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Some radar theory

How radar works
Radar transmits pulses of microwave energy by a rotating aerial and measures time of echo returns. The beam on its way meets different obstructions and part of microwave energy is reflected back.
Beams with high amounts of reflected energy are marked by red color.
The echo picture is displayed on the radar screen called PPI (Plan Position Indicator).
The intensity of echo returns depends on many factors, main of which are
  • Reflecting material capability. Good microwave reflectors are metal, rock and water. Wood and plastics are poor ones.
  • Reflecting surface geometry, more precise, on the angle between the beam and the surface normal at the hit point (picture below).
On the picture, the radar beam which is more perpendicular to the surface (case 2) produces more intensive echo returns.
  • The beam terminates (or loses part of its energy) once it hits a surface, so all objects behind become invisible (or hardly visible). In addition, some surfaces are simply cannot be hit by the beam, as they are shielded by the object body, what makes houses look in radar pictures as "ticks" (not the whole contour) generated by reflections from forward walls.
  • On this picture, the beam issued from ship 1 cannot reach target 2 behind the radar horizon.
The common formula for radar horizon distance is

R = 2.2 h 

where h is radar antenna height in metres with the distance R in nautical miles.
Switching power on

Radar equipment is usually provided with a power button which can be marked by POWER, or maybe by red color. Some radar sets require a single hit to switch them on, some require press and hold for a few seconds.

Magnetron is a device that emits pulses of microwave energy and receives reflected energy. On switching power on, some time is required to warm the magnetron. In reality, this process may take up to several minutes. After the magnetron has been warmed, the radar set goes into a stand-by mode. In stand-by mode, no energy is transmitted. There is normally a [TRANSMIT] button to start radar transmission.

These two simple features are missing in the online simulator.

Switching radar off Most radars require to "press and hold" the power button with a countdown displayed on the screen. PPI The PPI (plan position indicator) shows a radar picture. It is equipped with a cursor and various radar instruments. Gain adjustment The gain control changes the power of the emitted signal. When too high, the radar image is bright and heavily speckled. When too low, small targets are not visible. In practice, it is advised to set gain level to retain some speckles on the screen and not to remove them completely. Try to set the right gain level. With this radar set, it is accomplished by the [GAIN] slider. Click or drag the mouse to change the gain level. Setting range Range is the radius of area currently shown on a PPI. Its current value is now displayed at an upper line of radar display. Below it, you can see the distance between the rings. Use [RANGE] button to switch ranges and look how radar picture changes. Most commonly, the term "range" is a distance from the ship in nautical miles. Interpreting radar picture The brightest echoes are produced by shore lines which are closest to the ship and perpendicular to beam direction. You may not expect to see the whole contour, but only parts which are hit by the beam. Heading, course and track angle
Angles (directions) are measured from the north direction clockwise, all angles being trimmed to the range 0..360°.
Heading is the direction to which the stem of the ship is pointing.
Course is an angle in which the ship is steered.
Track angle is the direction of real velocity vector, pointing to the direction in which the ship is actually moving. If the velocity vector relates to the ground, the track angle is called COG (Course Over Ground) or CMG (Course Made Good); in case it is measured relative to water (water itself may move due to tidal streams or a current), the track angle is called water track angle.
North-up and head-up modes There are two typical ways for display of radar picture :
- head-up, when the heading direction vector always points upwards, and you see all the radar objects relatively to the ship
- north-up, the radar picture looks like a chart with its top at north and the heading line shows current ship heading on this chart
Please note that the radar "must know" the heading value to correctly display its heading scale around the screen and display its picture in north-up mode, so a radar must be always connected to a compass. Ship heading line (SHL) Typically, the ship is at the centre of radar image, and a bright line (SHL - ship heading line) or ship heading marker (SHM) shows direction of ship's heading. On most radars, this line can be hidden temporarily, usually for a short period of time e.g. while a button is pressed. Normally, this line is always shown, because it is always important to see the current ship heading. Range rings Range rings are used to estimate distance from the ship to targets. Distance between rings is usually preset for each range value but may be changeable on some radar sets.
Sometimes it is desirable to hide range rings not to obscure targets on radar image, so radar rings can be hidden or shown. Tuning Receiver sensitivity changes while the magnetron operates. Many radars have automatic tuning adjustment, while others have both automatic and manual control of tuning. This parameter in a sense resembles gain : changing this, the picture quality may be improved. To set up tuning for this radar set, press the [TUNE] slider. Brilliance Changes brightness of the picture. Excessive brightness decreases lifetime of the display. Brightness for this set can be changed the same way as tuning. Wave clutter Echoes from waves produce a star-burst pattern around the ship position. It is less pronounced at the windward side of display, due to the generated shape of waves. The radius of wave clutter is up to 2-3 miles from the centre. Weak echoes from targets may be hidden by the clutter, so all radars have a control to suppress it. By increasing its value, echoes from waves first are cleared from the centre and targets hidden by the wave echoes appear. The excessive value of this control hides weak targets and makes the coastline broken up into pieces. Use [SEA] slider to control wave clutter suppression. Rain clutter Rain clutter is produced by clouds making the echo looking like wool. There are two types of controls to suppress such types of spurious echoes :
- near the ship (rain clutter control)
- remote rain clutter (FTC, short for Fast Time Constant)
Like the sea clutter control, it should be used with care not to hide important contacts. FTC (fast time constant) This is a special kind of rain clutter suppression for distant clouds. In contrast to the rain clutter suppression (sometimes called slow fast constant or STC), the FTC uniformly affects radar image over the full screen. FTC tends to break up radar images, not only rain. The closest edges of targets are usually not affected. FTC, like STC, reduces sensitivity of the receiver.
The mechanism of FTC is differentiation of the reflected signal. Interference Radar interference from nearby radars and other transmitters may appear on radar PPI as multiple arch-shaped spurious echoes. Most radars have special controls to suppress them. Buoys and beacons

