Flying your drone at low altitude over water can be hazardous to its health. However, there are times when professional aerial drone photographers and videographers will have to do precisely that. The drones I fly are manufactured by DJI, which discourages flying over water.
DJI Manufacturer Statements Include:
The Vision System requires clear pattern variations, with light conditions greater than 100 lux. Further, they state that users should operate the aircraft with great caution when flying over water or transparent surfaces. And specifically, Vision Positioning may not function properly when the aircraft is flying over water.
What is a Vision System?
Vision systems may use ultrasonic sensors, infrared sensors, and cameras to detect objects in close proximity to the drone. Several drone models use combinations of sensors to accurately hold altitude and to enable object tracking functions.
How does a Vision System work?
Vision systems are used for short range detection and ranging. Ultrasonic-based sensors operate at about 40 kHz and use pulsed sonar techniques to detect the nearest object. Camera-based sensors employ image processing to determine objects that not only include the ground below but also people and moving vehicles.
DJI states their vision systems rely on very sophisticated image processing to detect nearby objects. Two models that I have owned are the Phantom 3 Professional and the Phantom 4 Professional Version 2. Both employ a combination of ultrasonic and camera sensors to determine their altitude above ground.
At low altitude, the fusion algorithm prioritizes the camera sensor above the ultrasonic sensor and the altimeter (barometer). e.g. the drone’s control system maintains a certain altitude that is stabilized by its downlooking camera. The problem with flying over water is that the downlooking camera sees what the human eye will see, including objects below the surface such as the bottom.
When hovering over water (or any other transparent object), the processor may be fooled into thinking the drone is flying too high, so it orders the drone to decrease its altitude. This happens fairly quickly, which risks the drone dropping into the water.
When you Absolutely Must Fly over Water . . .
If you have to fly below 2 meters for a special shot, we recommend that you turn off the Vision System to avoid unstable movements by the drone. DJI recommends that you fly the drone at low speed and stay alert to adjust altitude.
What else could possibly cause Altitude Issues over Water?
Other physical effects could fool the drone’s processor into decreasing altitude, but only one scenario seems to fit. For an ultrasonic sonar sensor, the sensitivity time control could cause a near-water second echo to be larger than the first echo, which would read a higher altitude.
What about Sonar Returns off the Sediment?
Although sound waves can penetrate the air-water interface, the transmission loss is about 99.95% – each way! Remember, the sound has to go back through the water-air interface for another loss of 99.95%. There’s just not enough signal return to fool the sensor.
You can likewise rule out other physical effects because they would lead the processor to read a lower altitude. These include: ground effect on the barometer and the increased sound speed caused by prop wash water vapor. In other words, the processor would be fooled into increasing the drone’s altitude.
Flying your drone at low altitude over water can risk losing your aircraft. If you must fly under these conditions, then turn off the Vision Positioning System, maintain a minimum altitude of 2 meters, and keep a close watch on your drone.