Gillham-Code

The Gillham code is a Gray code with exactly twelve binary digits, which is used in the aviation field for transponders for coding and for the automatic transmission of the barometric altitude of the flying object to ground stations. The code is set by the Federal Aviation Administration (FAA) in its construction.

The Gillham code is like any Gray code a " single step code" (English: one-step code). This means that adjacent altitude levels always distinguish only a single binary digit in the binary representation of the height information. If in the transfer more than one location have changed, the value at the receiver can be discarded because the last value obtained was then corrupted.

The altitude reading is done in the traditional aviation in the outdated unit length feet (ft), with one foot corresponds to about 0.3048 m. The thereby transmitting values ​​refer to 1013.25 hPa and range from -365.76 m to 38.618 km in steps of 30.48 m. The coding is done according to the following scheme: