Altitude Encoders Collision Avoidance Systems Traffic Collision Avoidance Systems (TCAS)  11-58 Figure 11-130.  Modern altitude encoders for general aviation  aircraft.  Figure 11-129.  A handheld transponder test unit.  Altitude encoders convert the aircraft’s pressure altitude  into a code sent by the transponder to ATC. Increments of  100 feet are usually reported. Encoders have varied over the  years. Some are built into the altimeter instrument used in the  instrument panel and connected by wires to the transponder.  Others are mounted out of sight on an avionics rack or similar  out of the way place. These are known as blind encoders. On  transport category aircraft, the altitude encoder may be a large  black box with a static line connection to an internal aneroid.  The ever increasing volume of air traffic has caused a  corresponding increase in concern over collision avoidance.  Ground-based radar, traffic control, and visual vigilance are  no longer adequate in today’s increasingly crowded skies.  Onboard collision avoidance equipment, long a staple in  larger aircraft, is now common in general aviation aircraft.  New applications of electronic technology combined with  lower costs make this possible. Traffic collision avoidance systems (TCAS) are transponder  based air-to-air traffic monitoring and alerting systems.  When a transponder selector is set on ALT, the digital pulse  message sent in response to the secondary surveillance  radar interrogation becomes the digital representation of  the pressure altitude of the aircraft. There are 1280 altitude  codes, one for each 100 feet of altitude between 1200 feet  mean sea level (MSL) and 126,700 feet MSL. Each altitude  increment is assigned a code. While these would be 1280 of  the same codes used for location and IDENT, the Mode C  (or S) interrogation deactivates the 4096 location codes and  causes the encoder to become active. The correct altitude  code is sent to the transponder that replies to the interrogation.  The SSR receiver recognized this as a response to a Mode C  (or S) interrogation and interprets the code as altitude code.  Operating a transponder in a hangar or on the ramp does not  immunize it from interrogation and reply. Transmission of  certain codes reserved for emergencies or military activity  must be avoided. The procedure to select a code during  ground operation is to do so with the transponder in the OFF  or STANDBY mode to avoid inadvertent transmission. Code  0000 is reserved for military use and is a transmittable code.  Code 7500 is used in a hijack situation and 7600 and 7700  are also reserved for emergency use. Even the inadvertent  transmission of code 1200 reserved for VFR flight not under  ATC direction could result in evasion action. All signals  received from a radar beacon transponder are taken seriously  by ATC.  test in accordance with Appendix F. Only an appropriately  rated repair station, the aircraft manufacturer (if it installed  transponder), and holders of a continuous airworthy program  are approved to conduct the procedures. As with many radio- electronic devices, test equipment exists to test airworthy  operation of a transponder.  [Figure 11-129] Modern general aviation encoders are smaller and more  lightweight, but still often feature an internal aneroid and  static line connection. Some encoders use microtransistors  and are completely solid-state including the pressure sensing  device from which the altitude is derived. No static port  connection is required. Data exchange with GPS and other  systems is becoming common.  [Figure 11-130] A1 A2 A4 B1 B2 B4 C1 C2 C4 D4 PIN 1 PIN 4 PIN 7 PIN 8 PIN 14 POWER MOTION PIN 15