Tuesday Views 21 September 2021

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Drone in Fog –

Spoiler: No bueno.

Drones can take advantage of diffused lighting for aerial shots. There are no sharp shadows, no murky surfaces and, more often that not, a chance of flying with very little wind. Surfaces look softer, saturated with color. I flew briefly in fog to show what it does to your drone.

Moisture in the air shows up as a bluish haze, and moisture and electronics seldom mix. Water vapor refracts scatters and refracts light. Pro drone avionics might be “waterproofed”, but the aerosol might throw off sensors and positioning data, and it can also dull the image by scattering the light and absorbing it. Fog is a big deal. It can be pleasing and a problem at the same time. Without thermal cameras, my drone is sort of stumbling through an ever-changing medium. Fog is difficult to test because, really, what is standard fog, anyways? What is the gold standard of “fog”. The relatively stochastic nature of fog make forgetting about fog extremely attractive, but dangerous.

If you happen to fly through temperature inversions, you also need to consider icing on propellers, cameras, sensors, and avionics.

Another problem I have found with flying in fog is a loss of signal, and that affects control options for shots. Your range is cut, and your duration is cut. What is the fog like above me? If fog is in between the drone, the controller, me, and a satellite array overhead, how do you quantify and react to random dissipative events like fog? There seems to be a bit of latency. In any case, if you are interested in image segmentation, here is a rabbit hole for you.

Figure 1, Trailing edge turbulence due to flow separation on aerodynamic door handle

Unnecessary Prizes! Woo hoo! This door handle (Figure 1) was on a car that moved through a pea soup fog for five minutes at 20 – 40 mph. Water condensed on the chrome finish of the plastic handle. So, one would expect points on a drone body or chassis to also gather water due to flow separation of moist, turbulent air.

So, I flew for nearly nine minutes, aware of the risk. My drone uses cameras above and below its chassis to triangulate its position in relation to other objects down to a certain resolution. As drops form, combine, and slide across the camera lenses, the drone will consume more resources to account for this noise. It might arrive at computation that ‘thinks’ a tendril or unusual concentration of fog is as solid as a person, and suddenly avoid it.

Fog Creates Droplets to Form and Gather as a Result of Flow Separation

If I had tufts attached all over the drone, theoretically, long enough to matter, but incapable of tangling the props or motors, I would be able to observe more clearly how the fog wets the drone (Figures 2,3).

Figure 2. Flow separation over rear CCW propeller where turbulence sweats the drone within dense fog just like it
Figure 3. Condensation directly over front right camera from CW turning propeller.

Almost all of the maneuvers involved straight, forward motion, at around 2/3 max throttle. This filming session occurred alongside a railroad track sandwiched between a steeply graded riverbank, and a long, tall, abandoned brick structure.

Notice the Doppler effect shift in pitch as the two locomotives pass alongside the structure and squishes the sound waves together (Figure 4).

Figure 4. Union Station in thick fog

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About Me

I became a drone technology service integration researcher in 2017 while in university. When I am not working on drones, I am researching related scientific reports and industry news, flying drones, or tinkering with electronics. Contact me!

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