# How Passive Satellite Navigation Works Without Sending Signals > A 1970s system for finding your location on Earth by listening to satellite signals without ever having to transmit a signal yourself. - **Patent:** US 3789409 - **Original title:** Navigation system using satellites and passive ranging techniques - **Owner:** Individual - **Granted:** 1974 - **Status:** Public domain (expired) - **Times cited:** 31 - **Field:** aerospace, telecommunications, mechanical ## What it does This patent describes a way to find a location using satellites that only broadcast signals, rather than requiring the user to send a signal back. The system relies on two extremely stable oscillators—one on the satellite and one at the user's station—to keep time perfectly. By comparing the phase (the timing of the wave cycle) of the signals received from the satellite against the signals generated by the user's own equipment, the system calculates the distance to the satellite. Because the user only listens and never transmits, their position remains secret, which is the key feature of this passive ranging technique. ## What it does NOT cover - Does not cover active radar systems where the user must send a signal to be reflected back. - Does not cover systems that rely on signal strength (RSSI) rather than phase comparison of multifrequency signals. - Does not cover navigation methods that require the navigator to transmit an interrogation signal to the satellite. ## The clever bit The system uses multifrequency signals to resolve distance ambiguity: the low-frequency signal provides a rough estimate, while the high-frequency signal provides the precision needed for an accurate location fix. ## Real-world examples 1. Early satellite-based passive tracking systems 2. Military covert navigation hardware 3. Foundational architectures for modern GNSS receivers ## Why it matters This patent represents a foundational concept for covert navigation. By allowing a navigator to determine their position without emitting any radio frequency energy, it provided a blueprint for military and intelligence applications where maintaining radio silence is a matter of survival. It predates the widespread civilian adoption of GPS and highlights the transition from active, detectable navigation to passive, stealthy positioning. ## Frequently asked questions ### What does How Passive Satellite Navigation Works Without Sending Signals cover? A 1970s system for finding your location on Earth by listening to satellite signals without ever having to transmit a signal yourself. ### Who owns patent US 3789409? Individual 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 3789409 cited by? This patent has been cited by 31 later patents that build on its ideas. ### What problem does this patent solve? This patent represents a foundational concept for covert navigation. By allowing a navigator to determine their position without emitting any radio frequency energy, it provided a blueprint for military and intelligence applications where maintaining radio silence is a matter of survival. It predates the widespread civilian adoption of GPS and highlights the transition from active, detectable navigation to passive, stealthy positioning. ### What does this patent NOT cover? Does not cover active radar systems where the user must send a signal to be reflected back. **Full plain-English explainer:** https://patentbrief.org/patent/us/3789409/gps-satellite-navigation **Original patent:** https://patents.google.com/patent/US3789409 --- _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 to Calculate Distance to a Radio Signal Using Two Antennas](https://patentbrief.org/patent/us/3789410/gps-timation-navigation) — 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. - [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 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 Bluetooth Creates Wireless Networks with Unique Addresses](https://patentbrief.org/patent/us/6590928/bluetooth-frequency-hopping) — This 2003 patent describes how Bluetooth devices use a master device's address and clock to create a unique, hopping radio channel for communication and build a network map. - [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.