Some radar theory
|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
|On the picture, the radar beam which is more perpendicular to the surface (case 2) produces more intensive echo returns.|
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.
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.
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.
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.
|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.
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 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.
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 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.
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.
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).
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.