Using IA for drones
Flying drones indoors and outdoors using Inverse Architecture (IA) is possible. However, it is trickier than with NIA or MF NIA. Thus, we recommend it only to experienced users who clearly understand the technology.
The apparent issue with IA and drones is that rotors and motors generate wideband noise that can easily transition from the audible spectrum to ultrasound, blocking the reception of the ultrasound signal by the mobile beacon on the drone.
The problem, per se, is not that drones are noisy. The problem is that the stationary beacons are located far away (meters or tens of meters). At the same time, the rotors and motors are very close to the mobile beacons (centimeters or small tens of centimeters). Thus, the ultrasound pulses from the stationary beacons are significantly more attenuated as they propagate in the air than the noise from the very close rotors.
However, the problem is not the level of noise until it becomes so powerful that the microphone is completely saturated. The potential problem is the signal-to-noise ratio.
Fortunately, special measures can be taken to enhance the signal-to-noise ratio, thereby making IA with drones feasible.
Noise-reflecting shield
The most straightforward solution is to install a noise-reflecting shield protecting the microphone from the noise coming from the propellers and motors:
- Ultrasound is well reflected even by very thin materials, for example, thin plastic. Even a sheet of paper can create a good ultrasound shield. The chosen variant for protection is mostly driven by the mechanics and robustness of the shield
- The shield must be clearly above the rotors
- Stationary beacons must always be above the shield because they would be equally shielded otherwise
- Other drones must not fly too close or above the drone
Other key solutions
- Special sharp and narrow digital filters. The drone’s rotors can produce a powerful noise, but it is primarily within the audible part of the spectrum and is wideband. Special narrow and sharp digital filters inside the mobile beacons cut that noise off, and only the portion that falls in the reception band of each frequency can potentially be harmed by reducing the signal-to-noise ratio
- Play with ultrasound frequencies. The wideband noise from robots and motors extends to ultrasound, but its level drops rapidly with increasing frequency. Thus, the noise level at 25kHz can be expected to be higher than that at 45kHz. However, depending on the frequency, the ultrasound is also attenuated in the air. The higher the frequency, the higher the attenuation; the law is pretty complex. Additionally, the sensitivity of the microphones and the power transmission of the transducers are frequency-dependent. Thus, it isn’t easy to forecast which frequency will give the best signal-to-noise ratio in a particular setup (given the distances). Thus, practical testing is highly advisable, particularly because it’s very easy and very fast (seconds) with the help of an embedded Oscilloscope. More about different ultrasound frequencies
Inverse Architecture (IA) is a powerful solution that can provide a high location update rate for even swarming drones and drone shows.