MTI and Pulse Doppler Radar

• The doppler frequency shift produced by a moving target may be• The doppler frequency shift produced by a moving target may be used in a pulse radar, to determine the relative velocity of a target– Or to separate desired moving targets from undesired stationary objects (clutter)

– Such a pulse radar that utilizes the doppler frequency shift as a means for discriminating moving target from fixed one is called an MTI (Moving Target Indication) or pulse doppler radar.g ) p pp

• The two are based on same physical principle, but practically they differ to each other– The MTI radar usually operates with ambiguous doppler measurement (blind

speed) but with unambiguous range measurement

hil di l d l d i i ll hi h h– While regarding pulse doppler radar, its PRF is usually high enough to operate with unambiguous doppler (no blind speed) but at the expense of range ambiguities

MTI Radar

• MTI is a necessity in high-quality air-surveillance radars that operate in the presence of clutter

• The CW radar may be converted into a pulse radar by providing a power amplifier and a modulator to turn the amplifier on or off for the purpose of generating pulsesfor the purpose of generating pulses

• figure

– The major difference between this pulse radar and the one described in– The major difference between this pulse radar and the one described in Chapter-1 is that a small portion of the CW oscillator power that generates the transmitted pulses are diverted to the Rx to take the place of the LO

– The CW signal act as the coherent reference needed to detect the dopplerThe CW signal act as the coherent reference needed to detect the doppler frequency shift

• Coherent means that the phase of the transmitted signal is preserved in the reference signalg

– The reference signal is the distinguishing feature of coherent MTI radar.

a) Simple CW radar) pb) Pulse radar using doppler information

• If the oscillator voltage is represented as A1sin2πftt, where A1 is the amplitude and ft is the carrier frequency, the reference signal is

V = A sin2πf tVref = A2sin2πfttand the doppler-shifted echo signal voltage is

Vecho = A3sin[2π(ft ± fd)t- 4πftR0/c]Vecho 3s [ π( t d)t π t 0/c]where, A2 = amplitude of reference signal

A3 = amplitude of signal received from a target at a range R0

fd = doppler frequency shift

• The reference signal and the echo signal are heterodyned in the mixer of the Rx.– Only the low frequency component from the mixer is of interest, given by

Vdiff = A4sin(2πfdt- 4πf R0/c]Vdiff A4sin(2πfdt- 4πftR0/c]

• For stationary targets the f will be zero• For stationary targets, the fd will be zero– Hence, Vdiff will not vary with time, and take on any constant value from

+A4 to -A4 including zero

• Moving targets may be distinguished from stationary targets by observing the video output on an A-scope

• figure

• This sweep shows various fixed targets and two moving targets i di d b hindicated by the two arrows

• On the basis of a single sweep, moving targets cannot be di ti i h d f fi d t tdistinguished from fixed targets– Thus, successive A-scope sweeps are shown in figure

A i di t th• Arrow indicates the position of moving target

• (a-e) Successive sweeps of an MTI radar A-scope display

• (f) superposition of many sweeps

• Echoes from fixed targets remain constant throughoutEchoes from fixed targets remain constant throughout– But echoes from moving targets vary in amplitude from sweep to sweep at

a rate corresponding to the doppler frequency

• The superposition of the successive A-scope is shown.– The moving target produce, with time, a butterfly effect

• In order to extract doppler information, we have to pass the signal after the Rx to the Delay-Line Canceller (DLC).– It act as a filter to eliminate the d-c component of fixed targets– It act as a filter to eliminate the d-c component of fixed targets– and, to pass the a-c components of moving targets

• figure

Th id ti f th R i di id d i t t h l• The video portion of the Rx is divided into two channels– One is normal video channel– The other video signal experiences a time delay equal to one pulse-

repetition period

• The output from the two channels are subtracted from one another– The fixed target with unchanging amplitudes from pulse to pulse are

cancelled on subtraction– However, the amplitudes of the moving target echoes are not constant from

pulse to pulse, and subtraction results in an un-cancelled residues

• Finally, it is converted to unipolar voltages by a full-wave rectifier

• The simple MTI radar shown earlier is not necessarily the mostThe simple MTI radar shown earlier is not necessarily the most typical one

