Performance of the GBT at 20.0 GHz

Measurements done March 28, 2002 by Ron Maddalena.
Here transcribed by F. Ghigo 28 July 2005.

The active surface setting model in normal use at K band did not change between March 2002 and December 14, 2006, thus the gain curve should be the same throughout that period.

We think that only the LCP channel was used for the gain curve.

The raw project data is in /home/archive/early-data/tape-1/pnt_highgreg_17/

Freq(GHz)	BW(MHz)	Date of Observations	Pol	Tcal(K)	Tsys(K)	Tau	Gmax (K/Jy)	Polynomial	Coefficients
20.0	320	March 28, 2002 with FEM surface corrections	LCP RCP	5.2 4.5	19.5 19.5	0.02 0.02	1.78 1.78	A0 A1 A2 A3 A4	0.7804 0.02135 -1.1260e-3 2.3733e-5 -1.6419e-7
20.0	320	March 28, 2002 with no surface corrections	LCP RCP	5.2 4.5	19.5 19.5	0.02 0.02	1.71 1.71	A0 A1 A2	0.444 0.0279 -3.650e-4

Cross scans were done on source NGC7027 in beam switching mode. Hence the Tcals are in effect the average of Tcals of the two beams. The assumed flux density of NGC7027 is 5.568 Jy at 20.0 GHz, based on Ott et al 1994 (Astron.Astrophys. 284, 331).

For any antenna the relation between flux density (S) of an unresolved radio source and the antenna temperature (Ta) is given by:

  
    S = 2kTa/(e A)

  where k is Boltzmann's constant, e is the aperture efficiency,
  and A is the aperture geometric area. 

  Converting to units in which S is in Janskys, Ta in Kelvin, 
  and A in sq. meters, we have:

    S(Jy) = 2761 Ta/(e A)

  Defining the Gain (G) as: G = Ta/S, then we have, for the GBT
  with A=7854 sq.m.:

    G = 2.845 e

The gain G as function of elevation with the active surface control engaged is plotted here.

Fit to Uncorrected surface