsub_z Displacement of the subreflector along the z axis. [double]

focus Displacement of the feed along the feed axis. A positive value moves the feed
closer to the subreflector. [double]

rfeed_x
Rotation of the feed in degrees about the x axis. A positive value will rotate
from the z axis through the y axis. [double]

rfeed_y
Rotation of the feed in degrees about the y axis. A positive value will rotate
from the x axis through the z axis. [double]

rfeed_z
Rotation of the feed in degrees about the z axis. A positive value will rotate
from the y axis through the x axis. [double]

rsub_x Rotation of the subreflector in degrees about the x axis. A positive value will
rotate from the z axis through the y axis. [double]

rsub_y Rotation of the subreflector in degrees about the y axis. A positive value will
rotate from the x axis through the z axis. [double]

rsub_z Rotation of the subreflector in degrees about the z axis. A positive value will
rotate from the y axis through the x axis. [double]

subrotpoint
Defines the point about which the rotation of the subreflector is performed. The
contents of the vector depend on the number of elements are provided: either only
the z value, or the x and y values, or the x, y, and z values. [vector (1 or 2 or
3)]

Operating condition parameters
compute
A string to tell what output to produce. The string can be `all', `none', or a
string containing flag characters. The default value is `all', meaning produce all
possible output. `none' will produce only messages on the screen and no output
files. The characters of the general string mean the following:

a Save the aperture images;

j Save the Jones matrices in a table;

p Save the parameters;

s Save the polarized beams.

Note that the string is case insensitive. [string]

diffeff
A user supplied diffraction efficiency. If none is provided, an internal algorithm
that is not very good is used. This needs to be upgraded! [double]

freq The frequency in GHz at which the calculation will be run. [double, > 0]

gridsize
Specifies a fixed grid size. If odd, the next even number will be used. This
option overrides any setting of oversamp and is the preferred method of setting the
grid size. Setting it to a value less than 32 will result in a grid size of 32.
[integer, >= 32]

leggroundscatter
The fraction of power that scatters off the struts toward the ground. The default
value is 0.2. [double, >= 0, <= 1]

misceff
A factor of the efficiency calculation that contains ``everything else''. The user
is responsible for choosing a realistic value for this. A default of 1 (i.e.,
100%) is assumed if this parameter is not provided. [double, >= 0, <= 1]

out The prefix for all output files. The default is cassbeam. A dot will always
separate the prefix from any trailing characters. [string]

oversamp
One way of specifying the grid size. This option will make the grid on the primary
fine enough to accommodate 4*oversamp*R/lambda points. The default is 1. Note
that vastly ``undersampling'' is fine as the field is never calculated anywhere
between the feed and the aperture plane. Normally blockage calculations and
constancy of the illumination will dictate the required sampling. See gridsize for
an alternate way of specifying the grid. This parameter is ignored if gridsize is
set. [double, > 0]

pixelsperbeam
This is the approximate number of pixels that the core of the beam will occupy in
the output images. [int, > 0]

Tground
The temperature in Kelvin of the ground. The default value is 290. [double, > 0]

Trec The equivalent temperature of the receiver. This adds into the system temperature.
The default value is 50. [double, > 0]

Tsky The temperature in Kelvin of the sky. The default value is 3 for frequencies over
1 GHz, and 3 * 10^-2.5 nu for frequencies below 1 GHz. [double, > 0]

OUTPUT FILES

Up to 12 output files are generated depending on which compute options were selected at
run time. These files are listed below. The letter in brackets in the section headings
indicate which option is used to enable this file to be written. All output files begin
with the value of the input parameter out. Currently all output images are in PGM format,
which is a very simple greyscale image format supported by most unix-based image viewers.

Aperture images [a]
Three images are generated that allow the aperture field to be examined qualitatively. If
quantitative numbers are needed, the source code should be modified to export the
illumination parameters.

out.illumamp.pgm
Raster image showing the amplitude of the illumination pattern of the primary. No
blockage is done at this point. The scale is linear in flux.

out.illumphase.pgm
Raster image showing the net phase (pathlength multiplied by wave vector) at each
point on the primary. A phase gradient is removed. Portions of the image that
correspond to zero flux have an arbitrary phase.

out.illumblock.pgm
Raster image showing the blocked portion of the aperture. White means that this
part of the dish is experiences either plane wave blockage from the sky or
spherical wave blockage from the feed, and thus does not contribute to the gain of
the antenna.

Jones matrix file [j]
The Jones matrix file, out.jones.dat contains the Jones matrix (see Hamaker et al. 1996
for details) corresponding to the effect of the antenna on the incoming radiation as a
function of position on the sky. The file is organized as an eight column ascii. The
first row corresponds to the point on the image with smallest l and m. The rastering then
proceeds first with increasing l, and then with increasing m. There are a total of n^2
rows, where n is the smallest odd number greater than or equal to the gridsize used. The
matrices are rastered on a sine-projected coordinate system tangent to the sky at the beam
center, which corresponds to row number (n^2+1)/2. At the beam center the pixel scale is
given by the output parameter beampixelscale, which is stored in the output file
out.params described below.

Parameter file [p]
The parameter file, out.params is an output file in the same format as the input file,
containing all of the input parameters that were specified (even if on the command line)
as well as many output values. They are:

Aeff The effective area of the antenna [m^2]. [double]

Aeff_Tsys
The effective area of the antenna divided by the system temperature [m^2/K].
[double]

ampeff The amplitude efficiency. [double]

beampixelscale
The scale of the generated beam images [deg/pixel]. [double]

blockeff
The blockage efficiency. [double]

diffeff
The diffraction efficiency. [double]

fwhm_l The full width at half max of the beam in the l direction. [double]

fwhm_m The full width at half max of the beam in the m direction. [double]

gain The gain G of the antenna. [double]

illumeff
The illumination efficiency. [double]

peaksidelobe
The directivity of the greatest sidelobe relative to the peak directivity of the
beam. [double]

phaseeff
The phase efficiency. [double]

point_l
The l component of the pointing offset from the z axis measured in the image plane.
[double]

point_m
The m component of the pointing offset from the z axis measured in the image plane.
[double]

prispilleff
The primary spillover efficiency. [double]

program
The name of the program run, which is cassbeam. [string]

misceff
The miscellaneous efficiency. [double]

spilleff
The spillover efficiency. [double]

subspilleff
The subreflector spillover efficiency. [double]

surfeff
The surface efficiency. [double]

totaleff
The total efficiency calculated for the antenna. [double]

Tsys The system temperature. [double]

version
The software version number. [string]

Polarized beam images [s]
With the s option, cassbeam will produce 7 images of the beam showing in the four Stokes
parameters the response to an unpolarized source as a function of the position of the
source on the sky. This information is derived from the Jones matrices which are saved in
out.jones.dat. These images are meant for qualitative inspection. The Jones matrices
contain the formal output.

out.I.pgm
Stokes I - total intensity;

out.Q.pgm
Stokes Q - excess linear polarization e_1 over e_2;

out.U.pgm
Stokes U - excess linear polarization in e'_1 over e'_2

out.V.pgm
Stokes V - excess right circular polarzation over left circular polarization;

out.QI.pgm
The ratio of the Stokes Q image to the Stokes I image;

out.UI.pgm
The ratio of the Sytokes U image to the Stokes I image;

out.VI.pgm
The ratio of the Stokes V image to the Stokes I image;

AUTHOR