{
  "patent_number": "US 3789409",
  "country": "US",
  "title": "How Passive Satellite Navigation Works Without Sending Signals",
  "original_title": "Navigation system using satellites and passive ranging techniques",
  "summary": "A 1970s system for finding your location on Earth by listening to satellite signals without ever having to transmit a signal yourself.",
  "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."
  ],
  "filed": "1970-10-08",
  "granted": "1974-01-29",
  "expires": "1991-01-29",
  "status": "expired",
  "holder": "Individual",
  "holder_url": "https://patentbrief.org/company/individual",
  "inventors": [
    {
      "name": "R Easton",
      "url": "https://patentbrief.org/inventor/r-easton"
    }
  ],
  "times_cited": 31,
  "tags": [
    "aerospace",
    "telecommunications",
    "mechanical"
  ],
  "abstract": "A navigation system wherein the navigator''s location is obtained by determining the navigator''s distance (or range) from one or more satellites of known location. Each satellite transmits multifrequency signals that are derived from a stable oscillator which is phase synchronized with the navigator''s equipment that produces similar multifrequency signals. Phase comparison between the signals received from the satellites and the locally produced signals indicates both the distance between the navigator and the satellites and the navigator''s location. In determining his location, the presence of the navigator is not revealed since no interrogatory transmission by him is required.",
  "url": "https://patentbrief.org/patent/us/3789409/gps-satellite-navigation",
  "markdown_url": "https://patentbrief.org/patent/us/3789409/gps-satellite-navigation/md",
  "google_patents_url": "https://patents.google.com/patent/US3789409",
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}