satellite communication

1. 14. Doppler Radar and MTI

2. 5. Doppler radar and MTI
5.1 Introduction
5.2 The Doppler effect
5.3 Delay-line canceller
5.4 Blind speeds
5.5 Clutter effects
5.6 Cascaded MTI filters
5.7 Staggered PRFs
5.8 Clutter locking
5.9 MTI improvement factor
5.10 Moving Target Detection

3. Introduction
Doppler – 1!
!
• In many radar applications there is a relative movement between
the radar and the target to be detected. Examples include, Air
Traffic Control, Battlefield Surveillance, Weapon Locating, all
Airborne Radars, SAR and ISAR as well as many others.!
• Consider the example of an Air Traffic Control radar. This has to
detect incoming and outgoing aircraft in the presence of a clutter
background. We have already seen that clutter can be
distinguished from receiver noise by virtue of its narrower, low
frequency spectrum. How can this be automatically exploited?!

4. Introduction
Doppler - 2!
• Targets can be distinguished from background clutter by virtue of
their motion. This enables a track history to be built up. !
• More useful, however, is exploitation of the Doppler effect which
enables moving targets to be filtered such that clutter is rejected
based upon the differing velocities of the two received signal
components.!
• A processor that distinguishes moving targets from clutter by
virtue of differences in their spectra is called a Moving Target
Indicator or MTI. MTI processors can take a number of forms.!

5. The Doppler effect
The Doppler effect - 1
• The Doppler effect was first recognized by Christian Johann
Doppler, who observed that the color of a luminous body and
the pitch of a sounding body are changed by the relative
motions of the body and observer.
• A very common example is the change in pitch (not the
frequency) of an approaching or receding vehicle with respect
to yourself. The pitch rises as the on coming vehicle gets
nearer, goes to zero as the vehicle passes (i.e. there is a zero
relative velocity) and then starts to fall as it recedes.

6. Christian Doppler‘s Birthplace in Salzburg,
Austria

7. Light from moving objects will appear to have different wavelengths
depending on the relative motion of the source and the observer. !
!
!
!
!
!
!
!
!
!
!
Observers looking at an object that is moving away from them see light
that has a longer wavelength than it had when it was emitted (a redshift),
while observers looking at an approaching source see light that is shifted
to shorter wavelength (a blueshift).!
The Doppler effect

8. The schematic diagram below shows a galactic star at the bottom left with its spectrum
on the bottom right. The spectrum shows the dark absorption lines first seen by
Fraunhofer. These lines can be used to identify the chemical elements in distant stars,
but they also tell us the radial velocity. The other three spectra and pictures from bottom
to top show a nearby galaxy, a medium distance galaxy, and a distant galaxy. The
pictures on the left are negatives, of course, so the brightest parts of the galaxies are
black. Notice how the pattern of absorption lines shifts to the red as the galaxies get
fainter. The numbers above and below the spectra are the measured wavelengths in
nm.!
!
!
!
!
!
!
!
!
!
By measuring the amount of the shift to the red, we can determine that the bright galaxy
is moving away at 3,000 km/sec, which is 1 percent of the speed of light, because its
lines are shifted in wavelength by 1 percent to the red. !
Measuring the speed of galaxies

9. The Doppler effect - 2
• Consider a stationary ground based radar observing an approaching aircraft.
• As the aircraft approaches each radar pulse travels a shorter and shorter
distance, consequently the phase of the signal is constantly changing with
each pulse or target position.
• The faster the aircraft approaches the radar the faster the rate of change of
the phase of the reflected signal.
• Thus the rate of change of the measured phase to the approaching aircraft is
relative to the velocity of the aircraft.
Pulse 1
Pulse 2
The Doppler effect

10. The Doppler effect
target!
r
The phase represented by the two-way path from radar to target is!
!
!
!
The Doppler shift is !
!
(- sign because an increase in path length !
represents a phase lag)!
2
2
r
φ π
λ
=
1
2
D
d
f
dt
φ
π
= −
021 4 2
2
vfd r dr
dt dt c
π
π λ λ
⎛ ⎞
= − = − = −⎜ ⎟
⎝ ⎠
velocity!
= v!
Radar!

