You can freely build on How Passive Satellite Navigation Works Without Sending Signals
This patent expired in 1991. Every claim — 0 independent, 1 dependent — is now unenforceable. Anyone can use, reproduce, manufacture, sell, or offer for sale this technology without a license.
Original assignee
Individual
Patent granted
1974
Expired
1991
Forward citations
31
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.
What is now free to use
All 1 claims of US 3789409 are in the public domain. Specifically:
The 1 dependent claim add narrowing limitations and are also free.
What is NOT covered
Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.
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.
Who is building on this today
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.
Products built on expired version of this technology
Early satellite-based passive tracking systems
Military covert navigation hardware
Foundational architectures for modern GNSS receivers
How to cite this patent in your documentation
Individual. US Patent 3789409. Navigation system using satellites and passive ranging techniques. Granted 1974, expired 1991. Now in the public domain.
Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.
PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.