Use of GPS for navigation and guidance is usually associated with large vehicles such as cars, trucks, or jet airliners. This article, however, describes the development of micro aerial vehicles (MAVs), a category of aircraft about the size of small birds. As the smallest, powered aircraft, MAVs can carry various sensors as payload to support a variety of civil and military applications.
Micro aerial vehicle (MAV) describes a category of aircraft with dimensions roughly comparable to small birds. As the smallest, powered aircraft, MAVs can carry various sensors as payload to support such civil and military missions as traffic monitoring, weather observation, and enemy surveillance during military conflicts. By next year, for instance, Germany's Federal Armed Forces could send the first operational MAVs to the field.
Much faster and cheaper than conventional reconnaissance aircraft, a MAV equipped with a miniaturized video camera could reconnoiter nearby enemy troop positions or, outfitted with highly sensitive sensors, could locate chemical weapons.
On the civil side, most requests for scientific MAV applications currently come from meteorologists seeking to measure temperature, humidity, and, most importantly, the speed and direction of wind. The weather researchers want a cost-effective, mobile, and reusable measurement platform that can replace non-returning radiosonde balloons whose onboard equipment is lost after a mission.
Other typical civil applications include traffic observation and control using mobile airborne camera platforms. Fixed traffic control systems installed along main highways, such as the German Autobahn, could be supplemented by MAVs to help facilitate optimal traffic flow not only on the main traffic routes, but also on side roads in case of closed highways.
Companies have already requested MAVs for observing their factory sites from the air. Search and rescue services are interested in MAVs to obtain a rapid overview of disaster areas--during forest fires (localization of the origin of fire), floods, and chemical or nuclear catastrophes--without endangering personnel in manned vehicles such as helicopters. Moreover, MAVs are ideal for providing information quickly in the wake of terrorist attacks.
Further applications include the reconnaissance of demonstrations, the creation of georeferenced maps, and determination of the maturity of farm crops in the field. Further in the future, MAV "swarms" will enable new applications such as providing mobile airborne communication networks or 3D scientific measurements and images. The number of new MAV applications is steadily increasing.
One measure of the international interest in this new field of research was manifested in July, when more than 100 researchers from around the world came to Braunschweig, Germany, to visit the First European Micro Air Vehicle Conference (EMAV 2004). In addition to presentations about every aspect of MAVs--such as guidance, navigation, and control (GN & C); aerodynamics, and propulsion--a flight competition took place at the research airport Forschungsflughafen Braunschweig.
Researchers presented both indoor and outdoor MAVs, including fixed, rotary and flapping wing designs. The flight performances demonstrated that the limiting factor in current MAVs, given their small size and weight, is ensuring their robust and stable operation, especially against strong wind gusts. However, outdoor fixed-wing aircraft in particular have reached an advanced development stage that supports operations even at storm forces 4 to 6 (Beaufort Scale) of 20-50 kilometers per hour. The second EMAV conference will take place in Braunschweig in summer 2006.
In Germany, current MAV activities are funded within the German National Aerospace Program 2003-2007. One project supported by these funds is AutoMAV, which is being conducted by a team of leading German MAV research institutions led by Stefan Winkler from the Institute of Aerospace Systems of the Technical University of Braunschweig (TU Braunschweig). The goal of the project is development of a very agile, fully autonomous fixed-wing outdoor MAV for observing airports and other critical facilities.
This article will provide an overview of MAV technical development trends and discuss progress on a particular MAV initiative, the Carolo project, under way by researchers from the TU Braunschweig, Germany.
Growing Interest in MAVs
Today, autonomous MAVs typically have a mass of less than 500 grams and a maximum length under 50 centimeters. They have a flight endurance ranging from several minutes (rotary-wing MAVs) up to about half an hour (fixed-wing MAVs). For these aircraft, remote control, as known from model aircraft design and operation, is no longer necessary.
Current fixed-wing MAVs, including the German Carolo, have been developed mainly for outdoor applications such as providing real-time video from the area surrounding a ground station, a commercial tablet PC. Current rotary-wing MAVs mainly concentrate on indoor applications because of their hover ability. The combination of both, fast outdoor reconnaissance and hover ability, which a flapping-wing aircraft could deliver, is still a challenge for such tiny dimensions.
MAV development activities involve not only established companies but also small start-ups such as Mavionics GmbH, founded by the developers of the MAV Carolo from the Institute of Aerospace Systems of TU of Braunschweig. Nonetheless, a large gap remains between the requirements of MAV end users and the technology's state of the art.
The primary technical goal in MAV development is to design an aerial vehicle capable of carrying out its mission fully autonomously, without pilot and remote control. Typically, such autonomous operations would involve the following steps. Before the flight starts, the operator defines a mission on the ground station, which typically is a tablet PC, laptop, or convertible computer. This involves using the cursor to "click" waypoints onto a digital map on the ground station. For each waypoint, a special maneuver--such as circling--can be defined.
The feasibility of the mission is then checked to ensure that the MAV route will not cross no-fly zones, will not encounter any obstacles (such as a tower), or exceed its flight-mechanical design parameters, and thus can achieve the planned range, endurance and curve radii. Once mission feasibility is assured, operators transfer the instruction set to the MAV via telemetry link. Then the MAV is hand-launched and executes the assigned operations. In general, the user is still able to adapt the current mission on-line, moving, deleting, or adding waypoints and maneuvers. For outdoor MAVs, the waypoints are typically defined by GPS coordinates.
MAV Design Approaches
In general, engineers have developed two different technologies for carrying out the autonomous flight of outdoor MAVs. In the first, a continuous telemetry connection between the MAV and its ground station is used to communicate most of the flight GN & C instructions based on algorithms running on a powerful computer on the ground. This approach has the advantage of avoiding the mass and power consumption required by a high-capacity on-board computer. A primary disadvantage, however, is that the aircraft cannot survive in case of interruption of the telemetry link. Such an aircraft is said to be a semiautonomous MAV.
The second approach is the fully autonomous MAV that carries a powerful computer on board running all the necessary flight GN & C algorithms. This advanced version of a MAV can still carry out its mission in case of telemetry link loss. Typical outdoor MAV sensors include GPS, inertial sensors, barometric and dynamic pressure sensors, and horizon sensors.
Although such on-board computers are sufficient for outdoor MAVs, their computational power currently is not really adequate for autonomous indoor operations. Here advanced image processing must be carried out to fly through windows and doors and to avoid collisions with obstacles. Current miniaturized off-the-shelf computers cannot handle that computational load. At the moment, the aircraft state--position, velocity and attitude (3D orientation)--is typically measured indoors by inertial sensors, ultrasonic sensors, and cameras equipped with image processing.