11. Doppler Ambiguity
The PRF must sample at twice the highest Doppler frequency i.e
to avoid aliasing.
4vf0
c

12. The Range-Doppler Map

13. The Range-Doppler Map

14. Police microwave and laser radars measure the relative speed a vehicle is
approaching (or receding) the radar. If a vehicle is traveling directly (collision course)
at the radar, the relative speed is actual speed. If the vehicle is not traveling directly
toward (or away) the radar but slightly off to avoid a collision, the relative speed with
respect to the radar is slightly lower than actual speed. The phenomenon is called
the Cosine Effect because the measured speed is directly related to the cosine of
the angle (alpha) between the radar and vehicle direction of travel (see figure
below). The greater the angle, the greater the speed error (the lower the measured
speed). A cosine angle of 90° has 100% error (speed measures 0).
Vo = Traffic Speed
R = Traffic Range to Radar
d = Antenna Distance to Traffic Lane (center)
LOS = Line-of-Sight
Police Speed Gun radars

15. 2D - Doppler Radar

16. dd
dd
ff
ff
findsent tobetoneedpulsesmany/1
quicklyfoundbecan/1
→
→>
< τ
τ
Finding the Doppler Frequency Shift

17. The delay line canceller
• MTI systems attempt to maximize signal to clutter ratios and are
consequently dependent of the correlation of the clutter. The
delay-line MTI subtracts an echo from that of the preceding pulse. !
• For moving targets the received signal will change from pulse to
pulse, so the output from the delay-line MTI will be non-zero. !
• Echoes from stationary clutter will be constant and thus
suppressed. !
• MTI systems can use phase, amplitude or phase and amplitude to
improve the signal to clutter ratio. Systems that use only phase or
amplitude do not match the performance of systems using both.!
• Virtually all modern systems use quadrature channel processing
which is equivalent to amplitude and phase processing!

18. The delay line canceller
Phase processing MTI!
!
Here the system distinguishes moving targets by virtue of the targets
Doppler frequency. Consequently phase coherence within the radar
system must be held in close tolerance. This coherence is provided by
a Stable Local Oscillator (STALO). If there are to be two down
conversion stages then a Coherent Oscillator (COHO) is used that
establishes an intermediate frequency.!
!
The STALO translates the signal from Radio Frequency (RF) to an
Intermediate Frequency (IF). The COHO provides a reference signals
that effectively ‘remembers’ the phase of each transmitted and
received pulse. In the simplest processor this phase detected signal is
processed in a delay line canceller that forms the difference of two
signals separated in time corresponding to an inter-pulse period. !

19. The delay line canceller
display!
video !
amplifier!
target!
StAble Local Oscillator!
Coherent transmitter oscillator !
delay line !
delay = T !
(one p.r.i.)!
+
+
-
subtractor!

20. The delay line canceller
The frequency response of the two-pulse delay-line canceller can be
derived as follows :!
!
In the time domain we have;!
!
so in the frequency domain this is; !
!
The frequency response is therefore given by;!
!
!
!
so
( ) ( ) ( )y t x t x t T= − −
( ) ( ) ( )1 expY X j Tω ω ω= − −⎡ ⎤⎣ ⎦
( )
( )
( )
exp exp exp
2 2 2
Y j T j T j T
H
X
ω ω ω ω
ω
ω
⎡ ⎤⎛ ⎞ ⎛ ⎞ ⎛ ⎞
= = − − −⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎢ ⎥
⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎣ ⎦
exp 2 sin
2 2
j T T
j
ω ω⎛ ⎞ ⎛ ⎞
= −⎜ ⎟ ⎜ ⎟
⎝ ⎠ ⎝ ⎠
( ) 2 sin
2
T
H
ω
ω =

21. The delay line canceller
The delay line filter! +
-
This has a frequency response:!
Doppler
0 1/T 2/T 3/T 4/T

22. Blind speeds
It can be seen that the frequency response has zeroes at Doppler
shifts corresponding to :!
!
!
!
!
or!
!
These can also be thought of in terms of target movement such that
the pulse-to-pulse phase changes by 2pi, in other words that the
target range changes by l/2 in one PRI. !
. ( integer)d
n
f n PRF n
PRI
= =
0
. .
2
blind
n c PRF
v
f
=
0
. 2 . .
2
blind
n n c PRF
v
PRI f
λ
= =

