Branchoria Sky Detectors Sky Detectors

How serious UFO detectors turn strange sightings into measurable sky data.

Within Radar

How FM radio signals can reveal sky objects

FM transmitters can act as free illuminators, but terrain, clutter and weak echoes decide whether a station gets useful data.

On this page

  • Why FM stations can illuminate the sky
  • Direct signal leakage and urban clutter
  • When FM echoes become useful tracks
Preview for How FM radio signals can reveal sky objects

Introduction

FM broadcast stations can act as free illuminators for passive radar, making them one of the most practical radio sources for citizen-built sky monitoring systems. Instead of transmitting any signal, a passive receiver compares the strong, direct FM broadcast with weaker copies that have reflected from aircraft or other objects. For an automated instrumented UAP detector, this approach can provide an independent measurement of motion without requiring licensed radar transmissions. However, its usefulness depends far more on geometry, terrain, clutter suppression and signal processing than on simply having a nearby radio station. In practice, FM-based passive radar is best viewed as a complementary sensing channel that can confirm or reject optical observations rather than as a standalone detector. [IET+2NATO STO]theiet.orgIETFM radio based bistatic radarJune 22, 2021 — by PE Howland · 2005 · Cited by 913 — Having adaptively filtered out the direct path sign…Published: June 22, 2021

FM Echoes illustration 1

Why FM stations can illuminate the sky

Commercial FM broadcasters continuously transmit high-power signals across the VHF band (roughly 88–108 MHz). These transmissions were designed for audio broadcasting, not radar, yet they provide a stable source of radio energy that fills large areas.

A passive radar receiver typically uses two receiving paths: [youtube.com]youtube.comPassive Radar Detection | John Sahr - YouTube Tim Ventura…

  • A reference antenna aimed towards the FM transmitter to capture the direct broadcast.
  • A surveillance antenna aimed at the sky to receive reflections from moving objects.

Signal processing then compares the two. If an aircraft or another reflective object lies in a suitable location, a delayed and Doppler-shifted copy of the FM signal appears in the surveillance channel. Measuring the delay estimates the additional travel distance, while the Doppler shift indicates radial motion relative to the transmitter and receiver. [IET]theiet.orgIETFM radio based bistatic radarJune 22, 2021 — by PE Howland · 2005 · Cited by 913 — Having adaptively filtered out the direct path sign…Published: June 22, 2021

Unlike conventional monostatic radar, the transmitter and receiver are separated. This creates a bistatic geometry in which detection depends not only on the object itself but also on where it sits relative to both the broadcaster and the receiver.

Direct signal leakage and urban clutter

The largest engineering challenge is rarely detecting echoes from moving targets. It is removing the overwhelming direct FM signal.

The broadcast arriving directly from the transmitter is often many orders of magnitude stronger than any reflected echo. Unless sophisticated cancellation methods are applied, the receiver simply becomes saturated by the broadcast itself.

In addition to direct leakage, the receiver also sees reflections from:

  • buildings,
  • hills,
  • towers,
  • power lines,
  • stationary infrastructure,
  • nearby vehicles.

These stationary reflections produce clutter concentrated near zero Doppler, masking weaker targets. Modern passive radar systems therefore devote much of their computation to adaptive filtering that estimates and subtracts direct-path interference and persistent clutter before attempting target detection. This processing is widely regarded as one of the most critical components of an FM passive radar system. [FCC de Castro+2ResearchGate]fccdecastro.com.brFCC de Castro Efficient Direct Signal Cancellation for FM-based Passive RadarFCC de CastroEfficient Direct Signal Cancellation for FM-based Passive RadarMarch 4, 2021 — by C Schüpbach · Cited by 8 — Abstract—The pe…Published: March 4, 2021

Urban environments are especially difficult because dense multipath creates numerous delayed copies of the broadcast signal. Rural sites often provide cleaner propagation, although local terrain can also block useful transmitter illumination.

Inventor says his new radar network could spot UAPs | Reality Check

Channel: NewsNation · Views: 174.9K · Uploaded: July 2025

Open on YouTube

When FM echoes become useful tracks

Once the dominant interference has been removed, moving objects begin to appear as peaks in delay-Doppler space.

For citizen UAP sensing, this has several practical implications.

