Gillham Code
Encyclopedia
Gillham code is a digital
code
using an eleven-wire interface that is used to transmit uncorrected barometric
altitude
between an encoding altimeter or analog air data computer
and a transponder
. It is a modified form of a Gray code
and is sometimes referred to as a "Gray code" in avionics literature.
The code was named in memory of the late Ronald Lionel Gillham, the UK's representative to the IATA committee developing the specification for the second generation of Air Traffic Control System, known in the UK as "Plan Ahead", who died suddenly in March 1968. Mr Gillham was said to have had the idea of using a modified Gray code while at a family dinner.
The Gillham code's only application is in the transmission of height information from an altitude encoder to an secondary surveillance radar (SSR) transponder as used in aircraft.
A common configuration is for the transponder to supply power to the encoder only in the altitude reporting (mode C) setting. This could be problematic if mode C is selected in flight as the warm-up time would start from that point. Height information may not then be transmitted to the SSR station for up to 10 minutes. The purpose of the encoder supply switching via the transponder is to reduce power consumption. Typically an encoder will require about 5 W whilst the heater is engaged. The heater cycles on and off during operation and is controlled by the conditioning circuitry. The duty cycle varies according to the ambient temperature conditions.
Light aircraft electrical systems are typically 12 V or 28 V. To allow seamless integration with either, the encoder uses a number of open-collector (open-drain) transistors to interface to the transponder. The height information is represented as 11 binary digits in a parallel form using 11 separate lines designated D2 D4 A1 A2 A4 B1 B2 B4 C1 C2 C4. The Gillham code contains a D1 bit but this is unused in practical applications.
Different classes of altitude encoder do not use all of the available bits. All use the A, B and C bits; increasing altitude limits require more of the D bits. Up to and including 30700 ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4. Up to and including 126700 ft requires D4 and D2. Note that D1 is never used.
The datum used by altitude encoders is −1200 ft although many will not output anything lower than −1000 ft. Negative flight level
s are included in the code to permit altitude measurement at low levels when the ambient pressure is high.
Note that the altitude code output by a standard altitude encoder is a pressure altitude. That is to say, it is always with respect to a pressure datum of 1013.2 mBar (hectopascals) or 29.92 inHg. It does not indicate the height above sea level (altitude) or the ground (height). Pressure altitudes are referred to as flight levels and are expressed to the nearest 100 ft. For clarity here is a sample of the Gillham code from 0 ft to 1000 ft; note that 1000 ft is equivalent to a flight level of 10.
Bits D1–B4 use a standard Gray code to store the number of 500 ft increments.
Bits C1–C4 use a non-linear reflected Gray code to store the number of 100 ft increments +1. The values when converted to decimal follow this repeating pattern: 1 2 3 4 7 7 4 3 2 1 1 2 3 4 7 ....
The C bits must be converted from Gray to standard binary, the 7 changed to a 5 and the reflected order changed when the 500 ft increment is even.
The number resulting from combining the above gives the flight level above the minimum datum (−1200 ft). This offset must be removed to give a corrected altitude value.
The Gillham code can be decoded using various methods. Standard techniques use hardware or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
using an eleven-wire interface that is used to transmit uncorrected barometric
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...
altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
between an encoding altimeter or analog air data computer
Air data computer
An air data computer is an essential avionics component found in modern glass cockpits. This computer, rather than individual instruments, can determine the calibrated airspeed, Mach number, altitude, and altitude trend from input data from sensors such as an aircraft's pitot-static system,...
and a transponder
Transponder (aviation)
A transponder is an electronic device that produces a response when it receives a radio-frequency interrogation...
. It is a modified form of a Gray code
Gray code
The reflected binary code, also known as Gray code after Frank Gray, is a binary numeral system where two successive values differ in only one bit. It is a non-weighted code....
and is sometimes referred to as a "Gray code" in avionics literature.
