# Can radar detect a small flying device with MEMS microengines?

I have seen some MEMS micro engines and some micro turbines and I would guess they can be attached to some payload to fly. Is it possible for commercial/military radars to detect those micro flying devices?

Obviously the MEMS engine itself is not going to be detected: they're only a few millimetres across. The question entirely depends on what you attach it to. Conventional radars have trouble detecting UAVs less than a couple of metres long, and replacing their power plants with micro turbines won't change that.

My neighbours at Aveillant already have a holographic radar system that can detect smaller targets such as UAVs more accurately. Again, changing the power plant technology won't change that.

Micro-turbines are not intended to replace propulsion systems, but to replace batteries, allowing them to use denser hydrogen, methane, or methanol fuel. As far as I've been able to find, they don't produce break-even power yet, but it's envisioned that they will become commercially relevant once they can produce more than 10 W (about .01 hp), in a few years' time. For comparison, a current, small, military UAV might generate tens of horsepower, and some experimental super-lightweight UAVs need as little as 5 hp, but with no payload or cargo.

TL;DR: (a) MEMS turbines aren't going to be relevant for UAVs for some years, and they'll have to be much bigger than they are now; (b) replacing the power plant won't make much difference to the radar signature.

Radar detection depends on the scattering of radar waves on objects. If we use the common X-band (8-12 GHz frequency) as an example, the wavelength $$\lambda$$ of the radar waves is in the order of 3 cm. Now the size of the target comes into play, expressed as a ratio α, defined as $$\alpha = \frac{\pi\cdot D}{\lambda}$$ with $$D$$ the diameter of the object to be detected.

α ≪ 1: Rayleigh scattering (small particle compared to wavelength of light). Radiation energy is diffused without a preferred direction, strongly dependent on wavelength.

α ≈ 1: Mie scattering (particle about the same size as wavelength of light, valid only for spheres)

α ≫ 1: geometric scattering (particle much larger than wavelength of light). Radiation energy is reflected with the angle of reflection equal to the angle of incidence.

Only for large α you will get a return signal which can be used for detection. Then we speak no longer of scattering, but of reflection. If your MEMS engine is just a few mm across and attached to a plastic airframe (which should be transparent to radar waves), it should be impossible to detect with conventional radars. Since the atmosphere is rather opaque for microwave radiation above 20 GHz, infrared or optical detectors will be needed to detect such an aircraft.

Also, the software driving the radar display is an important factor. Common aviation radars will filter out small, slow-moving objects, so even a metallic or carbon-composite airframe several centimeters across will not be detected.

• Could you please explain the difference between scattering and reflexion, in this context? Commented Mar 13, 2022 at 20:50

It does not matter what type of engine is used; if the Radar is powerful enough and the flying object is near enough (making allowances for any low observable design) i.e. based on the Radar Cross Section, it will be detected by the Radar.

In case of small flying objects, the main problem is not in detection (Radars usually pick up bugs and such) but in recognizing the object in background clutter.

In this case, the military Radars have an edge as they are more powerful and are optimized for detection of small targets in ground clutter.

• Some have even received anti-UAV updates (software mostly) in the past few years. Commented Sep 25, 2015 at 15:45