TABLE OF CONTENTS Inertial navigation systems (INS) alone cannot be used for estimating altitude or vertical speed (rate of change of altitude) because of the associated numerical instability. Without an external aid, the computations of the INS vertical channel variables are bypassed and replaced with nominal values for altitude and vertical speed.
Alternatively, an air data system would complement the inertial navigation system for estimating the vertical channel variables. Since the early days of flight, air data systems have been part of aircraft navigation and virtually all aircraft are equipped with them. They have preceded the employment of inertial navigation systems as they provide crucial parameters pressure altitude, vertical speed, density altitude and relative to wind airspeed without which air navigation is impractical or unsafe. Thus we can see that the air data system complements and integrates well with the inertial navigation system.
Typically, an air data suite would comprise a free stream air pressure sensor, a pitot tube and an outside air temperature probe mounted on board of craft. The sensor data, when processed, can provide the craft altitude and the air speed. The free stream air pressure - also called the static pressure - is completely oblivious to the presence of any moving object and varies nonlinearly with the craft's altitude above ground. On the other hand, the air pressure that results from the air impact on the nose of a flying aircraft called the stagnation pressure as it brings the air velocity to zero ...
