3.3 Gyrocompass & Heading Feedback
Key Takeaways
- The gyrocompass provides heading and rate-of-turn feedback essential for yaw control and for rotating body-frame thruster commands into earth axes
- Class 2/3 vessels typically fit three gyros so 2-out-of-3 (2oo3) voting can reject a single failed heading sensor
- Loss or gross error of the selected heading source can degrade or destabilise yaw control and corrupt position geometry that depends on heading
- Speed and latitude errors are classic gyro compassing error sources at the conceptual level tested on DPO exams
- Heading redundancy is as safety-critical as PRS diversity: without trustworthy heading, Auto DP cannot hold a commanded yaw reliably
Heading Is Not Optional on DP
Dynamic positioning controls surge, sway, and yaw. Position references mainly answer where the vessel is; the gyrocompass answers which way it points and how fast heading is changing (rate of turn). Without heading, the controller cannot:
- hold a heading setpoint,
- convert thruster forces from vessel axes into earth-fixed north/east commands correctly,
- interpret some relative reference geometries that depend on antenna offsets and heading,
- coordinate a controlled turn while staying on position.
On the exam, treat the gyro family as a critical sensor group, not as a navigation nicety only used for chart work.
What the Gyro Supplies to DP
| Signal | Use in DP |
|---|---|
| Heading | Yaw feedback vs heading setpoint; axis transformation |
| Rate of turn (ROT) | Damping and prediction in yaw control |
| Status/quality | Voting, alarms, selection of healthy sensor |
Magnetic compasses are generally not the primary DP heading source offshore. North-seeking gyrocompasses (and sometimes high-grade heading from integrated navigation systems interfaced as gyro equivalents) provide the stable, continuous heading DP needs. Exact manufacturer interfaces vary; the principle does not: DP needs reliable heading feedback.
Redundancy and 2-out-of-3 Voting
IMO equipment class philosophy for redundant DP vessels requires that a single failure should not cause loss of position-keeping capability. For heading sensors that means multiple independent gyros. In practice, three gyros are common on Class 2/3 vessels so the system can apply 2-out-of-3 (2oo3) voting:
- All three report heading.
- If one diverges beyond tolerance, it is rejected as faulty.
- Control continues on the two that still agree.
- The DPO is alarmed to investigate the failed unit.
| Configuration | Typical implication |
|---|---|
| One gyro | Single point of failure for heading (more Class 1-like risk profile) |
| Two gyros | Detect disagreement, but may not know which is correct without other cues |
| Three gyros (2oo3) | Majority vote identifies the odd one out |
This same 2oo3 idea appears again for wind sensors, MRUs, and other Class 2/3 sensor groups — learn it once, reuse it across the syllabus.
Gyro Failure Effects on Yaw Control
If the selected or voted heading becomes wrong or freezes:
- Yaw control may drive thrusters to chase a false heading error (vessel starts to spin or snake).
- Position control can suffer because force allocation in earth axes depends on heading.
- Some PRS lever-arm corrections (antenna positions relative to the CRP) become wrong if heading is wrong, injecting a false position component that grows with lever-arm length.
Operator response themes (aligned with later sensor-voting and operations chapters):
- Acknowledge heading alarms immediately.
- Confirm which gyro is selected / which has been voted out.
- Do not ignore a slow drift between gyros during critical work — slow faults are harder than hard-overs.
- Be ready to reduce operational risk (stop critical activity, increase watchfulness) if heading redundancy is lost (e.g., only one healthy gyro left).
Alignment and Common Error Concepts (DPO Level)
You do not need a full naval architecture course on gyro theory, but Induction-level items often touch conceptual compassing errors:
| Error / topic | Conceptual meaning for DPOs |
|---|---|
| Speed error | Incorrect speed input to the gyro can bias the north-seeking result |
| Latitude error | Incorrect latitude input similarly degrades compassing accuracy |
| Alignment / installation offset | A constant heading bias if the gyro is not aligned to the vessel reference line |
| Settling after manoeuvre | Rapid turns and acceleration can temporarily disturb performance |
| Power / warm-up | Gyros need stable power and time; UPS-backed supplies protect critical units |
For DP operations, a constant small offset may be tolerable if known and consistent across systems, but a sudden change relative to other gyros is a fault signature. During setup and trials, heading alignment against known references matters; during operations, cross-checking the three gyros is the practical daily defence.
Heading Versus Other Sensors (Avoid Mix-Ups)
Exam distractors often swap sensor roles. Keep this table cold:
| Sensor | Primary DP contribution |
|---|---|
| Gyrocompass | Heading and rate of turn |
| MRU | Roll, pitch, heave (geometry compensation) |
| Wind sensor | Wind speed/direction for feed-forward |
| PRS | Position (absolute or relative) |
| UPS | Ride-through power for control electronics |
A failed MRU does not replace heading. A wind sensor does not provide yaw feedback. Only heading sensors close the yaw loop.
Operational Scenarios Worth Memorising
| Scenario | Likely effect | DPO focus |
|---|---|---|
| One of three gyros hard-over | Voted out by 2oo3; Auto DP continues | Restore redundancy; log fault |
| Two gyros disagree, third offline | Ambiguous heading truth | High risk — reduce work, manual awareness |
| Heading jumps during Auto DP | Yaw thruster demand spike | Check gyro status/voting |
| Controlled heading change on DP | Temporary thruster activity | Allow settle; watch position footprint |
| Entering Auto DP with wrong heading selected | Immediate yaw error | Verify sensor selection before critical modes |
Link Forward to Class and FMEA Chapters
When you later study Class 2/3, FMEA, and sensor voting, gyros are a textbook example: redundancy is not “spare parts in a drawer,” it is online, independent, voted sensors that keep the controller honest after a single failure. When you study drive-off, remember that a corrupted heading can be as dangerous as a corrupted position reference because thrusters will confidently push in the wrong direction.
Bottom Line for Assessment
- Gyro = heading + ROT for yaw control and axis transforms.
- Class 2 typically → three gyros, 2oo3 voting concept.
- Failure of heading feedback → yaw (and possibly position) degradation.
- Know speed/latitude/alignment as conceptual error sources.
- Never confuse gyro with MRU, wind, or PRS roles.
What primary information does the gyrocompass provide to the DP system?
Why are three gyrocompasses commonly fitted on a DP Class 2/3 vessel?
A sudden large heading error on the selected gyro during Auto DP is most likely to affect which controlled degree of freedom first and most directly?
At DPO conceptual level, incorrect speed or latitude input to a gyrocompass is associated with: