Subsea Cable-Lay and Repair Ship Activity Monitoring
Fewer than 60 purpose-built cable-lay and repair vessels operate globally, making individual ship movements strategically significant. Satellite SAR, spaceborne AIS and high-resolution optical combine to track where these vessels go, how fast, and whether their behaviour matches any declared cable route.
Sensors
- Sentinel-1 SAR (C-band, ESA): Detects vessels as bright radar returns against sea clutter at 5 x 20 m resolution in IW mode. Revisit is 6 days at the equator for a single satellite, 3 days with both Sentinel-1A and 1B operational. Works through cloud and at night, which is essential for high-latitude cable corridors. Cannot confirm vessel identity or read deck markings.
- Spire spaceborne AIS: Spire's constellation of over 100 LEO satellites collects AIS transponder messages globally, with typical message latency under 30 minutes and multiple passes per day per location. Provides MMSI, vessel name, position, speed over ground and heading. Collision or deliberate deactivation of AIS is detectable by gap analysis.
- exactEarth spaceborne AIS: Second independent AIS collection stream, useful for cross-validating Spire detections and reducing missed-message probability in congested or high-latitude regions. The two feeds together substantially reduce the window in which a vessel can go undetected while its transponder is active.
- Planet SkySat (optical, 50 cm resolution): Tasked optical imagery at 50 cm resolution can confirm vessel identity from hull markings, deck equipment configuration and cable drum presence. Revisit on a tasked basis is typically same-day to next-day for mid-latitude targets. Cloud cover is a hard limit; no thermal or SAR capability.
- Sentinel-1 SAR (GRDH mode, wide-area): 250 km swath in Extra Wide mode at 20 x 40 m resolution enables systematic surveillance of entire cable corridors in a single pass, useful for flagging any vessel present in a zone of interest before committing a tasked optical collect.
Why a handful of ships warrant systematic surveillance
The global fleet of purpose-built cable-lay vessels numbers somewhere between 40 and 60 hulls, depending on how repair-only ships are counted. That scarcity is itself the intelligence signal. When one of these vessels departs port, the question of where it is heading and on whose behalf is not academic. Subsea cables carry an estimated 95 per cent of international internet traffic by volume. Damage to a single trunk route can degrade connectivity for entire regions for weeks, as the 2022 Tonga cable cut demonstrated.
Monitoring this fleet is tractable in a way that monitoring, say, the global bulk-carrier fleet is not. The vessels are identifiable by name, flag and MMSI. Their operational signatures are distinctive. And the routes they work are, at least in part, a matter of public record through cable landing station databases and ITU filings. Anomalies stand out precisely because the baseline is so well defined.
The behavioural signature of a vessel paying out cable
A cable-lay vessel on an active lay operates at between 1 and 3 knots, maintaining a heading that follows the pre-surveyed cable route with minimal deviation. That combination, very slow speed and very straight track over many hours, is unusual in open ocean and essentially unique among large commercial vessels. A tanker or bulk carrier drifting at 2 knots is almost certainly waiting at anchor or in difficulty. A cable ship doing the same over 200 nautical miles in a straight line is doing exactly its job.
AIS speed-over-ground and course-over-ground fields capture this directly. When spaceborne AIS from Spire or exactEarth shows a known cable vessel holding 1.8 knots on a consistent bearing for 18 hours, the operational inference is straightforward. The complication arises when AIS is switched off. SAR then becomes the primary detection layer: Sentinel-1 can detect a vessel of cable-ship displacement (typically 10,000 to 25,000 gross tonnes) reliably in sea states up to around Beaufort 5 or 6, though smaller repair vessels near 3,000 GT approach the practical detection floor in rougher conditions.
Matching tracks against declared routes, and what divergence means
Published cable route data from sources such as TeleGeography's Submarine Cable Map, combined with ITU coordination filings, gives a reference geometry against which observed vessel tracks can be compared. A cable ship transiting within a few kilometres of a known route, at laying speed, is almost certainly working that route. A vessel exhibiting the same behavioural signature 80 kilometres from any filed route is a different matter entirely.
Divergence can have innocent explanations: emergency repairs to an undisclosed segment, survey work ahead of a new lay, or simply a route deviation to avoid a hazard. It can also indicate undisclosed cable work, which may be of interest to coastal states whose exclusive economic zones are crossed, to competing operators, or to national security agencies monitoring infrastructure in strategically sensitive straits. The analytical task is to flag the divergence and characterise it, not to adjudicate intent. That judgement belongs to the client.
Resolution floors, cloud, and the limits of the method
Sentinel-1 SAR at IW mode resolution cannot read a vessel's name or confirm its identity from imagery alone. A 5 x 20 m pixel gives you a bright elongated return and a rough length estimate; it does not give you 'CS Dependable' painted on the hull. Identity confirmation requires either an AIS match within a plausible position window, or a tasked optical collect at 50 cm or better. SkySat can resolve deck equipment and, in good conditions, hull text, but it is a pointed instrument with finite daily capacity and it cannot see through cloud.
