Primary Pulse Radar operates on Bearing by Search light principle and Range by echo principle
Pulse Radar transmits and receives the reflected pulses from a target using the same radar antenna
Direction of the target is obtained by calculating the direction from where pulse is reflected
The time delay for the pulse to go up to the target and return to the radar is converted into range of the target in pulse radar
Primary Continuous wave radar transmits and receives its reflected continuous wave transmission
Range of the target is obtained by calculating the phase difference between the phase at the time of transmission and reception
Secondary Radar transmits a pulse to the target and the target cooperates by receiving the signal and replying on a different frequency
Parabolic Dish Antenna
Parabolic Dish Antenna has a parabolic dish reflector behind the antenna designed to generate narrow parallel beams towards the target
Focal Point of the antenna or the point of energy transmission is equidistant from any point on the dish reflector
Since the same antenna is used for transmission and reception the transmitter has to be switched off during reception
Parabolic Antenna has a problem of side lobes of lesser strength which light up targets outside the required area
Flat Plate Planar Slotted Array Antenna
Flat Plate is normally used in Airborne Radars since they require lesser energy for transmission
The energy of transmission enhanced by series of antenna of half wavelength fed in phase through slots in flat metal plate
Flat plate antenna produces narrow beams with lesser side lobe reduced problem
Pulse Width (PW)
Pulse Width or Pulse Length is the duration of transmission of a single pulse determining the minimum range of radar
If we consider a pulse width of 1 micro second and one way distance to the target lesser than 150 m the radar would be in transmission mode and receiver switched off when the front of the pulse returns to the radar
Therefore, the one way minimum range would be 150 m or the pulse width determines the minimum range of a primary radar
Pulse Recurrence Frequency (PRF)
Pulse Recurrence Frequency (PRF) or Pulse Recurrence Rate (PRR) is the number of pulses transmitted per second
Pulse Recurrence Interval (PRI) or Pulse Recurrence Period (PRP) is the time interval between the start of successive pulses
PRI and PRF are inversely proportional to each other
Pulse Recurrence Interval = 1 / Pulse Recurrence Frequency
PRF determines the maximum theoretical range of the radar or lower PRF produces greater maximum range
Radar Mile
Slant Range calculated for a time gap of 12.36 micro seconds would be 1 NM based on Distance = Speed x Time
Radar mile is the time taken for the radio wave to travel 1 NM which would be 12.36 micro-seconds
Fly Back Time
Fly-back Time (Dead-time) is the time taken by dot in CRT screen to return back to its original position
The maximum practical range reduces as compared to maximum theoretical range due to fly-back time of the radar scope
Factors affecting Radar Range
Pulse width and PRF determine the minimum and maximum theoretical range of the radar
Inverse square law of electro magnetic radiation states that to double range power needs to be increased by 4 times
Since the pulse has to travel 2 ways to double the range power has to be increased 16 times
Line of sight equation modified by atmospheric refraction also affects the practical range
Target characteristics like material, size and shape of aircraft improve the visibility of target
Mountain shadow or similar terrain would stop radar waves and hence would reduce effective range of radar
Rainfall would attenuate radar waves less than 4 cm wave length since the wave length would be nearly equal to diameter of rain drops
Super-refraction and sub-refraction may increase range sporadically but would not provide accurate bearing of target
Distortion of Target on a Radar Scope
Beam Width, Pulse Length and Spot Size distorts the target painted on a radar scope
Half of the beam width would be added on either side of the target which would increases with range of the target
Radar cannot distinguish between two targets lesser than distance representing half the Pulse Length since half of the pulse length is added behind the target
Spot Size Distortion is added on all sides due to scale used in cathode ray tube in the monitor
Distance represented would be spot size x representative fraction
If a spot of size is 0.5 mm in diameter on a scale of 1:1 million the distance represented would be 500 meters
Overall distortion of radar picture will include combined effects of Beam width, Pulse length and Spot size
Advanced Radar Techniques
Moving Target Indication (MTI) eliminates ground returns since they are designed to display only moving targets using doppler technique
Jittering the PRF Technique changes the PRF at random intervals to avoid false targets due second trace returns from preceding pulse that are beyond the maximum range