[evlatests] Tsys variation with elevation

[Rick Perley]

     Observations taken overnight on 10 February were primarily intended 
to measure the antenna gain variation with elevation. But these are also 
useful to measuring the rise in Tsys with elevation.   The Tsys system 
worked very well for nearly all antennas and on all bands (notable 
exceptions will be identified later).  Weather was clear and calm 
throughout.  I observed two sources rising from 10 to 90 degrees, and 
two others setting from 90 to 10 degrees, interleaved.  Agreement 
between ascending and descending sources was impressively good.

     I plotted up the Tsys vs elevation curves.  The rise with 
decreasing elevation is of course quite obvious -- this is due to a 
combination of increasing atmospheric emission and ground spillover.  It 
is harder than you may think to separate these... (see the old memos by 
Bob Hayward and me on this topic).

     Below is a short table, giving the typical *increase* in system 
temperature at elevations of 10, 20 and 30 degrees compared to 90 
degrees.  The ranges given mostly reflect the uncertainty in the Tcal 
values, except at Q-band, where it is due to the rapidly increasing 
opacity at the upper end of the band.  (Observations at Q-band were made 
at 42 and 48 GHz).

Band      T10          T20              T30

------------------------------------------------------

L            12 -- 15    5 -- 6           2 -- 3

S            18 -- 25    5 -- 9           2 -- 4

C            15 -- 18    4 -- 5           2 -- 3

X            10 -- 15    3 -- 5           2 -- 3

Ku          17 -- 20    6 -- 7           3 -- 4

K            30 -- 40    11 -- 14       5 -- 8

Ka          35 -- 45     10 -- 16      4 -- 9

Q            60 -- 120   25 -- 60     12 -- 35

----------------------------------------------------------

A few clearly erroneous values are noted:

1) ea08 at C-band claims a system temperature of ~15K.  It should be 
nearly twice that -- the Tcal values must be wrong.  (Also, the 
atmospheric rise is about half of what it should be).

2) ea26 at Ku-band claims a system temperature of 70K, and its 
temperature rise with elevation is about twice the other antennas.  
Thus, the Tcal values here are incorrect.

3) ea03 at Q-band claims a system temperature of about 400K. Since the 
atmospheric component here is about right,  the problem must lie with 
the receiver.