New Applications of Downlink Airborne Parameters

The more parameters the better? A study on DAPs seeking a balance between supply and requirements

Dutch air traffic controllers currently use three Downlink Airborne Parameters, or DAPs. These are handy as they provide useful information about the aircraft: selected altitude, indicated airspeed and magnetic heading. These parameters are only a fraction of the wide range of DAPs available. How can air traffic control make even better use of the information available in the cockpit?

A little bit of theory
DAPs are extracted from the Mode S Enhanced Surveillance transponder, abbreviated to the Mode S EHS transponder. In Dutch airspace, aircraft must be equipped with this kind of transponder if they fly at an altitude in excess of 7.5 kilometres (FL245). Air Traffic Control the Netherlands (LVNL) can use its radar to interrogate aircraft and receive specific DAPs from the aircraft. 

Seeking a balance between supply and requirements
KDC foundation tasked MovingDot and NLR experts with seeking new applications for DAPs. Project leader Michel Koeslag: “On the one hand we studied what is available: what are the options and what can you do with them? But we also looked at the air traffic controllers’ requirements. Which parameters would they find handy? And can they be made available? 

Three likely DAPs
Three likely candidates remained out of the 50 parameters studied. These are mainly aimed at improving safety. Koeslag: “The three are easily available for both old and new aircraft. The parameters also relate to different aspects of the tasks of air traffic control. This makes the parameters interesting and widely usable.” 

  • DAP 1: Roll angle/track angle

This parameter enables air traffic controllers to obtain an early warning that an aircraft is about to make a turn. This DAP could be a handy tool for the control tower in the case of poor visibility if aircraft start simultaneously or immediately after one another. Air traffic controllers can then establish more quickly that the aircraft are flying the correct route.

  • DAP 2: Barometric altitude rate/inertial vertical velocity

This DAP is useful for the en-route and ACC radar controllers who control traffic at higher altitudes. It enables them to monitor the vertical velocity. Air traffic controllers can then act more quickly to prevent aircraft from deviating from the indicated altitude (level bust).

  • DAP 3: Barometric pressure setting

The barometric pressure setting is a handy addition to the current pilot selected level. The approach controller then sees immediately whether the pilot switches on time between local pressure (QNH) and standard pressure (QNE) when ascending or descending. This third DAP is currently less easy to use due to implementation discrepancies among aircraft manufacturers.

More DAPs in future?
LVNL is currently satisfied with its use of existing DAPs. Koeslag expects to be able to use the new DAPs from 2016. “New EU legislation aims to make new aircraft ready for this from this date. Existing aircraft will first need to be converted. These will also be able to use the parameters from 2020. Further research will determine whether we really do take the new DAPs into use.”

Parties involved
LVNL, MovingDot, NLR

Download full report
In Dutch