sbn cal values measured 15mar06
11apr06
Why the cals were remeasured.
An experiment was done with the mars express spacecraft
using 2290 Mhz. The old cal values were only measured at 2380 Mhz (probably
around 1995). On 14oct03 the polarization stubs for the turnstile were
switched so that the OC polarization fed the amp with the lower system
temperature. When this was done the cal values for the 2 polarization's
were switched (rather than being remeasured).
The cals were measured on 15mar06. They were installed
on 10apr06. They were backdated to be valid from 14oct03 (when the
pols were switched).
Measuring the cal using
sky and absorber: (top)
Sbn has a single diode (diode 1->polA and polB). There
are no combinations of cals (like the other receivers). We used the
sky/absorber
technique
to measure the cal values. The absorber was done starting at 10:30 am.
The sky measurement followed. The observations used 3 second calon followed
by 3 second cal off. For the sky observations, blank sky was tracked. A
25 Mhz band was measured at a time (I actually thought i was using a 100
Mhz band, but i forgot that the 260 IF only supports a single band). At
2280,2270 a 100 Mhz filter centered at 2275.
The temperatures used in the computation were:
| Tabsorber |
298 K |
| Tsky+Tscattered |
18 K |
Treceiver PolA
2255-2394 PolB |
7.5 - 6.5
11.5 - 7.8 |
The 25 bands were measured 20 times at each of the
frequencies on absorber and again on sky. The ratio (CalOn-CalOff)/CallOff
was then computed for the data. A 3rd order polynomial was then fit for
all of the data (2255 to 2395). The fit was iterated throwing away outliers.
The Trcvr and Tsky + Tscattered were adjusted so
that the Tcal from sky, absorber, and skyAbsorber ratio gave consistent
results. For best alignment, a linear ramp (between 2255 and 2395
Mhz) for Trcvr was used. This only affects Tcal from sky and absorber
(Trcvr cancels in the Tcal from sky,absorber). The Cal value measured from
the sky absorber ratio was then used for the cal values.
The plots show the
results of the cal measurements (.ps 1.9mb) (.pdf
1mb):
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PAGE 1: This shows the the caldeflection/Tsys spectra for all of
the measurement. The top two are on the sky. The bottom two are on the
absorber. The vertical units are Tsys. The green line is the 3rd order
polynomial fit ( i know there are a few missing points in the middle...).
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PAGE 2: The top 2 plots show the cal value in kelvins versus
frequency. This comes from the 3rd order polynomial fit of the previous
page.
-
The black line comes from the sky absorber ratio. This was used as the
sbn cal values.
-
The red line is the Tcal measured using only
the absorber. It lies on top of Tcal from the ratio. It is not sensitive
to Tsky+Tscattered (since they aren't included). It is not sensitive to
Trcvr since Tabsorber is so much larger.
-
The blue line is Tcal from the sky. It is
sensitive to Tsky+Tscattered and to Trcvr.
-
The bottom plot is Tsys from the calOff sky measurements. Black in
polA, and red is polB.
Discussion:
The cal values decreased by about 5%. This will
lower Tsys by 5%. It will also lower the sbn gain by 5%.
The illumination of the sbn horn appeared to be lower
than the other receivers (see hpbw
measurements of the various receivers). This would give us a
lower Tsys (less spill over) and a lower gain. When plotting the
gain of all of the receivers at AO (see the second page) the
sbn receiver's gain was on the high side. This differed from the
hpbw values. The difference could be explained by the cals being a bit
high. The hpbw measurements do not use the cal values, the gain curves
do.
So the good news is that Tsys has gone down
(the bad news is that the radar equation goes as the gain^2 ).
processing: x101/sbn/cals/mar06/hcorcal/sbninp.pro,sbncmp.pro,sbnfit.pro,sbnplot.pro
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