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 a receiver. This reflected energy produces a bright “echo” or “target” on a cathode ray tube.

Secondary Surveillance Radar (ssr)

The SSR system provides for six modes; only two modes 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 identifying aircraft. Identification is achieved by providing the controller with a specific radar beacon target identity of aircraft. A total of 4096 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 indicator) 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.

Visual Aids for Navigation

Additional visual aids to navigation consist of markings on the aerodromes. These markings comprise single lines or rows of lines which, for the pilot, are very important for holding positions, runway thresholds, the runway centre lines, the sides of the runways, etc.

However, at night or during poor visibility by day, lights are required. To be effective lights must be of adequate intensity. At certain aerodromes the controller can vary the intensity of some of the lights so that they can be reduced not to blind the pilot and strong enough so that he can see them in bad weather.

The first lights a pilot sees on approach is generally the aerodrome beacon. It may rotate and can be seen at a great distance. There might be an identification beacon which shows green flashes of light. Red lights, the usual danger signal, warn pilots of the obstacles such as hangars and other high buildings, telephone poles, etc. Runway edge lights identify the runway and approach lights assist the pilot to align himself with the runway.

Lights may also be used to provide a glidepath similar to what an ILS provide electronically. The Visual Approach Slope Indicator System (VASIS) is a beam of light having a white colour in its upper part and a red colour in its lower part. A pilot of an aeroplane during an approach will:

when above the approach slope, see the lights to be white in colour;

when on the approach slope, see the lights to be pink in colour; and

when below the approach slope, see the lights to be red in colour.

By reference to vasis, combined with ILS, the pilot can bring an aircraft down safely almost to touchdown by day or night.

After landing, he follows the blue taxi lights along the taxiway to the apron and the service areas.

At the service area a marshaller, with illuminated wands, directs the aircraft with signals to its proper position for unloading and, finally, signals pilot to cut the engines.