User Tools

Site Tools


operations:tsys_method

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

operations:tsys_method [2011/10/26 06:37] (current)
Line 1: Line 1:
 +====== Tsys and Tcal calculations based on Y-factor data ======
 +
 +System temperature includes a number of contributions:​ receiver, atmosphere, CMB, plus antenna effects such as groundspill,​ surface imperfections etc. It can be measured by comparing observations of the sky and of a known load,
 +
 +(T<​sub>​load</​sub>​ + T<​sub>​LNA</​sub>​) / T<​sub>​sys</​sub>​ = Y
 +
 +where the measured Y-factor is given in terms of the difference in measured power (in decibels),
 +
 +Y = 10<​sup>​0.1(P<​sub>​load</​sub>​-P<​sub>​sky</​sub>​)</​sup>​
 +
 +Typical values are P<​sub>​load</​sub>​-P<​sub>​sky</​sub>​ = 4.5 - 6 and hence Y = 2.8 - 4.
 +
 +Hence T<​sub>​sys</​sub>​ = (T<​sub>​load</​sub>​ + T<​sub>​LNA</​sub>​) / Y
 +
 +Load temperature is just the ambient temperature (in Kelvin), and the LNA temperature is assumed to be 45K.
 +
 +With the system temperature known, there are two ways of calculating the noise diode (cal) temperature. When measurements of the sky and load are both made with the diode included,
 +
 +T<​sub>​cal</​sub>​ = (T<​sub>​load</​sub>​ + T<​sub>​LNA</​sub>​ - Y T<​sub>​sys</​sub>​) / (Y-1)
 +
 +When the diode in included in the load measurement,​ but not in the sky-only measurement,​
 +
 +T<​sub>​cal,​2</​sub>​ = Y T<​sub>​sys</​sub>​ - T<​sub>​load</​sub>​ - T<​sub>​LNA</​sub>​
  
/home/www/auscope/opswiki/data/pages/operations/tsys_method.txt · Last modified: 2011/10/26 06:37 (external edit)