Radar Navigation Notes

Radar as a Navigational Aid

Precautions in Taking Radar Fixes and Accuracy of Obtained Range and Bearing

• Radar Reflectivity of Coast:
  • Radar reflectivity depends on the height of coastal features, their composition, nature of the surface, and aspect.
  • The radar picture (PPI) might show gaps in the coastline because the detection range in some parts may be less than the actual range from the ship.
• Height of Tide:
  • Charts are drawn for the water level at Mean High Water Ordinary Springs (MHWOS).
  • Radar shows the coastline as it is at the present state of tide, which is probably not MHWOS.
  • A gently sloping coastline may not change much in shape but can change considerably in position due to small tide changes.
  • A rocky coast's radar picture can change significantly in position and shape due to tide, as rocks may cover at high water and uncover at low water.
• Shadow Areas:
  • The radar picture gives a bird’s eye view based on horizontal travel of radar waves.
  • Shadow areas exist behind large targets where smaller targets don’t show up, and these change with aspect.
• Shapes and Sizes of Targets on Accuracy of Obtained Range and Bearing:
  • Charts show the correct shapes and sizes, while radar screens do not.
  • Targets on the PPI are subject to distortion in azimuth due to Horizontal Beam Width (HBW) and the scale size of the spot.
  • Radar pulses reflect from the nearer edge of each target and can't explore the farther side.
  • The reflecting edge of each target appears on the PPI to have a radial depth of $(1/2 \times PL + \text{scale size of the spot})$, regardless of whether it's a buoy or a building.
  • A stepped target has many reflecting surfaces at different ranges, so its paint would have greater radial depth than a target with a single vertical edge.

Factors That Might Cause Faulty Interpretation (Spurious Echoes)

• Indirect Echoes:
  • Shipboard obstructions (masts, funnels, etc.) may reflect radar energy onto a target in another direction.
  • The echo returns along the same path and paints in the direction of the obstruction at the correct range.
  • These are called indirect or false echoes.
  • The normal (direct) echo also paints at the correct range and bearing.
  • Indirect echoes are not common on modern ships with properly sited radar scanners.
• Side Lobe Echoes:
  • Commercial marine radar sets have limitations that prevent them from sending all energy in a single narrow beam.
  • Some energy radiates as weak beams (side lobes) at various angles on either side of the main beam.
  • These side lobes have extremely small energy content.
  • Only very close-by objects give a strong enough response to side lobes.
  • Side lobe echoes are considerably reduced when using a slotted waveguide scanner.
• Multiple Echoes:
  • When two ships pass beam to beam on parallel courses at close range (less than a mile), a second echo may appear on the bearing of the other ship at double the range.
  • Occasionally, a series of such echoes may appear at equal range intervals.
  • The closest echo represents the correct position of the target.
• Second Trace Echoes:
  • Echoes from targets just outside the range scale are not registered because the spot has returned to the screen's center.
  • If the target is far away and super- refraction is present, a strong echo may arrive after the next trace has started.
  • It will be painted on the PPI at the correct bearing but at a wrong range.
  • This is called a second trace echo because the echo of the first pulse paints on the second trace.
  • The echo of the first pulse should arrive during the second trace, i.e., after the center spot has left the center, on its second run, but before it has reached the edge of the screen.
• Spoking:
  • Unwanted radial lines that sometimes appear on the screen are called spoking.
  • These spokes may occur all around the screen or in certain arcs.
  • They may appear on every rotation or intermittently.
  • They may be complete or incomplete radial lines.
  • Common causes include dirty contacts in the heading marker circuit or slip rings of the rotating deflection coils.
  • Spoking can also be caused by heavy sparking of motors from nearby instruments like clear-view screens or echo sounders.
• Starring:
  • Echoes sometimes appear as curves or spirals of dotted lines (starring).
  • This changes position with every rotation of the scanner.
  • Caused by another ship’s radar operating in the vicinity whose transmitting frequency is within the bandwidth of the own ship’s radar.
  • If the other ship puts her set on standby, starring will disappear.
  • Starring can occur even if only one other ship is detected on the PPI.
  • The other ship may not be operating her radar; the starring may be caused by a third ship too far away to show up on the own ship’s screen but close enough to cause starring.

