Radar Systems β Unit 3 Full Overview for u
- 1. Radar Systems UNIT 3 By E V Naga Lakshmi Assistant Professor ECE Department MVSR Engineering College
- 4. Low, High and medium PRF
- 8. β’ Moving targets can be distinguished from stationary targets by observing the video on an A scope (amplitude vs range). β’ A single sweep might appear as in Fig. 4.3 a.
- 9. Delay line cancellor The delay line canceller acts as a filter to eliminate DC. Fixed targets with unchanging amplitudes pulse to pulse are cancelled. The amplitudes of the moving targets are not constant pulse to pulse and subtraction results in an uncancelled residue. The rectifier provides the video for the PPI.
- 10. β’ The Doppler shift produced by a moving target may be used in pulse radar: (1) To determine the relative velocity of the target or (2) To separate desired moving targets from undesired stationary clutter. β’ The MTI radar usually operates with ambiguous Doppler measurements (blind speeds)but with unambiguous range (no second time around echoes). β’ The pulse Doppler radar has a high enough PRF to operate with unambiguous Doppler, but at the expense of range ambiguities.
- 11. MTI Radar β’MTI Radar means moving target indication radar. This is one form of pulsed radar. MTI radar is characterized by its very low PRF and hence no range ambiguity in MTI Radar. The un- ambigues range is given by Runamb = vo/fp Where vo = velocity of electromagnetic wave in free space fp = pulse repetition frequency in Hz β’Moving target indication is the process of rejecting fixed or slowly moving clutter while passing echoes from targets moving at significant velocities. β’Moving Target Indication (MTI) radar: A delay line canceller filter to isolate moving targets from non moving background i. Ambiguous velocity ii. Unambiguous range
- 12. Types of MTI Radar: 1.Area MTI Radar 2.Coherent MTI Radar 3.Non-coherent MTI Radar 4.Airborne Moving Target Indicator (AMTI) Radar 5.Digital MTI (DMTI) Radar.
- 13. MTI Radar Block Diagram ( Power Amplifier)
- 14. 1. It can be easier to reach the desired performance i.e., in terms of line width, wavelength tuning range, beam quality or pulse duration. This is because various performance aspects are decoupled from the generation of high powers which gives the extra flexibility. 2. Low power seed laser can be modulated i.e., an optical modulator can be used between seed laser and power amplifier. 3. The combination of an existing laser with an existing amplifier may be simpler than developing a new laser with high output power. 4.Optical intensities are lower compared to intraacavity intensities in an amplifier. Advantages of MOPA (Master Oscillator Power Amplifier):
- 15. DISADVANTAGES OF MOPA β’ The complexity of the setup is higher. β’ The wall-plug efficiency is often lower. β’ The resulting laser noise tends to be higher. β’ A MOPA can be sensitive to back reflections which are amplified again before entering the master laser.
- 16. APPLICATIONS OF MOPA: β’ In pulsed laser sources it can be used as a reservoir of energy. β’ This can be used in the deformation of the temporal pulse shape.
- 17. Delay Line Cancellers: Filter Characteristics of a Delay Line Canceler: The canceler acts as a filter to reject DC clutter but because of its periodic nature it also rejects energy near the PRF and its harmonics. The video received from a target at range R0 is The signal from the previous transmission which is delayed by a time T = PRI is The output from the pulse canceler is The frequency response of the single delay-line canceller is H(f) = 2k sin(ΟfdT). The response of the single delay line canceller will be zero when the argument ΟfdT is 0, Ο, 2Οβ¦etc.
