Executive Summary
Engineers at Thales Alenia Space have proposed relocating core SBAS functions from geostationary satellites into a distributed low Earth orbit network, removing the regional ceiling imposed by ground reference station coverage. For offshore operators, the significant change is not accuracy but continuous global integrity monitoring – the capability DP and survey most need at high latitude and far from shore. The concept is credible but unstandardised for marine use, and it must not be treated as a drop-in that collapses the independent correction sources DP redundancy depends on.
A proposal to lift SBAS off the ground
A team at Thales Alenia Space in Toulouse – Sébastien Trilles, Thierry Authié, Xavier Vasseur and Marie Abbal – has published a case in Inside GNSS for what they call spatialising Satellite Based Augmentation Systems. In plain terms, the proposal moves the core functions of an SBAS off the ground and into a distributed network of satellites in low Earth orbit.
The reasoning starts from how SBAS was built. Civil aviation defined it to do two things: compute correction messages so aircraft can use GNSS for precise positioning even when the constellation carries intentional or unintentional degradations, and monitor the broadcast navigation data in real time to detect anomalies and warn users within a timeframe matched to their flight phase. Because aviation needs continental coverage, dissemination went to geostationary satellites. The classic architecture is a network of ground reference stations collecting GNSS measurements, central processing that builds the augmentation messages, and transmission stations that uplink the signal to the GEO satellite.
Today’s operational systems augment single-constellation GPS on L1, using the L/NAV message, and broadcast orbital corrections, clock corrections and an ionospheric grid model on a dedicated PRN. The next generation, Dual Frequency Multiple Constellations (DFMC), serves L1/E1 and L5/E5a users, carries L/NAV for GPS and F/NAV for Galileo, broadcasts on L5, and provides orbital and clock corrections across constellations. The authors identify the binding constraint clearly: the regional footprint of the ground reference network does not allow continuous monitoring of every satellite in the navigation constellation, so an SBAS must constantly manage satellite visibility. That constraint is what a LEO architecture is meant to remove.
What the architecture actually changes
It helps to separate the two SBAS missions, because they behave very differently offshore. Correction is the part everyone talks about – decomposing the error budget into satellite clock, orbit and ionospheric contributions and rebroadcasting them so the receiver can reassemble a correction for its own location. Integrity is the part that decides whether you can trust the answer. An SBAS computes protection levels and guarantees a time-to-alert if a fault develops. Aviation lives or dies by that second function; a horizontal or vertical position that drifts silently is far more dangerous than one that is merely imprecise.
The ground-station limitation bites hardest on integrity. A regional reference network can only characterise a satellite while it is in view of that network. Corrections for a satellite rising over a remote ocean basin are extrapolated, not observed, and the confidence bound has to widen to cover the uncertainty. Protection levels inflate, availability of the tighter service degrades, and the very geometry that offshore work depends on – low-elevation satellites at range from any monitoring station – is the geometry the system knows least about.
Spatialising the reference and monitoring function is an answer to this. Put the observers in orbit, distributed globally, and every navigation satellite is under continuous watch from somewhere, all the time. The correction estimate stops being a regional extrapolation and becomes a global observation. A second benefit is dissemination. A GEO sits far out in geostationary orbit and, above a certain latitude, sinks toward or below the horizon – which is precisely why GEO-delivered augmentation has always been weak in the Arctic and unusable near the poles. A LEO pass is an order of magnitude closer and comes overhead at high latitude. The signal is stronger and the coverage geometry no longer collapses where the work is moving.
There is a further property that matters for convergence. LEO satellites cross the sky quickly, so the line-of-sight geometry to a stationary or slow-moving user changes rapidly. Fast geometry change is what lets a precise solution resolve its ambiguities and pull in quickly rather than sitting through a long settling period. For anyone who has watched a PPP solution take tens of minutes to reach centimetre-level after a signal outage, that is not a small detail.
Why this lands on the offshore desk
The reality for marine work is that most offshore DP and survey operations do not use SBAS as a primary reference at all. WAAS, EGNOS, MSAS and GAGAN are regional, aviation-owned services with continental footprints, and once you are a few hundred kilometres offshore or working at high latitude, the tighter service simply is not there. The industry solved augmentation commercially instead – satellite-delivered DGNSS and, increasingly, PPP services with global correction streams. SBAS has been, for our sector, largely a coastal convenience rather than a working tool.
A globally spatialised SBAS changes that calculus, and it does so on the axis we care about most. DP position reference integrity is a genuine weakness in current practice. A DP system consumes a position and, at best, a quality figure; it does not receive a guaranteed protection level with a bounded time-to-alert in the aviation sense. The classic DP hazard is a slow, undetected drift in a GNSS reference that the vessel’s model absorbs and then drives against – the position reference telling a confident lie. An augmentation layer that delivers a real integrity bound, continuously and globally, addresses the failure mode that FMEA studies keep flagging and that IMCA guidance on position reference systems has long treated with suspicion.
