atmosphere

An atmosphere is provided which models the air through which simulated platforms move. The default definition of atmosphere is the 1976 standard atmosphere. However, MIL-STD-210A defines several non-standard atmosphere types enumerated as Hot, Tropical, Cold, and Polar. One other atmosphere type is also allowed which elevates or depresses the standard atmosphere temperature at all altitudes below 100000 ft, in order to match a desired density altitude* at a particular geopotential altitude. The WSF simulation loader will permit the following keywords or input blocks to be specified, describing the atmosphere type(s) to be used for the scenario:

Note

The MIL-STD-210A Non-Standard Atmospheres only extend up to 100000 ft, by definition. The standard atmosphere model continues upward to approximately 200000 ft (61 km), above which the returned air density is zero. In this model, the non-standard atmospheres are extended to the same 61 km altitude, but blended beyond 100000 ft to reach the same atmosphere terminus condition. At 47 km in standard atmosphere, there is a layer where temperature does not change with increasing altitude. By assumption, all non-standard atmospheres uniformly blend their temperature profile from 100000 ft upward, to match the standard temperature at 47 km (270.65 deg K), and then ending with 249.1 deg K at 61 km.

default_atmosphere_type [standard_day | hot_day | tropical_day | cold_day | polar_day | custom | simple_dt <temperature value>]

The “standard_day” atmosphere is supplied by default. This command is global in scope, and should be specified before any types are defined that use an atmosphere. Each instance of “simple_dt” type atmosphere must use the same (static) temperature deviation from “standard_day” atmosphere, and is supplied using the global syntax above for “simple_dt”. The default for this value is +10 deg K.

In addition, any individual instance of atmosphere may override the supplied global default. Individual instances of atmospheres may apply to specific sensors and movers. Because platforms and sensors can be declared with the different atmospheres, users should use caution to ensure that they declare atmospheres which are consistent with the purposes of specific studies. To invoke a specific atmosphere for a mover or sensor definition, the ‘atmosphere_type’ keyword should be inserted into the relevant sensor or mover block. Individual instances of atmospheres are invoked by specifying:

atmosphere_type [default | standard_day | hot_day | tropical_day | cold_day | polar_day | custom | simple_dt]

The context of this command is local to an atmosphere instance. It will override the global default atmosphere type. Supplying the parameter “default” is the do-nothing option, the atmosphere type will become the default atmosphere type. Supplying the parameter “simple_dt” will specify the use of that atmosphere type, however the value of temperature deviation “dT” is global for all instances of this atmosphere type, and cannot be overridden here.

Note

Be aware that once an individual atmosphere is declared to be of a specific type, the default_atmosphere_type will not be used, and the simulation may have conflicting atmospheric attributes.

This command creates an output file called “UtAtmosphereTables.lis” in the current directory that tabulates on even increments of either 500 meters or 5000 feet. The tabulation includes altitudes, temperatures, pressures, density, sonic velocity, and water density.

atmosphere_table … end_atmosphere_table
atmosphere_table
   <height-m> <temperature-K> <pressure-Pa>
   <height-m> <temperature-K> <pressure-Pa> <density-kg-per-m3>
   <height-m> <temperature-K> <pressure-Pa> <density-kg-per-m3> <sonic-speed-mps>
end_atmosphere_table

This block sets the default_atmosphere_type to ‘custom’, and will build up an atmosphere from scratch based on the table input. For ease of both input and processing, SI units are assumed. A maximum of 512 altitudes can be defined, and they must be defined in order of ascending altitude.

The fourth and fifth columns are optional for any given datum. If explicit density and/or speed-of-sound values are not provided for an altitude, the data will be estimated based on the input temperature and pressure, in combination with the standard sea level information.

contrailing_altitude_floor <length-value>
contrailing_altitude_ceiling <length-value>

The previous two commands adjust the band of the atmosphere where contrails may be formed. By default, this band resides between 26,000 feet and 35,000 feet above the ellipsoid. There are no guards against unusual inputs, but setting contrailing_altitude_floor above contrailing_altitude_ceiling will effectively eliminate the contrailing band.

atmosphere_calibration … end_atmosphere_calibration
atmosphere_calibration
   altitude <length-value>
   density <mass-density-value>
   temperature <temperature-value>
end_atmosphere_calibration
altitude <length-value>
density <mass-density-value>
temperature <temperature-value>

The preceding block type will set the default_atmosphere_type to ‘simple_dt’, and will choose a temperature deviation from standard that provides the desired air density OR temperature at the specified geopotential altitude. (This utility is provided to assist in matching actual flight test data.) Following is a working example of the above block, and corresponding simulation output:

atmosphere_calibration
  # This block will set the default_atmosphere_type_ to simple_dt, and choose  the delta temperature
  # needed to match the given air density or temperature at a specified altitude.   A printed block
  # in the output stream will indicate the delta temperature (from standard) value selected.
  # "altitude" is required, and one only of either "density" or "temperature" is required.
  altitude  11000 ft
  density   0.85896 kg/m3
  #temperature 20 F
end_atmosphere_calibration

Note

Per the atmosphere_calibration block provided, selected default atmosphere is simple_dt 5.46253 deg K