AIS vessel tracking and dark-ship detection
Space-based AIS receivers capture VHF transponder broadcasts from vessels worldwide, but gaps in that record are where the real intelligence begins. Cross-referencing AIS silence against SAR and optical detections exposes vessels that would prefer not to be seen.
Sensors
- Spire Maritime (LEO AIS constellation): Over 110 LEO satellites carrying VHF AIS receivers on 161.975 MHz and 162.025 MHz. Store-and-forward latency typically 20–90 minutes depending on orbital geometry; near-real-time delivery available from multi-satellite passes. Global coverage with revisit measured in minutes at high latitudes, longer near the equator.
- exactEarth (SAT-AIS): Commercial LEO AIS constellation providing global vessel position data. Offers historical AIS archive and streaming feeds. Latency comparable to other LEO store-and-forward systems; the company quotes sub-30-minute global latency for its enhanced service tiers.
- Sentinel-1 SAR (ESA Copernicus): C-band (5.405 GHz) synthetic aperture radar. Detects vessel-sized objects at 5–20 m resolution depending on acquisition mode (IW mode: 5×20 m). Imaging is independent of cloud cover and daylight. Revisit 6–12 days at mid-latitudes for a single satellite; 3–6 days with both Sentinel-1A and 1B active. Ship detection sensitivity depends on vessel size and sea state; small vessels below roughly 30 m length become difficult in high sea states.
- Planet SkySat (optical cross-reference): Sub-metre resolution (0.5 m panchromatic) optical imagery for visual confirmation of vessel identity, heading and configuration. Cloud-dependent. Tasked on demand; latency from tasking to delivery typically same-day to 48 hours. Useful for positive identification once a SAR anomaly has been flagged, not for wide-area persistent surveillance.
What AIS actually is, and what it was never designed to do
The Automatic Identification System is a VHF radio protocol mandated by the International Maritime Organisation under SOLAS for vessels over 300 gross tonnes on international voyages, cargo vessels over 500 GT, and all passenger ships regardless of size. Each transponder broadcasts a vessel's MMSI number, position (from its own GNSS receiver), speed, heading, draught and destination at intervals ranging from two seconds (for vessels underway at speed) to three minutes (at anchor). The protocol was designed for collision avoidance between ships and for port authority traffic management. It was not designed to be a surveillance system, and that design choice has consequences.
Terrestrial AIS receivers work well in coastal waters and busy ports. In open ocean, the signal simply travels beyond any shore station. Space-based receivers solve the coverage problem by placing VHF antennas on LEO satellites, but they introduce a different one: message collision. In a dense shipping lane, hundreds of transponders broadcast on the same two channels simultaneously. A satellite pass over the Singapore Strait or the English Channel receives a cacophony of overlapping packets. Advanced multi-channel receivers and signal-processing techniques (including successive interference cancellation) recover more messages from the collision, but some are always lost. Reported message-recovery rates in very high-density areas can fall below 50 % without these techniques.
Latency, revisit and the gap between knowing and acting
A LEO AIS satellite receives messages only during its overpass, which lasts a few minutes over any given ocean area. In a store-and-forward architecture, those messages are downlinked at the next ground station contact, adding latency. For a constellation of around 110 satellites such as Spire's, the median latency globally is roughly 20–90 minutes, with better performance at high latitudes where orbital tracks converge and passes are more frequent. Near the equator, the geometry is less favourable.
For fisheries monitoring or sanctions compliance, 90-minute latency is often acceptable. For active interdiction or search-and-rescue, it is not. Some operators address this by combining LEO AIS with geostationary VHF receivers or by using multi-satellite constellations dense enough to achieve near-continuous coverage over specific regions of interest. The honest position is that no current commercial SAT-AIS service provides truly continuous, real-time global coverage. The operational question is always: how stale can the data be before it stops being useful for this specific decision?
Dark ships: what the absence of a signal means
A vessel that switches off its AIS transponder, or never had one, creates a gap in the position record. That gap is not itself evidence of wrongdoing. Transponders fail, batteries die, and some flag states have poor compliance. But certain patterns of AIS absence are operationally significant: a tanker that goes dark near a sanctioned port and reappears days later with a changed draught; a fishing vessel that disables its transponder in a protected zone; a vessel that transmits a position inconsistent with its reported speed.
The detection method is straightforward in principle. SAR imagery of a sea area produces a list of radar-bright objects consistent with vessels. That list is compared against the AIS position record for the same time window. Objects present in SAR but absent from AIS are candidates for dark-ship investigation. Sentinel-1's IW mode covers 250 km swaths, making it practical for wide-area screening. The limit is vessel size: a 300-metre VLCC is an obvious SAR target; a 15-metre fishing dhow in a 2-metre swell may not be. False positives from wave clutter, offshore platforms and sea ice require filtering, and that filtering requires either manual review or a well-trained classifier.