23. Blind speeds – velocity ambiguities
• This also says that to keep the first blind speed above the
highest likely target velocity, it is necessary to use a high
PRF. This conflicts with the requirement to use a low PRF in
order to avoid range ambiguities.
• This is an example of the tradeoffs involved in the choice of
PRF in the design of the radar modulation scheme, and is
covered in more detail when considering the ambiguity
function.

24. Blind speeds –
radial and tangential velocities
aircraft track!
velocity too low to be detectable!
Note: !
!
• It is the radial component
of target velocity that
causes the Doppler shift. !
• A t a r g e t m o v i n g
tangentially to the range
direction will have zero
Doppler, and hence will not
be detected by an MTI
processor.!

25. The delay line canceller
Amplitude processing MTI
• The amplitude processing MTI is exploited by a non-coherent radar
system.
• An advantage of this is that the local oscillator need not be as stable (and
therefore as expensive). However the clutter must be significantly above
the noise and it must be relatively stable.
• Here the cancellation circuit takes successive amplitude differences. If the
clutter amplitude is stable it will be subtracted leaving the fluctuating target
signal to be detected.
• This can be depicted vectorially.
Ec (clutter voltage)
Es (signal voltage)
θ
2/θΔ
2/θΔ
E1
Es(t+T)
θΔ
E2

26. The delay line canceller
Thus we have, for the resultant vector for the first and second pulses!
)2/cos(2
)2/cos(2
2
1
θθ
θθ
Δ+−+=
Δ−−+=
scsc
scsc
EEEEE
and
EEEEE
If the detector prior to the canceller is a square law detector then the !
canceller will form the difference between the square of the two resultants.!
)2/)(sin(sin42
2
2
1 θθ Δ=−= Scr EEEEE
however! od ff /2πθ =Δ
therefore! ))(sin/(sin4 θπ odscr ffEEE =
This shows that blind speeds are also present in amplitude canceller
MTI systems!

27. The delay line canceller
Quadrature channel processing MTI!
The quadrature processor has two channels identical except that
one contains a ninety degree phase shift. The two channel outputs
are called I (in phase) and Q (quadrature phase). The vector sum
gives the amplitude of the signal and their arctan the phase. The
advent of high speed A/D converters makes possible a digital
implementation of MTI. This is increasingly becoming the norm.!
phase
Amplitude
I
Q

28. Typical cluttered scenario

29. Clutter effects
Clutter spectrum!
Many types of clutter (sea, wind-blown vegetation) have a finite and
variable Doppler spectrum. Thus the basic two-pulse canceller does not
achieve perfect clutter suppression. With a knowledge of the clutter
spectrum and the canceller frequency response, the actual degree of
suppression can be calculated.!
0
Doppler!
Clutter!
spectrum!
Clutter residue!
Amplitude!

30. Cascaded MTI filter
+
-
+
-
delay, T delay, T
For improved performance, two cascaded cancellers can be used. !
!
The frequency response is the square of the single delay line canceller :!
!
!
!
!
This gives better suppression close to zero Doppler!
0 f
1
f
2
f
3
Doppler!
( )
2
2
2 sin 4sin
2 2
T T
H
ω ω
ω
⎛ ⎞
= =⎜ ⎟
⎝ ⎠

31. Staggered PRFs
time!
transmitted !
pulses!
T1 T2 T1 T2 T1 T2 etc
To address the problem of blind speeds, it is possible to use a
“staggered PRF”. The diagram below shows a two-PRF stagger
pattern. The two PRFs are related by an integer ratio n : m.!

32. Staggered PRFs
1/T1
2/T1
3/T1
4/T1
f
f
1/T2 2/T2 3/T2 4/T2 5/T2
The composite frequency response is the sum of the responses
for the individual PRIs. In this example the two PRIs are related in
the ratio 4 : 5.!

