ANTENNA RECEIVER :
The radar antenna receiver system helps in detecting the emissions from the radio of radar systems . The radar receivers help in amplifying the signals received preventing it from noise and any type of distortion . It also optimizes the detection probability by increasing its bandwidth characteristics and provides a vast range to keep cluster signals . The antenna receiver helps in optical detection by deletion of large interfering signals . Radar signals require :
- The minimum detectable signal ( MDS )
- Dynamic range
1.The minimum detectable signal ( MDS ) : Minimum receivable power is important because this is one of the factor or characteristics which describes the maximum range performance of the radar . Based on certain requirements the radars are designed to have a sensitivity level . A radar having greater sensitivity than required will limit the bandwidth and will start processing the signals it has no interest with . The higher the power level given at the time of processing , fewer number of false alarms are processed and hence the detection of good noise signal will reduce .
2. Bandwidth : Receiver noise is the most important factor of radar receiver . The antenna receiver adds some noise to signal , very similar to that of other receivers because noise due to thermal action is unavoidable . Hence the receiver bandwith is directly proportional to certain amount of thermal noise . Hence the reduction of bandwith acts as a proper solution to the problem of receiver noise
3. Dynamic range : The signal received must be amplified without distortion . If a large number of signals is send to saturation level , then a modification to the spectrum signal is the solution . The MTI improvement factor degrades because of this reduction of spectral content . Delay will also take place in target detection restoration if the receiver enters saturation level .
The transmitters and the radar receivers are not always in the same location . The receiving antenna that captures the radar signals are weak and can be made stronger by using amplifiers .
NOISE FIGURE :
A noise factor converted to decibel is called Noise figure ( NF) . The measure of defall of any signal to noise ratio components in radio frequency signal chain for a bandwidth given is Noise figure . Hence the noise power also increases from input to output . It is also defined as the amount of reduction that occurs in the noise ratio . It can be both expressed by ratio or in decibels. .
This stage helps in increasing the received frequency to any intermediate frequency . The input from local oscillator is also received by mixer . The input is obtained by heterodyning by beating the two signals together . The second frequency reception’s result is a mirror image around the intermediate frequency . This can be represented by :
- fIF = frx – flocal oscillator
- fIF = flocal oscillator –frx
DISPLAYS – TYPE A :
The type A cannot show the direction but can only show the range . it can be also called as R scope . It is an electroscopic oscilloscope which was used in World War 2 . it could produce an oscillating voltage signal from the receiver of radar . It represents any signal detected on vertical axis . It has its application in aircrafts and ships . An A scope display was also used in air surface vessel radars and it used two antennas with a common reflector in front . The main disadvantage is that it was unable to indicate the direction to target .
LONG WIRE ANTENNA :
Long wire antennas are simple antennas . They are mostly efficient like upto a half wave antenna . They are made longer and so are installed almost close to the ground . The main advantages are they are very simple and inexpensive . The main disadvantages are they are a single wire feeder and are dangerous at the time of lightning .
LOW NOISE RECEIVER FRONT END :
A single chip low noise receiver front end includes low noise amplifier and a mixer and is applied at 77 GHz in automotive radars . This uses a SiGe HBT Technology to be implemented . A minimum measured single side band noise figure is represented by the front end of 11.5 dB . The main advantage is that this can be linearly measured .
SYNTHETIC APERTURE RADAR AND CONTINUOUS APERTURE SOURCES :
The main characteristics of aperture radar is relative motion between the target region and antenna to provide long term signal variations that are supposed to be generated from spatial resolution . It is mostly used in a platform that is moving by mounting on it for example like aircraft or spacecraft and from here a target scene is illuminated again and again with radiowave pulses . It has been basically designed to collect more information . This uses the principle of combining many pulses to form a synthetic aperture but also includes some additional processing .
It’s images has many applications in sensing and mapping of both earth and other planet surfaces .
HF OVER THE HORIZON RADAR :
An ionospheric reflection is the most common type of OTH radar . Under some conditions the radio signals sent towards the ionosphere reflects back to the earth . and after reflection a small amount of this signal reflects back to the sky . hence the HF or high frequency behavior is exhibited by only a single range of frequency . Under these conditions the frequency range of these radio signals reflects back to the ground . and because the reflected back signal will be large than the signal that is reflected from target , these will require to differentiate background noise from the targets . Hence Doppler effect is used where frequency shift takes place by moving objects so that their velocity can be measured .This in a way defines OTH or over the horizon as the capability to detect the targets at large ranges like thousands of kilometers .
BISTATIC RADARS :
These type of radars consists of a transmitter and receiver separated by a distance that can be compared by the distance of the target . A monostatic radar is a radar in which both the transmitter and receiver are combined . There are specific classes of bistatic radars like :
Psedo-monostatic radars , forward scatter radars , multistatic radar , passive radar . The advantage of bistatic radars are :
- it has low maintenance cost
- It can operate without any clearance in frequency
- I converts the receiver operations
- I has light receivers and powerful and heavy transmitters .
- Due to geometrical effect the cross section of the radar is enhanced .
The disadvantages of this type of radar includes :
- The complexity of the systems
- The costs involved in giving communication between different sites
- There is no control over the transmitter
- It is very hard to deploy
It stands for airport surveillance radar and is used in airports for displaying and detecting the exact position of aircrafts .It uses an automated radar terminal system to display data . This system was developed to control air traffic , separation of aircrafts , and provide weather advices .
QUESTIONS AND ANSWERS :
QUE > DEFINE ASR ?
ANS > It stands for airport surveillance radar and it is used in airports to display and detect the exact position of aircrafts in various regions . It uses an automated radar terminal system that enables the display of data . The applications are control air traffic , separation of aircrafts , and provide weather advices .
QUE > WHAT ARE THE ADVANTAGES OF BISTATIC RADARS ?
ANS> the advantages of bistatic radars are :
- the maintenance cost is low
- will be able to operate without any clearance in frequency
- It converts the operations of receivers
- It has light weight receivers and powerful and heavy transmitters .
- Due to geometrical effect the cross section of the radar is enhanced .
QUE > WHAT IS DISPLAY TYPE A RADAR ?
ANS > The type A does not show the direction but can only show the range . It can be also called as R scope radar . It was used in World War 2 as it also works as an electronic oscilloscope . It could produce an oscillating voltage signal from the receiver of radar . It represents any signal detected on vertical axis . It is used in aircrafts and ships . An A scope display was also used in air surface vessel radars and it used two antennas with a common reflector in front . The main disadvantage is that it was unable to indicate the direction to target.