4. Radars

The principles of radar are not “new”: in fact, early experiments were made back in 1880s. For instance, in 1904 a German engineer had invented, as he explained, a “radio-echo collision prevention device”.

The word “radar” was originally derived from the descriptive phrase “Radio Detection and Ranging”.

The application of radar in the ATC system consists of 2 basic designs. The initial type of radar, called primary radar, began to be used for advanced ATC. When the word “radar” is used alone it usually includes both primary and secondary radar.

There are three additional forms associated with radars:

Radar Echo – the visual indication on display of a radar signal transmitted from an object.

Radar Response – the visual indication on display of a radar signal transmitted from an object in reply to an interrogation.

Radar Blip – the collective term meaning either echo or response.

PRIMARY RADAR.

In primary radar a beam of individual pulses of energy is transmitted from the ground equipment. These pulses hit the aircraft from 16 to 34 times each scan. An aircraft in the path of this radar beam will reflect back some of the pulses which are picked up by receiver. This reflected energy produces a bright “echo” or “target” on a cathode ray tube.

SECONDARY SURVEILLANCE RADAR.

The SSR system provides for six modes; only two of them are used in civil aviation:

• Mode A for civil and military identification;

• Mode C for automatic pressure altitude information.

The SSR is a valuable tool for automatically identity flying aircrafts. Identification is achieved by providing the controller with a specific radar beacon target identity of aircraft. A total of discrete reply codes are available for special position identification to be transmitted on request of a controller.

With SSR display the controller sees aircraft returns on his PPI (plan position indication) as two slashes, clearly distinguishing them from primary targets which are single blips. In modern systems different synthetic symbols are used to indicate a lot of additional information.

One of the problems, associated with radar is the amount of unwanted clutter on the radar scope which the scanner picks up. This unwanted clutter is mainly echoes of stationary objects, such as mountains and other elevated terrain, as well as other objects situated close to the transmitting/receiving antennae.

With too much clutter a controller can’t always see the aircraft, MTI allows him to reduce clutter. The MTI does this by not allowing echoes received from stationary objects to be displayed on the radar scope. A controller has an MTI switch at his console and with this he can control, to a degree, the amount of clutter caused by stationary objects.

5. Airspace classifications

There are two categories of airspace: controlled airspace and uncontrolled airspace. Within these two categories, various ‘Classes’ have been allocated in different parts, in line with a classification system for civil airspace recently introduced by the International Civil Aviation Organization (ICAO). This new system is designed to simplify airspace structure and establish commonality between countries.

The ICAO system grades airspace from A to G in order of importance. It begins with A, the higher status, which is allocated to the busiest controlled airspace.

• Controlled airspace.

When, in further development, it is decided that an aerodrome should handle IFR traffic, it becomes necessary to protect such traffic by extending control to IFR flights and by placing additional restrictions on VFR flights. To accomplish this, controlled airspace should be established to protect the arrival, departure, and holding paths of the IFR flights.

In controlled airspace Air Traffic Control is provided to all flights. It made up of various aerodrome Control Zones (CTR), Terminal Control Areas (TMA), Control Areas (CTA) and Airways.

A Control Zone – is airspace around certain aerodromes in which ATC is provided to all flights. A CTR extends from ground level to a specified altitude or a specified FL, depending on the height.

A Terminal Control Area (TMA) – is a Control Area established at the confluence of controlled airspace routes in the vicinity of one or more major aerodromes. TMA is sometimes abbreviated as TCA, but more commonly as TMA (from the earlier designation Terminal Manoeuvring Area).

A Control Area (CTA) – is a portion of airspace in which ATC is provided, and which extends upwards from a specified base altitude or FL to an upper limit expressed as a FL.

An Airway is a Control Area in the form of a corridor and is marked by radio navigation aids. Each airway has an identification code (A25 or Alpha Two Five) and extends 5nm each side of a straight line joining certain places, with specified vertical limits. All airways are Class A except where they pass through a TMA, CTA or CTR of lower status. They are used by airliners (and other IFR traffic) travelling between the principal aerodromes. As it approaches an aerodrome, the lower level of an Airway is usually stepped down to provide controlled airspace protection for air traffic on climb and descend.

• Uncontrolled Airspace.

Normally, uncontrolled airspace consists of Advisory Routes and Open-FIR. Open-FIR includes various areas and zones.

Advisory Routes.

Advisory Routes are allocated Class F. VFR operations on an Advisory Route require a flight plan to be compiled. En route, these flights then receive an Air Traffic Advisory Service from the Air Traffic Service Unit (ATSU), responsible for the route.

Advisory routes may be established instead of an airway in some less developed parts of the world where traffic is relatively light. It is still a narrow corridor but positive control is not imposed.

Controllers just pass information to help pilot maintain their own separation. Airways and Advisory Routes, as well as Arrival and Departure routes, are known as Air Traffic Services (ATS) Routes and each is identified by a unique designator allocated by ICAO.

Open-FIR

Open-FIR is a class G airspace. Air Traffic Services provided to flights in Open-FIR include:

• Information and warnings on meteorological conditions;

• Changes of serviceability in navigational and approach aids;

• Condition of aerodrome facilities;

• Aircraft proximity warnings;

• Other information pertinent to the safety of air navigation.

Special use airspace.

Aeronautical charts also depict Airspace Restrictions and Hazards. This include Danger Areas such as military weapons ranges, Prohibited Area around critical installations, Restricted Areas for various defence purposes, Military Training Areas, and Air Defence Identification Zones.

- A Danger Area is defined airspace in which activities dangerous to flight may occur:

- A Restricted Area is defined airspace in which flight is restricted according to certain conditions;

- A Prohibited Area is defined airspace in which flight is prohibited.