Drone Detection: Radar vs. RF Sensing (Radar Equation, RCS and Detection Math)

The radar range equation#

For a monostatic radar, the maximum detection range is set by the minimum signal-to-noise ratio the detector needs (Skolnik):

R_{m a x} = [\frac{P _{t} G ^{2} λ ^{2} σ}{( 4 π ) ^{3} k T _{0} B F L SNR _{m i n}}]^{1/4}

Everything except the target's radar cross section $σ$ is a property of the radar. So for a given radar, $R_{m a x} \propto σ^{1/4}$ : a target with one-hundredth the cross section is detected at about one-third the range.

How small is a small drone?#

Published measurements put small consumer quadcopters in the range of a few hundredths of a square metre at microwave frequencies. One study reports an average of −15.02 dBsm (about 0.03 m²) for a DJI Phantom 4 Pro at 15 GHz, VV polarisation; compact-range measurements at 15 and 25 GHz show strong variation with aspect angle. Relative to a 1 m² reference target, detection range scales as follows:

Target	σ (m²)	σ (dBsm)	Range relative to 1 m²
Reference target	1	0.0	1.00×
Small car (order of magnitude)	10	10.0	1.78×
Person (order of magnitude)	0.5	-3.0	0.84×
Phantom 4 Pro, 15 GHz VV (measured avg −15.02 dBsm)	0.0315	-15.0	0.42×
Small target, −20 dBsm	0.0100	-20.0	0.32×

The person and car rows are order-of-magnitude illustrations, not measurements. The drone sits close to many birds in cross section, which is why radar-only systems either miss small drones or alarm on birds. Classification — micro-Doppler from rotors, track behaviour, cueing a camera — matters as much as raw sensitivity.

How much SNR detection needs#

Albersheim's closed-form approximation gives the single-look SNR needed for a probability of detection $P_{d}$ at false-alarm probability $P_{f a}$ , non-coherently integrating $N$ pulses:

A = ln \frac{0.62}{P _{f a}}, B = ln \frac{P _{d}}{1 - P _{d}}

SNR_{dB} = - 5 lo g_{10} N + (6.2 + \frac{4.54}{N + 0.44}) lo g_{10} (A + 0.12 A B + 1.7 B)

P_d	P_fa	SNR, N = 1	SNR, N = 10	Range gain from integration
0.5	1e-6	11.2 dB	3.6 dB	1.56×
0.8	1e-6	12.5 dB	4.5 dB	1.58×
0.9	1e-6	13.1 dB	5.0 dB	1.60×
0.9	1e-4	11.7 dB	3.9 dB	1.57×
0.95	1e-6	13.6 dB	5.4 dB	1.61×

Integrating ten pulses cuts the required SNR by roughly 15 dB at $P_{d} = 0.9$ , which buys a factor of $1 0^{15.6/40} \approx 2.5$ in range — more than the factor a drone loses to its small cross section. Dwell time is the radar designer's main weapon against small targets, and it trades against update rate and coverage.

RF detection: seeing the link, not the aircraft#

RF sensors listen for the command, telemetry and video links between a drone and its controller. They can identify protocol families, sometimes make and model, and — with direction finding — locate the pilot, which radar cannot. Their blind spots are equally clear: a drone flying a pre-programmed route with its links silent, or using an unfamiliar protocol, is invisible to them. Some RF techniques also raise legal questions in the United States under interception statutes; the 2020 interagency advisory discusses this.

Layering#

Question	Radar	RF	EO/IR camera	Remote ID receiver
Is something there?	Yes	If transmitting	If cued	If compliant
Is it a drone?	With micro-Doppler/track	Yes	Yes	Yes
Where is the pilot?	No	Often	No	Take-off/control location
Works on silent autonomous drones?	Yes	No	If cued	No

Elevation helps every column: a radar or camera perched above ground clutter sees low-flying drones that a ground sensor loses behind buildings. That is the role Kestrel and Bulwark play in counter-UAS deployments, as cueing sensors for authorized responders. See the counter-drone briefing for program design.

Frequently asked questions#

Why is radar detection of drones difficult?

Small drones have radar cross sections of a few hundredths of a square metre, similar to birds, and detection range scales with the fourth root of cross section.

Can RF detectors find autonomous drones?

Not if the drone's radio links are silent. RF detection depends on transmissions; radar and cameras do not.

What is Albersheim's equation?

A closed-form approximation for the SNR required to achieve a given probability of detection and false alarm, for a non-fluctuating target with N pulses integrated non-coherently.

References#

M. I. Skolnik, Introduction to Radar Systems, 3rd ed., McGraw-Hill.
Albersheim's equation, as documented in MathWorks Phased Array System Toolbox, albersheim (required SNR for given P_d, P_fa and number of pulses; non-fluctuating target). mathworks.com
“Radar Cross Section Based Statistical Recognition of UAVs at Microwave Frequencies,” arXiv:2102.11954 (reports an average RCS of −15.02 dBsm for a DJI Phantom 4 Pro at 15 GHz, VV polarisation).
M. Ezuma, M. Funderburk, I. Guvenc, “Compact-Range RCS Measurements and Modeling of Small Drones at 15 GHz and 25 GHz,” arXiv:1911.05926.
C. J. Li, H. Ling, “Radar Signatures of Small Consumer Drones,” University of Texas at Austin.
U.S. DOJ, FAA, DHS and FCC, Advisory on the Application of Federal Laws to the Acquisition and Use of Technology to Detect and Mitigate Unmanned Aircraft Systems, August 2020. justice.gov

Drone detection: radar vs. RF sensing