Buoy is a floating device used to facilitate navigation, namely to mark hazards (cardinal buoy), passage channels (lateral buoy) etc. Buoys vary in shape (can, cone or sphere) and colors. They can be provided with a top mark to identify its type and light of a certain color and flash pattern.

Buoys are usually provided with radar reflectors to make them clearly visible on radar screens. The simplest type of radar reflector is a corner reflector based on a very simple principle of wave reflection : angle of incidence equals to the angle of reflection, so the beam, hitting the reflector, moves exactly back in opposite direction. The angle of the corner is the right angle.
In three dimensions, the construction is a cut of a cube, typically installed on buoys.

Beacons are not floating, but stationary navigation aids, sometimes very large towers, and equipped like buoys.

Racons Racon is a combination of words RAdar and beaCON. This is an electronic device wich is capable to emit on radar frequencies when triggered by a radar. It responds on the same frequency forming a line of dots and dashes radiating from the centre and making up a Morze character. Racons are used mostly to mark hazards. Most racons emit the signal only a part of time, e.g. for 20 seconds, and other 20 seconds do not, what makes 50% duty cycle. SART (Search And Rescue Transponder) This floating object is used to locate a survival craft or distressed vessel by creating a series of dots or arcs on a rescuing ship's radar display. It only responds to X-band radars. 12 dots transform into arcs on approaching to a SART. The dots or arcs are equally spaced along the radius of 0.64 NM. Measuring ranges and bearings
Range is a distance from the ship to a target or a landmark measured in nautical miles.
Bearings are measured in degrees from 0 to 360°. Bearings can be true (another term is “compass”) or relative (related to current ship heading). To convert a relative bearing to a compass bearing, add the ship heading to the relative bearing and trim the value to 0..360° range. When measuring bearing to an target, it is recommended to direct the bearing line through the centre of the contact.
On the picture,
- the ship heading is 76°
- true (compass) bearing to the target is 59°T
- relative bearing to the target is 343°R

On the radar indicators, bearings are marked by 'T' and 'R' characters after the degree sign.

