## 2 Hardware  
A multi-rotor UAV (OktoKopter Droidworx/Mikrokopter AD-8) currently being developed as a remote sensing platform by the _TerraLuma_ research group at the University of Tasmania is used for this study (Figure 1). Multi-rotor UAVs offer increased stability and decreased vibration in comparison to other platforms. This is a key consideration in the development of a mapping platform as any source of vibration equates to a source of error within measurements of position and orientation. The OktoKopter has 8 brushless motors which operate at different rotor speeds to achieve directional flight. There is however a minimum rotor speed required to achieve flight which will induce vibration. We isolate this vibration by ensuring the sensing payload is mounted in a rigid frame isolated from the OktoKopter airframe through the use of 4 silicon mounts. These mounts have been selected based on the mass of the payload and the minimum frequency vibration expected from the rotors. The OktoKopter has a standalone control system including an on-board navigation and autopilot system. This allows predefined flight paths to be followed ensuring maximum use of flight time and repeat surveys to be easily performed. Furthermore, the use of a VTOL UAV within forested environments is of high importance as cleared areas for use as runways are often not present. The main limitation of this platform is the small payload capacity and subsequently the reduced flight time. The electric OktoKopter is only capable of flight times between 3-5 min. The sensor payload is mounted on the UAV through a custom-designed rigid sensor framework with fixed lever arm offsets and boresight angles between all sensors. The framework also allows for an  
Figure 1: The multi-rotor Oktocopter UAV platform with the vibration isolated sensor frame, carrying the laser scanner, MEMS based IMU, GPS receiver and antenna and video camera. adequate sky-view for the accurate operation of both the GPS antennas and GPS enabled video camera. The primary sensor on-board the UAV is an Ibeo LUX laser scanner. The scanner is designed for automotive purposes and has a maximum range of 200 \(\mathrm{m}\) and scans in 4 parallel layers with a transversal beam divergence of 0.8\({}^{\circ}\) allowing complete coverage of the sensor's field of view. The sensor was set to have a scan frequency of 12.5 Hz and an angular resolution of 0.25\({}^{\circ}\). These settings were chosen primarily due to the limitations of the data logging computer. The maximum scanning range of the Ibeo LUX is 110\({}^{\circ}\), although this is restricted to \(\pm\)30\({}^{\circ}\) as large scan angles have been shown to have a significant impact on the derivation of key metrics used for forest investigation [30]. The LUX records ranges and intensities for up to three echoes per pulse, with a range repeatability of 10 cm and a resolution of 4 cm. Although the attributes of the scanner such as the wide beam divergence and low range resolution do not make it an ideal mapping sensor, its low power consumption and lightweight (approximately 1 \(\mathrm{kg}\)) allow its use on-board UAV platforms. The remaining sensors contained within the sensing payload belong to the Positioning and Orientation System (POS). The POS consists of a MEMS IMU (microstrain 3 DM-GX3 35), a dual frequency GPS receiver (Novat OEMV1-df) and lightweight antenna (Novatel ANT-A72 GA) and a high definition GPS enabled video camera. The IMU contains orthogonal sets of accelerometers, gyroscopes, and magnetometers as well as an internal GPS receiver allowing all observations to be synchronised to GPS time. The IMU, which weighs only 50 \(\mathrm{g}\), is set to observe angular rate and acceleration at a rate of 100 Hz. The key properties of the gyroscopes and accelerometers are summarised in Table 1. The IMU has been factory calibrated, however, to confirm the results the IMU was calibrated using the methods outlined in [31].