Sensors
- HawkEye 360 cluster satellites: Fly in formation triplets separated by tens of kilometres; measure time-difference and frequency-difference of arrival (TDOA/FDOA) of VHF/UHF signals across 100 MHz–6 GHz. Achieves positional fixes of roughly 5–15 km CEP on maritime VHF working channels depending on geometry and signal duration. Revisit over any given ocean point is approximately 3–6 times per day at mid-latitudes with the current constellation.
- Spire LEMUR-2: Multi-payload 3U CubeSats carrying AIS receivers alongside GNSS-RO sensors. Primarily used here for AIS message ingestion at sub-minute latency; constellation of more than 100 satellites gives global AIS revisit of roughly every 20–30 minutes in open ocean. Does not perform TDOA geolocation of voice channels but provides the AIS track against which RF fixes are compared.
- AIS Class A transponders (vessel-declared): Mandatory on SOLAS vessels above 300 GT. Broadcast position, speed, course and MMSI on 161.975 MHz and 162.025 MHz at intervals of 2–10 seconds when under way. The declared position is GPS-derived and self-reported; it is the data stream that a fraudulent operator can manipulate.
- VHF maritime working channels (156–174 MHz): ITU-R M.1084 allocates specific working channels for ship-to-ship and ship-to-shore voice. Channel 16 (156.8 MHz) is the international distress and calling channel; channels 6, 8, 9, 10, 13 and others carry routine traffic. Transmissions are typically 25 kHz FM, power 1–25 W, and last long enough (several seconds to minutes) for a TDOA fix from a passing satellite cluster.
Why voice radio is harder to fake than AIS
AIS position data is self-reported. The vessel's own GPS feeds its transponder, and nothing in the AIS protocol prevents an operator from injecting a false coordinate. Position relay fraud, where one vessel rebroadcasts another's MMSI from a different location, has been documented in IMO circulars and UNODC maritime crime assessments. The result is a track that looks legitimate to any shore-side traffic management system.
Voice radio is different. When a watchkeeper keys a VHF handset, the transmission originates from the physical antenna on that ship. The radio waves propagate outward at the speed of light from wherever the vessel actually sits. A satellite cluster overhead does not care what the AIS feed says; it measures the arrival time of those wavefronts at each spacecraft and computes a position from the differences. That physics-derived fix is independent of anything the crew chooses to declare.
How TDOA geolocation works from a LEO cluster
Time-difference of arrival geolocation requires at least three spatially separated receivers. A signal emitted by a ship reaches each satellite at a slightly different moment because the propagation paths differ in length. Each pair of satellites yields one hyperbolic surface on which the emitter must lie. Three satellites produce two independent hyperbolae; their intersection, projected onto the ocean surface, is the estimated emitter position.
HawkEye 360 publishes that its formation-flying triplets achieve positional accuracy in the range of 500 metres to several kilometres depending on signal duration, signal-to-noise ratio and the geometric dilution of precision imposed by the cluster's instantaneous geometry. For maritime VHF voice, where transmissions are typically a few seconds to a minute long and power is up to 25 W, the realistic CEP in open ocean is 5–15 km. That is not precise enough to identify a vessel among a dense anchorage, but it is more than sufficient to determine whether a ship claiming to be off the Azores is actually transmitting from the Gulf of Guinea.
FDOA, frequency-difference of arrival, adds a second observable. Each satellite's velocity relative to the emitter produces a Doppler shift; comparing those shifts across the cluster constrains the emitter's velocity as well as position. Combined TDOA/FDOA solutions can tighten the fix and help distinguish a stationary relay station from a moving vessel.
What a position discrepancy actually proves, and what it does not
A mismatch of 200 km between an AIS-declared position and a TDOA-derived RF fix is a strong indicator of fraud. It is not, by itself, legally conclusive attribution to a specific vessel. Several alternative explanations must be ruled out before an analyst can assert deliberate falsification with confidence.
Propagation anomalies are rare on VHF at low elevation angles but not impossible. Multipath over calm water can introduce timing errors of microseconds, translating to hundreds of metres of positional error, not tens of kilometres. A fix that diverges by less than 20 km from the AIS position sits in ambiguous territory and warrants corroboration from other sources such as SAR imagery or optical overpass.
MMSI collision, where two vessels inadvertently share an identifier due to misconfiguration, can make one ship's AIS track appear to originate from another's location. And a vessel may legitimately be relaying a distress call on behalf of a third party, causing the voice content to reference a position different from the relay vessel's own. Analysts must examine the content of the transmission, the MMSI history, and any available SAR or optical imagery of the declared and RF-derived positions before drawing conclusions.
Operational workflow: from satellite pass to discrepancy report
The process begins with AIS ingestion. A Spire LEMUR-2 pass or a terrestrial AIS aggregator captures the vessel's declared track in near real time. That track is stored with timestamps, MMSI, and the GPS-derived coordinates the vessel is broadcasting.