• The block diagram of a more common MTI radar employing aThe block diagram of a more common MTI radar employing a power amplifier is shown in figure– The difference from the earlier one is the manner in which the reference

signal is generatedsignal is generated

• The coherent reference is supplied by an oscillator called the cohowhich stands for coherent oscillatorwhich stands for coherent oscillator– The coho is a stable oscillator whose frequency is same as the intermediate

frequency used in the receiver

• The output of coho (fc) is also mixed with the LO frequency fl– The LO must also be a stable oscillator, and is called as stalo for Stable LO

• MTI radar with power amplifier Tx

• The RF echo signal is heterodyned with the stalo signal to produce h IF i l j i i l h d Rthe IF signal just as in conventional superheterodyne Rx

• The characteristic feature of coherent MTI radar is that the i d i l b h (i h ) i h h ftransmitted signal must be coherent (in phase) with the reference

signal in the Rx– This is done by generating the transmitted signal from the coho reference

signal

• The function of the stalo is to provide the necessary frequency l i f h IF h i d (RF) ftranslation from the IF to the transmitted (RF) frequency

– Although the phase of the stalo influences the phase of the transmitted signal, any stalo phase shift is cancelled on reception

b h l h h i d i l l h LO i h R• because the stalo that generates the transmitted signal also acts as the LO in the Rx

• The reference signal from the coho and the IF signal are both fed into i ll d h d t ta mixer called phase detector

• Any one of a number of transmitter-tube might be used as theAny one of a number of transmitter tube might be used as the power amplifier.

• A Tx which consists of a stable low-power oscillator followed byA Tx which consists of a stable low power oscillator followed by a power amplifier is sometimes called MOPA– which stands for Master-Oscillator Power Amplifier

fi– figure

• A portion of the transmitted signal is mixed with the stalo output to produce an IF beat signal whose phase is directly related to theto produce an IF beat signal whose phase is directly related to the phase of the Tx– This IF pulse is applied to the coho and causes the phase of the coho CW

oscillation to LOCK in step with the phase of the IF reference pulse• At the next transmission, another IF locking pulse is generated to relock the

phase of the CW coho.

Delay Line Canceller (DLC):Delay Line Canceller (DLC):• It is an example of a time-domain filter

– The capability of this device depends on the quality of the medium used as the delay line

• For ground-based air-surveillance radars, this might be several illi dmilliseconds

– Delay time of this magnitude cannot be achieved with practical EM transmission lines

• One of the advantage of a time-domain DLC as compared to the more conventional frequency domain filter is that– a single network operates at all the ranges and does not require a separate

filter for each range

Filter characteristic of the DLC:• The DLC acts as a filter which rejects the d-c component of clutter

– Because of its periodic nature, the filter also rejects energy in the vicinity of the PRF and its harmonicsthe PRF and its harmonics

• The video signal received from a particular target at a range R0 isV = k sin(2πf t - φ )V1 k sin(2πfdt - φ0)

• The signal from the previous transmission, which is delayed by a time T (=1/prf) istime T ( 1/prf), is

V2 = k sin[2πfd(t-T) - φ0]

Everything else is assumed to remain essentially constant over theEverything else is assumed to remain essentially constant over the interval T so that k is the same for both. The o/p from subtractor is

V = V1 - V2 = 2k sin(πfdT) cos[2πfd(t-T/2) - φ0]

• It is assumed that the gain through the DLC is unity• It is assumed that the gain through the DLC is unity