Spoofing is detectable, but the detection has its own limits
AIS spoofing, broadcasting a false position while the vessel is elsewhere, is technically simple. A vessel can transmit coordinates placing it in a permitted anchorage while physically operating somewhere else. It can clone another vessel's MMSI. It can replay a historical track. These manipulations are increasingly common in sanctions-evasion contexts, and the public record documents cases in the Persian Gulf, off the coast of North Korea and in the Black Sea.
Kinematic inconsistency is the primary detection signal. A vessel cannot physically travel 400 nautical miles in three hours at a reported speed of eight knots. Reported positions that require impossible accelerations, that jump discontinuously, or that place a vessel simultaneously in two locations are flagged by simple physics checks. SAR cross-referencing provides independent ground truth: if a vessel's AIS says it is in Rotterdam and a Sentinel-1 pass shows its hull in Bandar Abbas, the spoofing is confirmed. The limit of this approach is temporal. SAR revisit over any specific location is days, not hours, so a vessel that spoofs briefly and moves on may never be caught in the act by a satellite overpass.
Building a usable intelligence product from noisy inputs
Raw SAT-AIS data is a firehose. A global feed contains millions of position reports per day. The analytic value comes from filtering, normalisation and anomaly scoring against a baseline of expected behaviour. A vessel's historical track, its flag, its registered owner, its cargo type and its declared route all contribute to a behavioural prior. Deviations from that prior, a bulk carrier loitering in an area with no declared port call, a tanker making a ship-to-ship transfer in international waters, a vessel whose draught changes without a port visit, are the signals worth surfacing.
Satellize builds these detection pipelines on a combination of commercial AIS feeds, open Sentinel-1 SAR acquisitions and, where a client requires it, tasked very-high-resolution optical imagery for confirmation. The architecture is the same whether the client is a government maritime authority or a commodity trading desk that needs to know where a specific cargo actually is. One practical note for any buyer: the quality of the underlying AIS feed matters enormously and varies between providers, particularly in high-density areas and at high latitudes. Evaluating message-recovery rates in your specific region of interest before committing to a data contract is worth the effort.
Typical figures
| AIS frequency | VHF 161.975 MHz (Channel 87B) and 162.025 MHz (Channel 88B) |
| Typical SAT-AIS latency (LEO store-and-forward) | 20–90 minutes globally; shorter at high latitudes |
| SAT-AIS global revisit (Spire ~110-satellite constellation) | Multiple passes per hour at high latitudes; 1–3 passes per hour near equator |
| Sentinel-1 SAR spatial resolution (IW mode) | 5 m range × 20 m azimuth; 250 km swath |
| Sentinel-1 revisit (dual-satellite) | 3–6 days at mid-latitudes; 1–3 days at high latitudes |
| Minimum detectable vessel size (Sentinel-1 SAR) | Approximately 30–50 m in moderate sea states; smaller vessels detectable in calm conditions |
| SkySat optical resolution | 0.5 m panchromatic; 0.9 m multispectral |
| AIS archive depth (commercial providers) | Typically 5–10 years depending on provider; exactEarth and Spire both hold multi-year archives |
| Delivery formats | CSV/JSON position feeds, GeoJSON anomaly layers, GeoTIFF SAR detections, API streaming |
Analytics Satellize can run
| Dark-ship candidate list | SAR vessel detection (CFAR or ML classifier) cross-referenced against AIS position records for coincident time window | Ranked alert list with vessel coordinates, SAR chip, nearest AIS track and confidence score; delivered as GeoJSON or PDF report |
| AIS spoofing flag | Kinematic consistency check: reported speed and position sequence tested against physical navigation limits; SAR ground-truth comparison where imagery is available | Per-vessel anomaly score with flagged position jumps and SAR confirmation imagery where acquired; daily feed or on-demand report |
| Ship-to-ship transfer detection | Loitering detection from AIS track clustering in open water, combined with SAR detection of proximate vessels; optical tasking for confirmation | Event alert with time window, coordinates, vessel candidates and supporting imagery |
| Port call verification | AIS arrival/departure events correlated with Sentinel-1 SAR detections at port; draught-change inference from AIS metadata | Structured port-call log per vessel or fleet; exportable to compliance databases |
| Fleet behaviour baseline and deviation scoring | Historical AIS track modelling to establish expected routes, speeds and port sequences; statistical deviation scoring against rolling baseline | Weekly fleet intelligence summary; configurable alert thresholds for specific vessel types or flag states |
| Sanctioned-area proximity alert | Geofence logic applied to AIS position stream; dark-period detection within defined exclusion zones | Real-time or near-real-time alert via API webhook or email; GIS layer showing incursion events |
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.