Radar Fundamentals and ARPA Basics

What the rule requires

Radar detects the presence, direction, height and distance of objects by using reflected electromagnetic energy, and at the frequencies used it is unaffected by darkness and penetrates weather to some degree depending on frequency NEETS Module 18 — Learning Objectives ¶1. For a surface contact the set hands you two quantities and no more: range and bearing. On a plan position indicator, range is read as the distance of the pip from the centre of the screen and bearing as the radial angle of that pip NEETS Module 18 — Glossary ¶3.

Range is elapsed time, divided

Range to a target is obtained by measuring the time for a pulse to travel out to the target and return to the receiver, then dividing that elapsed time by 12.36 microseconds NEETS Module 18 — Function Installation Vehicle ¶3. That figure is the radar mile — the time for rf energy to go out one nautical mile and come back . It falls out of two numbers worth knowing: radiated energy travels about 984 feet per microsecond, and a nautical mile is about 6,080 feet .

Bearing depends on which reference you are using

The bearing angle is obtained by moving the antenna to the point of maximum signal return; at the position where the beam axis points directly at the target the echo strength is maximum, and search sets determine that point in the detection circuitry as the beam sweeps past NEETS Module 18 — Learning Objectives ¶3. Whether the resulting number is a true bearing or a relative bearing depends entirely on the reference, and that distinction is worked below.

What ARPA adds, and what it does not

ARPA (Automatic Radar Plotting Aid) computes target tracks automatically and displays predicted CPA and TCPA; it does not replace bridge watchkeeping, and it supplements the visual look-out Bowditch Ch. 13 §1303. Both halves of that sentence get tested. The first half is why a candidate should expect a question whose correct answer is "predicted closest point of approach and time to closest point of approach." The second half is why any option that has the equipment discharging the look-out is wrong on its face.

Your ARPA has three contacts acquired and tracking, all with CPAs over 2 nm. Does that satisfy the requirement for a look-out?

No. ARPA supplements the visual look-out and does not replace bridge watchkeeping . Tracked targets are additional information, not a substitute watchstander.</details>

Why a target may not appear at all

High-frequency energy does not normally bend to follow the curvature of the earth, so most radar systems cannot detect targets below the radar horizon . Distance to the radar horizon varies with the height of the antenna, and a target at a range greater than the radar horizon will not be detected unless it is high enough to be above the horizon . Antenna height and target height are therefore both factors in detection range.

Maximum range of a pulse radar depends on carrier frequency, peak power, pulse-repetition frequency and receiver sensitivity .

Antenna rotation rate also affects maximum detection range, and the relationship runs the opposite way to most candidates' first instinct: the slower the antenna rotates, the greater the detection range, because more pulses strike a given area and the number of hits per scan goes up . PRF and antenna speed have to be matched — at 20 rpm with a PRF of 200 pps the set puts so few pulses into each degree of azimuth that targets are missed entirely.

Resolution: what the set can separate

Range resolution is the ability to distinguish between two or more targets on the same bearing, and it depends primarily on pulse width. Bearing resolution is the ability to separate targets at the same range on different bearings, and it depends on beam width and range. Radar accuracy is largely dependent on resolution .

The atmosphere moves the horizon both ways

Under normal conditions the wavefront speed increases uniformly with altitude, which curves the travel path slightly downward and extends the radar horizon beyond a line tangent to the earth NEETS Module 18 — Learning Objectives ¶5. Temperature and moisture content normally decrease uniformly with height, but temperature may instead increase with height before it begins to decrease — a temperature inversion — and over large bodies of water the moisture content may decrease more rapidly just above the sea, which is moisture lapse. Either one, alone or in combination, can cause a large change in the refractive index of the lowest few hundred feet, producing greater bending of the radar waves. That increased bending is ducting, and it may extend or reduce the radar horizon depending on the direction the rays are bent.

Suspended particles work against you regardless. Water droplets and dust diffuse radar energy by absorption, reflection and scattering, so less energy reaches the target and the return echo is smaller; the usable range drops, and the higher the frequency of the set the more it is affected by rain or clouds.

Two more display artefacts a question can hang on. Sea clutter is unwanted echoes from the irregular surface of the sea appearing on the indicator . Ambiguous returns, also called second-sweep echoes, are echoes from targets that exceed the PRT of the set and produce false range readings .

The signal chain, in one pass

Six components make up a pulse radar. The synchronizer, also called the timer or keyer, supplies the signals that time the transmitted pulses and the indicator; the transmitter generates the energy as short, powerful pulses; the duplexer allows one antenna to both transmit and receive; the antenna radiates a highly directional beam and routes returning echoes to the receiver; the receiver amplifies the weak returns and reproduces them as video pulses . The indicator produces the visual indication of range and bearing .

Telling it apart

The two resolutions are separated by one question: where are the two contacts relative to each other?

Range resolution — the pair lie on the same bearing at different ranges. Governed primarily by pulse width NEETS Module 18 — Function Installation Vehicle ¶3. Most often misfiled: a tug and its barge in line ahead of each other, painting as one elongated pip.
Bearing resolution — the pair lie at the same range on different bearings. Governed by beam width and range. Most often misfiled: two contacts that merge into one pip at long range and separate as you close, which candidates read as a target manoeuvre rather than as resolution improving with decreasing range.

True bearing against relative bearing

The separating criterion is the reference direction the angle is measured from.

True bearing — measured clockwise from true north, in the horizontal plane, to the line of sight .
Relative bearing — measured clockwise from dead ahead of your own vessel NEETS Module 18 — Glossary ¶3.

Add ship's head to a relative bearing to get the true bearing, and subtract 360° if the sum exceeds it.

