4.2 Relative GNSS / RTK Concepts

Absolute Versus Relative Satellite Positioning

Section 4.1 treated DGNSS as an absolute geographic PRS: the vessel’s northing/easting in a world frame. Many DP tasks, however, care more about where the vessel is relative to a platform, FPSO, barge, or seabed structure than about the global coordinate of the CRP. Relative GNSS methods — including techniques marketed as RTK (Real-Time Kinematic) or similar high-precision relative solutions — focus on the vector between two GNSS antennas (or antenna and reference station) with centimetre-to-decimetre class relative accuracy when conditions allow.

You do not need to process carrier-phase equations on the Induction exam. You do need the operational concepts: baseline, relative versus absolute use cases, and failure modes near structures and in interference.

What “Relative” Means for the DPO

In a simple picture:

1. A base antenna is installed on a fixed or known point (platform, reference station, or carefully surveyed location).
2. A rover antenna is on the DP vessel.
3. The system solves the relative vector between base and rover at high rate.
4. The DP controller uses that relative position (often transformed into the vessel/control frame) for station-keeping relative to the structure.

Some installations also blend absolute and relative information, or use relative GNSS as one of several PRS inputs. Brand implementations differ; the exam cares about principle and limitations, not menu paths.

When Relative / RTK Is Preferred Over Absolute Alone

Absolute DGNSS is excellent in open water and for geographic setpoints. Relative methods shine when:

• Work requires a tight offset to a platform, jacket, or floating unit.
• Absolute coordinate biases (datum, residual atmospheric effects after standard DGNSS) matter less than relative stability between vessel and asset.
• The field design provides a reliable base antenna and radio/data link for corrections.
• Construction, gangway, or close-support tasks define success in metres relative to steel, not in global lat/long.

Relative GNSS is not the only relative PRS. Laser (Fanbeam/CyScan class), radar/microwave (RadaScan class), and taut wire are also relative by nature. Relative GNSS is simply the satellite-physics member of the relative family.

Baseline, Corrections, and Quality

Baseline length affects performance. Short baselines (vessel close to a platform base antenna) generally support better relative precision when sky view is good. Very long baselines behave more like wide-area differential problems and may not deliver classic RTK-class tightness. The correction stream from base to vessel must be continuous and timely — radio line-of-sight, interference, and range limits on the data link are operational constraints just as important as satellite count.

Both ends need adequate satellite visibility. If the base is shadowed by platform modules or the vessel antenna is masked by derrick and cranes, ambiguity resolution can fail, drop to float solutions, or become noisy. Quality indicators (fix status, residual, satellite count, baseline health) should be treated like any other PRS quality: green does not mean “ignore,” but red/amber means investigate before critical simultaneous operations.

Limitations Near Structures and in Interference

Relative GNSS remains a satellite system. Near large offshore structures you inherit many of the same problems as absolute DGNSS, sometimes worse:

Exam trap: “RTK always works better than DGNSS next to a platform.” Not always — structure multipath and masking can destroy relative solutions. That is why fields still fit laser and radar relative systems that do not depend on satellites for the relative measurement.

Independence and Class Thinking

A vessel with absolute DGNSS and relative GNSS still has two satellite-principle references if both depend on GNSS signals. For common-mode analysis:

• Absolute DGNSS + relative GNSS: shared vulnerability to jamming, scintillation, and major constellation events.
• Absolute DGNSS + laser to platform targets: different physics (satellite vs optical line-of-sight).
• Absolute DGNSS + USBL/LBL: different physics (satellite vs acoustic).

When writing or applying ASOG-style thinking, count principles, not product names. Relative GNSS is valuable, but it is not a substitute for non-satellite diversity on high-risk Class 2/3 tasks.

Operator Practices

• Confirm base station health, battery/power, and data-link status before approach.
• Verify the intended relative setpoint (offset to structure) matches the field plan — wrong target or wrong base ID is an operational own-goal.
• Allow the estimator to settle when enabling a relative GNSS PRS; do not thrash thrusters during the first noisy seconds.
• If relative solution drops fixed status near the worksite, do not force it; use remaining healthy independent PRS and follow degraded-mode criteria.
• After any change of base location or antenna, treat the system as requiring verification — baselines and lever arms may no longer match configuration.

Scenario Table

Bottom Line

Relative GNSS / RTK-class systems measure the vessel against a local baseline with high relative accuracy when sky, link, and multipath allow. Use them when the job is offset to a structure or unit; never confuse them with true independence from satellite failure modes. For assessment: baseline + base/rover + relative vs absolute use cases + structure/jamming limits + still one satellite principle for diversity counting.