Motion sensor location – why it matters and what to do about it
Practical guidance on choosing motion sensor locations to ensure accurate helideck motion measurements and regulatory compliance.
Purpose
This guide is published by WISE Group to help operators and installation owners understand why motion sensor placement matters and what the implications of different locations are. It covers sensor location guidance relevant to CAP 437, Norwegian BSL D 5-1 and Offshore Norge GL-074, and Brazilian NORMAM-223. If you are planning an installation or unsure about a location, contact WISE Group before making a final decision.
Why the location matters
A motion sensor measures movement at the point where it is physically installed. On a vessel or floating installation, every location moves slightly differently — a sensor mounted away from the helideck will not necessarily experience the same heave, pitch, or roll as the helideck itself. This matters because regulations require that displayed motion values represent conditions at the helideck landing area, not somewhere else on the vessel.
Placing the sensor at the helideck centre gives a direct measurement of helideck motion. Any distance between the sensor and the helideck centre introduces error into that measurement. The further away the sensor is, the greater the potential inaccuracy — and some regulatory frameworks explicitly reflect a preference for direct placement over corrected values from a remote location.
A further consideration is vibration. Machinery, generators, thrusters, and engines all produce vibration that inertial sensors pick up. A sensor mounted near engine rooms or on thin deck plate above machinery can show elevated readings that have nothing to do with actual vessel motion.
What the sensor actually measures
For moving helidecks, the HMS must report the following motion parameters:
- Pitch and roll — the angular motion of the helideck in the fore-aft and port-starboard directions
- Inclination — the maximum combined angular displacement at any given moment
- Significant Heave Rate (SHR) — the primary threshold used to determine whether conditions are within limits for helicopter landing
- Helideck accelerations — used in some configurations for motion sickness index (MSI) assessments
Pitch, roll, and inclination must be accurate to within ±0.1° RMS (root mean square); heave rate to within ±0.1 m/s RMS. These tolerances leave little margin for errors introduced by poor mounting or misalignment.
Good practice for sensor location
Mount on the underside of the helideck structure, directly below the centre point. This is the only location that gives a direct measurement of helideck motion with no corrections required. When the sensor is positioned directly below the helideck centre, the system output reflects the full native accuracy of the MRU. Any distance from that centre point degrades that accuracy. The sensor should be mounted as close to it as the installation allows.
Mount on rigid, load-bearing structure. The sensor must be fixed to the main hull or a primary structural member — not to handrails, secondary steelwork, thin deck plate, or equipment supports that can flex or vibrate independently. Structural flexibility introduces readings that do not represent actual vessel motion.
Align correctly with the vessel's longitudinal axis. Inertial sensors are directional. The sensor's reference axis must be aligned with the vessel's bow-forward heading, and the alignment angle entered into the sensor configuration at commissioning. An incorrectly aligned sensor will swap or offset pitch and roll values, producing misleading results even when everything else about the installation is correct.
Avoid vibration sources. Keep the sensor away from engine rooms, thruster tunnels, generator sets, and high-flow pipe runs. If no vibration-free location is available, use a vibration-isolating mount as specified by the sensor manufacturer, and document the arrangement.
Plan access for maintenance. The sensor will require periodic inspection, functional checks, and occasional replacement. Mount it in a location that can be reached safely, without climbing exposed structures or requiring confined-space entry.
Regulatory considerations
CAP 437 and HCA Rev 9c (international / UK sector)
CAP 437, implemented for moving helidecks through the Helideck Certification Agency document HCA Rev 9c (2023), requires that sensors are located in optimum positions to provide the required information relating to the helideck. Verification must be repeated every two years, or whenever accuracy is in doubt. HCA Rev 9c has been the recognised benchmark for moving helidecks since 1 October 2025.
HCA Rev 9c specifies MRU placement requirements in relation to the centre of the TDPM (Touchdown/Positioning Marking) circle:
Within 4.0 metres of the TDPM centre (preferred). If the MRU is mounted within a hemisphere of radius 4.0 m centred at the TDPM circle centre, no lever arm corrections are required. The MRU's native accuracy is sufficient provided it meets the stated performance requirements.
Beyond 4.0 metres from the TDPM centre (requires additional measures). If the 4.0-metre hemisphere cannot be achieved, HCA Rev 9c permits two alternative approaches: (a) apply lever arm corrections with a corresponding increase in MRU accuracy to compensate for the additional error introduced by the offset; or (b) mount the sensor at a point further from the vessel's pitch and roll axes than the TDPM centre itself, and vertically within 3.0 m of the helideck surface. If neither criterion is met, a sea trial is required to confirm that the system achieves the required measurement accuracy at the helideck.
