Signals of Opportunity: The Illuminators Powering Passive Radar (Part 2 of 4)

This is Part 2 of a 4-part series on passive radar. See Part 1: Introduction to Passive Radar for the foundation.

What Can We Borrow?

Passive radar systems need existing transmitters to illuminate targets—what engineers call “emitters of opportunity.” The options are surprisingly diverse: FM radio stations, digital television broadcasts (DVB-T), cellular base stations, GPS satellites, and even low-earth orbit constellations like Starlink.

Each signal type offers different characteristics. The question becomes: which signals work best for detecting aircraft or drone?

The Ideal Illuminator

Four factors determine how well a signal performs for passive radar:

High power means longer detection range. FM stations can transmit at 250 kW, while cellular towers operate around 100 W—a 2,500× difference that translates directly into coverage.

Wide bandwidth enables better range resolution. DVB-T’s 6-8 MHz bandwidth can distinguish targets separated by just 25 meters. FM’s narrow bandwidth cannot.

Continuous transmission ensures 24/7 surveillance capability. Broadcast signals never stop. Cellular traffic fluctuates with demand.

Favorable signal structure matters for extracting weak echoes. OFDM signals—used in digital TV—have mathematical properties that make target detection more reliable than analog transmissions.

Why Broadcast TV Dominates

Digital television emerged as the gold standard for passive radar. DVB-T combines all four requirements: wide bandwidth for resolution, OFDM structure for reliable detection, high transmit power for range, and continuous operation for persistent coverage.

This is not theoretical. Operational systems already exploit broadcast TV: Hensoldt’s TwinVis in Germany, ERA’s Vera-NG in the Czech Republic, and Silentium Defence’s Maverick in Australia—all using DVB-T. The United States and South Korea, now transitioning to ATSC 3.0, present unexplored territory.

But DVB-T has a limitation. Its pilot structure—the reference symbols broadcast engineers use for channel estimation—creates a side effect: false targets appearing every 12.8 km in range. This ambiguity constrains how far a single-transmitter system can reliably operate.

Next in this series: Why do these ghost targets appear, and how do engineers suppress them?