Tsys during az swings with dome at 19 deg, sun setting.

26dec,00

A number of birdies had popped up at lband close to 1420 Mhz. To get an idea of where the were coming from, a number of azimuth swings were done from azimuth 270 to 630 degrees with the dome at 19 degrees za. Lband narrow was used with a 3 MHz band about 1420 Mhz. After looking at the data some interesting system temperature dependencies with azimuth were found.
The azimuth spun at .4 degrees/second and data was integrated for 10 second dumps. A clean part of the band between 1419 and 1420 Mhz was used to measure the system temperature. Since no cals were used, I used 35 and 32 degrees K for the average system temperature at 19 degrees za.  The plots show the total power 1419-1420 Mhz in pol A and pol B for the multiple spins.
• Figure 1 top shows the system temperature versus AST hour of day. Sunset was about 17:55.
• Figure 1 bottom shows the system temperature plotted versus the dome azimuth angle. The positions perpendicular to the triangle sides are flagged ( the triangle is oriented 2.87 degrees east of north). There are increases in system temperature every 120 degrees that are perpendicular to the triangle sides as well as a large increase at about az  60 degrees. The system temperature increase every 120 degrees remains constant for many azimuth spins.
• Figure 2 top plots the zenith angle of the sun versus the azimuth of the sun while the spins were taken.
• Figure 2 middle shows the system temperature versus the sun azimuth angle.
• Figure 2 bottom is the system temperature versus the sun zenith angle.
• Figure 3 plots the system temperature versus the (dome az - sun az) and (dome za - sun za). The  tsys increases every 120 degrees do not repeat as well as when they are plotted versus azimuth. This implies that the reason is tied to the triangle and not the azimuth arm.
The large increase in tsys at AST 16.6 and 16.95 hours is caused by the sun. Some possible scattering mechanism for this to get into the receiver are:
1. The sun hits a side of the triangle and then scatters down into the beam.

2. The peaks increase as azsun approaches 62.87 degrees but they aren't symmetric about it. The platform is not blocked by the hills so it should continue scattering until sunset (about 17:55).
3. A specular reflection from the sun off of the ground screen goes into the dome or up to the platform then down into the beam.

4. The ground screen is tilted at 25 degrees from the vertical. The sun za was 74 and 78 degrees at the peaks. This would send a specular reflection from the sun to ( (25-16)+25)=34 deg and ((25-12)+25)=38 deg za. The edge of the dish is 35 degrees za so this would shoot up over the dome at 19 degrees and probably hit the platform. The ground screen went into shadow from the surrounding hills at about 17:10. This could explain why the peaks stopped after 17:00.
Number 2 above is the most likely explanation. It would be interesting to do some azimuth swings earlier in the day with the sun low in the horizon to see if the side of the triangle is involved in this scattering. The large standing waves that people have seen during sun rise and sunset are probably connected with this scattering (we should check to see if they correlate with azimuth angles perpendicular to the sides of the triangle).

The small bumps every 120 degrees do not decrease in amplitude even after the sun has set. They probably come from ground radiation scattering up off the triangle side and then into the beam. Az swings late at night could verify that the sun is not involved.

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