# How to Calculate Distance to a Radio Signal Using Two Antennas > A 1970s Navy system that calculates the distance to a radio-emitting target by measuring the tiny time and phase differences between signals arriving at two separate antennas. - **Patent:** US 3789410 - **Original title:** Passive ranging technique - **Owner:** US Department of Navy - **Granted:** 1974 - **Status:** Public domain (expired) - **Times cited:** 16 - **Field:** telecommunications, mechanical, aerospace ## What it does This system determines how far away a radio-emitting source is without the source knowing it is being tracked. It uses two antennas placed at a known distance from each other to capture incoming radio waves. By measuring the difference in the time it takes for a signal to reach each antenna, the system calculates the bearing (direction) to the source. It then uses a phase rate computer to analyze the 'beat frequency'—a pattern created by the interaction of the signals—to determine the distance. This allows a vehicle to locate a target passively, meaning it does not need to send out its own radar pulses that would reveal its own position. ## What it does NOT cover - Does not cover active radar systems that emit signals to detect targets. - Does not cover systems that use more than two antennas for triangulation. - Does not cover methods that rely on signal strength (RSSI) to estimate distance. - Does not cover systems that require the target to cooperate or transmit a specific identification code. ## The clever bit It uses the 'phase rate' of the incoming signal—the frequency shift caused by the vehicle's own movement relative to the target—to calculate distance from a single moving platform, effectively turning the vehicle's own motion into a synthetic baseline. ## Real-world examples 1. Passive electronic warfare suites on naval destroyers 2. Electronic intelligence (ELINT) gathering aircraft 3. Modern passive radar tracking systems ## Why it matters This technology was vital for military stealth. By allowing a vehicle to 'see' without being 'seen,' it provided a massive tactical advantage in electronic warfare. It laid the groundwork for modern passive electronic support measures used in maritime and aerial reconnaissance. ## Frequently asked questions ### What does How to Calculate Distance to a Radio Signal Using Two Antennas cover? A 1970s Navy system that calculates the distance to a radio-emitting target by measuring the tiny time and phase differences between signals arriving at two separate antennas. ### Who owns patent US 3789410? US Department of Navy owns this patent, granted in 1974. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3789410 cited by? This patent has been cited by 16 later patents that build on its ideas. ### What problem does this patent solve? This technology was vital for military stealth. By allowing a vehicle to 'see' without being 'seen,' it provided a massive tactical advantage in electronic warfare. It laid the groundwork for modern passive electronic support measures used in maritime and aerial reconnaissance. ### What does this patent NOT cover? Does not cover active radar systems that emit signals to detect targets. **Full plain-English explainer:** https://patentbrief.org/patent/us/3789410/gps-timation-navigation **Original patent:** https://patents.google.com/patent/US3789410 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Passive Satellite Navigation Works Without Sending Signals](https://patentbrief.org/patent/us/3789409/gps-satellite-navigation) — A 1970s system for finding your location on Earth by listening to satellite signals without ever having to transmit a signal yourself. - [How a Digital Phased Array Antenna Steers Radio Signals Electronically](https://patentbrief.org/patent/us/12738649/system-and-method-for-a-digitally-beamformed-phased-array-feed-12738649) — This patent describes a system that uses many small antennas and digital processing to precisely aim and receive radio signals, like a smart, electronic spotlight for radio waves. - [How a Digital Phased Array Antenna System Works](https://patentbrief.org/patent/us/12738648/system-and-method-for-a-digitally-beamformed-phased-array-feed) — This patent describes a compact, multi-band antenna system that uses digital technology to steer radio waves, allowing it to communicate in multiple directions and frequencies without physically moving. - [How a Compact Digital Phased Array Antenna System Works](https://patentbrief.org/patent/us/12738650/system-and-method-for-a-digitally-beamformed-phased-array-feed-12738650) — This patent describes a system for an advanced antenna that can steer radio signals using digital technology, packed into a compact, managed unit. - [A Digital Antenna System for Steering Radio Signals](https://patentbrief.org/patent/us/12738651/system-and-method-for-a-digitally-beamformed-phased-array-feed-12738651) — This patent describes a digital antenna system that uses multiple small antennas and digital processing to precisely aim radio signals across different frequency bands, while also managing power and heat.