5.3 Radar/Microwave Relative References (RadaScan)
Key Takeaways
- RadaScan-type systems are microwave (radar) relative PRS that measure range and bearing to responders or dedicated targets on a structure
- Microwave wavelengths generally perform better than laser in fog, rain, and spray that optically block Fanbeam/CyScan
- Active responders/targets and correct identification remain essential; interference and multipath can still degrade quality
- Like laser systems, radar relative PRS are relative to the structure—not absolute GNSS positions by themselves
- Comparing taut wire, laser, and radar helps the DPO choose diversity: mechanical seabed, optical, and microwave principles
Microwave Relative References: Why They Exist
Laser relative systems are precise in clear air but fragile in bad optical weather. Offshore DP needed a relative structure-based reference that could keep working when fog banks, rain squalls, and thruster spray blank the laser. Radar/microwave relative position-reference systems — commonly examined under the product name RadaScan and similar microwave concepts — fill that niche.
Exam recognition phrase: microwave / radar energy / responders / better in fog and rain than laser / relative to structure.
Principle of Operation
A microwave relative PRS typically:
- Emits radar/microwave energy from a vessel-mounted unit.
- Communicates with or reflects from responders / dedicated targets mounted on the platform, rig, or other reference structure.
- Measures range and bearing (and processes multi-target geometry where configured).
- Outputs a continuous relative position to the DP controller with quality indicators.
| Element | Role |
|---|---|
| Microwave sensor head | Transmits/receives radar energy; scans or tracks targets |
| Responders / active targets | Mounted on structure; provide reliable returns at microwave frequencies |
| Processor | Target ID, filtering, relative position solution |
| DP interface | Feeds relative offsets into voting and the estimator |
Active responders matter conceptually: the system is designed around cooperative targets, not random radar paint of the entire platform. Wrong target selection, dead batteries/power to a responder (where applicable), or mounting on a moving appendage creates the same class of operational errors seen with laser prisms — different physics, similar human-factor traps.
Better Poor-Visibility Performance Than Laser
The headline operational advantage over Fanbeam/CyScan:
| Condition | Laser relative | Microwave relative (RadaScan-type) |
|---|---|---|
| Clear air | Excellent | Excellent / very good |
| Fog / mist | Often poor | Generally better |
| Heavy rain | Often poor | Generally better |
| Sea spray | Often intermittent loss | Generally more robust |
| Needs LOS to targets | Yes (optical) | Yes (radar path, but different attenuation) |
| Absolute WGS-84 alone? | No | No |
This table is gold for exam stems that ask how RadaScan differs from Fanbeam/CyScan. The answer is almost always: microwave vs laser and better fog/rain/spray performance, not “measures depth,” not “uses seabed transponder arrays,” and not “provides satellite absolute position.”
Interference, Multipath, and Other Limits
Microwave is not magic immunity:
| Issue | Effect |
|---|---|
| Radio/radar interference | Noise, dropouts, false tracks if environment is electromagnetically hostile |
| Multipath / complex steel clutter | Reflections from adjacent structure can confuse geometry if not filtered |
| Target/responder failure | Loss of that relative PRS channel |
| Geometry / range limits | Too far, too close, or poor aspect angle reduces quality |
| Incorrect target ID | Relative hold to wrong object |
| Structure motion | Relative truth follows the structure (same as laser) |
The DPO should still maintain independent absolute or alternative relative references so a microwave outage does not leave the vessel single-threaded. Weather that helps microwave relative systems (fog) may also stress visual lookout — procedures and watchkeeping still apply.
Relative Frame Reminder
RadaScan-type systems are relative PRS:
- They answer “where am I relative to this structure’s responders?”
- They do not replace DGNSS for open-water absolute station-keeping without a structure.
- They do pair powerfully with DGNSS and HPR for principle diversity during platform work in dirty weather.
Comparison Table: Laser vs Radar vs Taut Wire
Use this master comparison for selection questions and oral/simulator explanations:
| Feature | Light taut wire | Laser (Fanbeam/CyScan) | Microwave (RadaScan-type) |
|---|---|---|---|
| Physical principle | Mechanical wire + clump weight | Optical laser to reflectors | Radar/microwave to responders |
| Reference frame | Relative to seabed weight | Relative to structure targets | Relative to structure targets |
| Needs structure targets? | No | Yes (prisms/reflectors) | Yes (responders/targets) |
| Needs seabed weight? | Yes | No | No |
| Best niche | Shallow–moderate depth | Close structure work, clear air | Close structure work, poor optical weather |
| Classic weakness | Depth, current, snag | Fog, rain, glare, spray | Interference, multipath, target issues |
| Deep water ideal? | No | Structure-dependent, not depth-based | Structure-dependent, not depth-based |
| Absolute GNSS? | No | No | No |
| Typical exam product names | Light taut wire / LTW | Fanbeam, CyScan | RadaScan |
How the Three Fit a Diverse Reference Mix
A robust platform-approach mix might look like:
- DGNSS — absolute earth frame and long-range approach.
- HPR/USBL or LBL — independent acoustic principle (where water depth and noise allow).
- Laser and/or microwave relative — tight relative offsets to the structure; microwave preferred if weather is optical-hostile; laser excellent in clear conditions.
- Taut wire — optional diversity in shallow fields if fitted and snag risk is acceptable.
You rarely need every sensor online at once, but you do need enough independent online references for Class 2/3 critical work, and you should not count two sensors that fail together as two principles.
Operator Practices for Microwave Relative PRS
- Confirm responders/targets are powered, correctly identified, and mounted on the intended structure element.
- Verify range/aspect is within system capability for the planned station-keeping box.
- Prefer microwave relative when fog/rain/spray is forecast and laser is likely to drop — but still keep non-optical diversity (DGNSS/HPR).
- Investigate interference sources if quality collapses without weather explanation (nearby radar, industrial RF, damaged cabling).
- On voting rejection, do not force a noisy microwave channel back as “must have three greens” without quality — three healthy independents beat three forced labels.
Exam Scenarios
| Scenario | Correct link |
|---|---|
| RadaScan vs Fanbeam principal difference | Microwave vs laser; better fog/rain/spray |
| Needs seabed transponder array | False for RadaScan — that is HPR/LBL |
| Measures water depth under keel | False — echo sounder, not relative radar PRS |
| Absolute satellite position | False — relative structure reference |
| Platform work in thick fog with laser failing | Microwave relative + other independents |
Bottom Line for Assessment
- RadaScan-type = microwave relative PRS to responders/targets.
- Better than laser in many poor optical visibility conditions.
- Still has interference, multipath, target limits.
- Relative, not absolute.
- Compare freely with taut wire and laser using principle, frame, and environment columns.
RadaScan differs from Fanbeam/CyScan principally because it:
Which statement correctly compares light taut wire, laser relative PRS, and microwave relative PRS?
During a platform standby in dense fog, the laser relative PRS repeatedly drops out while a microwave relative system remains stable. What is the best DPO interpretation?
A microwave relative PRS begins showing intermittent large residuals with no change in fog or rain. Which investigation theme is most appropriate?