# 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 Edwin Armstrong Invented the Superheterodyne Radio Receiver](https://patentbrief.org/patent/us/1342885/superheterodyne-radio-armstrong) — A foundational 1920 patent by Edwin Armstrong that describes the superheterodyne circuit, the technology that allowed radios to tune into specific stations clearly and reliably. - [How Radio Frequency Identification (RFID) Tags Were Invented](https://patentbrief.org/patent/us/3713148/rfid-transponder-cardullo) — A 1970 patent describing a remote tag that powers itself using incoming radio signals to read and write data, forming the foundation of modern RFID technology. - [How Early Cell Phones Handled Calls Across Different Towers](https://patentbrief.org/patent/us/3906166/cellular-mobile-phone-radio-telephone) — This patent describes a system for early portable phones to automatically find the strongest signal from a base station and switch channels as the user moves, reducing battery drain and interference. - [How the Theremin Makes Music Without Touching Anything](https://patentbrief.org/patent/us/1661058/theremin-leon-theremin) — Leon Theremin's 1928 patent for an electronic musical instrument that generates sound based on the proximity of a performer's hands to metal antennas.