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.
Original patent title: “Navigation system using satellites and passive ranging techniques”
A 1970s system for finding your location on Earth by listening to satellite signals without ever having to transmit a signal yourself. Granted to Individual in 1974 with 8 claims and 31 forward citations, and it is now in the public domain.
Coverage
What does this patent actually cover?
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.
The gap
What does this patent 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
Key facts
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Early satellite-based passive tracking systems
Military covert navigation hardware
Foundational architectures for modern GNSS receivers
Why it matters
The bigger picture
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.
Filed
October 8, 1970
Granted
January 29, 1974
Market context
Who's building on this
Companies in this space
The principles of passive ranging are now standard in the global navigation satellite system (GNSS) industry. Companies like Qualcomm, Broadcom, and major aerospace defense contractors continue to refine the signal processing techniques that allow modern devices to calculate location using passive satellite reception.
Market impact
This patent helped establish the technical feasibility of passive satellite navigation, moving the industry away from active interrogation methods. It influenced the development of secure, non-detectable positioning systems that are now essential for both modern defense infrastructure and the global logistics industry.
Claim 1 — Plain English
What this patent covers
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.
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.
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.
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
Patent enters public domain
This patent is in the public domain
See the Freedom to Build guide — what is free to use, what is not, and how to cite this patent.
PatentBrief Score
Impact Score
Early stage
Citation count
30/40
Moderately cited
Claim breadth
5/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$20K – $65K
Midpoint $41K · expired or expiring · industry ×1.5
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Patent Claims
0 independent claims · 1 dependent
Claims are the legal boundaries of the patent. An independent claim stands alone. A dependent claim adds limitations to its parent, narrowing — but not broadening — the scope.
The original legal language
Original claims
8 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Easton, R. (1974). How Passive Satellite Navigation Works Without Sending Signals (U.S. Patent No. 3,789,409). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3789409/gps-satellite-navigation
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
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.
Same assignee
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