Measuring ranges
Typically, there are the three ways to measure ranges from the ship :
- Using range rings. Range rings can be hidden and shown, on some radars with a softkey or a special button, on others from a setup menu. The value measured this way is not accurate.
- With a moveable cursor. Now radars have a cursor movable with a trackball or trackpad with a window displaying the range from the ship to the current cursor location.
- With a special VRM (Variable Range Marker) control, which looks like a ring of variable radius.
Measuring bearings

Measuring bearings is not a radar strong point because of
- non-zero horizontal beam width echoes blurred in peripheral direction
- its dependence upon knowledge of the vessel heading

Radar bearings can be measured by
- the cursor (most modern radars have a cursor and a window to display range and bearing to the current cursor location)
- electronic bearing line (EBL)
VRM and EBL radar controls are usually combined.

Collision avoidance with EBL

Electronic bearing line provides the most well-known way of collision avoidance : if the ship is on steady course and bearing to a target does not change, there is a certain possibility of collision. That is, if the target slides along the EBL line to the centre, it is a dangerous one.

Guard zones

Guard zone is an area near the ship bound by two radii and two concentric circles. This shape is specified by the user.
When a new object enters or exits a guard zone, an alarm (usually sound alarm) is issued. It is used to warn the user about new targets appeared near the ship.

Wakes

Another name for the wakes is trails. This is afterglow left on the screen by targets (and, unforunately, by landmasses, too). The length of wakes is usually set in minutes. Looking at the screen, it is easy to estimate relative speed and heading of any target. Projecting the heading line (wake) of any vessel, and estimating its velocity by wake length (which is specified in minutes), you can roughly get the CPA - the minimum distance separating two approaching ships, which is the most important parameter in collision avoidance.

Closest point of approach (CPA)
Two main parameters used in collision avoidance are illustrated on picture. Consider two ships approaching each other at constant courses with constant speeds. The red ship is faster. Points mark ship positions in time in minutes counted from the current positions.
The distance between ships decreases for approximately 2.5 minutes, then starts to grow. The minimum achievable distance between ships is called "closest point of approach" (CPA). The time elapsed to reach this closest position is called "time to closest point of approach" (TCPA).
Automatic Radar Plotting Aid (ARPA)

Some radars have ARPA - Automatic Radar Plotting Aid or MARPA (Mini Automatic Radar Plotting Aid). This is a basic instrument for tracking targets to prevent collisions.

Those targets whose CPA is small are considered dangerous. Safe and dangerous targets are shown on radar screen in different ways : safe targets are usually marked by circles, dangerous - by blinking triangles and sound alarms. For each target, its velocity vector (usually relative to the ship) is displayed, showing direction of an approaching ship like wakes; but, in addition, its length is proportional to the actual target speed. Numerical values of speed, course, CPA and TCPA for acquired targets can be displayed in separate windows on the screen.

The typical succession of operation when using ARPA is :
- Place the PPI cursor over a target.
- Select ARPA and acquire the target.
- Acquired targets are displayed as safe, marked by circles and dangerous, marked as triangles.
- If ARPA finds a target dangerous, a sound alarm is issued.

Target expansion Echoes from targets can be enlarged. Use [EXPANSION] button to turn expansion on/off. Horizontal beam width The edges of the radar beam diverge at a distance from the aerial. This is one of the factors which strongly influence the radar image. As a result, even small targets look large due to blurring in peripheral direction.
Horizontal beam width is a negative quality, as it makes the radar discrimination worse. The larger the scanner the smaller is the beam width.
For most radars the horizontal beam width is between 2 and 6°.
Non-zero horizontal beam width makes it hard to discriminate between close targets.
Another effect of non-zero horizontal beam width is that narrow entrances to harbours are not visible at all and they do not show up until you get close to them.

References 1. T. Bartlett. The RYA Book of Navigation. Adlard Coles Nautical, London
2. Wikipedia about radar
3. Wikipedia about radiation patterns