3.2.2. UAS Equipment

Three different types of UAS were tested in this study to assess the variety of UAS as a platform: one rotary-wing UAS (Inspire 2), one hybrid UAS (FireFLY 6) and one fixed-wing UAS (DT18). The consciously chosen platforms have different specifications in terms of operability, coverage, price, and necessity of ground control measurements. This study set-up reflected the broad spectrum of commonly used UAS and allowed to acknowledge these varieties within the assessment of the fitness of use. All platforms were equipped with an RGB sensor to capture nadir images (Table 3). The Inspire2 from DJI refers to semi-professional UAS with a focus for filmmaking. Both, the FireFLY6 and the DT18 PPK are survey-grade UAS of which the FireFLY6 presents a lower cost solution, and the DT18 PPK refers to a professional UAS with high-end components. The DT18 PPK is equipped with a combined Inertial Measurement Unit/ GNSS solution from Applanix (APX15) which allows direct georeferencing and minimizes the need for ground control measurements.


**Table 3.** Specifications of UAS used in this study.

During flight planning, the first step for the UAS data collection, areas for take-off and landing, the UAS trajectory and the flying height are specified. A typical procedure to create the flight trajectory with strips is 80% forward overlap and 40–80% side overlap [1] since redundancy can compensate for aircraft instabilities. The flight planning configurations in this study were constraint by the regulations (operational limitations), requirements for accurate data in an urban environment and external flight conditions such as wind and illumination. Therefore, taking all these factors into account, the flight missions were carried out with 80% forward and 70% side overlap. According to the regulations, flight height was set to 100m above the surface. All UAS were equipped with a RGB sensor and resulting ground sampling distances varied between 2–3cm depending on the specification of the camera.

To emphasize the diversity of possible flight configurations, data collection included three different contexts–one urban study area, one peri-urban and one rural study area. The location of the study sites is visualized in Figure 3.

**Figure 3.** Overview of UAS data collection sites in Rwanda.

Due to the limited availability of open spaces, a hybrid UAS for the data collection of the densely populated urban environment in Muhoza Sector was chosen. In contrast, the rotary wing UAS was used to collect the images of the peri-urban area in Busogo Sector, as it is located on volcano slopes and highly affected by local wind systems which strongly influences the stability of fixed-wing UAS. Due to the regulatory restrictions and the difficulty to find sufficient large landing sites, the fixed-wing flight was conducted from a cricket stadium 20 km south of the City of Kigali. At the time of data acquisition, the maximum lateral distance of the pilot to the UAS was limited to 300m. Since only one area provided sufficient space for landing, the DT18 could only capture images over the cricket stadium which is embedded in a rural area in Gahanga Sector. Both, the Inspire 2 and the FireFLY6 were equipped with a consumer-grade GNSS antenna allowing geotagging of all images. However, the measurements of additional Ground Control Points (GCPs) indispensable. In contrast, the DT18 stood out for its high-quality navigation sensor that records precise attitude logs including both, angular observations (< 10 arcmins) as well as camera positions (<2.3 cm) [41]. However, former test flights showed, that the DT18 requires additional GCPs to correct for (minor) systematic errors [41], particularly when no GNSS corrections are applied.
