ICAO Framework

Since the introduction of modern navigation technologies it is possible to determine the position of an aircraft in terms of Latitude and Longitude coordinates. The term Area Navigation (RNAV) defines the capability of an aircraft to navigate from two arbitrary geographical positions on the ground.

The US FAA regulation was the first example of a Performance Based Navigation (PBN). 

The progress of the navigation capabilities led the ICAO to introduce the term “Required Navigation Performance Capability” (RNPC) and to publish the first edition of Doc 9613, Manual on Required Navigation Performance.


The 4th edition of the Doc 9613 introduced a new navigation specification intended for helicopters flying at low altitude and speed much lower than jet-propelled aeroplanes and in fact it includes RNP 0.3 (i.e. maximum TSE not larger than 0.3 NM = 556 m) intended for helicopters flying at relatively low level (e.g. 3000 ft AGL) and of course at a speed much smaller than jet-propelled fixed wing aeroplanes.


ATS Routes

ATS Routes are “specified routes designed for channelling the flow of traffic as necessary for the provision of air traffic services”. This term includes the track to or from significant points (waypoints) and the lowest safe altitude (as determined by the competent Air Traffic Service Authority).

The routes flown by drones rely on satellite positioning, similarly to RNAV/RNP routes of the manned aviation, and GNSS availability is considered as mandatory in the concerned airspace.

As consequence, navigation based on performance requirements assumes relevance for drones operating along “drone routes” in specific scenarios, in which performance requirements are usually expressed in navigation specifications in terms of Accuracy, Availability, Integrity and Continuity. In addition, one should consider the “Flyability” of a “drone route” and the Interferences that cannot influence the signals.

These points were addressed in the Narrow Path study.

Narrow Path Project


Narrow Path delivered a feasibility study for the application of the PBN to small UAS flying at Low Level with the development of a simulator for the calculation of the FTE in real time with different drones available on the market.

The study focused on the FTE and NSE to explore the possibility to reduce the protection “buffer value” applied to a specific route. This would allow a small UAS to fly closer to obstacles, as appropriate for operations at Low level.

In particular, the study has explored the possibilities to:

  • Install advanced satellite navigation systems (i.e. GNSS – Galileo/GPS, EGNOS) on board of a small drone, in the form of a “GNSS UTM transponder” to reduce the NSE;
  • Reduce the FTE and the “buffer value” around the planned route, accordingly to high level of automation on board and the drone flight path;
  • Define the volume of the “buffer value” on the lateral sides of the route up possibly around 50m/100m each side of the instrument route.

The results of the study confirmed the possibility to have an agreed buffer value smaller than 50 meters with demanding environmental conditions (strong wind) and different commercial drone models, when designing instrument routes for UAS at LL or VLL.

The sensitivity analysis performed allowed to assess the Navigation System Error through:

  • GNSS / PBN literature and output of previous H2020 GNSS projects (e.g. Gauss/ Real);
  • Prototype of a GNSS UTM Transponder based on Galileo /EGNOS GNSS Receiver with NB-IoT / LTE connection;

The Flight Technical Error was assessed by means of a Simulator to evaluate Drone autopilot behavior (System response) in presence of strong wind with different Drone Ground speeds.

The Sensitivity Analysis has provided the required information to assess the FTE with different wind conditions and different drone ground speeds. One of the Drone used in the simulation was a 20 Kg hexcopter equipped with the an industrial grade autopilot system.


The Tests performed relied on a simulation system based on a COTS industrial drone, a flight controller program and a Ground Station. On this basis we developed our custom application that embeds the simulator engine.

We managed to measure some key parameters through a series of simulated flights and how they affect the components of TSE. We processed the collected dataset and found a sensitivity function.

Narrow Path project showed the possibility to achieve the Required Navigation Performance RNP 0.02 (2/100 of NM, approximately 37 m) in many operational conditions, but also RNP 0.01 (approximately 18 m) with some additional limitations


The risk assessment conducted with SORA methodology for the scenarios proposed confirmed that the operations can be still done under the conditions of Specific category (and not certified category).

The possibility to have an agreed buffer value not larger than 50-100 m when designing instrument routes for UAS at LL or VLL airspaces is confirmed. A tool for the calculation of FTE has been developed and it form the basis of the Narrow Path web solution.

For more information visit Narrow Path Project page on