## 2 Materials and Methods
### Single-Antenna versus Dual-Antenna GNSS/INS Heading Precision Experiment  
The UAS-lidar mapping system used in this study was an implementation of the open-source Open Mobile Mapping System (OpenMMS) (OpenMMS, Moose Jaw, Canada) [26] and consisted of an Applanix APX-18 UAV GNSS/INS sensor, two Trimble AV14 GNSS antennas (Trimble, Sunnyvale, USA) a Livox Mid-40 lidar sensor (Livox Lidar, Shenzhen, China), and a Sony \(\alpha\)6000 RGB camera (Sony, Tokyo, Japan) (Figure 6a). The mapping system was installed on a Freefly Systems Alta X UAS (Freefly Systems, Woodinville, USA) (Figure 6b) that operated autonomously during data collection via execution of a pre-planned mission. Raw GNSS and IMU observations were collected by the APX-18 at rates of 5 Hz and 200 Hz, respectively, and the integrated magnetometer was properly calibrated and enabled during the missions. The length of the GNSS compass baseline was measured to be 1.525 m. Lidar observations were collected by the Mid-40 at a rate of 100 kHz within a 38.4\({}^{\circ}\) circular field of view (FoV). The experiment involved collecting UAS-lidar mapping data during four separate flights (mission) as shown in Table 3. Each mission followed the same pre-planned path consisting of nine north-south (N-S) and eight east-west (E-W) flight lines over a project area located near Saskatoon, Saskatchewan, Canada. The project area contained a variety of natural and built features including forested areas, open fields, roofed buildings, and large concrete pads (Figure 7). The UAS-lidar system collected mapping data from a nominal height of 67 m above ground level (AGL) during each mission. However, each mission was  
Figure 6: (**a**) OpenMMS UAS-lidar mapping system used in this study. (**b**) Mapping system installed on a Freefly Systems Alta X UAS. (**c**) Internal configuration of the APX-18 GNSS/INS sensor. (**d**) Relative positions of the mapping sensors. flown at a different horizontal velocity in order to examine the effects of sensor dynamics on heading precision (Table 3). At the beginning and end of each flight, an INS initialization procedure was performed following the manufacturer's instructions [27]. A local GNSS reference station was established at a location within the project area and consisted of a Trimble R10 GNSS receiver/antenna installed on a calibrated tribrach and was secured to a standard wooden-legged tripod. The position of the reference station as shown in Figure 7 was verified to have not changed over the course of the fieldwork. The reference station collected dual-frequency observations from all applicable GNSS satellites based on a 7.5\({}^{\circ}\) elevation mask at a rate of 1 Hz for 6.5 h. Upon completion of the fieldwork, the static GNSS observations were uploaded to the online Precise Point Positioning (PPP) service provided by Natural Resources Canada (NRCan) to determine accurate horizontal and vertical datum coordinates for the reference station. While collecting the static observations, the reference station also served as a real-time kinematic (RTK) \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Flight Name** & **Horizontal Velocity** & **Duration 1** & **Wind Direction (from)** & **Wind Speeds** \\ \hline A & 2.5 m/s & 657 s & S & 2–3 m/s \\ B & 5 m/s & 351 s & S–SW & 1–2 m/s \\ C & 10 m/s & 227 s & S & 2–3 m/s \\ D & 15 m/s & 178 s & S & 2–3 m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristics of each mission. Figure 7: Project area located near Saskatoon, Saskatchewan. INS initialization segments are shown as white dashed lines and mission flight lines are shown in black. The GNSS reference station is shown as a triangle and the GCPs as circles. GNSS base station for a ground survey. The RTK \"rover\" consisted of a second Trimble R10 GNSS receiver/antenna installed on a calibrated fixed-height survey pole. The RTK survey precisely measured the relative distances (i.e., baselines) from the reference station to 14 ground control points (GCPs) within the project area (Figure 7).