Listening for the right radio signals could be an effective way to track small drones

The recent spate of unidentified drone sightings in the U.S., including some near sensitive locations such as airports and military installations, has caused significant public concern.

Some of this recent increase in activity may be related to a September 2023 change in U.S. Federal Aviation Administration regulations that now allow drone operators to fly at night. But most of the sightings are likely airplanes or helicopters rather than drones.

The inability of the U.S. government to definitively identify the aircraft in the recent incidents, however, has some people wondering, why can’t they?

I am an engineer who studies defense systems. I see radio frequency sensors as a promising approach to detecting, tracking and identifying drones, not least because drone detectors based on the technology are already available. But I also see challenges to using the detectors to comprehensively spot drones flying over American communities.

How drones are controlled

Operators communicate with drones from a distance using radio frequency signals. Radio frequency signals are widely used in everyday life such as in garage door openers, car key fobs and, of course, radios. Because the radio spectrum is used for so many different purposes, it is carefully regulated by the Federal Communications Commission.

Drone communications are only allowed in narrow bands around specific frequencies such as at 5 gigahertz. Each make and model of a drone uses unique communication protocols coded within the radio frequency signals to interpret instructions from an operator and to send data back to them. In this way, a drone pilot can instruct the drone to execute a flight maneuver, and the drone can inform the pilot where it is and how fast it is flying.

Identifying drones by radio signals

Radio frequency sensors can listen in to the well-known drone frequencies to detect communication protocols that are specific to each particular drone model. In a sense, these radio frequency signals represent a unique fingerprint of each type of drone.

In the best-case scenario, authorities can use the radio frequency signals to determine the drone’s location, range, speed and flight direction. These radio frequency devices are called passive sensors because they simply listen out for and receive signals without taking any active steps. The typical range limit for detecting signals is about 3 miles (4.8 kilometers) from the source.

These sensors do not represent advanced technology, and they are readily available. So, why haven’t authorities made wider use of them?

Challenges to using radio frequency sensors

While the monitoring of radio frequency signals is a promising approach to detecting and identifying drones, there are several challenges to doing so.

First, it’s only possible for a sensor to obtain detailed information on drones that the sensor knows the communication protocols for. Getting sensors that can detect a wide range of drones will require coordination between all drone manufacturers and some central registration entity.

In the absence of information that makes it possible to decode the radio frequency signals, all that can be inferred about a drone is a rough idea of its location and direction. This situation can be improved by deploying multiple sensors and coordinating their information.

Second, the detection approach works best in “quiet” radio frequency environments where there are no buildings, machinery or people. It’s not easy to confidently attribute the unique source of a radio frequency signal in urban settings and other cluttered environments. Radio frequency signals bounce off all solid surfaces, making it difficult to be sure where the original signal came from. Again, the use of multiple sensors around a particular location, and careful placement of those sensors, can help to alleviate this issue.

Third, a major part of the concern over the inability to detect and identify drones is that they may be operated by criminals or terrorists. If drone operators with malicious intent know that an area targeted for a drone operation is being monitored by radio frequency sensors, they may develop effective countermeasures. For example, they may use signal frequencies that lie outside the FCC-regulated parameters, and communication protocols that have not been registered. An even more effective countermeasure is to preprogram the flight path of a drone to completely avoid the use of any radio frequency communications between the operator and the drone.

Finally, widespread deployment of radio frequency sensors for tracking drones would be logistically complicated and financially expensive. There are likely thousands of locations in the U.S. alone that might require protection from hostile drone attacks. The cost of deploying a fully effective drone detection system would be significant.

There are other means of detecting drones, including radar systems and networks of acoustic sensors, which listen for the unique sounds drones generate. But radar systems are relatively expensive, and acoustic drone detection is a new technology.

The way forward

It was almost guaranteed that at some point the problem of unidentified drones would arise. People are operating drones more and more in regions of the airspace that have previously been very sparsely populated.

Perhaps the recent concerns over drone sightings are a wake-up call. The airspace is only going to become much more congested in the coming years as more consumers buy drones, drones are used for more commercial purposes, and air-taxis come into use. There’s only so much that drone detection technologies can do, and it might become necessary for the FAA to tighten regulation of the nation’s airspace by, for example, requiring drone operators to submit detailed flight plans.

In the meantime, don’t be too quick to assume those blinking lights you see in the night sky are drones.

Iain Boyd receives funding from the U.S. Department of Defense and NASA.