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:
Measures water depth under the keel using acoustic ranging
Requires a pre-calibrated seabed transponder array like LBL
Provides absolute geographic position solely from satellites
Uses microwave (radar) energy to responders/targets and typically performs better in fog, rain, and spray
Which statement correctly compares light taut wire, laser relative PRS, and microwave relative PRS?
All three provide absolute WGS-84 positions without any local target or weight
Taut wire is mechanical to a seabed weight; laser and microwave are relative to structure targets, with microwave usually better in poor optical weather
Laser works only in deep water; taut wire works only in fog; microwave works only with GNSS off
Microwave systems replace the gyrocompass and thruster allocation logic
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?
The vessel must immediately disable all relative references forever
Optical conditions are likely degrading the laser as expected; continue with microwave plus other independent PRS and monitor quality
Fog always invalidates DGNSS and HPR at the same time as laser
Microwave stability proves the taut-wire clump weight is on the platform deck
A microwave relative PRS begins showing intermittent large residuals with no change in fog or rain. Which investigation theme is most appropriate?
Possible interference, multipath, responder/target problems, or geometry limits — check target health and electromagnetic environment while peers support the estimate
Replace all gyrocompasses because heading is measured by RadaScan
Increase free daily AI limit settings on the bridge computer
Assume the mathematical model has deleted the need for any PRS
Sections you finish are checked off in the contents.