33. Staggered PRFs
f
which looks like :
The first blind Doppler has been increased to 4/T1 = 5/T2 .!
!
The choice of the ratio m : n is a compromise between flatness of
frequency response on one hand, and extension of the first blind
speed on the other. !
1 2 3 4 5

34. • Consider a radar mounted on an airborne platform moving with a
linear trajectory.!
• All stationary objects are now moving with respect to the radar
platform. !
• For a sideways looking radar these stationary objects will cause
different Doppler shifts between successive pulses as the aircraft
flies by, because their radial velocities are changing with respect to
the platform’s linear motion.!
• Detecting targets that have motion with respect to the ground then
becomes much more complex.!
Airborne MTI

35. Clutter locking
Clutter locking MTI - 1!
• The presence of a mean clutter velocity component can
dramatically effect MTI performance. !
• This mean velocity component can originate either from the
average motion of the clutter itself or from the motion of the
radar platform (such as an aircraft). !
• Clutter locking is a method for removing this average clutter
frequency or velocity. !

36. • The average inter-pulse phase change is measured and
averaged over several range bins. This provides an estimate of
the average clutter velocity. !
• This average velocity may then be compensated for in the
delay line circuit. !
• Clearly this process can be contaminated if the clutter exhibits
a spread of velocities or if the mean velocity of the target is
similar to that of the clutter. !
• Sometimes the clutter locking is bypassed of the clutter to
noise ratio is small.!
Clutter locking MTI - 2!
Clutter locking

37. Clutter locking
And the clutter
spectrum will be:
mainlobe
clutter
altitude
return
Doppler
frequency
02
cos
vf
c
θ+ 02vf
c
+
02vf
c
−
look-back look-ahead
0
Thus for a forward
looking radar:

38. Clutter locking
If the radar is looking for ground-based targets, the main-lobe clutter
Doppler can be used as a reference.!
02vf
c
+
02vf
c
−
Doppler ground
range
mainlobe
clutter

39. MTI improvement factor
There are two quantities usually used to measure MTI performance:!
!
Clutter Attenuation (CA) and the system Improvement factor (I). !
!
Clutter attenuation is defined as the ratio of the input clutter power
Ci to the output clutter power Co.!
!
! ! !CA=Ci/Co!
!
The system improvement factor is defined as the signal to clutter
ratio at the output of the MTI system compared with that at the
input. The signal is assumed to be averaged uniformly over all
radial velocities, i.e.!
ii
00
_
/CS
/CS
I = CA
S
S
I
i
_
0
=or

40. Moving Target Detection
• The Moving Target Detector (MTD) is an advance on the simpler
MTI system. !
• We could consider a cascaded MTI system as an N point FFT
performed on a time sequence of range bins. The zero Doppler
bin is then removed prior to detection processing. !
• However, we know that clutter doesn‘t always fall in the zero
Doppler bin and that targets themselves can have inconsistent
Doppler histories. !
• An MTD enables full advantage to be taken in both the Doppler
and range domains. !
• A ‘clutter map’ can be formed in range-Doppler space that is a
time averaged representation of the clutter in each range bin
and in each Doppler bin.!
!

41. Moving Target Detection
Doppler processing!
pulse no.
1
2
3
4
5
6
7
.
.
.
• Take slice through echoes from a burst at constant
range, then take FFT to give Doppler information.!
• Repeat at all ranges.!

42. Moving Target Detection
Range!
Frequency!

43. Moving Target Detection
155
0
Speed/knots
7.8Range / km
Target 1 Target 2
Target range / km
2.7 7.4
1.9 6.5
1.3 5.7
0.8 4.9

44. Moving Target Detection
Range/km0 7.8
Speed/knots
155
0.4 4.0
0.2 3.2
landed 2.3
Target 1 Target 2
Target range / km

45. Moving Target Detection
We can distinguish:!
!
• High PRF operation (unambiguous in Doppler but ambiguous in
range), !
• Low PRF operation (unambiguous in range but ambiguous in
Doppler), and !
• Medium PRF operation (ambiguous in both range and Doppler).!
!
High, Low and Medium PRF!