REFRACTIVITY FORECASTS USING ATMOSPHERIC REFRACTIVITY PROFILE GENERATOR (ARPGEN)

LEARNING OBJECTIVES: Identify applications, limitations, assumptions, and functional description of the ARPGEN product. The ARPGEN is used for two purposes:

2. To place it into the RDS for use by the electromagnetic propagation programs various

The RDS displays a profile of modified refractive index (M) with respect to height, the height of the evaporation duct, and the surface wind speed. The operator directly enters the necessary surface observation data for all except the historical option of the program; the historical option is retrieved from the permanent data base (PDB) files.

ARPGEN is used to create RDS. These data sets describe the effects of the environment on the propagation of electromagnetic (EM) energy in the microwave portion of the spectrum.

The restrictions as well as principles taken for granted in using the ARPGEN product are as follows:

. ARPGEN allows a maximum entry of 30 M-unit versus height pairs. Levels with heights >10,000 m are discarded due to insignificant refractive effects at higher altitudes.

. The standard atmospheric lapse rate is used to extrapolate for a sea-surface M-unit value.

. The evaporation duct-height algorithm assumes that entered surface weather observations are at a height of 6 m above the sea surface.

l The RDS menu selection can accommodate up to 10 refractivity data sets. As these sets are created, they are placed into higher numbered positions in the RDS. When 10 data sets are present, a newly created data set takes the place of the data set not accessed for the longest period of time.

. M-unit profiles must be entered in ascending order.

. For historical data sets, the M-unit profile is retrieved for the closest radiosonde station to the operator-selected location; the surface data are retrieved for the closest Marsden square containing data in the PDB. In many instances, these locations for data may be several hundred miles apart. Data base coverage maps are provided in the TESS (3) Operators Manual. 

. Four types of historical profiles can be created by this function; standard, surface-based duct, elevated duct, and combined surface-based and elevated duct.

. The position of the RDS (for nonhistorical profiles) is specified when the operator selects to compute rather than enter an evaporation duct height. This location will be associated with the operator-selected refractivity profile.

. The RDS (with the airborne microwave refractometer [AMR]) option accommodates five flights containing refractivity information. Different portions of a particular flight can be accessed to generate different refractivity profiles. This function will not appear in the menu if an AMR tape-reading device is not connected.

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