FPSO vessel heading and mooring status from SAR geometry
SAR imagery reveals FPSO heading and mooring geometry with enough precision to distinguish production, shutdown, and offloading events. Cross-validated against AIS, it gives commodity analysts a persistent watch on units that report selectively.
Sensors
- Sentinel-1 A/B SAR (C-band, 5.4 GHz): Interferometric Wide Swath mode delivers 10 m ground resolution across a 250 km swath. Revisit at mid-latitudes is 6 days per satellite; with both satellites operational it was 3 days, though Sentinel-1B was lost in 2021, leaving 6-day repeat until Sentinel-1C restores the pair. Sufficient for tracking heading changes between offloading cycles.
- ICEYE SAR constellation (X-band, ~9.65 GHz): Commercially taskable with sub-1 m resolution in Spot mode and revisit achievable in hours across a growing constellation of over 30 satellites. X-band returns from metal superstructure are strong and geometrically precise, making heading angle measurement more reliable than with coarser sensors.
- KOMPSAT-5 SAR (X-band): Korean national SAR satellite offering 1 m resolution in Fine mode and 3 m in Standard mode. Useful as a supplementary commercial tasking option where ICEYE coverage windows are suboptimal, though revisit is limited by single-satellite orbit.
- AIS (Automatic Identification System) positional data: Provides vessel identity, reported heading, speed over ground and position at message intervals of 2 to 10 seconds when active. Used to cross-validate SAR-derived heading angles and to flag periods of AIS silence that prompt SAR tasking. Spire Global and similar providers offer global AIS aggregation.
Why a moored vessel is a compass
An FPSO on a single-point mooring (SPM) or a turret mooring behaves like a weathervane. The hull aligns with the vector sum of the dominant environmental forces: current drag along the submerged hull, wind pressure on the topsides and, in shallow water, wave orbital forces. In practice, for most deepwater FPSOs, current dominates. The vessel's bow points into the strongest force. That alignment is not random and it is not chosen by the operator. It is physics.
This matters for remote sensing because SAR captures the vessel's long axis orientation with enough angular precision to measure heading to within a few degrees, depending on pixel spacing and vessel length. A 300 m FPSO imaged at 3 m resolution gives a long-axis fit with uncertainty well under five degrees. That is sufficient to distinguish a vessel weathervaning freely on its mooring from one that is constrained, beached, or alongside a terminal.
What heading change actually tells an analyst
A freely mooring FPSO in open ocean will rotate slowly as current and wind direction shift over days. Its heading in successive SAR passes should track those environmental vectors. If the vessel heading is stable across multiple passes while the oceanographic forcing has rotated, the mooring is constrained. That can indicate a tandem offloading operation, a maintenance intervention that has the vessel held by tugs, or a mooring fault.
The offloading signature is particularly legible. During a ship-to-ship transfer, a shuttle tanker positions stern-to-stern with the FPSO, connected by a floating hose. The shuttle tanker's heading is constrained by the hose geometry and the FPSO's position. SAR captures both hulls in the same image. The angular relationship between them, combined with the FPSO's deviation from its expected weathervane heading, is a reliable indicator of an active offloading event. Typical West African and Brazilian FPSO offloading cycles run every 7 to 14 days depending on production rate and cargo parcel size, so a 6-day SAR revisit catches most events with at least one image during the operation.
Shutdown is subtler. A non-producing FPSO may still weathervane normally, so heading alone does not confirm production. But heading combined with flare stack state (from nighttime optical or thermal sensors, covered in the refinery flare page in this library) and with AIS-reported speed of zero over an extended period gives a convergent picture.
The geometry of the measurement
SAR images a vessel from a side-looking geometry at incidence angles typically between 20 and 46 degrees in standard modes. The vessel appears as a bright elongated return from its metal hull and superstructure, with a radar shadow extending away from the sensor. The long-axis orientation of the bright return gives heading directly, though with a 180-degree ambiguity: SAR cannot distinguish bow from stern without a priori knowledge or a second look from a different geometry.
AIS resolves that ambiguity when active. When AIS is off, the ambiguity can often be broken by the known position of the turret (forward of midships on most turret-moored FPSOs) or by the asymmetric superstructure signature visible at sub-3 m resolution. ICEYE Spot mode at better than 1 m makes this structural disambiguation routine. At Sentinel-1's 10 m, it requires careful template matching against the known vessel profile.
One honest limit: in high sea states, wave clutter raises the noise floor on vessel edges and degrades the angular precision of the long-axis fit. Beaufort 6 and above introduces measurable error. For tropical FPSOs in benign sea states this is rarely a problem; for North Sea or FPSO units in high-latitude fields it is a real constraint that should be acknowledged in any monitoring specification.
AIS silence as a trigger, not a conclusion
Some operators disable or reduce AIS transmission deliberately. Reasons range from commercial confidentiality around cargo movements to regulatory non-compliance. AIS silence on a known FPSO position is therefore a trigger for SAR tasking, not itself a conclusion about vessel status.
