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GBT Gain Curves and Performance data.

Gain curve data is summarized here, and references to memos are given. Click the link under "Date of Observations" for more information. The gain in Kelvin/Jy as a function of Zenith Distance = 90-Elevation (in degrees) is given by a polynomial of the form
Gain(ZD) = Gmax * (A0 + A1*ZD + A2*ZD^2 + ...)
in which the coefficients are listed in the table.

We also list the efficiency (Eff) corresponding to Gmax: Eff = Gmax/2.845

A simple model for the atmosphere was fitted by least-squares to the data:
Tsys = T0 + Tatm (1 - exp(-Tau*A)),
in which airmass A = 1/sin(Elev), T0 is extrapolated noise temperature for airmass = 0 (sum of the receiver temperature, antenna and ground contribution, etc.), Tau is the atmospheric opacity at the zenith.
The gain curve is not corrected for atmospheric opacity for L, S, C, X, Ku bands.
The gain curve is corrected for atmospheric opacity for K band and Q band.

The difference in gain as well as opacity between the two polarizations is up to 10%, which is about the uncertainty in the noise calibration temperatures (Tcals), hence not significant.

Freq(GHz) BW(MHz) Date of Observations Pol Tcal(K) Tsys(K) Tau Eff Gmax (K/Jy) Polynomial Coefficients
1.438 20 April 21 & May 3, 2004 LCP
RCP 		1.13
2.13 		18.5
18.5 		0.0081
0.0076 		0.72
0.64 		2.06
1.81 		A0
A1
A2 		0.96575
1.8256e-03
-2.4477e-05
2.240 80 April 19, 2004 LCP
RCP 		2.10
2.15 		19.5
19.5 		0.0091
0.0086 		0.67
0.62 		1.91
1.75 		A0
A1
A2 		0.97935
1.1154e-03
-1.7833e-05
4.980 80 March 8, 2004 LCP
RCP 		2.80
3.29 		20
20 		0.0077
0.0074 		0.67
0.64 		1.91
1.82 		A0
A1
A2 		0.93999
3.4857e-03
-5.0846e-05
8.420 80 June 10, 2004 LCP
RCP 		3.39
3.10 		32
31 		0.015
0.014 		0.73
0.69 		2.08
1.97 		A0
A1
A2 		0.95899
2.6531e-03
-4.1662e-05
15.360 80 September 25, 2004 LCP
RCP 		2.75
1.61 		28
31 		0.022
0.025 		0.62
0.68 		1.78
1.93 		A0
A1
A2 		0.90235
5.2533e-03
-7.1736e-05
20.0 320 March 28, 2002 (with FEM surface, no Zernikes) LCP
RCP 		5.2
4.5 		19.5
--- 		?
? 		0.63
--- 		1.78 (#)
--- (#) 		A0
A1
A2
A3
A4 		0.7804 (#)
0.02135 (#)
-1.1260e-3 (#)
2.3733e-5 (#)
-1.6419e-7 (#)
22.236 320 December 28, 2005 (with FEM surface, no Zernikes) LCP
RCP 		5.213
4.333 		50
50 		0.066
0.064 		0.59
0.63 		1.68 (#)
1.81 (#) 		A0
A1
A2 		0.815 (#)
0.00970 (#)
-1.273e-4 (#)
22.236 320 April 8, 2008 (with FEM plus 2005WinterV2 Zernike model) LCP
RCP
50
50 		0.103
0.102 		0.61 A0
A1
A2
43.124 320 Nov 22, 2003 (FEM surface model)

Condon's memo (PTCS project note 31) ** 		LCP
RCP 		6.00
5.25 		55
55 		0.072
0.072 		0.43
0.43 		1.22 (#)
1.22 (#) 		A0
A1
A2
A3 		0.6425 (#)
0.01572 (#)
-8.7609e-5 (#)
-1.7117e-6 (#)
43.124 320 Apr 11, 2005 (with 2005SpringV1 Zernike model) LCP
RCP 		9.81, 5.73
9.37, 5.13 		80
70 		0.09
0.10 		0.44
0.46 		1.25 (#)
1.31 (#) 		A0
A1
A2 		0.7611 (#)
0.00985 (#)
-1.0186e-4 (#)

(#) The marked Gmax values and polynomial coefficients have been corrected for atmospheric absorption. The others are not.
** The results quoted here for 43 GHz are from Condon's memo. Curve at 43 GHz valid only for elevation > 10°


Note that at the higher frequencies (>10GHz) the model used to correct the active surface has a significant effect on the gain curves. When a change in the surface model happens, a new determination of the gain curve will need to be done.

More info for 40-50 GHz Observing may be found on Ron's Q-band Performance page.

-- YuriKovalev - 28 Dec 2005

-- FrankGhigo - 13 Dec 2007

Attachment: sort Action: Size: Date: Who: Comment:
efficiency050411.2005SpringV1.txt action 4907 29 Nov 2007 - 22:22 ToddHunter efficiency data for 2005SpringV1 Surf. model (BXN)

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