The code was named in memory of the late Ronald Lionel Gillham, the UK's representative to the IATA committee developing the specification for the second generation of Air Traffic Control System, known in the UK as "Plan Ahead", who died suddenly in March 1968. Mr Gillham was said to have had the idea of using a modified Gray code while at a family dinner.
The Gillham code's only application is in the transmission of height information from an altitude encoder to an secondary surveillance radar (SSR) transponder as used in aircraft.
Altitude encoder
An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics. The pressure sensor is often heated, which requires a warm-up time during which height information is either unavailable or inaccurate. Older style units can have a warm-up time of up to 10 minutes; more modern units warm up in less than 2 minutes. Some of the very latest encoders incorporate unheated 'instant on' type sensors. During the warm-up of older style units the height information may gradually increase until it settles at its final value. This is not normally a problem as the power would typically be applied before the aircraft enters the runway and so it would be transmitting correct height information soon after take-off.A common configuration is for the transponder to supply power to the encoder only in the altitude reporting (mode C) setting. This could be problematic if mode C is selected in flight as the warm-up time would start from that point. Height information may not then be transmitted to the SSR station for up to 10 minutes. The purpose of the encoder supply switching via the transponder is to reduce power consumption. Typically an encoder will require about 5 W whilst the heater is engaged. The heater cycles on and off during operation and is controlled by the conditioning circuitry. The duty cycle varies according to the ambient temperature conditions.
Light aircraft electrical systems are typically 12 V or 28 V. To allow seamless integration with either, the encoder uses a number of open-collector (open-drain) transistors to interface to the transponder. The height information is represented as 11 binary digits in a parallel form using 11 separate lines designated D2 D4 A1 A2 A4 B1 B2 B4 C1 C2 C4. The Gillham code contains a D1 bit but this is unused in practical applications.
Different classes of altitude encoder do not use all of the available bits. All use the A, B and C bits; increasing altitude limits require more of the D bits. Up to and including 30700 ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4. Up to and including 126700 ft requires D4 and D2. Note that D1 is never used.
The datum used by altitude encoders is −1200 ft although many will not output anything lower than −1000 ft. Negative flight level
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
s are included in the code to permit altitude measurement at low levels when the ambient pressure is high.
Note that the altitude code output by a standard altitude encoder is a pressure altitude. That is to say, it is always with respect to a pressure datum of 1013.2 mBar (hectopascals) or 29.92 inHg. It does not indicate the height above sea level (altitude) or the ground (height). Pressure altitudes are referred to as flight levels and are expressed to the nearest 100 ft. For clarity here is a sample of the Gillham code from 0 ft to 1000 ft; note that 1000 ft is equivalent to a flight level of 10.
| ALTITUDE | D1 | D2 | D4 | A1 | A2 | A4 | B1 | B2 | B4 | C1 | C2 | C4 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 0 |
| 100 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 |
| 200 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 0 | 0 |
| 300 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 0 | 0 |
| 400 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 |
| 500 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 |
| 600 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 1 |
| 700 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 |
| 800 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 1 |
| 900 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
| 1000 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 0 |
Decoding
The Gillham code is an unusual mix of codes. It is a parallel binary code that uses a Gray code to ensure that there are not multiple bit changes between adjacent altitudes. The bit pattern is split into those bits used to indicate the number of 500 ft increments and those used to indicate the number of 100 ft increments. The split is as follows.Bits D1–B4 use a standard Gray code to store the number of 500 ft increments.
Bits C1–C4 use a non-linear reflected Gray code to store the number of 100 ft increments +1. The values when converted to decimal follow this repeating pattern: 1 2 3 4 7 7 4 3 2 1 1 2 3 4 7 ....
The C bits must be converted from Gray to standard binary, the 7 changed to a 5 and the reflected order changed when the 500 ft increment is even.
The number resulting from combining the above gives the flight level above the minimum datum (−1200 ft). This offset must be removed to give a corrected altitude value.
The Gillham code can be decoded using various methods. Standard techniques use hardware or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.