Calculation of Ground and Sky Noise Components for the Outdoor Test Building

Introduction

This web page describes a program written in C/C++ that computes the expected noise components (cosmic background, galaxy, atmosphere, and ground radiation) seen by an antenna pointed at the zentith in the Green Bank Outdoor Test Building. The antenna's reception power pattern is an ASCII output file from CST in the format shown below. Two values of noise from ground radiation are computed, one assumes a constant elevation horizon that can be specified in an optional command line argument, and the other uses a surveyed horizon stored in a file that specifies the horizon height as a function of azimuth as seen from the top of the outdoor test building. The cosmic background temperature is assumed to be 2.72 K. The galaxy radiation is from the Haslam sky map at 408 MHz scaled to the specified frequency with a spectral index of -2.7. The atmospheric radiation is specified by a modified secant(Z) function with a value of 1.8 Kelvin at the zenith.

The code, executable, and data files can be found in a zip file (DipoleNoise.zip) on the Green Bank network at ~rfisher/Applications/PAFonGBT/Papillon. The executable in this zip file is compiled for a Windows computer, but it should be possible to compile it for Linux with minor modification.

Command Line Parameters and Defaults

Executing the program without any command line parameters with show the parameter options and defaults.

Usage: dipole_noise cst_file_name [-h d.d] [-t h.hhh] [-f mmmm.m] [-a ddd.d]
  Options:
    -h average elevation of horizon in degrees (default = 2.0)
    -t local siderial time in decimal hours (default = 0.0)
       used for galactic background calculation
    -f frequency in MHz (default = 1612.0)
       used to scale galactic background by f^-2.7 spectrum
    -a azimuth of antenna pattern zero point (phi = 0.0)
       in degrees (default = 0.0)
       used for galactic background calculation

The only parameter not defaulted is the name of the file that contains the power pattern of the antenna under test in a format exported by CST. Three example files in this format are included in the .zip file with names dipole20cm....txt.

The -h parameter is used only to compute the noise from the ground from an ideal horizon of constant elevation. Comparison of this noise temperature value with the on computed from the surveyed horizon shows how much noise may be coming from nearby objects that extend above the horizon. These objects are assumed to be black-body absorbers so their noise contribution is probably an upper limit. The -t parameter specifies the local siderial time of the noise measurement used to compute the orientation and therefore the noise contribution from the the Galaxy's non-thermal emission. The -f parameter is the measurement frequency used by the program to scale the non-thermal noise emission from the galaxy. Finally, the -a parameter can be used to specify the azimuthal orientation of the antenna under test. This is the direction relative to true north of the 0.0 degree Phi direction in the CST antenna pattern. East azimuth is +90 degrees.

Example Output

Below is an example command line and the resulting output. Note that the "Ground Temp (constant horizon)" is not included in the "Total".
dipole_noise dipole20cmshield450mmFFdata.txt -t 13.6 -f 1420

CST File: dipole20cmshield450mmFFdata.txt
Horizon:  2.0 degrees
LST:      13.6000 hours
Freq:     1420.0 MHz
PatAz:    0.0 degrees

Ground Temp:      1.27 K (constant horizon)
Ground Temp:      5.65 K (survey horizon)
Atmosphere Temp:  2.51 K
CBR Temp:         2.72 K
Galaxy Temp:      0.75 K
                -------
         Total:  11.63 K

CST File Format for Antenna Pattern

Theta [deg.]    Phi [deg.]     Dir.Abs [dBi  ]    Dir.Th [dBi  ]    Dir.Ph [dBi  ]      Left [dBi  ]     Right [dBi  ] 
--------------------------------------------------------------------------------------------------------------------------
   0.000           0.000           8.029e+000        8.029e+000       -4.369e+001        5.019e+000        5.018e+000
   5.000           0.000           8.781e+000        8.781e+000       -5.377e+001        5.771e+000        5.771e+000
  10.000           0.000           7.785e+000        7.785e+000       -5.255e+001        4.773e+000        4.776e+000
  15.000           0.000           8.246e+000        8.246e+000       -4.303e+001        5.237e+000        5.235e+000
  20.000           0.000           7.834e+000        7.834e+000       -5.193e+001        4.823e+000        4.823e+000
  25.000           0.000           6.694e+000        6.693e+000       -5.216e+001        3.685e+000        3.681e+000
  30.000           0.000           6.526e+000        6.526e+000       -4.600e+001        3.520e+000        3.511e+000
  35.000           0.000           4.836e+000        4.836e+000       -5.769e+001        1.824e+000        1.827e+000
  40.000           0.000           3.224e+000        3.224e+000       -5.414e+001        2.098e-001        2.176e-001
  45.000           0.000           2.317e+000        2.317e+000       -4.416e+001       -6.908e-001       -6.952e-001
etc., etc.