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In recent years, consumer drones have gained massive popularity for capturing breathtaking footage, delivering emergency supplies, and even competing in aerial races. Most modern drones rely on a range of sensing technologies to enable autonomous navigation, obstacle avoidance, and more. Among these, ultrasonic sensors play a crucial role, especially when it comes to landing, hovering, and ground tracking.
The drone's landing assistance system is designed to detect the distance between the drone and the landing surface, assess if the area is safe, and then guide the drone down smoothly. While GPS, barometric pressure sensors, and other technologies contribute to the landing process, ultrasonic sensing remains the most accurate and reliable method for measuring proximity during this critical phase. Many drones also use hover and ground tracking modes to maintain a stable height above the ground, which is essential for filming or navigating complex terrains—again, where ultrasonic sensors prove invaluable.
This blog post explores the reasons behind the widespread use of ultrasonic sensing in drone applications. It starts by explaining how ultrasonic waves work and then dives into the Time of Flight (ToF) principle used in drone landing systems.
Ultrasonic waves are sound waves with frequencies higher than the upper limit of human hearing, typically above 20 kHz. These waves can travel through various media, such as air, water, or solid objects, and are reflected when they encounter a surface with different acoustic impedance. The speed of sound in dry air at 20°C is approximately 343 meters per second. However, ultrasonic signals in air experience greater attenuation at higher frequencies and in humid conditions, which limits their effective range to below 500 kHz.
The ToF technique measures the time it takes for an ultrasonic pulse to travel from the sensor to an object and back. This information is used to calculate the distance between the drone and the surface below. The formula for calculating distance using ToF is:
**Distance (d) = (ToF × Speed of Sound) / 2**
This division by two accounts for the round-trip nature of the signal.
Ultrasonic sensing is particularly advantageous for drone landing due to its cost-effectiveness, reliability, and ability to work across a variety of surfaces. For instance, while light-based sensors may struggle with transparent materials like glass, ultrasonic waves can reliably reflect off such surfaces, making them ideal for detecting landing zones on buildings or other structures.
Texas Instruments’ PGA460 is an example of an integrated ultrasonic signal processor that enhances the performance of drone landing systems. It supports both half-bridge and H-bridge drive configurations, making it versatile for different sensor types. The device also includes an advanced analog front end for receiving and processing echoes, along with digital signal processing capabilities to accurately measure ToF.
Moreover, the PGA460 excels in near-field detection, allowing drones to sense objects as close as 5 cm. This is achieved by separating the transmitter and receiver, enabling continuous monitoring even during the sensor’s oscillation period.
As drone technology continues to evolve, the use of ultrasonic sensing is expanding beyond landing and hovering. Its combination of affordability, accuracy, and adaptability makes it a key player in the growing drone industry. Whether it's for commercial, recreational, or industrial purposes, ultrasonic sensing is proving to be an essential component in the future of aerial robotics.