Radar System

1.      What’s radar ?

The radar is radio equipment for receiving the electric wave which emitted the electric wave and was reflected in target, and knowing existence of a target and a target position ( a direction/distance).

The origin of a word of  a radar

R a d a r   : Radio Detection and Ranging

The kind of the radar

•   Vessel traffic service system
•   Airport Surveillance Radar
•   Meteorological Radar

 

A radar can detect a target throught clouds, fog, rain, etc. Night using the feature of an electric wave. More over, the important function of radar is able to “Measure the distance to a target, and direction”

Pulse Radar Outline

General

Most of the thing called present radar is a pulse radar.

An electric wave is devided and transmitted by the short pulse, the electric wave reflected in target is received, and it is displayed on an display as a pulse radar. Usually, a transmitting antenna and receiving antenna use the same thing. The pulse radar consist of an antenna, a transmitter (TX), a receiver (RX) and a display. The display indicate the output radar video signal from RX which is made to synchronize to “Trigger timing” and “Azimuth signal of antenna”. (PPI : Plan Position Indicator)

The measurement principle to a target

[ Measurement of distance ]

The distance to target is know by measuring time until a radiation pulse reflects in a target and returns.

If after a transmitting pulse is emitted until it receives a reflectives wave is set to At , the distance to the target will be found by the lower formula.

[ Measurement of direction ]

The direction to the target is known in the direction of an antenna to which the radiation puse reflected in the target and has returned.

The “Rotari Encoder” for detecting the direction of an antenna is built in the inside of an antenna , and the direction pulse corresponding to rotation of an antenna is outputted. There are  an “Azimuth Reference Pulse” (ARP) use the reference of a direction and an “Azimuth Changing Pulse” (ACP) for measuring a direction as direction pulse. With an indicator. The target direction can be known by count the ACP from timing of ARP.

The display method of radar video

The Radar Signal consisted of the “Trigger,Video,ACP and ARP”

The display indicate the radar video signal on the screen by use of the distance from the trigger “r” and direction from the ARP

The display method of radar video is called PPI.

The radar receiving video include an unnecessary reflection video except target reflection video. These reflection video indicate on the radar display with target reflection.

The general unnecessary radar video are followings.

•   Sea Cutter
•  Rain Clutter
•   Interference from other radar
•   Ground Clutter

(However, by the radar for marine observation, since a land echo does not influence direct observation, it omits explanation)

SEA CLUTTER

Sea clutter is reflection of radar radio wave from sea surface. The target in the sea clutter is difficult to detect by same level of sea cutter. Moreover, sea clutter indicate on the display according with the situation of sea surface.

The sea clutter is indicate to strong near the radar centre. Usually, the display have adjusting volume to reduce the sea clutter. (STC : Sencitive time control). The operator have to adjust the STC level to detect the target which is in the sea clutter.

Rain Clutter

Rain clutter is reflection of radar radio wave from the rain. The target in the Rain Clutter is dificult to detect by same level of rain clutter. Moreover, rain clutter indicate on the display according with the situation of a rain condition.

Usually, the radar display have adjusting volume to reduce the rain clutter. (FTC:Fast Time Constant)

The operator have to adjust the FTC level to detect the target which is in the rain clutter.

Radar interference

Radar interference is display on the screen of the radar display, when the receiver has received the same transmitting frequency radar radaio wave, Usually, the display have adjusting volume to reduce the interference (IR). The operator have to adjust the IR level to reduce influence of another radar interferenc. The interference will be different to pulse reputation frequency ration between own radar and another radar.

 Thankyou OJT 2009, Batam