  Aircraft trajectory prediction requires the determination of the atmospheric
properties (pressure, temperature, and density) encountered by the aircraft
during its flight. This is accomplished by employing a tabulated prediction
published by a meteorological service, a static atmosphere model that does not
consider the atmosphere variation with time or horizontal position, such as the
International Civil Aviation Organization (ICAO) Standard Atmosphere (ISA), or
a variation to the later so it better conforms with the expected flight
conditions. This article proposes an easy-to-use quasi static model that
introduces temperature and pressure variations while respecting all the
hypotheses of the ISA model, resulting in more realistic trajectory predictions
as the obtained atmospheric properties bear a higher resemblance with the local
conditions encountered during the flight. The proposed model relies on two
parameters, the temperature and pressure offsets, and converges to the ISA
model when both are zero. Expressions to obtain the atmospheric properties are
established, and their dependencies with both parameters explained. The author
calls this model INSA (ICAO Non Standard Atmosphere) and releases its C++
implementation as open-source software.
