UAS ATM Common Altitude Reference System (CARS)

Discussion document

This is one of a collection of three documents discussing distinct points raised in the 1st UAS-ATM Integration Workshop organised at EUROCONTROL Brussels in April 2017. The purpose of this workshop was to discuss the problems associated with integrating Unmanned Aircraft Systems (UAS) - colloquially called 'Drones' - including Remotely-Piloted Aircraft Systems (RPAS), into the airspace that also includes many manned flights and, therefore, presents a definite safety hazard. 

The three distinct points are:

  • Flight rules;
  • Airspace, or operating environment, assessment;
  • A common altitude reference system (this document);

The many activities for which unmanned air systems (UAS) are used – from military through commercial to leisure – can lead to their sharing airspace with conventional aircraft. For separation to be maintained between all users of this airspace, it is essential that the altitudes of all of these aircraft be known unambiguously.

However, whereas conventional manned aviation uses pressure altitude obtained from barometric readings, UAS often use other systems such as satellite-derived altitudes. While each of these different systems can enable safe separation on its own, they can each furnish different altitude values from each other. A common altitude reference system needs to be established.

This document provides a basis for discussion on such a system, following a workshop and a series of webinars organised by EUROCONTROL in collaboration with the European Aviation Safety Agency (EASA).

There are many advantages and disadvantages, economic and technical, to all of the altitude measurement systems available. In the conclusions of this document, a solution is proposed that will enable two different systems to co-habit safely.