1. Module 1. Principles of work, key parameters of radio location systems Topic 1.2. Principles of work and key parameters of active radars Lecture COHERENT METHODS OF LOCATION. DOPPLER’S EFFECT

2. In this lecture the “concept of coherence” is explained, a term which is often given as a feature of a radar system. A “coherent radar” is able to distinguish between fixed and moveable objects by analysis of the Doppler frequency.

3. Radar Clutter PPI screen of an ATC-radar with targets and clutter

4. Radar returns are produced from nearly all surfaces when illuminated by a radar. Therefore, in competition with the return from an aircraft, there are many sources of unwanted signals. Unwanted signals in a search radar are generally described as noise and clutter.

5. Clutter is the term used and includes ground returns, sea returns, weather, buildings, birds and insects. The definition of clutter depends on the function of the radar. Weather is not clutter in a weather detecting radar.

6. Since aircraft usually move much faster than weather or surface targets, velocity-sensitive radar can eliminate unwanted clutter from the radar indicator. Radar systems that detect and process only moving targets are called Moving-Target Indicators (MTI).

7. The basic types of clutter can be summarized as follows: Surface Clutter – Ground or sea returns are typical surface clutter. Returns from geographical land masses are generally stationary, however, the effect of wind on trees etc means that the target can introduce a Doppler Shift to the radar return. This Doppler shift is an important method of removing unwanted signals in the signal processing part of a radar system. Clutter returned from the sea generally also has movement associated with the waves.

8. Volume Clutter – Weather or chaff (дипольные отражатели) are typical volume clutter. In the air, the most significant problem is weather clutter. This can be produced from rain or snow and can have a significant Doppler content.   Point Clutter – Birds, windmills and individual tall buildings are typical point clutter and are not extended in nature. Moving point clutter is sometimes described as angels. Birds and insects produce clutter, which can be very difficult to remove because the characteristics are very much like aircraft.

9. Clutter can be fluctuating or non-fluctuating
Clutter can be fluctuating or non-fluctuating. Ground clutter is generally non- fluctuating in nature because the physical features are normally static. On the other hand, weather clutter is mobile under the influence of wind and is generally considered fluctuating in nature.

10. Clutter can be defined as homogeneous if the density of all the returns is uniform. Most types of surface and volume clutter are analysed on this basis, however, in practice this simplification does not hold good in all cases. Non-homogeneous clutter is non uniform clutter where the amplitude of the clutter varies significantly from cell to cell. Typically non-homogeneous clutter is generated by tall buildings in built up areas.

11. Sea-Clutter

12. Sea-clutter are disturbing radar-echoes of sea wave crests
Sea-clutter are disturbing radar-echoes of sea wave crests. This clutter gets also a Doppler- speed by the wind. This means, the scenario „moves away”, i.e. changes with time, while for ground clutter it stays the same. Therefore, in practice, Sea-clutter is very difficult to control without some loss in detection.

13. Sea-Clutter can be seen here in the picture
Sea-Clutter can be seen here in the picture. The wind comes either from about 310° (NO) or from the opposite direction. (Unfortunately, whether the Doppler frequency is positive or negative cannot be recognized on the PPI-Scope.) But this region, in which the radial speed of the waves is very small, is „cleaned” by the MTI system very clearly.

14. Doppler- Effect In radar technology the Doppler Effect is using for two tasks: Speed measuring and MTI - Moving Target Indication

15. The Doppler- Effect is the apparent change in frequency or pitch when a sound source moves either toward or away from the listener, or when the listener moves either toward or away from the sound source. This principle, discovered by the German physicist Christian Doppler, applies to all wave motion.

16. The apparent change in frequency between the source of a wave and the receiver of the wave is because of relative motion between the source and the receiver. To understand the Doppler effect, first assume that the frequency of a sound from a source is held constant.