Ordinary aircraft become valuable calibration targets. Their known flight paths, often available through ADS-B data, allow experimenters to verify that the passive radar is functioning correctly.

Moving objects separate naturally from stationary clutter. Doppler processing suppresses many fixed reflections, making genuine moving targets easier to identify.

Tracking improves over time. Individual detections are generally less useful than continuous tracks. Kalman filtering and related tracking algorithms can associate successive detections into a coherent trajectory, improving confidence and rejecting random noise. Experimental FM passive radar systems have demonstrated aircraft tracking over distances exceeding 150 km under favourable conditions. [ResearchGate+2IET]researchgate.netResearch Gate(PDF) FM radio based bistatic radarResearchGate(PDF) FM radio based bistatic radarJuly 3, 2005 — PDF | An experimental bistatic radar system is described that detects and t…Published: July 3, 2005

For an automated UAP observatory, a radio track that coincides with a camera observation is significantly more informative than either measurement alone.

FM Echoes illustration 2

Why transmitter choice matters

Not every FM station produces equally useful radar performance.

Important factors include:

  • Transmitter power. Higher effective radiated power generally increases available illumination.
  • Distance from the receiver. Too close and the direct signal may overwhelm the receiver; too distant and the illumination becomes weak.
  • Terrain. Hills may either shield unwanted direct-path interference or prevent useful illumination altogether.
  • Programme content. FM broadcasts continually change with speech and music. Their instantaneous waveform affects range resolution because the radar uses the broadcast itself as the probing signal.
  • Frequency congestion. Strong neighbouring stations can increase interference and complicate processing. [IET+2IET Research Journal]theiet.orgIETFM radio based bistatic radarJune 22, 2021 — by PE Howland · 2005 · Cited by 913 — Having adaptively filtered out the direct path sign…Published: June 22, 2021

Consequently, two observers using identical hardware may obtain very different results simply because they are located in different radio environments.

Retired U.S. Navy Rear Admiral Tim Gallaudet on Grusch, UAP claims: Full Interview | Reality Check

Channel: NewsNation

Open on YouTube

Geometry matters more than transmitter count

A common misconception is that more FM stations automatically produce better passive radar.

In reality, useful coverage depends on bistatic geometry.

An object reflects energy most effectively when it occupies favourable positions relative to both transmitter and receiver. If the geometry is poor, even a powerful broadcaster may contribute little usable information.

For this reason, multistatic systems—which combine measurements from multiple receivers and sometimes multiple transmitters—can outperform a single passive station. Independent delay and Doppler measurements from different baselines can be combined to estimate three-dimensional position and velocity rather than only detecting motion along a single bistatic path. [arXiv]arxiv.orgSkyWatch: A Passive Multistatic Radar Network for the Measurement of Object Position and VelocityMay 29, 2023…Published: May 29, 2023

Citizen Disclosure Part 1: Detection and Tracking of UAP with Mitch Randall

Channel: Reed Summers - EMERGENT

Open on YouTube

What FM passive radar can and cannot tell a citizen UAP detector

FM reflections can strengthen evidence but have important limitations.

Useful capabilities include:

  • confirming that an optical target also reflects radio energy;
  • estimating bistatic range and radial motion;
  • providing independent timing for sensor fusion;
  • helping distinguish genuine moving aerial objects from camera artefacts.

Important limitations remain:

  • many small drones, plastic balloons or low-reflectivity objects may produce little detectable echo;
  • low-altitude targets can disappear into terrain and ground clutter;
  • range estimates are bistatic rather than direct geometric distances;
  • a non-detection does not prove that no object was present.

These constraints mean that FM passive radar should be interpreted as one sensor within a larger observational system rather than as a decisive detector on its own. [NATO STO+2IET]sto.nato.intNATO STOPassive Radar Technology– Properties of different passive radars using different illuminators of opportunity like FM… – Signal…

FM Echoes illustration 3

Lessons from current UAP-oriented passive radar work

Recent citizen-science and academic projects have adapted FM passive radar specifically for anomalous aerial object monitoring. One prominent example is the SkyWatch multistatic passive radar concept developed within the Galileo Project, which uses geographically separated receivers exploiting commercial FM broadcasts. The design aims to combine passive radar with optical and infrared sensors so that independently measured positions, velocities and trajectories can be compared across multiple sensing modalities rather than relying on visual observations alone. The published design also discusses adapting operation to varying terrain, transmitter availability and direct-signal strength, including sharing reference signals between networked receivers when the local FM broadcast is weak. [arXiv+2ADS]arxiv.orgSkyWatch: A Passive Multistatic Radar Network for the Measurement of Object Position and VelocityMay 29, 2023…Published: May 29, 2023