• The output from the canceller consist of a cosine wave at the doppler frequency fd with an amplitude 2k sin(πfdT)doppler frequency fd with an amplitude 2k sin(πfdT)– Thus the amplitude of the cancelled video o/p is a function of the doppler

frequency shift and the pulse-repetition interval

Blind Speed:Blind Speed:• The response of the single DLC will be zero whenever the

argument πfdT in the amplitude factor is ‘nπ’ where n=0, 1,2…etc or when

fd = n/T = nfp

• The DLC not only eliminates the d-c component caused by clutter (n=0), but unfortunately it also rejects any moving target whose doppler frequency happens to be the same as the PRF or a multipledoppler frequency happens to be the same as the PRF or a multiple.– Those relative target velocities which results in zero MTI response are called

blind speed and are given by

λ/2T λf /2 1 2 3vn = nλ/2T = nλfp/2, n = 1,2,3,….where, vn is the nth blind speed.

• The blind speeds are one of the limitations of MTI radar which doThe blind speeds are one of the limitations of MTI radar which do not occur with CW radar.– They are present because doppler is measured by discrete samples (pulses)

at the PRF rather than contin o slat the PRF rather than continuously

• If the first blind speed is to be greater than the maximum radial velocity expected from the target the product λf must be largevelocity expected from the target, the product λfp must be large

• Thus, in order to reduce the effect of blind speed:– Radar must be operated at long wavelengths (low frequency)– Radar must be operated at long wavelengths (low frequency)

• Have the disadvantage that antenna beamwidth, for a given antenna, are wider than at the higher frequency

• and, would not be satisfactory where angular resolution is importantand, would not be satisfactory where angular resolution is important– Or, with high PRF

• But, it can’t be always varied over wide range since it primarily obtained by the unambiguous range requirementsthe unambiguous range requirements

– Or, both

Double Cancellation:Double Cancellation:• The frequency response of a single DLC does not always have as

broad a clutter-rejection null as might be desired in the vicinity of d-c– The clutter-rejection notches may be widened by passing the output of the

DLC through a second DLC– figure

• The output of the two single DLC in cascade is the square of that from a single canceller– Thus the frequency response is 4sin2(πfdT)– Thus also called as double cancellerThus, also called as double canceller

• The relative response of the double canceller compared with that of a single DLC is showng

Multiple Or Staggered PRF:Multiple Or Staggered PRF:• The use of more than one PRF offers additional flexibility in the

design of MTI doppler filters– It reduces the effect of blind speed

• The blind speeds of two independent radars operating at the same f ill b diff if h i difffrequency will be different if their PRFs are different– Therefore, if one radar were blind to moving targets, it would be unlikely

that the other radar would be blind also

• Thus, instead of using two separate radars, the same results can be obtained with one radar which time-shares its PRF between two or more different values (multiple PRFs)or more different values (multiple PRFs)– The PRF might be switched every other scan or every time the antenna is

scanned.Wh h i hi i l l i i k d PRF– When the switching is pulse to pulse, it is known as a staggered PRF

• As example of the composite response of an MTI radar operatingAs example of the composite response of an MTI radar operating with two separate PRF on a time-shared basis was shown– The PRF are in the ratio of 5:4

• The first blind speed of the composite response is increased several times over what it would be for a radar operating on only a single PRFa single PRF

• Zero response occurs only when the blind speeds of each PRF coincidecoincide– In given example, the blind speeds are coincident for 4/T1 = 5/T2

• Although the first blind speed may be extended by using more• Although the first blind speed may be extended by using more than one PRFs– The regions of low sensitivity might appear within the composite passband

Limitations to MTI Performance:Limitations to MTI Performance:• The improvement in signal-to-clutter ratio of an MTI is affected by

factors other than the design of the doppler signal processor

• MTI improvement factor (I)– The signal-to-clutter ratio at the output of the MTI system divided by the

i l l i h i d if l ll di lsignal-to-clutter ratio at the input, averaged uniformly over all target radial velocities of interest

• Subclutter visibility (SCV)Subclutter visibility (SCV)– The ratio by which the target echo power may be weaker than the coincident

clutter echo power, and still be detected with specified detection and false alarm probabilitiesalarm probabilities

