Optically coupled digital altitude encoder for general aviation altimeters

An optically coupled pressure altitude encoder which can be incorporated into commercially available inexpensive general aviation altimeters was successfully developed. The encoding of pressure altitude is accomplished in 100-ft (30.48-m) increments from -1000 to 20,000ft (-304.8 to 6096 m). The prototype encoders were retrofitted into two different internal altimeter configurations. A prototype encoder was checked for accuracy of transition points and environmental effects. Each altimeter configuration, with the encoder incorporated, was laboratory tested for performance and was subsequently flight-tested over the specified altitude range. With few exceptions, the assembled altimeter-encoder met aeronautical standards for altimeters and encoders. Design changes are suggested to improve performance to meet required standards consistently.