Downlink Airborne Parameters

LVNL is implementing Mode-S Enhanced Surveillance (EHS) to make the ATM system even safer and more efficient.

This technique consists of an aircraft transponder which is suitable for transmitting Mode-S data, and the radar on the ground that is suitable for receiving Mode-S data. AIC-A 01/05 of 2007 regulates which data from the aircraft must be available for transmission via Mode-S. These are so-called Downlink Airborne Parameters or Downlink Aircraft Parameters (DAPs).

The Challenge

The challenge was to find a suitable way to present Selected Altitude, Indicated Airspeed and Magnetic Heading to air traffic controllers without over-informing them. The expectation is that these DAPs could lead to a VEM (i.e. safety, efficiency and environment) improvement quickly.

Selected Altitude is the altitude parameter that the pilot sets on his “Mode Control Panel”. The pilot then also sees the value on the Primary Flight Display (PFD). Once the pilot has set this parameter, the plane will climb to this altitude.

The altitude set by the pilot is usually the altitude to which the aircraft has been cleared by the air traffic controller. If the controller can see the set altitude too, he’ll know whether the flight is moving towards the cleared altitude. If the controller observes that the pilot-set altitude differs from the cleared altitude, he is able to intervene directly. This may help to avoid “level busts”.

Indicated Airspeed and Magnetic Heading are parameters measured by the aircraft itself. The pilot sees these on his PFD. At present the air traffic controllers must ask the pilot to “report SPEED” or “report HEADING”. The pilot then reads back this data to the controller. If the controller can see this data, he will have to ask the pilot for them less frequently. This may yield a considerable reduction of the RT load for the pilot and the controller.

The Approach

On behalf of the KDC, a study was conducted on the application of these DAPs for air traffic control. LVNL and NLR have paid orientation visits to NATS and MUAC, where a number of DAPS have already been implemented to satisfaction.

Subsequently, simulations were conducted at NLR to test the applications. This has led to a proposal to present DAPs in two ways on the air traffic controllers’ radar screens.

The first is a warning signal when the set altitude or “Pilot Selected Level” (PSL) differs from the cleared altitude (EFL). During the KDC study, LVNL and NLR developed an algorithm to compare these two parameters. In case of a deviation, the air traffic controller sees the PSL in the label in red, next to the EFL. This happens automatically for each flight under the control of the LVNL.

The alarm will not work when an aircraft is not broadcasting Enhanced Mode-S data. This can occur when a transponder is broken, or when the airplane is not (yet) equipped with a suitable transponder. In that case, the EFL will automatically turn gray.

The second way is a separate a Mode-S part in the bottom of the radar screen. If the flight controller selects a flight, he sees the corresponding DAP data from this flight.

With the support of the KDC, LVNL has now started a project to build the DAPs into the ATM system. It is expected that the DAPs for ACC will be available in the ATM system at the end of July 2012. For APP, the implementation is expected by mid-2013.

KDC has thus contributed to a further increase of aviation safety in the Netherlands.

Parties Involved



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