The SET ATM 1985|2009|OLD|NEW command switches the ATM version and the ATM command computes the atmospheric parameters. In ATM 1985, the atmosphere properties other than temperature, pressure and humidity, are fixed while they can be changed for ATM 2009 through the following SIC variables with their default values, their units and their definitions
ATM%PROFILE%TYPE MIDLATSUMMER Atmospheric type ATM%PROFILE%HUMIDITY 1.000 [%] Ground relative humidity (indicative) ATM%PROFILE%WVSH 2.000 [km] Water vapor scale height ATM%PROFILE%TLR -5.600 [K/km] Tropospheric lapse rate ATM%PROFILE%TOP 48.000 [km] Upper atm. boundary for calculations ATM%PROFILE%DP 10.000 [hPa] Primary pressure step ATM%PROFILE%DP_FACTOR 1.200  Pressure step ratio between two consecutive layersThe other input parameters of the ATM command are the following SIC variables
TEMPERATURE Outside temperature ZERO_PRESSURE Pressure at sea level. AIRMASS Number of air masses WATER Precipitable water vapor FORWARD_EFF Forward efficiency GAIN_IMAGE Receiver gain ratio TREC Receiver temperature FREQ_SIG Signal frequency (in GHz) FREQ_IMA Image frequency (in GHz)These variables can be assigned using the LET command. The remaining input variable
ALTITUDE Site altitudeis modified by the OBSERVATORY command. The output of the ATM command consists in the following variables
TRUE_PRESSURE Local pressure, derived from the site altitude and the sea level pressure by the scale height of 5.5 km TSYS The system temperature in the signal sideband TAU_H2O The water vapor opacity TAU_O2 The dry atmosphere opacity (mostly due to Oxygen, but minor constituents like Ozone may also play a role) TAU_TOT The total opacity: H2O + O2 (+ minor components for new ATM versions) EMIS_SIG The atmospheric emission (Kelvin) in the signal side band EMIS_IMA The atmospheric emission (Kelvin) in the image sideband PATH_SIG The atmospheric excess optical path length (mm) in the signal sideband PATH_IMA The atmospheric excess optical path length (mm) in the image sidebandThese variables can then be used to produce plots of atmospheric transparency, system temperatures, etc...