High-latitude cable corridors, particularly trans-Arctic routes now attracting commercial interest, present a specific challenge: sea ice returns in SAR can mask or mimic vessel signatures, and cloud cover in subarctic regions reduces optical utility for days at a time. Revisit gaps matter too. A 3-day Sentinel-1 revisit means a vessel can complete a short repair segment and depart before being imaged. Closing that gap requires commercial SAR tasking from constellations such as ICEYE or Capella, which offer sub-daily revisit at a cost.
AIS itself is not immune to manipulation. Vessels can transmit false positions, replay historical tracks, or simply switch off. Cross-referencing AIS-reported position against SAR-detected position at the time of a satellite pass is the standard method for identifying discrepancies, and it is covered in detail in the sibling page on dark-ship detection.
What a persistent monitoring programme looks like in practice
A practical cable-ship monitoring programme combines three layers. First, a global AIS watch on a defined list of MMSI numbers corresponding to known cable vessels, with automated alerts when any vessel in the list enters a defined geographic zone or drops below 4 knots on an ocean heading. Second, systematic SAR coverage of priority cable corridors on each available Sentinel-1 pass, with vessel detections cross-referenced against the AIS watch list. Third, triggered optical tasking when the first two layers produce an anomaly that warrants visual confirmation.
The output is a track database: each vessel, its position history, its speed profile, and a flag indicating whether its behaviour is consistent with a known route or constitutes an anomaly requiring review. Satellize structures this kind of persistent watch as a recurring analytic feed rather than a one-off report, since the intelligence value accumulates over time as baseline behaviour becomes better characterised and deviations become easier to identify with confidence.
Clients who want to go further can request correlation against port departure records, satellite-derived vessel loading status before and after a transit, and cross-referencing against publicly available cable outage reports to test whether observed vessel activity precedes or follows reported faults.
Typical figures
| SAR spatial resolution (Sentinel-1 IW mode) | 5 x 20 m (ground range x azimuth) |
| SAR spatial resolution (Sentinel-1 EW mode) | 20 x 40 m, 250 km swath |
| Sentinel-1 revisit (dual-satellite) | 3 days at mid-latitudes when both satellites operational; 6 days single-satellite |
| Optical resolution (Planet SkySat) | 50 cm, tasked; same-day to next-day availability at mid-latitudes |
| AIS latency (Spire / exactEarth spaceborne) | Typically under 30 minutes from transmission to delivery |
| Minimum detectable vessel size (Sentinel-1 SAR) | Approximately 30 m length in moderate sea state; smaller vessels approach detection floor in Beaufort 5-6 |
| AIS archive depth | Multi-year historical records available from both Spire and exactEarth commercial APIs |
| SAR archive depth (Sentinel-1) | From October 2014 (Sentinel-1A launch); free via Copernicus Data Space |
| Delivery formats | GeoJSON vessel tracks, GeoTIFF SAR chips, CSV alert logs, PDF anomaly reports |
| Coverage | Global ocean; polar regions above 75° N/S covered with reduced revisit |
Analytics Satellize can run
| Named cable-vessel watch list with real-time AIS alerts | MMSI-based AIS filtering with speed-over-ground and geographic zone triggers | Automated alert feed (JSON or email) when a listed vessel enters a monitored zone or enters low-speed ocean transit |
| SAR vessel detection in defined cable corridors | CFAR (constant false alarm rate) detection on Sentinel-1 GRD imagery; standard published method for SAR ship detection | Georeferenced vessel detection layer (GeoJSON) per Sentinel-1 pass, with estimated vessel length and heading |
| AIS-to-SAR position cross-reference | Temporal and spatial matching of AIS-reported positions against SAR detections within a configurable position window | Match/no-match table per SAR pass; unmatched SAR detections flagged as potential dark vessels for review |
| Route-deviation anomaly scoring | Comparison of observed vessel track geometry against reference cable route polylines from public databases; perpendicular offset and speed-profile analysis | Weekly anomaly report listing vessels whose tracks deviate beyond a defined threshold from any known route, with map and narrative |
| Optical identity confirmation on anomalous detections | Triggered SkySat tasking; visual and automated hull-marking analysis at 50 cm resolution | Annotated image chip with vessel name, flag, and deck-equipment characterisation where cloud-free imagery is obtained |
| Historical transit database for a defined fleet or corridor | Retrospective AIS and SAR archive processing; track reconstruction and behavioural classification | GIS layer and CSV of all detected transits with timestamps, speeds, headings and route-match scores, covering up to 10 years of Sentinel-1 archive |
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.