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
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.
```
Topic revision: r1 - 2010-01-23, RemyIndebetouw
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