Abstract: A new and improved computing apparatus for determining aircraft altitude has a pressure transducer to sense atmospheric pressure at which the aircraft is located, and from this and a preset local ground level atmospheric pressure reading, actually solve a non-linear altitude-pressure equation. The computed altitude is used in conjunction with a time measurement to determine aircraft vertical rate. The computed altitude also specifies an address in a memory circuit, with the data stored at such address being the grey code which is transmitted to others for air traffic control purposes.