Fig. 1. Classification results of UAVs based on aerodynamics, landing, size, and performance ^[26-36]

Fig. 2. Photographs of 3D printed mini-MPC^[42]. (a) Seven nonintersecting-circle spot pattern; (b) ray trajectories between two spherical mirrors; (c) mini-MPC with a gas inlet and an outlet; (d) top view of the mini-MPC

Fig. 3. Photograph of QTF-thermal detector ^[50]

Fig. 4. External and internal images of non-distributed infrared sensor ^[43]

Fig. 5. Optical fiber coupled photoacoustic detection module^[64]. (a) CAD image of photoacoustic detection module; (b) photo of photoacoustic detection module

Fig. 6. A side-by-side comparison of conventional QEPAS and BF-QEPAS techniques^[66]

Fig. 7. Schematic of airborne sensing system

Fig. 8. A UAV carries a miniaturized methane gas sensor ^[82]

Fig. 9. Photographs of the gas sensing platform based on a helicopter UAV^[83]. (a) Multi-pass cell in the vertical cavity surface emitting lasers sensor; (b) helicopter UAV with VCSEL sensor onboard

Fig. 10. Photographs of non-dispersive infrared gas sensor based on UAV^[84]. (a) Components of the portable CO₂ sensor system setup; (b) selected multirotor UAV

Fig. 11. Photograph and schematic of non-dispersive infrared gas monitoring platform based on UAV^[85]