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Add table for clarity and additional information on transponder and encoder operation. Hopefully I'm getting the hang of this now :-) I will put some more ref & links in later. |
Add Ameriking & ACK references. |
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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.
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.
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