The word RADAR is an acronym for RAdio Detection And Ranging, and in its simplest form it consists of a transmitted radio signal aimed by an antenna in a particular direction, and a receiver that detects the echoes off any objects in the path of the signal, he says.
The transmitter consists of an electronic circuit that osciallates at a specific frequency, usually much higher than those frequencies used for radio or TV broadcasts, says Brooker. This signal is sent out in short bursts of electromagnetic energy, called pulses, through the antenna which produces a narrow beam like that of a torch. The distance to the target is determined from the time taken between transmitting the pulse and receiving the echo. This can be accurately determined because the radar signal travels at the speed of light, which is constant.
For air traffic control radars, the beam is shaped like a fan, narrow in the horizontal direction, and wide in the vertical direction, to accommodate high-flying planes. This beam scans around in a circle once every two or three seconds and echoes are displayed on a circular display called a plan-position indicator. The air traffic controller — or a computer — can track the echoes or 'blips' on the display to determine where the aircraft is heading.
This is called primary radar. This used to be called identification friend or foe, or IFF. Air traffic controllers mostly use secondary radar to track commercial aircraft and only use real radar in the case where transponders are not fitted, are turned off or are broken. If the aircraft transponder is switched off, it can be difficult to identify which one of the many primary radar "blips" on the air traffic control display corresponds to the aircraft you are interested in, says Brooker.
Most people will have heard the expression 'flying below the radar'. This is named after a true phenomenon, Dr Brooker explains.
If an aircraft is flying low enough, the beam hardly illuminates it and the range at which it can be seen is limited. There are also limits to the distance over which radar can be used. The main problem with radar for long distance operation is the fact that the amount of power required to send and receive the signal is dependent on the distance to the aircraft raised to the power of four, says Brooker. Typical radars used to track planes out to a range of kilometres or more transmit peak powers in the megawatts.
However, the transmitted pulse is short, typically one micro second or so, and they only occur a few hundred times per second, so the average power is quite low. For really long-range operation, the peak power required to send out the radar pulses become prohibitively large. This has resulted in the development of innovations such as phased arrays that consist of a large number of smaller transmitters and receivers on a planar surface that operate in unison and pulse compression, which allows longer and lower power encoded pulses to be generated while still maintaining good range accuracy.
Another limitation to long-range radar is caused by attenuation through the atmosphere — even in clear air, but worse in the rain. This is inversely related to the wavelength of the signal, so long range radars operate at low frequency.
Electromagnetic waves "bounce" off objects that conduct electricity, so old-fashioned aircraft made from wood and canvas didn't produce big radar echoes, says Brooker. The same applies to modern planes made from carbon fibre composites. Evidence has emerged to suggest that the Boeing ER aircraft flew on for hours after being reported missing.
The plane, with people on board, disappeared from air traffic control screens at approximately local time on Saturday 8 March - just an hour after leaving Kuala Lumpur bound for Beijing. The investigation is now focusing on the actions of the air crew, as officials believe that the plane was deliberately diverted, with key communications systems switched off.
So, how do you track a plane and what do we know about the movements of flight MH? Air traffic control - standard international practice is to monitor airspace using two radar systems: primary and secondary. Primary radar -based on the earliest form of radar developed in the s, detects and measures the approximate position of aircraft using reflected radio signals.
It does this whether or not the subject wants to be tracked. Secondary radar, which relies on targets being equipped with a transponder, also requests additional information from the aircraft - such as its identity and altitude.
All commercial aircraft are equipped with transponders an abbreviation of "transmitter responder" , which automatically transmit a unique four-digit code when they receive a radio signal sent by radar. The code gives the plane's identity and radar stations go on to establish speed and direction by monitoring successive transmissions.
This flight data is then relayed to air traffic controllers. However, once an aircraft is more than km miles out to sea, radar coverage fades and air crew keep in touch with air traffic control and other aircraft using high-frequency radio. The Boeing disappeared from air traffic control screens when its transponder signal stopped. The last definitive sighting on civilian radar showed the aircraft flying north east across the Gulf of Thailand. The final radio message received by air traffic control - "Alright, goodnight" - suggested everything was normal on board.
Military radar shows that the plane then turned and headed west across Malaysia towards the Andaman Sea. On 15 March, satellite data emerged to suggest the plane could be somewhere on an arc stretching either north up to to central Asia, or south, to the Indian Ocean and Australia. Yes, but while GPS Global Positioning System is a staple of modern life, the world's air traffic control network is still almost entirely radar-based. Aircraft use GPS to show pilots their position on a map, but this data is not usually shared with air traffic control.
Some of the most modern aircraft are able to "uplink" GPS data to satellite tracking services, but handling large volumes of flight data is expensive and such systems are usually only used in remote areas with no radar coverage. The satellite data which suggests flight MH flew on for several hours are basic 'pings' sent by the plane, and so far only help to identify two very approximate flight corridors north and south.
Over the next decade, a new system called ADS-B Automatic Dependent Surveillance Broadcast is expected to replace radar as the primary surveillance method for air traffic control.
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