"Exposure" calculation for the GBT.
-----------------------------------

The thermal rms noise level for GBT observations
(not corrected for atmospheric attenuation or antenna or beam efficiencies)
is given by 

    Trms(Kelvin) = K1 * Tsys / sqrt( K2 * teff * npol * (BW/Nchan) )

    Srms(Jansky) = Trms/Gain

    where 
	K1 depends on the backend sampling efficiency (see table)
	Tsys is the system temperature (typical values given below)
	K2 depends on channel weighting (1.21 for uniform, 2.0 for hanning)
	teff is the effective ingetration time.
	npol is the number of polarization channels to be averaged (1 or 2).
	BW is the bandwidth.
	Nchan is the number of spectral-line channels across the band.
	Gain is system gain in K/Jy
	

  K1 and K2 depend on the backend mode:
     -----------------------------------
     Backend               K1       K2
     ACS: 9level         1.032       2
     ACS: 3level         1.235       2
     SpectralProcessor   1.3       1.21
     DCR (continuum)     1.0       1.0
     -----------------------------------

 teff depends on switching mode:
    ----------------------------------------------------------
    Mode                                    teff
    ---------------------------     ------------------------
    On/Off position switching       ( ton * toff)/(ton+toff)
    In-band frequency switching      ton/sqrt(2)
    Nod between 2 beams              ton
    Mapping                          time per pixel
    ----------------------------------------------------------

 For continuum observations, Nchan=1, and BW depends on 
 IF filter choice and backend setup.  
 Possible values are:  
	12.5, 20, 40, 50, 80, 200, 240, 320, 800, 1280, 4000 MHz.

 Note that for continuum observations, 1/f noise in the receiver
 exceeds the thermal rms by more than a factor of 10.
	

 For spectral line observations, see the descriptions of the
spectral processor and spectrometer for the possible choices
of BW and Nchan.


---------------------------------------
  Typical Values of Tsys and Gain

  Obs.Frequency    Tsys      Gain
  -------------    ----      -----
   342 MHz          70 K      2.0 (K/Jy)
   450              57        2.0
   600              48        2.0
   800              25        2.0
  1070              22        2.0
  1-2 GHz           20        1.8
  2-3 GHz           22        1.9
  4-6 GHz           25        1.8
  8-10 GHz          30        1.8
  12-15 GHz         30        1.7
  18-22 GHz         35        1.5
  22-26 GHz         35        1.5
  30-37 GHz         45        1.3?
  43 GHz            60        1.0
  48 GHz            80        1.0
  50 GHz           130        1.0
---------------------------------------