Frames of Reference

As described in the Frames of Reference manual, the MRS system offers a flexible way to define the coordinate frames used in the system. This is enabled using geographical libraries in the real-world that map from latitude-longitude coordinates to the local Cartesian coordinate system. The MRS system uses the GeographicLib library to handle these transformations.

Custom Real World File

An example real world file is shown below. The file is divided into two main sections: world_origin and safety_area. The world_origin section defines the origin of the local Cartesian coordinate system. The safety_area section defines the safety area around the origin. The safety area is defined by a polygon in the horizontal plane and by the maximum and minimum heights in the vertical plane.

world_origin:

  units: "LATLON" # {"UTM, "LATLON"}

  origin_x: 50.0764041
  origin_y: 14.4180359

safety_area:

  enabled: true

  horizontal:

    # the frame of reference in which the points are expressed
    frame_name: "latlon_origin"

    # polygon
    #
    # x, y [m] for any frame_name except latlon_origin
    # x = latitude, y = longitude [deg]  for frame_name=="latlon_origin"
    points: [
      50.0765653, 14.4178914,
      50.0762102, 14.4179722,
      50.0763015, 14.4181847,
      50.0765825, 14.4181411,
    ]

  vertical:

    # the frame of reference in which the max&min z is expressed
    frame_name: "world_origin"

    max_z: 8.0
    min_z: 1.0

Choice of coordinate system

There are two choices available for the coordinate system in the real world file: LATLON and UTM. The LATLON system uses latitude and longitude coordinates, while the UTM system uses the Universal Transverse Mercator (UTM) coordinate system. The choice of coordinate system is made by setting the units parameter in the world_origin section.

For example, the latitude and longitude for our university yard can be specified as 50.076408, 14.418019 in the LATLON system. The same point can be specified as 458357.390, 5547288.346 in the UTM system.

This origin leads to the defintion of a plane which converts from either LATLON or UTM to the local cartesian plane with co-ordinates in meters.

Safety Area

Safety area is a polyhedron defined by vertically extruding a polygon to mark an area outside which the UAV should not fly. When specified and enabled, the system prevents goto commands and trajectory_references from taking the UAV outside the safety area.

This area can either be defined in co-ordinate frames such as local_origin or world_origin which uses the origin specified in the world to define the polygon in meters, or in latlon_origin which uses the latitude and longitude to define the polygon.

To define a new polygon, simply add the co-ordinates of the vertices in the points parameter. Order the co-ordinates in a clockwise or anti-clockwise manner to define the polygon. The last point is connected to the first point to close the polygon.

The vertical limit of the safety area can only be defined in meters by specifying the max_z and min_z parameters. The max_z parameter specifies the maximum height, while the min_z parameter specifies the minimum height. When a UAV is initialised, its altitude is set to zero, and is further incremented/decremented based on sensors like GNSS | Laser altimeter | Barometer, and therefore, the UAV might intend to land below its takeoff altitude. To enable autonomous operations in these cases, min_z can be set to a negative value to allow the UAV to land below its takeoff altitude.