Field Deployment Strategies for UAV-Based Chemical Sensors: Systematic Evaluation of Direct Drop, Tethered Hovering, and Low-Altitude Placement in Hazardous Chemical Scenarios

Unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs) equipped with chemical sensors hold great promise for hazardous incident reconnaissance, yet practical deployment is hindered by sensor survival upon impact, rotor downwash interference, and terrain-induced trade-offs between accuracy and efficiency. In this study, we focus on UAV-based deployment but also consider UGVs as complementary platforms for ground-level placement and long-term monitoring. To address these challenges for typical operational scenarios, we first implemented two customized engineering improvements: a carbon-fiber anti-downwash plate and a reinforced tether–release mechanism. Using a custom hexacopter with a 1 kg sensor prototype, we then systematically evaluated three deployment strategies. Direct free-fall from 5 m resulted in 0% survival due to housing fracture and circuit damage. The anti-downwash plate reduced local airflow by 70%, establishing a 6 m safe tether length. Tethered hovering achieved 100% survival, ~3 min/point mission time, and strong terrain adaptability, albeit with moderate dispersion. Low-altitude gentle placement delivered the highest accuracy and 100% survival but required 8 min/point and flat terrain. Based on these experimental results, we derive a mission-adaptive decision matrix for four representative chemical scenarios, recommending tethered hovering as the primary strategy for complex terrains, low-altitude placement for fixed long-term monitoring on flat surfaces, and direct drop only if cushioning is added and height is reduced to ≤3 m. We further discuss how the decision matrix can be extended to multi-sensor UAVs, UGVs, and UAV–UGV collaborative systems. Our findings offer a practical, evidence-based operational framework tailored to unmanned chemical reconnaissance.