17. The wavelength of the sound will also remain constant
The wavelength of the sound will also remain constant. If both the source and the receiver of the sound remain stationary, the receiver will hear the same frequency sound produced by the source. This is because the receiver is receiving the same number of waves per second that the source is producing.

18. Doppler effect

19. Now, if either the source or the receiver or both move toward the other, the receiver will perceive a higher frequency sound. This is because the receiver will receive a greater number of sound waves per second and interpret the greater number of waves as a higher frequency sound.

20. Conversely, if the source and the receiver are moving apart, the receiver will receive a smaller number of sound waves per second and will perceive a lower frequency sound. In both cases, the frequency of the sound produced by the source will have remained constant.

21. For example, the frequency of the whistle on a fast-moving car sounds increasingly higher in pitch as the car is approaching than when the car is departing. Although the whistle is generating sound waves of a constant frequency, and though they travel through the air at the same velocity in all directions, the distance between the approaching car and the listener is decreasing. As a result, each wave has less distance to travel to reach the observer than the wave preceding it. Thus, the waves arrive with decreasing intervals of time between them.

22. fD =2·v /λ   fD - Doppler Frequency [Hz] λ - wavelength [m] v - speed of the wave-source [m/s]

23. This equation is valid, if the speed if the source of a wave is like the radial speed. But the airplane usually flies in another direction than the direction towards to the radar. Only the radial speed is then also measured. However, this is different from the aim speed so that the following equation is valid:

24. fD =2·v· cos α /λ   fD - Doppler Frequency [Hz] λ - wavelength [m] v - speed of the aircraft [m/s] α - angle between the direction of the transmitted/reflected signal and the direction of flight of the target

25. Derivation of the Doppler-frequency formula The phase shifting φ of an electromagnetic wave from the radar antenna to the aim and back results from the ratio of the covered distance and the wavelength of the transmitted energy multiplied with the scale of the full circle (2·π):

26. The phase shifting of the received signal

27. φ = 2r · 2π /λ φ - phase-difference between the transmitted and the received signal 2r - the distance: the way and the way back 2π - 360°: the period of an oscillation λ - wavelength of the transmitted energy

28. If the aim has the radial speed vr = d(r)/dt then the value of the phase changes to d(φ)/ dt =- 4π · vr /λ

29. This is equivalent to the Doppler- frequency fD according to: fD = (1/ 2π )· (d(φ) /dt)= (1/ 2π )· (- 4π · vr /λ )   | fD| = 2 · vr /λ = 2 · vr· ftx /c0 where: ftx - is the transmitters frequency c0 - is the speed of the light in vacuum vr - is the radial speed of the aim

30. This means: In practice the Doppler- frequency occurs twice at a radar
This means: In practice the Doppler- frequency occurs twice at a radar. Once on the way from the radar to the aim, and then for the reflected (and already afflicted by a Doppler-shift) energy on the way back.

31. Pulse-Pair Processing To distinguish a moving target of a fixed object with help of the Doppler frequency, at least two periods of the deflection must be compared with each other. Since the Doppler- frequency (few Hertz) is small relatively to the transmitted frequency (much Mega-Hertz), therefore a phase comparison is more easily to carry out than a direct frequency comparison technically.

32. The storage of a deflection is carried out in suitable memory media, in the past in special analogous vacuum memory tubes, later also with a chain of condensers (distance: digital, signal: analogous) and today only in digital memory cells.

33. Functional block circuit diagram of a coherent receiver

34. Fixed target suppression happens by the phase comparison of the echoes received by several pulse periods (pulse- pair processing). If the phase relationship is always equal, then there isn't any phase difference and the target will be suppressed. If the target has moved, the phase difference is unequally zero and the target will be shown on the screen.

