I want to get an idea of the 'radio shad ... / 2014-04-16

2014-04-16 I want to get an idea of the 'radio shad ... 6 years ago
I want to get an idea of the 'radio shadow' around our backyard to get a better idea of the minimum elevation to receive from and transmit to amateur radio satellites. Since there still is a gps receiver on the roof of the shed and the earlier ntp experiments aren't running at the moment I decided to stop ntp and log the $GPGSV GPS satellites in view messages from the gps unit. My idea is that the radio signals from GPS satellites get obstructed by houses at least the same as UHF signals, so a GPS satellite reception plot will be interesting. Something like the VisualGPS plot I made at a previous house with a different GPS unit. Note that the plotted satellite tracks are way outside the plotted contour which I recall was a nice approximation of the view during the test.

Now to get this data plotted with gnuplot in a polar plot. I found out the orientation of $GPGSV messages (true north is 0 degrees, east is 90 degrees, south is 180 degrees, west is 270 degrees) does not match the azimuth range available by the polar plot in gnuplot (0 degrees is to the right, 90 degrees is up, 180 degrees is to the left). And the horizon is 0 in $GPGSV messages and maximum range in gnuplot. Time for some perl massaging of the $GPGSV lines to gnuplot orientation:
#!/usr/bin/perl -w

use strict;

while (<>){
    chomp;
    if (/^\$GPGSV,\d+,\d+,\d+,([\d,]+)\*[0-9A-Z]{2}$/){
        my @fields=split(/,/,$1);
        while ($#fields>0){
            my $sv=shift @fields;
            my $elevation=shift @fields;
            my $azimuth=shift @fields;
            my $signal=shift @fields;
            if ($signal){
                warn sprintf "SV %d elevation %d azimuth %d signal %d\n",$sv,$elevation,$azimuth,$signal;
                $azimuth=90-$azimuth;
                if ($azimuth<0) {
                    $azimuth+=360;
                }
                printf "%3d %3d\n",$azimuth,90-$elevation;
            }                   
        }               
    }           
}       
And indeed we have data:
SV 33 elevation 27 azimuth 205 signal 38
SV 29 elevation 83 azimuth 100 signal 44
SV 31 elevation 48 azimuth 227 signal 45
SV 21 elevation 47 azimuth 169 signal 44
SV 25 elevation 29 azimuth 122 signal 41
And azimuth/elevation in a file that gnuplot can handle:
245  63
  8   9
283  40
226  44
326  64
The azimuth/elevation data, modified for gnuplot. And the next step is a gnuplot plotscript:
set size square
set angles degrees
set polar
set grid polar 30
set xtics 30
unset border
unset param
set xrange[-90:90]
set yrange[-90:90]
set rrange[0:360]
set trange[0:90]

set title "GPS satellite tracks"
set xlabel "Azimuth"
set ylabel "Elevation"
set terminal png size 600,600
set output "gpsazel.png"
plot "gpsazel.dat" using 1:2 notitle
Which indeed gives a nice plot of some recent data.

Main conclusion: this sirf star II gps is 'too good' for this application. For example, one measurement:
SV 5 elevation 4 azimuth 86 signal 37
Satellite 5 seen at an elevation of 4 degrees above the horizon in easterly direction with a signal/noise ratio of 37 dB. There are high buildings (4 floors) in the easterly direction so I think I'm seeing the gps receiver being way too good at this.

The good part is that I'm not the first one to think of this: GPS Skyline: A Panorama in 1.6GHz Microwave-"Light" which suggests I need to find the right cutoff value for my type of GPS unit.

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