Drone detection radar
Radar detects the aircraft itself rather than its transmissions, so autonomy does not defeat it. Its difficulties are physical, and siting matters more than the specification sheet.
Key facts
- Radar detects the aircraft as a physical object, so an aircraft flying a pre-programmed route with no radio link is still detected, which is the principal case radio-frequency sensing misses.
- Small unmanned aircraft return very little energy to a radar, and the returns are similar in size to birds, which is why classification rather than raw detection is the hard engineering problem.
- Ground clutter from buildings, vehicles, terrain and vegetation is the dominant siting constraint, so mounting position and height often affect real performance more than the radar model does.
- Published detection ranges are obtained in favourable conditions against a stated target size, so a range figure is only meaningful alongside the target it was measured against.
- Radar detection is lawful for private operators in the United States, as it neither transmits at the aircraft in the sense of interfering with it nor acts on it.
What radar does that the alternatives do not
Radio-frequency detection is cheaper, passive, and often locates the operator. It has one structural weakness: it depends on the aircraft transmitting. An aircraft flying a stored route with its radio link off is silent, and radio-frequency sensing does not see it.
Radar detects the object. Autonomy, radio silence and darkness make no difference to it. For a site whose concern is someone deliberate rather than someone careless, that is usually the deciding property.
Why small aircraft are hard
A radar measures energy returned from a target. A small unmanned aircraft made largely of plastic returns very little, and what it does return resembles a bird.
The difficult engineering is therefore classification rather than detection: deciding whether a small, slow, low return is an aircraft of interest or a bird. Systems do this using movement characteristics and signal features, and they do it well enough to be operationally useful. False classification rates are a legitimate question for any supplier, and the answer is only meaningful when measured in an environment resembling yours.
Siting usually beats specification
The dominant constraint at most sites is ground clutter. Buildings, vehicles, terrain and vegetation all return energy, and a radar looking across a cluttered environment has to separate a faint target from a loud and complicated background.
The practical consequence is that mounting position and height frequently affect delivered performance more than the choice between two comparable radars does. A well-sited average radar outperforms a well-specified badly-sited one, which is why coverage design is not an optional preliminary.
Reading a range figure
Published ranges are measured in favourable conditions against a stated target. A range quoted without the target size it was measured against conveys very little, and the same radar will show materially different figures against a large fixed-wing aircraft and a small consumer quadcopter.
The useful specification question is not how far the radar reaches but whether the resulting coverage, at this site, detects the aircraft that matters early enough for the response to happen. Those are different questions, and only the second one has operational consequences.
Related work
FAQ
▸Why use radar when radio-frequency detection is cheaper?
Because they fail differently. Radio-frequency sensing depends on the aircraft transmitting, so an autonomous aircraft on a stored route is invisible to it. Radar does not care whether the aircraft is transmitting. Sites that face a deliberate intruder rather than a careless hobbyist tend to need the method that autonomy does not defeat.
▸Can radar tell a drone from a bird?
Modern systems classify using movement characteristics and signal features rather than size alone, and they do it well enough to be useful. It is not perfect, and false classification rates are a fair question to put to any supplier, tested against your own environment rather than a demonstration site.
▸Does radar need a licence?
Radar transmits, so the equipment must comply with the Communications Act and the applicable equipment authorisation rules. That is a different and lighter question than the operating authority required for mitigation, but it should be confirmed for the specific system before purchase.
▸How many radars does a site need?
That is what a coverage design determines, and it depends on the approaches that matter, the structures in the way and the standoff distance required. A figure quoted before anyone has looked at the site is a guess presented as a specification.