An autonomous aircraft is an aircraft which flies under the control of on-board autonomous robotic systems and needs no intervention from a human pilot or remote control. Most contemporary autonomous aircraft are unmanned aerial vehicles (drones) with pre-programmed algorithms to perform designated tasks, but advancements in artificial intelligence technologies (e.g. machine learning) mean that autonomous control systems are reaching a point where several air taxis and associated regulatory regimes are being developed.
See main article: History of unmanned aerial vehicles. The earliest recorded use of an unmanned aerial vehicle for warfighting occurred in July 1849,[1] serving as a balloon carrier (the precursor to the aircraft carrier)[2] Significant development of radio-controlled drones started in the early 1900s, and originally focused on providing practice targets for training military personnel. The earliest attempt at a powered UAV was A. M. Low's "Aerial Target" in 1916.[3]
Autonomous features such as the autopilot and automated navigation were developed progressively through the twentieth century, although techniques such as terrain contour matching (TERCOM) were applied mainly to cruise missiles.
Some modern drones have a high degree of autonomy, although they are not fully capable and the regulatory environment prohibits their widespread use in civil aviation. However some limited trials have been undertaken.
As flight, navigation and communications systems have become more sophisticated, safely carrying passengers has emerged as a practical possibility. Autopilot systems are relieving the human pilot of progressively more duties, but the pilot currently remains necessary.
A number of air taxis are under development and larger autonomous transports are also being planned. The personal air vehicle is another class where from one to four passengers are not expected to be able to pilot the aircraft and autonomy is seen as necessary for widespread adoption.
The computing capability of aircraft flight and navigation systems followed the advances of computing technology, beginning with analog controls and evolving into microcontrollers, then system-on-a-chip (SOC) and single-board computers (SBC).
Position and movement sensors give information about the aircraft state. Exteroceptive sensors deal with external information like distance measurements, while exproprioceptive ones correlate internal and external states.
Non-cooperative sensors are able to detect targets autonomously so they are used for separation assurance and collision avoidance.[4]
Degrees of freedom (DOF) refers to both the amount and quality of sensors on board: 6 DOF implies 3-axis gyroscopes and accelerometers (a typical inertial measurement unit IMU), 9 DOF refers to an IMU plus a compass, 10 DOF adds a barometer and 11 DOF usually adds a GPS receiver.[5]
UAV actuators include digital electronic speed controllers (which control the RPM of the motors) linked to motors/engines and propellers, servomotors (for planes and helicopters mostly), weapons, payload actuators, LEDs and speakers.
UAV software called the flight stack or autopilot. The purpose of the flight stack is to obtain data from sensors, control motors to ensure UAV stability, and facilitate ground control and mission planning communication.[6]
UAVs are real-time systems that require rapid response to changing sensor data. As a result, UAVs rely on single-board computers for their computational needs. Examples of such single-board computers include Raspberry Pis, Beagleboards, etc. shielded with NavIO, PXFMini, etc. or designed from scratch such as NuttX, preemptive-RT Linux, Xenomai, Orocos-Robot Operating System or DDS-ROS 2.0.
| Layer | Requirement | Operations | Example | |
|---|---|---|---|---|
| Firmware | Time-critical | From machine code to processor execution, memory access | ArduCopter-v1, PX4 | |
| Middleware | Time-critical | Flight control, navigation, radio management | PX4, Cleanflight, ArduPilot | |
| Operating system | Computer-intensive | Optical flow, obstacle avoidance, SLAM, decision-making | ROS, Nuttx, Linux distributions, Microsoft IOT |
Civil-use open-source stacks include:
Due to the open-source nature of UAV software, they can be customized to fit specific applications. For example, researchers from the Technical University of Košice have replaced the default control algorithm of the PX4 autopilot.[7] This flexibility and collaborative effort has led to a large number of different open-source stacks, some of which are forked from others, such as CleanFlight, which is forked from BaseFlight and from which three other stacks are forked from.
UAVs employ open-loop, closed-loop or hybrid control architectures.