When SAR confirms the vessel is present and heading is consistent with free weathervaning, the most likely explanation is simply that AIS is off. When SAR shows a constrained heading with a second vessel alongside, the probability of an offloading event is high. When the FPSO is absent from its licensed block position entirely, that is a different and more significant finding. The analytic value comes from combining the two data streams: AIS provides high-frequency positional continuity; SAR provides the geometric ground truth that AIS cannot fake.
Practical limits and what they mean for a monitoring programme
Sentinel-1's 6-day revisit (single satellite) means that a fast offloading event of 24 to 36 hours duration may be missed entirely in any given cycle. Commercial tasking through ICEYE reduces that risk substantially: with a constellation of over 30 satellites, same-day or next-day imaging is achievable for most ocean basins. The cost calculus is straightforward: Sentinel-1 is free and persistent but probabilistic; ICEYE is paid and targeted but near-certain.
The method also cannot directly measure cargo volume transferred during an offloading event. It confirms that an event occurred and, from the duration of the constrained-heading signature across multiple passes, can bracket the event window. Volume inference requires either AIS-reported shuttle tanker draft change (covered separately in the crude tanker cargo page in this library) or independent production reporting.
Satellize runs this heading-and-mooring analysis as part of its broader offshore energy monitoring capability, applying the same SAR geometry methods it uses in its analytics work, including the Tonga crop-estimation programme, to demonstrate that open-constellation data combined with targeted commercial tasking can answer specific operational questions without requiring proprietary sensors.
Building a watch-list: from single vessel to fleet
The practical application for most clients is not a single FPSO but a watch-list of 10 to 50 units across a basin, such as West Africa, Brazil pre-salt, or Southeast Asian fields. At that scale, Sentinel-1's free archive and systematic coverage make it the backbone. Commercial tasking is reserved for high-priority events: AIS silence, unexpected heading deviation, or a known offloading window for a cargo the client is tracking.
Heading angles from each SAR pass are stored as a time series per vessel. Deviations beyond a threshold from the expected weathervane direction, calculated from publicly available ocean current and wind reanalysis products such as ERA5, trigger an alert. The alert carries the SAR image chip, the measured heading, the expected heading, and the AIS status at acquisition time. That is a complete picture an analyst can act on in minutes rather than hours.
Typical figures
| Spatial resolution (Sentinel-1 IW mode) | 10 m ground range, 20 m azimuth (multi-looked) |
| Spatial resolution (ICEYE Spot mode) | Better than 1 m (published specification) |
| Heading measurement precision | Approximately 2 to 5 degrees for vessels over 200 m at 3 m resolution; degrades in sea states above Beaufort 6 |
| Revisit (Sentinel-1, single satellite) | 6 days at mid-latitudes; more frequent near poles due to orbit convergence |
| Revisit (ICEYE constellation) | Hours to 1 day with tasking, basin-dependent |
| SAR frequency | C-band (5.4 GHz, Sentinel-1); X-band (~9.65 GHz, ICEYE, KOMPSAT-5) |
| Minimum detectable vessel length | Approximately 50 m at 10 m resolution; smaller vessels detectable at sub-3 m resolution |
| AIS cross-validation latency | Near-real-time when using aggregated terrestrial and satellite AIS feeds |
| SAR archive depth (Sentinel-1) | From 2014 (Sentinel-1A launch); accessible via Copernicus Data Space |
| Delivery format | Georeferenced image chips (GeoTIFF), heading time-series (CSV/GeoJSON), alert feed (JSON API or email) |
Analytics Satellize can run
| FPSO heading time series | SAR long-axis orientation extraction via principal component or template-matching fit to vessel backscatter outline | Per-vessel CSV with acquisition timestamp, measured heading, expected weathervane heading from ERA5 reanalysis, and deviation flag |
| Offloading event detection | Dual-vessel geometry analysis: constrained FPSO heading plus shuttle tanker presence and angular relationship in same SAR scene | Event alert with SAR image chip, vessel identities (AIS-matched where available), estimated event window, and confidence rating |
| AIS-dark vessel watch | Cross-referencing satellite AIS gaps against SAR-confirmed vessel presence at licensed block position | Weekly report flagging AIS-silent periods per vessel, with SAR confirmation of presence or absence |
| Mooring constraint classification | Heading deviation from modelled weathervane direction, thresholded against sea-state-dependent uncertainty bounds | Binary free/constrained classification per SAR pass, stored in vessel activity database with confidence interval |
| Fleet-level basin summary | Aggregation of per-vessel heading and offloading signals across a defined watch-list, with Sentinel-1 as baseline and ICEYE tasking for priority units | Weekly basin dashboard (PDF or web) showing active, constrained, and silent units with trend indicators |
| Historical offloading frequency reconstruction | Retrospective analysis of Sentinel-1 archive from 2014 to present, fitting heading-deviation signatures to known offloading event patterns | Per-vessel offloading frequency table for up to 10 years, usable as a production-rate proxy for due diligence or commodity research |
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.