A range figure can look wonderfully decisive on a specification page. Twenty kilometres: there it is, measured and printed. It is tempting to turn that number into a circle on a map and call everything inside it reachable.

That is not what the number means.

What the manufacturer actually measured

DJI lists a maximum transmission distance of 20 km for the Air 3S under the FCC standard, and 10 km under CE, SRRC and MIC standards. The footnote matters more than the headline: the result was measured outdoors, unobstructed and free of interference. DJI describes it as the farthest communication range for a one-way, non-return flight and tells pilots to watch the app’s Return-to-Home reminders.

The same official specification page gives much shorter reference ranges when interference or obstacles enter the picture. Under FCC test conditions it quotes roughly 1.5–4 km in strong urban interference. With buildings obstructing the path, the reference range falls to 0–0.5 km. DJI also says those figures do not guarantee an actual transmission distance.

That gap is not a contradiction. A radio link is affected by the regulatory region, frequency use, antennas, interference, terrain, buildings, trees and the orientation of the aircraft and controller. A laboratory-style maximum answers a narrow engineering question. It does not describe the route you are about to fly.

The legal limit may arrive long before the radio limit

In the United States, the FAA tells both recreational and Part 107 pilots to keep the drone within visual line of sight. For recreational flying, the operator or a co-located visual observer must be able to see the aircraft and the surrounding airspace throughout the flight. The FAA’s recreational flyer guidance and Part 107 overview do not turn a strong video feed into permission to fly farther.

The UK CAA makes the same practical distinction. Its BVLOS guidance says that a pilot normally must keep the drone in direct sight. A clear image on the controller shows what the camera sees; it does not give the pilot a complete view of the airspace around the aircraft.

A useful way to read the range figure

Treat maximum transmission distance as evidence about the radio system, not as a route-planning allowance. Before take-off, the more useful questions are smaller and local:

• Can I keep the aircraft and the surrounding airspace in view for the whole route?
• What buildings, trees or terrain will interrupt the path between the controller and the drone?
• What interference is likely here, rather than at the manufacturer’s test site?
• How much battery must remain for the return leg, including wind and a diversion?
• What will the aircraft do if the control link is lost, and is that setting safe for this location?

A link that stays connected is valuable. It gives the pilot better telemetry, video and control. But connection is only one layer of a safe flight. Sight, airspace, battery, weather and a clear recovery plan can all set a shorter limit—and usually should.