The broader lesson is that FM broadcast reflections are most valuable when integrated into a calibrated, multi-sensor observatory. Their greatest contribution is not finding extraordinary objects in isolation, but reducing uncertainty by adding an independent radio measurement alongside cameras, aircraft identification receivers and environmental sensors.

Amazon book picks

Further Reading

Books and field guides related to How FM radio signals can reveal sky objects. Use these as the next step if you want deeper reading beyond the article.

Browse more on Amazon: to Radar Systems books The Hobbyist's to the RTL SDR books Digital signal processing

As an Amazon Associate I earn from qualifying purchases.

eBay marketplace picks

Marketplace Samples

Example marketplace items related to this page. Use the search link to explore similar finds on eBay.

Using USA
Browse more on eBay.co.uk

Example items shown for inspiration; availability and pricing can change. Branchoria may earn a commission if you purchase through outbound eBay links.

Endnotes

  1. Source: sto.nato.int
    Link: https://www.sto.nato.int/document/passive-radar-technology/
    Source snippet

    NATO STOPassive Radar Technology– Properties of different passive radars using different illuminators of opportunity like FM... – Signal...

  2. Source: researchgate.net
    Link: https://www.researchgate.net/publication/224699001_Analysis_and_Emulation_of_FM_Radio_Signals_for_Passive_Radar
    Source snippet

    Analysis and Emulation of FM Radio Signals for Passive...The paper examines the problem of cancellation of direct signal, multipath and...

  3. Source: researchgate.net
    Title: Research Gate(PDF) FM radio based bistatic radar
    Link: https://www.researchgate.net/publication/3357841_FM_radio_based_bistatic_radar
    Source snippet

    ResearchGate(PDF) FM radio based bistatic radarJuly 3, 2005 — PDF | An experimental bistatic radar system is described that detects and t...

    Published: July 3, 2005

  4. Source: arxiv.org
    Link: https://arxiv.org/abs/2305.18562
    Source snippet

    SkyWatch: A Passive Multistatic Radar Network for the Measurement of Object Position and VelocityMay 29, 2023...

    Published: May 29, 2023

  5. Source: arxiv.org
    Link: https://arxiv.org/abs/1805.01533

  6. Source: researchgate.net
    Link: https://www.researchgate.net/publication/366121932_SkyWatch_A_Passive_Multistatic_Radar_Network_for_the_Measurement_of_Object_Position_and_Velocity
    Source snippet

    A Passive Multistatic Radar Network for the Measurement...25 Apr 2026 — SkyWatch, a passive multistatic radar system based on commercial...

  7. Source: arxiv.org
    Link: https://arxiv.org/html/2411.07956v1
    Source snippet

    1 Introduction12 Nov 2024 — In 2023, NASA released an independent study Team (2023) which emphasized how studying the phenomenon using pa...

  8. Source: publications.sto.nato.int
    Link: https://publications.sto.nato.int/publications/STO%20Educational%20Notes/STO-EN-SET-243/EN-SET-243-07.pdf
    Source snippet

    Fm Radio Based Bistatic...

  9. Source: sto.nato.int
    Title: passive radar for satellite detection and tracking
    Link: https://www.sto.nato.int/document/passive-radar-for-satellite-detection-and-tracking/
    Source snippet

    Radar for Satellite Detection and Tracking20 Jan 2026 — This lecture presents an overview of passive radar techniques for the detection a...

  10. Source: theiet.org
    Link: https://www.theiet.org/media/11278/fm-radio-based-bistatic-radar.pdf
    Source snippet

    IETFM radio based bistatic radarJune 22, 2021 — by PE Howland · 2005 · Cited by 913 — Having adaptively filtered out the direct path sign...

