sbn cal values measured 15mar06


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):


    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/,,,