How to Calculate Distance to a Radio Signal Using Two Antennas
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.
Original patent title: “Passive ranging technique”
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. Granted to US Department of Navy in 1974 with 6 claims and 16 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
What does this patent NOT cover?
- 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
It uses the 'phase rate' of the incoming signal—the frequency shift caused by the vehicle's own movement relative to the target—to calculate distance from a single moving platform, effectively turning the vehicle's own motion into a synthetic baseline.
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
Passive electronic warfare suites on naval destroyers
Electronic intelligence (ELINT) gathering aircraft
Modern passive radar tracking systems
Why it matters
The bigger picture
This technology was vital for military stealth. By allowing a vehicle to 'see' without being 'seen,' it provided a massive tactical advantage in electronic warfare. It laid the groundwork for modern passive electronic support measures used in maritime and aerial reconnaissance.
Filed
January 7, 1972
Granted
January 29, 1974
Market context
Who's building on this
Companies in this space
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.
Market impact
This patent represents an early milestone in the shift toward passive sensing in military hardware. It helped define the category of Electronic Support Measures (ESM), which is now a standard requirement for modern combat platforms to avoid detection while maintaining situational awareness.
Claim 1 — Plain English
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.
The clever bit
It uses the 'phase rate' of the incoming signal—the frequency shift caused by the vehicle's own movement relative to the target—to calculate distance from a single moving platform, effectively turning the vehicle's own motion into a synthetic baseline.
What it does not cover
- 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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Early stage
Citation count
25/40
Moderately cited
Claim breadth
4/20
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
$11K – $36K
Midpoint $23K · expired or expiring · industry baseline
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
The original legal language
Original claims
6 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Smith, L., & Sayano, K. (1974). How to Calculate Distance to a Radio Signal Using Two Antennas (U.S. Patent No. 3,789,410). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3789410/gps-timation-navigation
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
Embed
Add this patent to your site
Drop this plain-English patent card into any blog post or article — free, no signup. It always links back to the full breakdown here.
<div data-patentlens-widget data-patent-number="US3789410"></div> <script src="https://patentbrief.org/embed.js" async></script>
Stay in the loop
Get a weekly digest of new patents.
One email per week. No spam. Unsubscribe anytime.
Keep exploring
Related patents you should know
US 4683195 · 1987
How to Make Billions of Copies of a DNA Segment
This patent describes the Polymerase Chain Reaction (PCR), a method to rapidly create many copies of a specific piece of DNA or RNA, enabling its detection and analysis.
Cetus Corp
US 8697359 · 2014
How to Edit Genes in Human Cells Using an Engineered CRISPR System
This patent describes an engineered CRISPR-Cas9 system for precisely cutting DNA in eukaryotic cells to change how genes work, opening the door for gene editing in complex organisms.
Massachusetts Institute of Technology
US 7657849 · 2010
How the iPhone's Slide-to-Unlock Gesture Works
Apple's 2010 patent describes unlocking a device by dragging a specific graphical image across the touchscreen along a predefined path, a gesture that became iconic with the original iPhone.
Apple Inc
US 4733665 · 1988
How Doctors Implant a Permanent Stent Using a Balloon
This patent describes the method for placing a permanent, expandable wire mesh tube inside a blood vessel or other body tube using a balloon-tipped catheter to widen it and keep it open.
Expandable Grafts Partnership
US 4405829 · 1983
How RSA Public-Key Encryption Keeps Digital Messages Secret
This patent describes the foundational RSA algorithm, a method for securely sending messages where anyone can encrypt a message using a public key, but only the intended recipient can decrypt it using a secret private key.
Massachusetts Institute of Technology
US 4575330 · 1986
How 3D Printers Build Objects Layer by Layer from Liquid
This patent describes the foundational method for 3D printing, where a machine builds a three-dimensional object layer by layer by hardening a liquid material with light or other energy.
UVP Inc
More to explore
More in Telecom & Wireless
US 5347632 · 1994 · Prodigy Services Co
Prodigy's System for Interactive Online Information and Shopping
US 3906166 · 1975 · Motorola Inc
How Early Cell Phones Handled Calls Across Different Towers
US 4063220 · 1977 · Xerox Corp
How Multiple Computers Share a Network Cable Without Crashing
US 4200770 · 1980 · Leland Stanford Junior University
How to Create a Secret Code Key Without Meeting First
New to patents?
Common Questions
Frequently Asked Questions
What does How to Calculate Distance to a Radio Signal Using Two Antennas cover?
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.
Who owns patent US 3789410?
US Department of Navy 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 3789410 cited by?
This patent has been cited by 16 later patents that build on its ideas.
What problem does this patent solve?
This technology was vital for military stealth. By allowing a vehicle to 'see' without being 'seen,' it provided a massive tactical advantage in electronic warfare. It laid the groundwork for modern passive electronic support measures used in maritime and aerial reconnaissance.
What does this patent NOT cover?
Does not cover active radar systems that emit signals to detect targets.
Patent monitoring