    Published: June 22, 2021

  11. Source: fccdecastro.com.br
    Title: FCC de Castro Efficient Direct Signal Cancellation for FM-based Passive Radar
    Link: https://www.fccdecastro.com.br/pdf/EDSCFMPR.pdf
    Source snippet

    FCC de CastroEfficient Direct Signal Cancellation for FM-based Passive RadarMarch 4, 2021 — by C Schüpbach · Cited by 8 — Abstract—The pe...

    Published: March 4, 2021

  12. Source: ietresearch.onlinelibrary.wiley.com
    Title: iet rsn.2013.0185
    Link: https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-rsn.2013.0185
    Source snippet

    IET Research JournalAnalysis of detection range of FM‐based passive radar1 Feb 2014 — This study presents an analysis of the detection ra...

  13. Source: fccdecastro.com.br
    Link: https://www.fccdecastro.com.br/pdf/AFDPMIC.pdf
    Source snippet

    an be designed to eliminate unwanted echoes affecting the signal collected at the surveillance antenna.Read more...

  14. Source: tv.theiet.org
    Link: https://tv.theiet.org/video/land_and_sea_clutter_from_fm-based_passive_bistatic_radars/cd7260ed-39e8-41b7-b537-573251d3f7db
    Source snippet

    Duration: 16 mins. D marine scene navigation Duration: 16 mins...

Additional References

  1. Source: github.com
    Link: https://github.com/Max-Manning/passiveRadar
    Source snippet

    Max-Manning/passiveRadar: Processing code for a FM...Passive radars don't transmit any signals of their own - instead, they locate targe...

  2. Source: avi-loeb.medium.com
    Link: https://avi-loeb.medium.com/commissioning-data-on-half-a-million-objects-in-the-sky-from-the-galileo-project-observatory-are-a23bd084233a
    Source snippet

    Data on Half a Million Objects in the Sky from...Commissioning Data on Half a Million Objects in the Sky from the Galileo Project Observ...

  3. Source: discovery.ucl.ac.uk
    Link: https://discovery.ucl.ac.uk/1397756/4/JBrown_Airborne_Passive_Radar.pdf
    Source snippet

    Airborne Passive RadarThe airborne application of Passive Bistatic Radar (PBR) is the latest evolution of the now established internation...

  4. Source: scivision.dev
    Link: https://www.scivision.dev/pdf/passivefmradar-pres.pdf
    Source snippet

    rplanes, ionosphere). Page 8. Aircraft and Meteor Hard...Read more...

  5. Source: ijcaonline.org
    Link: https://ijcaonline.org/archives/volume119/number10/21101-3815/
    Source snippet

    The illuminating signal used is an FM-Radio Broadcast signal...

  6. Source: youtube.com
    Link: https://www.youtube.com/watch?v=Qh_h5OUMKIE
    Source snippet

    Passive Radar Detection | John Sahr - YouTube Tim Ventura...

  7. Source: elib.dlr.de
    Title: Keywords: Bistatic
    Link: https://elib.dlr.de/55970/1/thesis.pdf
    Source snippet

    Parameter Estimation using Bistatic Polarimetric X-...by K Ben Khadhra · 2008 · Cited by 21 — Finally, the first results and validations...

  8. Source: pure.bit.edu.cn
    Link: https://pure.bit.edu.cn/en/publications/study-of-detection-performance-of-passive-bistatic-radars-based-o
    Source snippet

    of detection performance of passive bistatic radars...by T Shan · 2007 · Cited by 15 — Abstract: The passive bistatic radar based on the...

  9. Source: youtube.com
    Title: More “Reality Check” episodes
    Link: https://www.youtube.com/watch?v=rEYDY2EMm8c
    Source snippet

    "Retired U.S. Navy Rear Admiral Tim Gallaudet on Grusch, UAP claims: Full Interview | Reality Check: [https://www.youtube.com/watch?v=Qh_h5..."](https://www.youtube.com/watch?v=Qh_h5...")...

  10. Source: youtube.com
    Title: Inventor says his new radar network could spot UAPs | Reality Check
    Link: https://www.youtube.com/watch?v=mwsEatnZ358
    Source snippet

    Citizen [Disclosure]({{ 'disclosure/' | relative_url }}) Part 1: Detection and Tracking of UAP with Mitch Randall...

Topic Tree

Follow this branch

Parent topic

Radar Can Radar Make UAP Tracks Clearer?

Related pages 5