### ASTRO interface

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 layers

The 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 altitude

is 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 sideband

These variables can then be used to produce plots of atmospheric transparency, system temperatures, etc...