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Sense and Avoid (SAA) technologies for Safe integration of Unmanned Aircraft Systems (UAS) into Civil Airspace

Unmanned Aerial Vehicles (UAVs) are growing at frentic pace driven by civil, consumer and military requirements. In order for a UAS to safely navigate in the already crowded aerial environment of the modern world, the U.S. Federal Aviation Administration (FAA) and other international organizations have mandated that unmanned aircraft must have an on-board Sense and Avoid (SAA).


Safe and efficient integration of Unmanned Aircraft Systems (UAS) into Civil Airspace is a key challenge for unleashing their potential for non-military applications  “without reducing existing capacity, decreasing safety, impacting current operators, or placing other airspace users or persons and property on the ground at increased risk”


UAS Traffic Management (UTM) technologies and the associated regulatory framework to allow unrestricted access of UAS to all classes of airspace, including very low-level and Beyond-Line-of-Sight (BLoS) operations. Recent developments in communications, navigation and Sense-and-Avoid (SAA) technology are progressively supporting UAS operations in medium-to-high density operational environments, including urban environments.


Several research initiatives are currently underway in the US, Europe and Australia to help the FAA, EUROCONTROL and CASA define safety thresholds and develop policies, procedures and systems that would make UAS unrestricted airspace access a reality. Before unrestricted UAS operations become possible in the US, European and Australian airspaces, assurances must be made that they can operate safely.


Of the many challenges facing the aviation research community, developing a certifiable Sense and Avoid (SAA) capability for UAS is viewed as one of the most fundamental and yet most elusive tasks to be accomplished. For manned aircraft, one of the basic obligations of the pilot is to “see-and-avoid other aircraft.” The see-and-avoid procedure has the advantage of not relying on any cooperative equipment in the threat aircraft. While see-and-avoid is subject to human limitations, it is proving difficult to develop a practical suite of sensors/systems that can provide anything nearly equivalent to human vision and associated decision making.


SAA can be defined as the capability of a UAS to remain well clear from and avoid collisions with other airborne traffic. SAA provides the intended functions of self-separation and collision avoidance as a means of compliance with the regulatory requirements to “see and avoid” compatible with expected behaviour of aircraft operating in the airspace system.



From a conceptual point of view the SAA capability performs the following sub-functions:

  • Detect – Determine presence of aircraft or other potential hazards
  • Track – Estimates position and velocity of intruders based on surveillance reports
  • Evaluate – Assess collision risk based on intruder and UA positions and velocities
  • Prioritize – Determine which intruder tracks have met a collision risk threshold
  • Declare – Decide that action is needed
  • Determine Action – Decide on what action is required
  • Command – Communicate determination action
  • Execute – Respond to the commanded action



Sense and avoid technology  enables Drones  to detect aircraft & obstacles within the vicinity of the UAV and to execute manoeuvres to restore a safe situation if needed. In addition UASs must ensure that they can avoid the terrain and land without operator intervention, react to contingencies such as engine out and lost link scenarios, and Be reliable and cost-effective.


Sense and Avoid may utilize  Stereo Vision, Monocular Vision, Ultrasonic, Infrared, Time-of-Flight and Lidar sensors to detect and avoid obstacles. Manufacturers are also using multiple sensors fusing them together to create the obstacle detection and collision avoidance systems.This obstacle detection and avoidance technology started with sensors detecting objects in front of the drone. Now the latest drones from DJI, Walkera, Yuneec and others have front, back, below and side obstacle avoidance sensors.


 Sense and Avoid Technologies

Sense and avoid is a sequence of functions which, using a combination of airborne and ground-based sensors, are able to perform manoeuvres to avoid collisions and serve as a UAV replacement for the tradition “see and avoid” capability for manned aircraft. The Sense and avoid technology is of two types, one is for drones called manual sense and avoid which relays information to the UAV pilot and the second is the development of completely autonomous sense and avoid technology which removes the need for a pilot altogether. The cooperative technologies depend on cooperation from other aircrafts to know their distance, velocity and altitude and avoid collisions, while noncooperative technologies use active and passive sensors  to determine these parameters  on their own.


Noncooperative technologies benefit from the fact that they can be used to detect ground-based obstacles as well as those that are airborne.Generally sense and avoid consists of two components, such as separation assurance and collision avoidance. The first function reduces the probability of a collision by ensuring that the aircraft remain “well clear” of each other thereby assuring safe separation, while collision avoidance is related to extreme maneuvers just prior to closest point of approach to prevent collisions in cases where safe separation is lost.

Cooperative technologies

Cooperative technologies are widely used in manned aircraft and have a reliable track record in regards to reducing the number of midair collisions. One of the most desirable options is constructed using T-CAS or ADS-B transponders. Traffic Collision Avoidance Systems (T-CAS) or Automatic Dependent Surveillance-Broadcast (ADS-B) transmit information on an aircraft’s altitude, velocity and distance to other aircraft within a certain range. Whilst far more accurate than radar, systems such as T-CAS and ADS-B will only constitute an effective sense and avoid system once all aircraft are equipped with them.