- 18. Blind Speed 1. Blind speed is defined as the radial velocity of the target at which the MTI response is zero. 2. It is also defined as the radial velocity of the target which results in a phase difference of exactly 2Ο radians between successive pulses. 3. Blind speed is defined as the radial velocity of the target at which no shift appears making the target appearing stationary and echows from the targets are cancelled. For Ξ» in metres, fp in Hz and vn in knots, n=0,1,2,3,.... The first blind speeds in knots is given by vb1 = 0.97Ξ»fp = Ξ»fp If the first blind speed is to be greater than the expected maximum radial speed, then Ξ»fp must be large. Hence the MTI must operate at a long wavelength or high PRF or both. However there are other constraints on Ξ» and fp and blind speeds are not easily avoided. There are four methods to reduce the effect of blind speeds by operating the radar at: a. Long wavelength b. High PRF c. More than one PRF and d. More than one wavelength. The blind speed is dependent on the transmitted frequency and on the pulse repetition frequency of the radar unit. The effect of blind speeds can be reduced by operating with more than one PRF (staggered PRF MTI).
- 21. Multiple (Staggered) PRF β’Multiple PRFs reduce the effect of blind speeds and also allow a sharper low frequency cutoff. The blind speeds of two independent radars will be different if their PRFs are different. This same result can be achieved with one radar which shares its PRFs between 2 or more values. β’PRF can be switched every other scan, every time the antenna is scanned half a beam width or pulse to pulse (staggered PRF). Fig. 4.16 shows the composite response of MTI with two separate PRFs with a ratio of 5:4 β’Note: The first blind speed of the composite is greatly increased (i.e. at fd = 4/T1 = 5/T2). But regions of low sensitivity appear. β’The closer the ratio T1: T2 approaches unity, the greater the frequency y of the first blind speed, and the deeper he first null in the vicinity of fd - 1/T1. The null depth can be reduced and the first blind speed increased by operating with more than 2 PRFs.
- 24. Range gated Doppler filter If there is more than one target in the smeared region they can not be resolved. Even for one target, noise from other range cells will interfere (collapsing loss) resulting in reduction in sensitivity. The loss of range information and collapsing loss can be eliminated by quantizing the range into small intervals (range gating). The width of the range gate is usually of the order of the pulse width. The range resolution is established by range gating. Once the return is quantized, the output from each gate is applied to a narrow band filter since the pulse shape. Collapsing loss does not take place since noise from other range intervals is excluded.
- 25. DIFFERENCE BETWEEM PULSE DOPLER RADAR AND MTI RADAR: 4. Pulse Doppler radar has another interest, it is interested in the changes happen to the transmitted wave (DOPPLER SHIFT) either it will be compressed if the target moving toward the radar Example: received frequency may change from transmitted 6000MHz to 6010MHZ), or may stretch if the target are going away from the radar ( the 6000MHz will be 5990MHz 5. The PD radar are not interested in the transmitted frequency any more after it has been transmitted but it does set filters around it at the expected reflected frequency Example:5970,5980,5990,6010,6020,6030. If there are moving targets then the filters will receive power and that is an indication of presence of target.
- 26. DIFFERENCE BETWEEM PULSE DOPLER RADAR AND MTI RADAR: 1 Pulse Doppler radar, in this the radar send the pulse train to detect the position of target and MTI (moving target indicator) in which it detect the target which is moving. 2. Pulse doppler radar and MTI radar both are used to find the target range by using doppler effect( doppler shift) .but MTI radar uses low PRF whereas pulse doppler uses high PRF. 3. The different between MTI radar and PD radar is a unique even though they all relay on Doppler principle , but the MTI radar use Delay line canceller ( determine moving targets by detecting the phase and amplitude of the received wave and compare it with saved replica of the original transmitted wave but at opposite phase , so if the targets are not moving then the phase and amplitude of the 2 signals will match but at different value will result of cancelling each other, but if the 2 signals are not matched they will give positive or negative value and that is indication of moving target).
- 27. Difference between MTI Radar and CW Radar:
- 28. . Blind speeds Blind phases 1.The relative velocities of the target at which MTI response is zero are called as blind speeds 1. The blind phases are due to the presence of and are given by sampling pulses at the same point in the 2. Due to the presence of blind speeds within a Doppler cycle at each sampling instant. 2. When the Doppler frequency is half of the Doppler frequency band, the capability of PRF, blind phases with single has serious 3. By operating with more than one PRF or operating at more than one RF frequency it can be reduced 3. By using quadrature, we can eliminate the blind phases 4. Effects more in M TI Radar 4. Effects more in M TI Radar