A fall detector device for determining a height metric of an object in an environment using 3D coordinates associated with the object in the environment for determining a fall condition of the object, comprising: a 3D reflected wave measuring device having a 3D coordinate system for measuring 3D coordinates of an object in the environment for determining the height metric of the object in the environment, the 3D reflected wave measuring device having a first frame of reference; an orientation sensing device to indicate an orientation of the 3D reflected wave measuring device in an environment for aligning the first frame of reference with a second frame of reference that is associated with the environment; a processor coupled to the orientation sensing device and configured to receive orientation data indicative of an orientation of the device, the processor being configured to align the first frame of reference with the second frame of reference by using the orientation data to compensate for an angular difference between the first frame of reference and the second frame of reference, wherein the fall detector device is mountable on a wall within the environment and the second frame of reference comprises a horizontal plane associated with the environment, and wherein the processor is configured to determine the height metric of the object in the environment relative to the second frame of reference using 3D reflected wave measurements received by the 3D reflected wave measuring device. a 3D reflected wave measuring device having a 3D coordinate system for measuring 3D coordinates of an object in the environment for determining the height metric of the object in the environment, the 3D reflected wave measuring device having a first frame of reference; an orientation sensing device to indicate an orientation of the 3D reflected wave measuring device in an environment for aligning the first frame of reference with a second frame of reference that is associated with the environment; a processor coupled to the orientation sensing device and configured to receive orientation data indicative of an orientation of the device, the processor being configured to align the first frame of reference with the second frame of reference by using the orientation data to compensate for an angular difference between the first frame of reference and the second frame of reference, wherein the fall detector device is mountable on a wall within the environment and the second frame of reference comprises a horizontal plane associated with the environment, and wherein the processor is configured to determine the height metric of the object in the environment relative to the second frame of reference using 3D reflected wave measurements received by the 3D reflected wave measuring device. wherein the fall detector device is mountable on a wall within the environment and the second frame of reference comprises a horizontal plane associated with the environment, and wherein the processor is configured to determine the height metric of the object in the environment relative to the second frame of reference using 3D reflected wave measurements received by the 3D reflected wave measuring device.