## No Nitrogen available : TREC Mode

This is what happens in OBS when used in TREC mode (specified by OBSSET CAL TREC) and when you do a CAL command without the argument COLD. In this mode, the receiver temperature is used to determine the gain (conversion from counts to temperature) of the receiver.

With this method, the parameters which have to be specified are thus

- , the hot load (also called chopper) temperature.
- , the receiver noise (DSB) temperature.
- , the external pressure.
- , the outside pressure.
- , the forward sky coupling efficiency.
- , the ratio of the gain in image band to that in the signal band.

plus a number of (semi) constant parameters (, site altitude, etc...). The elevation (and hence ) is read by the control program, as are the frequencies of the signal and image bands. All other parameters are derived by the calibration procedure.

The weather parameters may be supplied by the meteo station, specified by the OBSSET AMBIENT AUTO command. If this automatic station does not work, you have to supply by hand and . This is done by the OBSSET AMBIENT P_amb T_amb Humidity command. The is not used for the calibration, but by the drive program for the refraction correction to the telescope pointing.