science

Secondary surveillance radar

Secondary surveillance radar is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Secondary surveillance radar rather than just read about it. In short: Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC) that, unlike Primary Surveillance Radar (PSR) systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply the Mode that an interrogation pulse code corresponds to, by transmitting a pulse telegram containing, e.g. the iden…

Key takeaways

Secondary surveillance radar belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Secondary surveillance radar to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Secondary surveillance radar from memory before moving on to harder problems.

Reference excerpt

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC) that, unlike Primary Surveillance Radar (PSR) systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply the Mode that an interrogation pulse code corresponds to, by transmitting a pulse telegram containing, e.g. the identity code as 4 digit octal number in Mode A, the aircraft's altitude from the barometric pressure sensor of an aircraft in Mode C and a unique 24-bit address and further information in other Modes. SSR is based on the military Identification Friend or Foe (IFF) initially standardized in the U.S. and later adopted by NATO (North Atlantic Treaty Organization); however, the first IFF systems date back to World War II. SSR Mode A, C and S are compatible with IFF Mode 3A, C and S. SSR Mode B and D are no longer specified in ICAO Annex 10. Additional SSR Mode S based systems are ADS-B, TCAS, Multilateration Systems (MLAT) and satellite based surveillance using ADS-B messages.

Overview

Primary radar

The rapid wartime development of Primary Surveillance Radar (PSR) had obvious applications for Air Defense (AD) for the detection of approaching enemies as well as Air Traffic Control (ATC) as a means of providing continuous detection of surveillance of air traffic disposition. Precise knowledge of the positions of enemy aircraft allowed AD to guide fighter aircraft towards unfriendly aircraft, while for ATC it permitted a reduction in the normal procedural separation standards which in turn promised considerable increases in the efficiency of the airways system as well as during approach to air fields for Ground Controlled Approach (GCA) by using Precision Approach Radar (PAR). 7.8, p.248 ff. PSR can detect and report the position of any object that reflects a sufficient amount of the transmitted radar energy back towards a PSR Radar sensor. Depending on the PSR design moving and/or stationary objects, e.g. aircraft, ships, birds, rain-clouds, other weather phenomena, land features or man made objects, can be detected. For air traffic control purposes this is both an advantage and a disadvantage. The advantage is that targets do not have to co-operate, they only have to be within its coverage and be able to reflect radio waves, the disadvantage a PSR Radar Sensor can only measure the slant range distance between the PSR sensor and an object. The azimuth is derived from the azimuth the antenna is pointing to. Specialized 3D-Radars allow within limits the detection of the aircraft height. While this allows to detect the position of the targets relative to the PSR sensor, it does not allow to identify if they are friend or foe, or which aircraft has been detected. When primary radar was the only type of radar available, the correlation of individual radar returns with specific aircraft typically was achieved by the controller observing a directed turn by the aircraft. Primary radar is still used by ATC as primary means surveillance as backup in case a detection of aircraft by SSR failed, e.g. because the SSR detection failure / complementary system to secondary surveillance radar, for cases when aircraft is not equipped with SSR, has a defective transponder, or when the transponder was intentionally manipulated or destroyed., ,

Secondary radar

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Illustrations

Worked examples

Example 1 — a first encounter with Secondary surveillance radar

Start with the simplest possible case. Write down what Secondary surveillance radar claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Secondary surveillance radar before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Secondary surveillance radar ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Secondary surveillance radar

In research: Secondary surveillance radar appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Secondary surveillance radar in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Secondary surveillance radar is common in secondary-school and first-year university syllabi. It links to neighbouring topics History of air traffic control, Radar theory, Radio stations and systems ITU, so understanding it makes those chapters shorter.
In everyday life: Look for Secondary surveillance radar outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Secondary surveillance radar in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Secondary surveillance radar means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Secondary surveillance radar out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Secondary surveillance radar in simple terms?

Why does Secondary surveillance radar matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Secondary surveillance radar?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Secondary surveillance radar.