Radars

Introduction:
Radar is an object detection system that works on radio waves used to determine angle, velocity and range. It can be used to detect aircraft, airships, missiles, weather formations and terrain. The first radar was invented in World War II for military purpose now it is being used by civil aircraft as well. There are two basic types of radar; primary and secondary.  

Primary Radar:

The first radar that was developed was primary radar. Primary radars work on reflections i.e. an electromagnetic wave(radio wave) is transmitted and is reflected back by an object which is then received. It is used to find the distance. The total time of the radio wave is measured which is then multiplied by the speed of radio wave (radio waves travel with speed of light). The time which is measured is the total time i.e. transmitted and received, only the received time is required so it is divided by 2.

Primary radar on an aircraft:

Weather Radar:

Weather Radar is used to locate precipitation, its motion and its type (rain, snow etc.). Weather Radar sends directional pulses, the transmitted pulses are then reflected back and the total time for the pulses to return is measured and the distance is calculated. Radar returns are basically described by the colors or levels. There are three levels:
Level 1: corresponds to a green radar return, indicating light precipitation and a little turbulence.
Level 2: corresponds to a yellow radar return, indicating moderate precipitation, low visibility and moderate turbulence.
Level 3: corresponds to a red return, indicating heavy precipitation, possibility of thunderstorms and severe turbulence. This can also damage aircraft.

Low Range Radio Altimeter (LRRA):

A radio altimeter is an airborne electronic device capable of measuring the height of the aircraft above terrain immediately below the aircraft. The picture below is the display of LRRA.

Secondary Radar:
Unlike primary radars that are reflection based radars, secondary radars work on the concept of point to point communication. Secondary radar sends out a signal and rather than receiving the reflected signal, the transmitted signal interrogates the receiving vehicle, which then responds automatically some useful
information. This information can tell the height and position of the aircraft, its status etc.

 

Distance Measuring Equipment (DME):

DME allows the aircraft to measure the distance from a ground reference. The distance is determined by measuring the propagation and delay of a RF pulse, which is emitted by the aircraft transmitter and returned at a different frequency by the ground station after reception. When a signal is transmitted, DME starts counting the time until it gets a reply from the ground station. The time measured is then used for the calculation of the distance between the ground and the aircraft. The display of DME is shown in below picture;

 

Traffic Collision and Avoidance System (TCAS):

Collision Avoidance System is basically the communication between two aircraft to avoid any accident. It transmits a signal which is then received by another aircraft. The received signal interrogates the aircraft which then responds providing its information about the altitude and speed. There are 3 types of TCAS out of which only 2 are used.

1. TCAS I: It assists the crew visually location and identifying an intruder aircraft by issuing traffic advisory (TA).
2. TCAS II: It not only provides traffic advisory but also provides vertical flight manoeuvre guidance to the crew in the form of resolution advisory (RA). Resolution advisory aids the pilot in maintaining the vertical separation between the aircraft.
3.  TCAS III: It was intended to give pilot lateral guidance in addition to RA and TA, but the idea was suspended and was replaced by ADS-B (Automatic Dependent Surveillance - Broadcast).