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== Altitude Encoder ==
An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics.<ref name="Ameriking Encoder"> [http://www.ameri-king.com/altitude_encoder.html Ameriking AK-350 Altitude Encoder] </ref><ref name="ACK Encoder"> [http://www.ackavionics.com/products.htm ACK A-30 Altitude Encoder] </ref> 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. 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.<ref name="Shadin Manual">[http://www.shadin.com/service/manuals/OP8800TC.pdf Shadin 8800-T Altitude Encoder Operating Manual]</ref>
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 5W whilst the heater is engaged. The heater cycles on an off during operation and is controlled by the conditioning circuitry. The duty cycle varies according to the ambient temperature conditions.{{Citation needed|date=December 2010}}
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Light aircraft electrical systems are typically 12V or 28V. 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.<ref name="Code Explanation"> [http://www.airsport-corp.com/modec.htm Explanation of Transponder Coding]</ref> 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 & C bits, increasing altitude limits require more of the D bits. Up to and including 30700ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700ft requires D4. Up to and including 126700ft requires D4 & D2. Note that D1 is never used.<ref name="Code List"
The datum used by altitude encoders is -1200ft although many will not output anything lower than -1000ft. Negative flight levels are included in the code to permit altitude measurement at low levels when the ambient pressure is high.{{Citation needed|date=December 2010}}
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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 500ft increments and those used to indicate the number of 100ft increments. The split is as follows.{{Citation needed|date=December 2010}}
Bits D1 - B4 use a standard Gray code to store the number of 500ft increments.<ref name="Code Explanation"
Bits C1 - C4 use a non-linear reflected Gray code to store the number of 100ft 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 ....{{Citation needed|date=December 2010}}
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