code as of Jan 2010:
   double Airmass = 1.0 / sinAltitude;

   double Tamb = 270; // Ambient temperature (260 - 280 K)
   double eta_feed = 0.95; // forward efficiency
   double Trx = ReceiverTemperature.getReceiverTemperature(frequencyGHz);
   Data tmp = atbl.lookup((float)frequencyGHz, wvindex);
   if (tmp == null)
     return 1.e9; // Screwy number equivalent to "no receiver"
   double Tatm = tmp.getTatm();
   double tau_zero = tmp.getTau();
   
   double f = Math.exp(tau_zero * Airmass);
   double Tcmb = 2.725; // [K]
   
   Trx  = planck(frequencyGHz, Trx);
   Tatm = planck(frequencyGHz, Tatm);
   Tamb = planck(frequencyGHz, Tamb);

   double Tsys =
      (Trx
         + Tatm
         * eta_feed
         * (1.0 - 1 / f)
         + Tamb * (1.0 - eta_feed));
   // GHz, K
   Tsys = f * Tsys + Tcmb;

comment Dec 2009:
In the ETC what is labelled "Tatm" should be called "Tsky", 
Tatm is generally understood as the physical temperature of the air column.

I don't understand the algorithm for calculating Tsys.  At fixed frequency 91 GHz, 
I vary the water vapour content which changes the opacity.  
The ETC returns the following results:

mmH20     tau    "Tatm"       Tsys
5.186    .047    14.713       54.717
2.738    .030    10.473       53.184
.4722    .016     6.807       52.053

I think I understand the variation of tau and "Tatm" as the sum of a thermal 
component and the wing of the O2 lines, keeping tau and Tatm above 0 with no mmH20.

But the span of "Tatm" is 7.9 K, so Tsys should vary by at least
this much, since Tsys = (Trcvr+"Tatm")/e^(-tau), no?  So I'm puzzled.

This topic: ALMA > WebHome > NAASC > DataServicesGroup > Jan2010Wkshop > ExposureTimeFormula
Topic revision: 2010-01-23, RemyIndebetouw
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