Radar Display Formats

• Radar users must understand what they are seeing.
• North up relative motion is the normal default display format.
• Relative and true vector and trails can be selected.
• North Up Mode:
  • Shows targets in their true (compass) directions from own ship, with North maintained up on the screen.
  • The heading marker changes direction according to the ship’s heading (gyro stabilized).
• Relative Motion Display:
  • Head Up display.
  • North Up Display.
  • Comparison between Head Up and North Up Display.
  • Course Up Display.
  • Off-Centre RM Displays.

Orientation, Motion, and Stabilization Modes

• Orientation, motion, and stabilization modes are the three different areas when using Radar.
• Orientation Mode:
  • Defines how the ‘vertical’ direction of the display aligns with the outside world horizontal (azimuthal) direction.
• Motion Mode:
  • Defines how the own-vessel moves with respect to the display.
• Stabilization Mode:
  • Defines how absolute movement is referenced – relative to the ground or relative to the sea.

Display Feature Comparison

Relative Motion (RM) vs. True Motion (TM) Display

• Relative Motion (RM) Display:
  • The Electronic Centre (EC), representing Own Ship, remains stationary, while all targets move relatively across the radar screen.
  • Radar users must clearly understand what they are seeing.
  • North up relative motion is the normal default radar display format.
• True Motion (TM) Display:
  • Own ship and other moving targets move according to their course and speed.
  • Fixed targets such as landmasses appear as stationary echoes.
  • The presentation mode used is North Up Relative Motion in the pictures above.

Sea vs. Ground Stabilization Mode

• Sea Stabilization Mode:
  • Vessel’s speed and course through the water are considered.
  • Assumes vessels are unaffected by the actual movement of the body of water (current).
  • All movement – Own ship, fixed objects, moving objects – is their movement through the water.
  • Ideal for collision avoiding actions, the display must always be on Sea Stabilised Mode for the ARPA calculations to be accurate.
• Ground Stabilisation:
  • Vessel’s position, Speed and Course made good (actual track) are obtained from GPS.
  • Shows the vessel’s track in relation to the ground after taking into account current, drift, etc.
  • The heading marker will always show the course steered (Gyro Course).
  • Fixed objects will not have any movement over the ground.
  • Ground Stabilisation mode must not be used for collision avoidance as incorrect ARPA readings may result, particularly where strong currents exist.

Parallel Index Technique

• Parallel Indexing allows the watchkeeper to react almost instantly to any deviation from the planned track.
• It also monitors whether the vessel is ‘right of the track’, ‘left of the track’ or ‘on track’.
• PI relies on the fact that the relative track of a fixed object is the reciprocal of the vessel’s ground track.
• During passage planning, certain fixed charted objects are chosen as indexing targets or references.
• They must be good radar targets, clearly visible on display at the appropriate ranges.
• As the vessel proceeds along its track, the PI moves with it, maintaining its position on the target.
• Should the vessel move off its track, the PI will also move away from the reference target.
• This prompts the operator to make timely adjustments to the heading to bring the vessel back on track and the PI back to the correct reference target.
• Parallel Index Techniques can be used effectively on the Radar while altering course while coasting in congested waters; for arriving at a pre-determined anchorage position and for collision avoidance.
• As it depends solely on Radar referencing, any malfunction/erroneous inputs from the Gyro and GPS on the ECDIS can be immediately detected.

Wheel Over Positions and Safety Margins

• Wheel Over Point (WOP):
  • Specific point on the charted course indicating where the helm must be executed to change heading to arrive on the new course without overshooting or undershooting the Waypoint (WP).
  • The WOP depends on the turning circle of the ship/Rate of Turn, which in turn depends on the LOA, Speed of the Ship, the draft (Loaded/in Ballast), the amount of helm, water depth, weather, among others.
• When planning/charting course on the Chart (or ENC in the case of ECDIS), WOPs must be marked for every alteration of course on the intended voyage.
• On ECDIS, the WOPs are automatically marked as an option, taking the data provided by the user (Speed, Rate of Turn, Turning Radius).
• Safety Margins:
  • Safety margins must be allowed for, and each situation must be handled accordingly, keeping in mind safety of navigation at all times.
  • On the ECDIS - Areas outside the Navigation Corridor (on ECDIS Charts) up to the No-Go areas are considered the Safety Margins available for unforeseen circumstances.
  • Entry into these Safety Margin areas must be done with great caution as these areas are not usually pre-checked or verified electronically by ECDIS for dangers.