Radar Observer

Relative motion:

Own ship is fixed at the center of the display and every echo moves according to its motion relative to yours. Stationary objects appear to move (in the reciprocal of own ship's course), so it can be hard to tell fixed from moving targets at a glance.

True motion:

Both own ship and targets move at their actual courses and speeds, and stationary objects stay put. This makes it easy to distinguish moving from fixed targets. True motion requires speed and heading inputs.

Orientation modes:

- Head-up: the ship's heading is at the top of the display. Simple, but unstabilized — the picture smears when the ship yaws. - North-up (stabilized): true north is always at the top, fed by the gyro; the heading marker rotates as the ship swings. Steady picture, easy to relate to the chart. - Course-up: the set course is frozen at the top, combining a steady picture with a heading-oriented view.

Stabilization:

A gyro-stabilized display (north-up or course-up) holds the picture steady during yaw; an unstabilized head-up display blurs echoes and bearings when the ship swings.

Gain:

Sets receiver sensitivity. Too low and weak targets are lost; too high and the screen fills with noise that can mask targets. Adjust for a light background speckle.

Tuning:

Matches the receiver to the transmitted frequency for the strongest return. A mistuned set shows weak echoes and reduced detection range.

Sensitivity Time Control (STC) — sea clutter:

Progressively reduces receiver gain at close range to suppress the bright clutter of waves near own ship. Overuse erases small close targets, so apply only as much as needed.

Fast Time Constant (FTC) — rain clutter:

Displays only the leading edge of echoes, breaking up the solid mass of rain so discrete targets within or behind it can be seen. Overuse weakens legitimate returns.

Performance monitor:

An echo box or performance monitor paints a known pattern whose extent shows whether transmitter power and receiver sensitivity meet standard — a check that the radar is fit before relying on it.

Range resolution:

The ability to display two targets on the SAME bearing as separate echoes. Governed chiefly by pulse length — a shorter pulse separates targets close together in range.

Bearing (angular) resolution:

The ability to separate two targets at the SAME range on slightly different bearings. Governed by horizontal beam width — a narrow beam separates them; a wide beam paints them as one arc and also makes targets look wider than they are.

Minimum range:

While a pulse is transmitting the receiver is blanked, so very close echoes are not seen until the pulse ends and the receiver recovers. A shorter pulse gives a smaller minimum range.

Radar horizon:

Standard atmospheric refraction bends radar energy slightly downward, so the radar horizon lies a little beyond the optical horizon. Detection range increases with the square root of antenna and target height.

Propagation anomalies:

- Super-refraction (ducting): beam bends down, range increases beyond normal; can cause second-trace echoes. - Sub-refraction: beam bends up, range decreases.

Shadow (blind) sectors:

Masts, stacks, and superstructure block or weaken the beam in certain bearings, leaving sectors where targets are faint or missing. These must be known and covered by other lookout means.

Side-lobe echoes:

Energy in the antenna's side lobes returns from a strong, close target and is painted in an arc at the same range on either side of the true echo. Reducing gain helps suppress them.

Indirect (reflected) echoes:

The beam bounces off a structure to a target and back, placing the echo at a false bearing (often in line with the reflector). The false echo shifts or disappears as the geometry changes.

Multiple / second-trace echoes:

Under ducting, a distant target's echo can arrive after the next pulse is fired and be plotted at a false short range. Changing the range scale (and thus the pulse repetition rate) reveals it, as the false echo moves while real ones stay put.

Target detection factors:

Return strength depends on size, material, and aspect. Large metallic surfaces facing the beam squarely reflect strongly; small, non-metallic, or sharply angled targets reflect poorly and may be missed.

Systematic observation:

Take ranges and bearings of a contact at regular timed intervals (commonly every 3 or 6 minutes). Even intervals establish the relative-motion line and the rates needed to compute CPA and TCPA. Acting on scanty radar information is specifically warned against.

CPA and TCPA:

- CPA (Closest Point of Approach): drop a perpendicular from own ship (display center) to the extended relative-motion line. A small CPA indicates risk of collision. - TCPA (Time to CPA): the time for the target to reach that closest point.

The relative (vector) triangle — WO/OA/WA:

- W-O: own ship's true motion over the plotting interval. - O-A: the target's relative motion (along the relative-motion line). - W-A: the target's true course and speed. Solving the triangle yields the target's true track plus the CPA and TCPA.

DRM and SRM:

Direction of Relative Movement and Speed of Relative Movement — the direction and speed the target appears to travel relative to own ship along the relative-motion line.

Aspect:

The angle at which the target presents to you — whether you see its bow, beam, or stern — which helps assess how the situation will develop.

ARPA tracking:

An ARPA acquires a target and tracks its successive positions over several scans, combining them with own-ship inputs to compute true course/speed, CPA, and TCPA. It needs time (about 3 minutes) for a steady solution.

Vectors:

- Relative vector: shows how the target moves relative to own ship (a vector aimed at own ship means small CPA / risk). - True vector: shows the target's actual course and speed over the ground. Always know which vector mode is selected.

ARPA limitations:

- Target swap: when two tracked echoes pass close together, the tracker may swap tracks, giving a sudden false vector. - Tracking can be lost in heavy clutter or for weak targets. - Trial maneuver: simulates a proposed course/speed change against all tracked targets before it is ordered, with a settable start delay.

AIS vs radar:

AIS gives identity and reported course/speed but only for vessels transmitting it, and the data can be wrong. Radar detects all reflective targets. They are complementary; neither replaces a proper lookout.

COLREGS Rule 7 (risk of collision):

Proper use must be made of radar, including long-range scanning and radar plotting or equivalent systematic observation. Risk is deemed to exist if a compass bearing of an approaching vessel does not appreciably change. Assumptions must not be made on scanty information.