• A SCV of 30dB implies that a moving target can be detected in the presence of clutter even though the clutter echo power is 1000 times the target echo power

• Clutter visibility factorClutter visibility factor– The signal-to-clutter ratio, after cancellation or doppler filtering, that

provides stated probabilities of detection and false alarms

• Clutter attenuation– The ratio of clutter power at the canceller input to the cluster residue at the

output normalized to the attenuation of a single pulse passing through theoutput, normalized to the attenuation of a single pulse passing through the unprocessed channel of the canceller

• Cancellation ratio– The ratio of canceller voltage amplification for the fixed-target echoes

received with a fixed antenna, to the gain for a single pulse passing through the unprocessed channel of the canceller

• The term I is equal to the SCV times the clutter visibilityThe term I is equal to the SCV times the clutter visibility factor (Voc)– In decibels, I = SCV + Voc (al in dB)– When the MTI is limited by noise-like system instabilities, the clutter

visibility factor should be chosen as is the SNR• The improvement factor (I) is the preferred measure of MTI radar

fperformance

NonCoherent MTI:NonCoherent MTI:• The composite echo signal from a moving target and clutter

fluctuates in both phase and amplitude.

• The coherent MTI and the pulse-doppler radar make use of the phase fluctuations in the echo signal to recognize the doppler

d d b icomponent produced by a moving target– Here, the amplitude fluctuations are removed by the phase detector– Radars working on this principle are called as Coherent MTI,g p p ,

• and depend upon a reference signal at the radar Rx that is coherent with the Tx signal

• It is also possible to use the amplitude fluctuations to recognize the doppler component produced by a moving target– The MTI radar which uses amplitude instead of phase fluctuations is called

noncoherent

Block Diagram of a NonCoherent MTI radar

• The noncoherent MTI radar does not require an internal coherent reference signal or a phase detector as does the coherent MTIg p– Amplitude limiting cannot be employed in the noncoherent MTI Rx, else the

desired amplitude fluctuations would be lost• Therefore the IF amplifier must be linearp

• The detector following the IF amplifier is a conventional amplitude detector– The phase detector is not used since the phase information is of no interest– The LO does not have to be as frequency-stable as in the coherent MTI

• The output of the amplitude detector is followed by an MTI processor such as a DLC

• The doppler component contained in the amplitude fluctuations may also be detected by applying the output of the amplitude detector to an A-scopep– Amplitude fluctuations due to doppler produce a butterfly modulation

• Its major limitation is that the target must be in the presence ofIts major limitation is that the target must be in the presence of relatively large clutter signals– If moving target detection is to take place

• The clutter serves the same function as does the reference signal in the coherent MTI

If l tt t t th d i d t t ld t b d t t d– If clutter were not present, the desired targets would not be detected

• It is possible to provide a switch to disconnect the noncoherent operation and revert to normal radaroperation and revert to normal radar– whenever sufficient clutter echoes are not present

Pulse Doppler Radar (PDR):Pulse Doppler Radar (PDR):• A pulse radar that extracts the doppler frequency shift for the

purpose of detecting moving targets in the presence of clutter is either an MTI radar or a PDR– The distinction between them is based on the fact that in a sampled

measurement system, like a pulse radar, ambiguities can arise in both the doppler frequency (relative velocity) and the range (time delay)

• Range ambiguities are avoided with a low sampling rate (low PRF)g g p g ( )– While doppler frequency ambiguities are avoided with a high sampling rate

• Therefore a compromise must be made and the nature of the• Therefore a compromise must be made, and the nature of the compromise generally determines whether the radar is called an MTI or a pulse doppler

• MTI usually refers to a radar in which the PRF is chosen lowMTI usually refers to a radar in which the PRF is chosen low enough to avoid ambiguities in range– But results in blind speed

• The PDR has a high PRF that avoids blind speeds– But it experience ambiguities in range