When a HawkEye 360 cluster passes over and detects a VHF transmission in the 156–174 MHz band, the raw TDOA/FDOA observables are downlinked and processed into a geolocation fix with an associated uncertainty ellipse. The fix timestamp is matched to the AIS record for the same period. If the RF-derived position falls outside the AIS-declared position by more than the combined uncertainty budget, a discrepancy flag is raised.
The flag is not an alert; it is a hypothesis. A human analyst or an automated scoring model then assesses the magnitude of the discrepancy, the vessel's historical AIS behaviour, whether the MMSI has appeared in known fraud databases, and whether any independent imagery is available for either location. Only after that triage does the finding graduate to a reportable event. Satellize structures this triage workflow for clients who want a scored, evidence-graded output rather than a raw data feed.
Honest limits of the method
Revisit is the most significant constraint. A HawkEye 360 cluster passes a given ocean region roughly three to six times per day. A vessel conducting illicit activity between passes leaves no RF record in this system. The method catches opportunistic transmissions, not silence.
The 5–15 km positional uncertainty means the technique is useful for ocean-basin-scale discrepancy detection, not for identifying which of two vessels anchored 3 km apart is the actual emitter. Dense anchorages and busy straits produce overlapping signals that can confuse TDOA solvers, particularly if multiple vessels transmit simultaneously on the same channel.
Attribution to a specific vessel, rather than a geographic area, requires corroborating data. A SAR image of the RF-derived location, an optical pass, or a port-state inspection record transforms a probabilistic RF finding into an actionable intelligence product. RF-only evidence has been accepted in maritime enforcement contexts, but prosecutors and flag-state authorities consistently require corroboration. That is not a weakness unique to this method; it is the standard evidentiary bar for any remote-sensing technique used in legal proceedings.
Typical figures
| VHF frequency coverage | 156–174 MHz (ITU maritime working channels); HawkEye 360 sensors cover 100 MHz–6 GHz broadband |
| TDOA positional accuracy (CEP) | 5–15 km typical in open ocean; sub-5 km achievable with long, high-SNR transmissions and favourable cluster geometry |
| AIS revisit (Spire LEMUR-2) | Approximately every 20–30 minutes globally in open ocean; shorter near polar regions |
| RF geolocation revisit (HawkEye 360) | Approximately 3–6 passes per day at mid-latitudes; variable by latitude and constellation growth |
| Minimum detectable transmission duration | Approximately 1–2 seconds for a TDOA fix; longer transmissions improve accuracy |
| Minimum detectable signal power | Practical floor around 1 W EIRP for VHF maritime handsets at LEO ranges; 25 W ship transceivers are well above threshold |
| Discrepancy detection threshold | Operationally meaningful at divergences greater than 20–30 km; ambiguous below that given combined uncertainty budgets |
| Latency from satellite pass to processed fix | Typically 1–6 hours depending on downlink schedule and processing pipeline; near-real-time options exist at premium tasking rates |
| Archive depth (AIS) | Commercial AIS archives extend to 2009 or earlier through providers such as Spire and exactEarth |
| Deliverable formats | GeoJSON discrepancy events, CSV track comparisons, scored PDF intelligence summaries, GIS-compatible shapefiles |
Analytics Satellize can run
| AIS-versus-RF discrepancy map | TDOA geolocation fix matched temporally and spatially to AIS-declared position; Euclidean separation computed with uncertainty propagation | GeoJSON layer of flagged events with discrepancy magnitude, uncertainty ellipse, and vessel MMSI; updated per satellite pass |
| Vessel fraud-risk score | Bayesian scoring model combining discrepancy frequency, MMSI history, flag-state risk tier, and route plausibility against known trade lanes | Ranked watchlist report (PDF and CSV) refreshed weekly or on-demand |
| Relay fraud pattern detection | Temporal clustering of AIS messages and RF fixes; identification of cases where a single MMSI appears at two geographically separated RF sources within the same time window | Alert feed with event timestamps, both candidate locations, and confidence rating |
| Dark-period RF activity flag | Cross-reference of AIS gap periods (vessel transponder off) against RF detections in the same ocean region during the gap | Incident report linking AIS gap metadata to any co-located RF fix; supports port-state control referrals |
| Historical track reconstruction | Retrospective fusion of archived AIS data with archived RF geolocation fixes to reconstruct probable true vessel path during a period of suspected falsification | Annotated track shapefile with timeline of AIS-declared versus RF-inferred positions; suitable for inclusion in enforcement dossiers |
| Channel-specific activity density map | Aggregation of VHF detections by ITU channel number across a defined ocean region and time window; hotspot identification using kernel density estimation | Raster heatmap (GeoTIFF) of voice radio activity by channel; useful for maritime domain awareness and spectrum management |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.