The four pulse-timing terms

Pulse width — the duration between the leading and trailing edges of one pulse .
PRT — the interval from the start of one pulse to the start of the next; the reciprocal of PRF.
PRF — the rate at which pulses are transmitted, in hertz or pulses per second; interchangeable with pulse-repetition rate NEETS Module 18 — Learning Objectives ¶3.
Duty cycle — the product of pulse width and PRF; the ratio of transmitter time on to time off .

Working a question

A pip appears on the PPI. The set shows the echo returning 61.8 microseconds after the pulse left the antenna, and the pip lies 035° clockwise from the heading marker. Own ship's head is 250° T.

Establish what the display is telling you. Distance of the pip from the centre is range; the radial angle of the pip is bearing NEETS Module 18 — Glossary ¶3. Nothing on the screen tells you the target's course or speed.
Convert time to range. Divide elapsed time by 12.36 microseconds per nautical mile: 61.8 ÷ 12.36 = 5.0 nm NEETS Module 18 — Function Installation Vehicle ¶3.
Decide which bearing you have. The heading marker is dead ahead, so 035° measured from it is a relative bearing, not a true one.
Convert to true. 250° + 035° = 285° T. Had the sum passed 360°, subtract 360°.
Test the range for ambiguity. If the round-trip time of an echo exceeds the PRT of the set, that echo arrives after the next pulse has gone out and is displayed at a false, shorter range. A 61.8 microsecond round trip is well inside any normal PRT, so the reading stands.

A power question runs on the same discipline — identify which quantity the instrument actually measured before you reach for a formula. Most instruments read average power directly, so peak power is the one you calculate NEETS Module 18 — Learning Objectives ¶3. Given a meter reading of 3 W average, a pulse width of 1 microsecond and a PRF of 1,000 pps:

Duty cycle = pulse width × PRF = 1 µs × 1,000 = 0.001.
Average power = peak power × duty cycle, so peak power = average ÷ duty cycle.
Peak power = 3 ÷ 0.001 = 3,000 W.

An echo returns 24.72 microseconds after transmission. What is the range?

2.0 nm. 24.72 ÷ 12.36 = 2 .</details>

Where candidates lose the point

Choosing the option that has the equipment keeping the look-out, usually phrased as tracked targets or an active guard alarm being sufficient. ARPA supplements the visual look-out and does not replace bridge watchkeeping Bowditch Ch. 13 §1303.
Assigning beam width to range resolution. Beam width and range set bearing resolution; pulse width is what merges two contacts lying on the same bearing NEETS Module 18 — Function Installation Vehicle ¶3.
Treating bearing resolution as a fixed property of the set. It depends on beam width and range, so the same two contacts that paint as one pip far out will separate as the range closes.
Answering that speeding up the antenna improves long-range detection, because faster scanning sounds like more information. The slower the antenna rotates, the larger the hits per scan and the greater the detection range NEETS Module 18 — Learning Objectives ¶3.
Over-reading "radar penetrates weather" and picking the answer that rain does not degrade performance. Penetration is partial and depends on frequency NEETS Module 18 — Learning Objectives ¶1; droplets and dust reduce the usable range, and the higher the frequency the worse the effect NEETS Module 18 — Learning Objectives ¶5.
Reporting an angle taken off the heading marker as a true bearing. That angle is relative, measured clockwise from dead ahead NEETS Module 18 — Glossary ¶3.
Assuming ducting can only extend range. It may extend or reduce the radar horizon, depending on which way the rays are bent .

Check yourself

Your ARPA displays a contact with a CPA of 0.3 nm and a TCPA of 8 minutes. What has the equipment done, and what has it not done?

It has computed the target's track automatically and displayed the predicted CPA and TCPA. It has not taken over the watch: ARPA supplements the visual look-out and does not replace bridge watchkeeping Bowditch Ch. 13 §1303.</details>

A tug and the barge astern of her, both nearly on the same bearing from you, paint as a single elongated pip. Which characteristic of your set limits your ability to separate them?

Pulse width. Distinguishing two or more targets on the same bearing is range resolution, which depends primarily on pulse width NEETS Module 18 — Function Installation Vehicle ¶3.</details>

Two contacts at the same range, a few degrees apart in bearing, show as one pip at 9 nm and as two pips at 3 nm. Why?

Bearing resolution depends on beam width and range. The beam width has not changed; the range has, so the same angular separation becomes a resolvable distance as you close .</details>

Ship's head is 310° T. A pip lies 070° clockwise from the heading marker. What is the true bearing of the contact?

020° T. The angle from the heading marker is a relative bearing, measured clockwise from dead ahead NEETS Module 18 — Glossary ¶3; 310° + 070° = 380°, less 360° = 020° T.</details>

Overnight, with warm air over a cooler sea, you begin holding land at ranges well beyond what the antenna height would give you. What is happening, and what else can the same condition do?

A temperature inversion or moisture lapse has changed the refractive index of the lowest few hundred feet enough to bend the radar waves more than normal — ducting. It can just as easily reduce the radar horizon, depending on the direction the rays are bent NEETS Module 18 — Learning Objectives ¶5.</details>

A vessel you expect to be 14 nm off does not appear on the screen, though the set checks out. What is the first explanation to consider?

She is below the radar horizon. High-frequency energy does not bend to follow the curvature of the earth, so a target beyond the radar horizon is not detected unless it is high enough to be above it; distance to the horizon varies with antenna height , NEETS Module 18 — Learning Objectives ¶3.</details>

An echo appears at 1.5 nm on a bearing where you can see nothing, and the round-trip time works out longer than your set's PRT. What are you looking at?

An ambiguous return, also called a second-sweep echo — an echo from a target whose round trip exceeds the PRT, displayed at a false range , .</details>