Hi BOAC,
Full details are in 737-SL-34-175 “AIR TRAFFIC CONTROL (ATC) SYSTEM ELEMENTARY SURVEILLANCE, ENHANCED SURVEILLANCE, AND EXTENDED SQUITTER UPGRADE”
To summarise: Mode S is being introduced in 2 phases - Elementary Surveillance (ELE) and Enhanced Surveillance (EHS).
In ELE the Flight ID, number or call sign if not a number eg “EZY2AB”, is used to supplement the mode A code because ATC are running out of numbers! From the SB “Other components of ELE also require the transponder to support Surveillance Identifier (SI), Mode S address reporting, transponder capability reporting, flight status reporting, altitude reporting in 25-foot resolution, and TCAS Resolution Advisory (RA) downlink functionality.” All aircraft should have this fitted by now.
EHS I will just quote from the SB:
“EHS refers to the transmission of aircraft state and intent data upon interrogation from a Mode S radar. This transmission is also known as the downlink of aircraft parameters (DAP). EHS provides air traffic controllers pertinent information to better manage air traffic. Aircraft parameters that are downlinked are as follows:
• Magnetic Heading
• Air Speed (Indicated Airspeed and Mach Number)
• Selected Altitude
• Vertical Rate
• Roll Angle
• Track Angle Rate
• Ground Speed
• True Track Angle
Mode S transponders that are EHS-compliant adhere to the ARINC 718A specification. Mandates for EHS will be effective March 31, 2005, for new production airplanes and March 31, 2007, for in-service airplanes.”
All data comes from the ADIRU except selected altitude (MCP) and Flight ID (FMC). I think all U10.x FMCs can send the Flight ID to the transponder but it requires the ARINC transponder to send it down to ATC.
Hope this helps
S&L
PS Not all ATC units can read these new mode S parameters. Some can read them but have not got time - so I am told.