Wednesday, June 16, 2010

Radar and its types

TYPES OF RADARS



A variety of radars have been developed and they are classified based on
1. The signal transmitted.
2. The target of interest and
3. The frequency of operation.

Some of the commonly used radars are discussed as follows.

CONTIUOUS WAVE (CW) RADAR



The radar transmitter transmits a continuous signal rather than a pulse one. The echo signal can be easily distinguished from the transmitted signal as there will be a shift in the frequency of the echo signal due to the DOPPLER EFFECT. Also, the echo signal’s strength is many times lower than the transmitted signal strength.
Using a CW radar, the relative velocity of a target can be determined. The drawback of a CW radar is that the range of the target cannot be determined.

MOVING TARGET INDICATION (MTI) RADAR

The MTI radar transmits a train of electromagnetic pulses. This radar system is capable of distinguishing a moving target from the non-moving objects. The non-moving objects in atmosphere that intercept the transmitted signal and scatter it are called CLUTTER. Since there will not be any DOPPLER frequency shift from fixed targets, as there is no relative motion, they are easily identified from the moving target. The echo from the moving target has the varying frequencies due to DOPPLER EFFECT.

TRACKING RADAR

A tracking radar system locks on to a particular target, measures its coordinates and provides data from which the future course of the target can be predicted.
The tracking radar is similar to other radars except that the antenna beam of a tracking radar is made to look at the target always. This is achieved by a servomechanism activated by error signals. The error signals is obtained using several techniques, viz. sequential lobing, conical scan and simultaneous lobing.

APPLICATIONS OF RADAR

Radar systems find their applications on the ground, in the air, on the sea and in space. The ground based radars have been used in the detection, location and tracking of aircraft or space targets. The shipboard radars are used to navigate the ships and to locate buoys, shorelines and other ships. They are also used to observe aircraft. The airborne radars are used to detect land vehicles, ships and aircraft. However, the principal application of airborne radar is for mapping of land storm avoidance and navigation. In space, radar is used for remote sensing purposes.

Megger(working principle)

MEGGER


Megger is the most portable insulation tester. It is used to remove very high resistances of the order of meg ohms.

PRINCIPLE:
The instrument works on the principle of ratiometer/ohmmeter. The required deflection torque is produced by both the system voltage and the current. Due to interaction between the magnetic fields produced by the voltage and the current, the deflecting torque is produced. The required coils are so positioned that the deflecting torque is proportional to the ratio, V/I.

CONSTUCTION:
It consists of
i. A small hand drives d.c. generator
ii. A moving element which has 2 coils, a deflecting coil (or current coil) and a controlling coil (or potential coil)
iii. Calibrated scale in mega ohms.
iv. Pointer and
v. Permanent Magnet.
The two coils are rigidly mounted at right angles to each other. They are connected to the small hand driven generator. The coils move in the air gap of a permanent magnet. To protect the coils under short circuit, a limiting resistor is connected in series with the coils.




OPERATION:
Resistance to be measured is connected across the test terminals i.e. connected in series with the deflecting coil and across the generator. When currents are supplied to the coils, then they have torques in opposite directions.
If the resistance to be measured is high, no current will flow through the deflecting coil. The controlling coil will, therefore, set itself perpendicular to the magnetic axis and hence, sets the pointer at infinity.
If the resistance to be measured is small, a high current flow through the deflecting coil and the resulting torque sets the pointer to zero.
For intermediate values of resistances, depending upon the torque production, the pointer is set at a point between zero and infinity.
The hand driven generator is of permanent magnet type and it is designed to generate from 500 to 2500 volts.

Fax(Fascimile)


FAX - Facsimile


This is a telecommunication service for the transmission and reception of picture information like photographs, drawing, weather maps, etc. The picture or any document to be transmitted is mounted on a cylinder and it is scanned by a photocell linked to the cylinder. The photocell produces an electrical analog signal as a voltage variation depending upon the intensity of the light and dark spots on the document. The electrical signal thus produced is converted into frequency variations and transmitted through a telephone line. At the receiving end, the frequency variations are converted back into corresponding voltage variations that is given to a plotter for reconstructing the original picture or document. Thus a photocopy of the original picture is obtained at a distance. Similar to a telephone signal, the FAX message also occupies a bandwidth of 4 kHz.

CARD READERS



Computers have undergone revolutionary changes with the advent of integrated circuit technology. Before the appearance of these modern technologies, one of the most popular input devices was the CARD READER. The data to be fed into computer are punched on these cards using punching machines. Computer punch card is a special card of size of 7.5” x 3.25” x 0.007”. It has 12 rows and 80 columns. A special device called punching machine, whose key board is similar to the key board of modern computers, was used to punch data and information on the card. Each character punched will produce a unique set of holes, row-wise in the particular column where the character is punched. The advent of modern technologies has made cards and card readers obsolete.

BROWSING ON INTERNET



For browsing on the internet, the following tools are available.
1. Who is – refers to identification of persons.
2. Finger – provides information like username, who has last logged in, how long his session had been idle, when he has viewed his last E-mail etc.
3. UU commands – information could be received about machines connected to Internet by UUCP (Unix to Unix copy).
UU Hosts – information is received about a particular site, who is responsible for it and what sort of UUCP connections it has with other sites.
UU Where – displays the path between the machine, the user is currently using and the site machine.
4. Gopher – Gopher is a software protocol designed to search, retrieve and display documents from remote sites on internet.
5. WAIS – Wide Area Information Service. It is a software protocol having the capability of searching more than one data line. The user has the provision of choosing the data for his requirement. Then he submits the query and gets the response for the query.
6. World Wide Web (WWW) – It is the most important aspect of Internet and has accelerated the growth of Internet. It is an easy to use, point and click graphical interface. It is highly interactive having graphics, text, sound and animation, etc. It is used as a market place, art gallery, library, community centre, school, publishing house, etc. There are search tools like web index and search engines to enable the information searches and discover them more effectively.