The deployment of unmanned aircraft has increased in recent years. These unmanned aerial vehicles (UAVs) are also commonly referred to as drones, unmanned aircraft systems (UAS), or remotely operated aircraft systems (RPAS). For purposes of the following description, these terms will be used interchangeably. Drones of all shapes and sizes have been used by the military for many years. Since then, the benefits of increasingly efficient and adaptive manufacturing procedures, such as 3D printing, and the miniaturization of electronic components have made the deployment of drones in civilian environments more feasible. Now, the technology is so simple that the average consumer can buy a drone, practice flying it and attach cameras and other sensors to take pictures of their surroundings.
Drone Navigation Requirements
UAVs can be fixed-wing, monoplane or multi-rotor aerial platforms controlled remotely by ground operators. Currently, in the civilian field in many flying jurisdictions around the world, drones are required to fly within the operator's naked eye (line of sight - VLOS). Such regulations are still evolving and more and more cases are now permitted to operate beyond the line of site (BLOS). When considering professional and high-end consumer drones, most drone platforms will use Global Positioning System (GPS). Many will also include at least a low-level inertial measurement unit (IMU) in their flight controllers for flight navigation and control.
GPS is generally the general term used to describe the Global Navigation Satellite System (GNSS) technology used to calculate position on the Earth's surface from timing information received from a network of GNSS satellites constantly orbiting the Earth. The constellation of satellites accessed by the GNSS system may include the original US-led Global Positioning System (GPS) project, the Russian GLONASS constellation, or one of the other constellations under development, such as the European-led GALILEO or China's BeiDou project. Using GNSS technology, the position of the UAV can be automatically calculated and positioned according to the actual coordinate system.
The IMU is assembled from a gyroscope and accelerometer. On drone platforms, these IMU components are increasingly fabricated using microelectromechanical (MEMS) technology. The IMU will provide data related to the drone's linear acceleration in three axes, and to the drone's rotational measurements in terms of roll, pitch, and yaw.
The flight controller on the drone will use the GPS data to provide the coordinates of the drone's position at a specific point in time. Data from the IMU will tell the flight controller if the drone is level, if it is spinning, and how stable it is during flight. If the flight controller uses both inertial and GPS information, it can provide the drone and its operator with the necessary feedback for safe operation in VLOS and BLOS scenarios.
Challenges of Aerial Mapping
Cameras mounted on aircraft have been used for surveillance or mapping purposes since the mid-1900s. When images from aerial cameras are used to use photogrammetry data to map terrain or land cover, or to calculate slope angles or mine volumes, it is important to know not only the specifications (internal model) of the camera or sensor being used, but also:
How the external orientation of the imaging sensor changes (i.e. how the sensor moves relative to the ground).
Influence of inherent 3D ground geometry on measurements and distances of mapping output when imaged from above.
The aircraft's navigation system provides information about the flight route and the location of the overflight. In order to create and update maps, camera photo locations and footprints need to be located with known coordinates of landscape features (Ground Control Points - GCPs) seen in the photo. Undulations in ground terrain are derived by applying aerotriangulation equations to relate features in photographs and images to CSP x, y, z coordinates. However, this is a labor-intensive and costly process, and the quality of the results depends on factors such as the number of GcPs used and the layout pattern of the GCPs.
This method of georeferencing against known global climate Cp coordinates is primarily used to provide survey control for drone aerial photography. As the application market of the drone platform becomes wider and wider, the cost of the computing system will also decrease, and the pricing system of the platform itself will be more favorable. Methods to further optimize GPS readings on UAV platforms such as receiving atmospheric and timing corrections from local base stations via a real-time radio link (real-time kinematic GPS) or via a post-processed differential GPS workflow are commonplace. However, this still does not negate the use of global climate plans, and it is easy to overlook the time required to establish measurement controls or conduct these necessary aerial measurement procedures.
