You can freely build on How to Calculate Distance to a Radio Signal Using Two Antennas
This patent expired in 1992. 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
US Department of Navy
Patent granted
1974
Expired
1992
Forward citations
16
What this patent covers
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 is now free to use
All 1 claims of US 3789410 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 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.
Who is building on this today
Major defense contractors like Lockheed Martin, Northrop Grumman, and Raytheon continue to refine passive sensing technologies. These companies build upon the fundamental principles of interferometry and phase-difference measurement established in early patents like this one.
Products built on expired version of this technology
Passive electronic warfare suites on naval destroyers
Electronic intelligence (ELINT) gathering aircraft
Modern passive radar tracking systems
How to cite this patent in your documentation
US Department of Navy. US Patent 3789410. Passive ranging technique. Granted 1974, expired 1992. 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.