Using atmospheric weather models to improve radar measured altitude

This report discusses how the atmosphere affects radar and barometric measurement of altitudes of airborne vessels. One may transform each kind of measurement to a corrected dataset in units of meters using atmospheric predictions.

Calibration of barometers are relative to mean sea level and ‘standard atmosphere’. By employment of a weather-modelled relationship between latitude, longitude, barometric calibrated pressure and geometric height in units of meters, barometer pressure converts to correct reading in meters.

Radar beams bend through the atmosphere due to a vertical refractive index gradient. If weather predictions are unavailable, the standard so-called ‘effective earth radius’ compensation is usually employed, which corresponds to a ‘standard’ constant refractive index gradient. We compare usage of this constant to employing an averaged weather predicted refractive index gradient between radar and target.

We also make calculations for illustration of the effect of relative positioning of radar and air vessels at different altitudes and distances. At long distances between radar and observed air vessel we find that use of atmospheric prediction data correction often means the order of 1000 meters less subtraction of altitude than by using ‘standard’ refractive index gradient correction, i.e. the altitude of the air vessel is in reality up to 1000 meters higher. In general, a large error and difference between barometric and radar based measurement is present if actual atmospheric conditions are not accounted for.