Many manned aircraft have transponders that offer cooperative detection by ground-based radar and airborne collision avoidance systems. Existing regulations require the use of a transponder in certain airspace and when operating under instrument flight rules. There is, however, substantial airspace where transponders are not required for operations. There are also many aircraft registered in Australia, in the United States and in Europe without a transponder or an altitude reporting transponder, either because their aircraft cannot support them or their owners elected not to equip. About half of these aircraft do not have electrical systems (e.g., some gliders, balloons, and classic aircraft). These non-cooperative aircraft are largely relegated to flying under visual flight rules and in lower altitude airspace (gliders are an exception), where many small UAS will likely want to fly. Even if UAS could count on all aircraft having transponders, cooperative detection of these transponders based on the TCAS design would require active interrogations and a directional antenna, both of which are likely not practical on a space and power limited small unmanned platform.


Another and perhaps more practical approach to SAA is through the adoption of ADS-B cooperative surveillance. ADS-B can be thought as an electronic beacon that is continually broadcasting position, velocity and ID information for the benefit of any receiver in range. If UAS could rely on all aircraft to have such a beacon, the physics of the SAA detect/track tasks would be reduced to a receiver that decodes the ADS-B messages and a computing system that processes ADS-B surveillance reports in order to allow the other SAA sub-functions (evaluate, prioritize, declare, determine action, command and execute). Conceptually, these sub-functions are applicable to the entire UAS and, therefore, may be allocated to any element within the UAS to include the following:

  • Unmanned Aircraft (UA)
  • Control station and associated data and communication links
  • UAS flight crew (UAS pilot/observer – visual or electronic)
  • Associated procedures needed to operate in the airspace
  • Equipment needed to manoeuvre the UA


As a consequence, until T-CAS and ADS-B become mandatory for all aircraft researchers are looking toward other options for sense and avoid. In addition, cooperative technologies provide no SAA capabilities against collisions with ground-based obstacles such as terrain features, towers, or power lines.

Detect and Avoid System Installed on National Guard UAS

General Atomics Aeronautical Systems, Inc. (GA-ASI) has entered into an agreement with the United States Air National Guard (ANG) to provide a Detect and Avoid System (DAAS) for one MQ-9 Block 1 and one MQ-9 Block 5 unmanned aerial system (UAS). The DAAS consists of a GA-ASI-developed Due Regard Radar (DRR), a processor and a Traffic Alert and Collision Avoidance System (TCAS). GA-ASI will upgrade the software in the DRR for this particular application to add a tactical weather mode, in addition to the air traffic surveillance capability. The system will provide the MQ-9 UAS with safe access to uncontrolled airspace and will comply with Due Regard procedure when operating in international airspace.


According to the company, the DAAS avionics will be integrated into the new Centerline Avionics Bay (CAB). The CAB provides additional volume and mission infrastructure for integrating future capabilities. The CAB’s modular design and additional infrastructure are designed to enable the MQ-9 Block 1 and Block 5 aircraft to be a more open and extensible platform for integration of other emerging capabilities.


The DAAS avionics will be integrated into the MQ-9 Block 1 and Block 5 unmanned aircraft’s new Centerline Avionics Bay (CAB), which features a modular design and provides additional space and mission infrastructure for integrating future payloads and capabilities. David R. Alexander, president of GA-ASI, commented: “Our Detect and Avoid System has been extensively tested and not only meets, but exceeds the capabilities of the air traffic awareness and avoidance systems used on today’s commercial airliners. The DAAS is an integral part of the certification effort that will allow RPA from GA-ASI to fly in non-segregated, controlled airspace.”


Volpe’s Ground-Based Sense and Avoid (GBSAA) capability

The ground-based sense and avoid (GBSAA) capability uses existing air traffic data from multiple sources to provide UAS operators with a real-time display of aircraft in the surrounding airspace. GBSAA alerts operators to potential conflicts with neighboring aircraft and recommends avoidance maneuvers for UAS in the event that a conflict does occur.


The primary component of GBSAA is a modified FAA terminal automation system that ingests and displays surrounding aircraft to a UAS operator. GBSAA leverages existing NAS radar equipment and infrastructure to locate these surrounding aircraft, and also has the ability to track “non-cooperative aircraft,” or aircraft lacking electronic means of broadcasting their position or speed.


Volpe has worked with industry collaborators from the MITRE Corporation, MIT Lincoln Laboratory, and Raytheon to develop and deploy a low-cost automated solution that enables UAS operators to “sense and avoid” other aircraft. The Air Force uses GBSAA for its UAS missions at Cannon AFB to ensure separation and conflict avoidance in relation to civilian air traffic.


Collision-avoidance radar for UAVs gets radome upgrade

Radar and sensor maker Hensoldt has  announced an upgrade to its collision-avoidance radar system for unmanned aerial vehicles (UAVs): The company says that its enhanced radome technology protects the radar from mechanical environmental influences — such as bird strikes or lightning — while minimally affecting the radar’s functionality.


Hensoldt is demonstrating its detect-and-avoid radar system, which uses AESA [active electronically scanned array] radar technology to detect objects in the flight path of a UAV and to give early warning of any threat of collision following precise evaluation of the flight direction. The sensors used for this purpose also can function, say company officials, as a weather radar system.


The AESA radar, say company officials, is equally suitable for military and civilian UAVs, for example, for the delivery of cargo. This AESA radar system — designed to replace the pilot’s visual assessment of the situation — is built into the UAV’s nose and must be protected by a radome that is electrically transparent, has exactly the same thickness across the board, and is adapted to the aerodynamics of the platform.



About Rajesh Uppal

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