Norway – BSL D 5-1 and Offshore Norge GL-074
BSL D 5-1 (§ 39) requires that moving helidecks are equipped with instruments capable of providing continuous registration of helideck movements: pitch, roll, inclination, and heave rate. Crew must have access to this data both before flight planning and immediately before landing. Recordings must be retained for a minimum of 30 days. Instruments must be calibrated and maintained in accordance with the manufacturer's instructions.
Offshore Norge Guideline 074 (Helideck Manual), Rev 4, provides the technical standard for Norwegian Continental Shelf (NCS) operations through Appendix K — Standard Measuring Equipment for Helideck Monitoring System (HMS) and Weather Data (Rev 9.2, 2024). Appendix K states that measuring equipment sensors shall be located in optimum positions to provide relevant information relating to the helideck, and that helideck heave data must be representative of the centre of the helideck.
On placement, Appendix K includes an explicit recommendation: for new designs, the motion sensor should be located within 4 metres from the helideck centre, in order to meet a possible future requirement for measurement of Motion Severity Index (MSI). This aligns with the 4.0-metre hemisphere criterion in HCA Rev 9c, though in the Norwegian framework it is currently expressed as a recommendation rather than a binding placement rule.
Accuracy requirements are identical to those in HCA Rev 9c: pitch, roll, and inclination must be accurate to within ±0.1° RMS; heave rate to within ±0.1 m/s RMS. Verification must be carried out initially and periodically, at a minimum of every three years, following the manufacturer's procedure. Test instruments must hold traceable calibration certificates, and must be positioned at the centre of the helideck during the verification test.
Brazil – NORMAM-223
NORMAM-223 requires that every ETEX M (offshore mobile unit) operating in Brazilian jurisdictional waters (AJB) is equipped with an HMS. On sensor placement, NORMAM-223 is explicit: motion sensors must be positioned on the helideck floor. Where this is not possible, the displayed values for pitch, roll, heave, heave rate, SHR, and inclination must be corrected for the height and position of the helideck. Temperature and wind sensors must be installed close to the helideck as a mandatory requirement.
Calibration must be carried out in accordance with the manufacturer's specifications. Where the manufacturer defines no interval, calibration must be performed every 24 months, and must comply with the standards of the Rede Brasileira de Calibração (RBC) — Brazil's INMETRO-accredited calibration network. Under Annex 1-F of NORMAM-223, MRU sensors (pitch, roll, and heave) that are uncalibrated or have expired calibration constitute an Exigência Impeditiva — an imperative impediment that prevents helideck certification until resolved.
Managing non-ideal locations
When the helideck centre is not available as a mounting position, the priority is to get as close to it as possible while still meeting the structural and vibration requirements above.
Stay within 4.0 metres of the TDPM centre where possible. HCA Rev 9c treats the 4.0-metre hemisphere as the boundary below which no additional corrections are needed. Remaining within this radius while mounting on the underside of the helideck structure is the most straightforward path to compliance.
If the centre is unavailable, favour the outboard side. If the sensor cannot be positioned within the 4.0-metre hemisphere, HCA Rev 9c permits an outboard mounting provided it is further from the vessel's pitch and roll axes than the TDPM centre and vertically within 3.0 m of the helideck surface. This approach avoids the need for lever arm corrections. A sensor positioned outboard experiences greater motion than the helideck centre itself; a sensor positioned inboard experiences less. Correcting downward from a stronger signal is more reliable than correcting upward from a weaker one, which amplifies noise and small positioning errors.
If neither criterion can be met, lever arm corrections or a sea trial are required. Positions that fall outside the 4.0-metre hemisphere and do not satisfy the outboard and vertical constraints require either lever arm corrections with increased MRU accuracy, or a sea trial to confirm the system meets the required measurement accuracy at the helideck. These are workable solutions, but they add cost and complexity that careful placement planning can often avoid.
Contact WISE Group before finalising the location. Non-ideal placements have implications for measurement accuracy and regulatory compliance that are easier to address before installation than after. WISE Group can advise on the best available option for the specific vessel and help ensure the chosen location meets the applicable requirements.
Key takeaway
The best location for a motion sensor is on the underside of the helideck structure, directly below the centre point, on rigid structure, correctly aligned. Any distance from the helideck centre has implications for measurement accuracy and regulatory compliance. Contact WISE Group before selecting a location — getting it right from the start is considerably easier than correcting it afterwards.