The operational cases follow directly from this. Arctic and sub-Arctic construction, pipelay and survey have poor or no GEO SBAS today; overhead LEO coverage removes the latitude ceiling. Deepwater DP drilling and heavy-lift far from any reference network gain continuously monitored corrections rather than extrapolated ones. Survey acquisition graded against IHO S-44 Edition 6 – where a Special or Exclusive Order job demands tight, defensible total propagated uncertainty – benefits from both the accuracy and, more importantly, a documented integrity chain that stands up in the survey report. IOGP guidance on GNSS positioning for the offshore energy sector already pushes toward independent verification and stated uncertainty; a global integrity service fits that direction of travel.
Where the concept is not yet a tool
We would caution strongly against reading an architectural proposal as an available capability. Several things stand between this and a specification line in a DP or survey scope.
1. The standards are aviation’s, and they are not written for us
SBAS interoperability rests on ICAO Annex 10 and the RTCA and EUROCAE Minimum Operational Performance Standards (MOPS) lineage, with DFMC standardisation still maturing. Those documents encode aviation’s requirements – vertical guidance, LPV (Localizer Performance with Vertical guidance)-style alert limits, a six-second time-to-alert for the approach phase. Marine DP and hydrographic survey have different needs: horizontal accuracy dominates, the dynamics are slow, and there is no agreed marine integrity standard equivalent to an aviation alert limit. A LEO-delivered service will be certified, if at all, against aviation criteria first. Translating that into a meaningful DP or S-44 integrity statement is unfinished work, and no marine class or IMCA framework currently defines how a DP system should consume a protection level.
2. Receiver and DP integration is not a firmware toggle
Current SBAS lives on specific PRNs and frequency bands with defined message formats. A spatialised architecture implies new signals, new message content and possibly new dissemination bands. Marine GNSS receivers and the DP position reference interfaces behind them would need to support it, and the DP control system would need to do something intelligent with an integrity flag rather than discard it. None of that exists in the deployed fleet today.
3. Common-mode failure is the trap
The single most likely mistake is architectural. DP2 and DP3 redundancy depends on position references that fail independently – which is why sound practice runs GNSS units on different correction services so a fault in one correction stream cannot take down every reference at once. A single, elegant, global augmentation source is a common-mode failure waiting to happen if operators let it feed every reference. The value of a global integrity service does not remove the requirement for diverse, independent references; it sits alongside it. Any FMEA that treats a spatialised SBAS as the one true position source has misunderstood the redundancy principle it is meant to protect.
4. The service is proposed, not proven
The published case is an argument for a change in framework, not a qualified system with demonstrated availability, continuity and integrity numbers. A LEO constellation carries its own questions – satellite lifetime, replenishment, cross-link and uplink capacity, and the governance and funding model behind a global public service. Until those are settled and the performance is demonstrated across a full range of latitudes and conditions, it is a promising direction, not a procurement option.
What to do now
The concept is worth engaging with early, because the offshore integrity gap it targets is real. Concrete steps for survey managers and positioning engineers:
- Treat it as an integrity layer, never a single source. In any future adoption, keep at least two independent DGNSS/PPP correction services across DP references so no single augmentation fault is common to all of them. State this explicitly in the DP FMEA and verify it in annual trials per IMCA M 190 practice.
- Write integrity into position reference procurement. Specify that the receiver output includes protection levels and a time-to-alert, and that the DP or survey acquisition system can log and act on that integrity flag. A quality number without a bounded alert is not integrity.
- Map the requirement to your accuracy grade. For survey, tie any augmentation performance to the S-44 order you are contracted to deliver, and demand a stated convergence budget after signal interruption. For construction DP, define the maximum tolerable position error in the activity-specific operating guidelines rather than accepting a generic figure.
- Prioritise the high-latitude case. If your portfolio includes Arctic or far-northern work, this is where a spatialised architecture would change what is technically possible, because GEO SBAS is unusable there. Track the development against that operational need specifically.
- Verify against a known reference before you trust it. When any LEO-augmented service becomes available, run static and dynamic comparison trials against an independent, established reference before it carries operational weight, consistent with IOGP verification practice. Do not accept a positioning source into the DP reference set on the strength of the vendor’s own numbers.
- Engage the standards bodies now. The marine sector was largely absent when SBAS integrity criteria were written for aviation. If a global augmentation layer is coming, IMCA and the hydrographic community should be defining what a marine integrity requirement looks like while the architecture is still on the drawing board, not after it flies.
The honest assessment is that Thales Alenia Space has identified the right constraint – regional monitoring caps global integrity – and proposed a defensible way around it. For our sector the prize is not another decimetre of accuracy; commercial PPP already delivers that. The prize is continuous, global, monitored integrity in exactly the remote and high-latitude places where DP has always had to trust references it could not fully verify. That is worth watching closely, and worth arguing our requirements into, before the standards harden around someone else’s mission.
Based on: The Case for Spatializing Global SBAS