Most UAVs use a radio for remote control and exchange of video and other data. Early UAVs had only narrowband uplink. Downlinks came later. These bi-directional narrowband radio links carried command and control (C&C) and telemetry data about the status of aircraft systems to the remote operator. For very long range flights, military UAVs also use satellite receivers as part of satellite navigation systems. In cases when video transmission was required, the UAVs will implement a separate analog video radio link.
In most modern autonomous applications, video transmission is required. A broadband link is used to carry all types of data on a single radio link. These broadband links can leverage quality of service techniques to optimize the C&C traffic for low latency. Usually, these broadband links carry TCP/IP traffic that can be routed over the Internet.
Communications can be established with:
As mobile networks have increased in performance and reliability over the years, drones have begun to use mobile networks for communication. Mobile networks can be used for drone tracking, remote piloting, over the air updates,[13] and cloud computing.[14]
Modern networking standards have explicitly considered autonomous aircraft and therefore include optimizations. The 5G standard has mandated reduced user plane latency to 1ms while using ultra-reliable and low-latency communications.[15]
Basic autonomy comes from proprioceptive sensors. Advanced autonomy calls for situational awareness, knowledge about the environment surrounding the aircraft from exteroceptive sensors: sensor fusion integrates information from multiple sensors.[16]
One way to achieve autonomous control employs multiple control-loop layers, as in hierarchical control systems. As of 2016 the low-layer loops (i.e. for flight control) tick as fast as 32,000 times per second, while higher-level loops may cycle once per second. The principle is to decompose the aircraft's behavior into manageable "chunks", or states, with known transitions. Hierarchical control system types range from simple scripts to finite state machines, behavior trees and hierarchical task planners. The most common control mechanism used in these layers is the PID controller which can be used to achieve hover for a quadcopter by using data from the IMU to calculate precise inputs for the electronic speed controllers and motors.
Examples of mid-layer algorithms:
Evolved UAV hierarchical task planners use methods like state tree searches or genetic algorithms.[19]
UAV manufacturers often build in specific autonomous operations, such as:
Full autonomy is available for specific tasks, such as airborne refueling[20] or ground-based battery switching; but higher-level tasks call for greater computing, sensing and actuating capabilities. One approach to quantifying autonomous capabilities is based on OODA terminology, as suggested by a 2002 US Air Force Research Laboratory, and used in the table below:[21]
| Level | Level descriptor | Observe | Orient | Decide | Act | |
|---|---|---|---|---|---|---|
| Perception/Situational awareness | Analysis/Coordination | Decision making | Capability | |||
| 10 | Fully Autonomous | Cognizant of all within battlespace | Coordinates as necessary | Capable of total independence | Requires little guidance to do job | |
| 9 | Battlespace Swarm Cognizance | Battlespace inference – Intent of self and others (allied and foes).Complex/Intense environment – on-board tracking | Strategic group goals assignedEnemy strategy inferred | Distributed tactical group planningIndividual determination of tactical goal Individual task planning/execution Choose tactical targets | Group accomplishment of strategic goal with no supervisory assistance | |
| 8 | Battlespace Cognizance | Proximity inference – Intent of self and others (allied and foes)Reduces dependence upon off-board data | Strategic group goals assignedEnemy tactics inferred ATR | Coordinated tactical group planning Individual task planning/execution Choose target of opportunity | Group accomplishment of strategic goal with minimal supervisory assistance(example: go SCUD hunting) | |
| 7 | Battlespace Knowledge | Short track awareness – History and predictive battlespaceData in limited range, timeframe and numbers Limited inference supplemented by off-board data | Tactical group goals assignedEnemy trajectory estimated | Individual task planning/execution to meet goals | Group accomplishment of tactical goals with minimal supervisory assistance | |
| 6 | Real TimeMulti-Vehicle Cooperation | Ranged awareness – on-board sensing for long range,supplemented by off-board data | Tactical group goals assignedEnemy trajectory sensed/estimated | Coordinated trajectory planning and execution to meet goals group optimization | Group accomplishment of tactical goals with minimal supervisory assistancePossible: close air space separation (+/-100yds) for AAR, formation in non-threat conditions | |
| 5 | Real Time Multi-Vehicle Coordination | Sensed awareness – Local sensors to detect others,Fused with off-board data | Tactical group plan assignedRT Health Diagnosis Ability to compensate for most failures and flight conditions; Ability to predict onset of failures (e.g. Prognostic Health Mgmt) Group diagnosis and resource management | On-board trajectory replanning – optimizes for current and predictive conditionsCollision avoidance | Self accomplishment of tactical plan as externally assignedMedium vehicle airspace separation (hundreds of yds) | |
| 4 | Fault/Event AdaptativeVehicle | Deliberate awareness – allies communicate data | Tactical group plan assignedAssigned Rules of Engagement RT Health Diagnosis; Ability to compensate for most failures and flight conditions inner loop changes reflected in outer loop performance | On-board trajectory replanning – event drivenSelf resource management Deconfliction | Self accomplishment of tactical plan as externally assignedMedium vehicle airspace separation (hundreds of yds) | |
| 3 | Robust Response to Real Time Faults/Events | Health/status history & models | Tactical group plan assignedRT Health Diagnosis (What is the extent of the problems?) Ability to compensate for most failures and flight conditions (i.e. adaptative inner loop control) | Evaluate status vs required mission capabilitiesAbort/RTB is insufficient | Self accomplishment of tactical plan as externally assigned | |
| 2 | Changeable mission | Health/status sensors | RT Health diagnosis (Do I have problems?)Off-board replan (as required) | Execute preprogrammed or uploaded plansin response to mission and health conditions | Self accomplishment of tactical plan as externally assigned | |
| 1 | Execute PreplannedMission | Preloaded mission dataFlight Control and Navigation Sensing | Pre/Post flight BITReport status | Preprogrammed mission and abort plans | Wide airspace separation requirements (miles) | |
| 0 | RemotelyPiloted Vehicle | Flight Control (attitude, rates) sensingNose camera | Telemetered dataRemote pilot commands | N/A | Control by remote pilot | |
See also: Perceptual control theory.
Reactive autonomy, such as collective flight, real-time collision avoidance, wall following and corridor centring, relies on telecommunication and situational awareness provided by range sensors: optic flow,[22] lidars (light radars), radars, sonars.
Most range sensors analyze electromagnetic radiation, reflected off the environment and coming to the sensor. The cameras (for visual flow) act as simple receivers. Lidars, radars and sonars (with sound mechanical waves) emit and receive waves, measuring the round-trip transit time. UAV cameras do not require emitting power, reducing total consumption.
Radars and sonars are mostly used for military applications.
Reactive autonomy has in some forms already reached consumer markets: it may be widely available in less than a decade.[16]
SLAM combines odometry and external data to represent the world and the position of the UAV in it in three dimensions. High-altitude outdoor navigation does not require large vertical fields-of-view and can rely on GPS coordinates (which makes it simple mapping rather than SLAM).[23]
Two related research fields are photogrammetry and LIDAR, especially in low-altitude and indoor 3D environments.
Robot swarming refers to networks of agents able to dynamically reconfigure as elements leave or enter the network. They provide greater flexibility than multi-agent cooperation. Swarming may open the path to data fusion. Some bio-inspired flight swarms use steering behaviors and flocking.
In the military sector, American Predators and Reapers are made for counterterrorism operations and in war zones in which the enemy lacks sufficient firepower to shoot them down. They are not designed to withstand antiaircraft defenses or air-to-air combat. In September 2013, the chief of the US Air Combat Command stated that current UAVs were "useless in a contested environment" unless crewed aircraft were there to protect them. A 2012 Congressional Research Service (CRS) report speculated that in the future, UAVs may be able to perform tasks beyond intelligence, surveillance, reconnaissance and strikes; the CRS report listed air-to-air combat ("a more difficult future task") as possible future undertakings. The Department of Defense's Unmanned Systems Integrated Roadmap FY2013-2038 foresees a more important place for UAVs in combat. Issues include extended capabilities, human-UAV interaction, managing increased information flux, increased autonomy and developing UAV-specific munitions. DARPA's project of systems of systems,[29] or General Atomics work may augur future warfare scenarios, the latter disclosing Avenger swarms equipped with High Energy Liquid Laser Area Defense System (HELLADS).
Cognitive radio technology may have UAV applications.[30]
UAVs may exploit distributed neural networks.[16]