35. To get the necessary frequency-reference for the phase-detector, a high correct coherent oscillator (called: „Coho”) is synchronized with the down converted on the IF- frequency transmitting pulse

36. Oscillogram of an output signal of a phase-detector

37. Echoes signals of fixed clutter have got the same amplitude pulse to pulse and can be cancelled: magenta: output of the phase-detector (actually period) green: output of the memory (delayed period) blue: cancelled video

38. The echo signal of a moving target at the output of the phase-detector changes it's value and also the polarity in every pulse period. A fixed cluttersignal will keep it's value and polarity in every pulse period.

39. A pulse period is stored in a memory
A pulse period is stored in a memory. This memory stage has got a memorycell for each rangecell and delays the whole scan for one pulse period (PRT). Both periods, the actually period and its predator, are led to an extractor. The output of this stage is the difference of both input-signals. Clutter with constant amplitude will be eleminated. Moving targets pass this stage. On this way the moving target produce an output signal and the fixed clutter don't do this.

40. COHERENT RADAR Concept of Coherence Non-coherent pulses with random phase from pulse to pulse

41. What is Coherent Radar? The transmitted pulse's of coherent radar have all defined phase angles to a reference. Whether a radar set is coherent or non-coherent always depend on the transmitter. As a transmitter different systems are used in radar.

42. Non-coherent Radar Processing One of the transmitting systems is the POT (Power Oscillator Transmitter) which is self oscillating. When such a device is switched on and off as a result of modulation by the rectangular modulating pulse, the starting phase of each pulse is not the same for the different successive pulses. The starting phase is a random function related to the start up process of the oscillator. Notice: Self oscillating transmitter gives random phase pulse to pulse and is not coherent!

43. Coherent Radar Processing Pulse to pulse phase coherence

44. Another transmitter-system is the PAT (Power-Amplifier-Transmitter)
Another transmitter-system is the PAT (Power-Amplifier-Transmitter). In this case, the high-power amplifier is driven by a highly stable continuous RF source, called the waveform generator. Modulating the output stage in response to the PRF does not affect the phase of the driver/RF source. Assuming the RF is a multiple of the PRF (as is normally the case), each pulse starts with the same phase.

45. Systems, which inherently maintain a high level of phase coherence from pulse to pulse, are termed fully coherent. Note that phase coherence is maintained even if the PRF and RF are not locked together (provided the RF source is phase stable). As stated, it is common practice to lock the PRF to the RF phase and this assumption makes it easier to understand the concept of coherence.

46. Notice: Low Power oscillator and amplifier give same phase pulse to pulse and are a coherent system! The most important benefit of this system is the ability to differentiate relatively small differences in velocity (which correspond to small differences in phase). This coherent target processing offers Doppler resolution/estimation and provides less interference and signal/noise benefits relative to non-coherent processing.

48. The block diagram on the figure illustrates the principle of fully coherent radar. The fundamental feature is that all signals are derived at low level and the output device serves only as an amplifier. All the signals are generated by one master timing source, usually a synthesiser, which provides the optimum phase coherence for the whole system. The output device would typically be a klystron, TWT or solid state. Fully coherent radars exhibit none of the drawbacks of the pseudo-coherent radars, which we studied in the previous section.

49. Functional Characteristics Duplexer The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.

50. Low Noise Preamplifier The Low-Noise Preamplifier (LNA) amplifies the very weak backscatter signals. The low noise characteristic is very important: all following amplifiers will amplify the added noise of the LNA! The amplifier has a gain of  dB. A higher gain would be possible, but this decreases the dynamic of the receiver.

51. Mixer Stage The function of the mixer stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.

52. Amplifier After conversion to the intermediate frequency, the signal is amplified in several IF-amplifier stages. Most of the gain of the receiver is developed in the IF-amplifier stages. The first IF- amplifier has got a wide bandwidth and suppress the influence of mirror-frequencies. The center frequency is relatively high, up to 450 MHz nominally. The overall bandwidth of the receiver is often determined by the bandwidth of the stages of the second IF amplifier. The center frequency is about 75 MHz nominally.

53. Power Amplifier In this system the transmitting pulse is caused with a small performance in a waveform generator. It is taken to the necessary power with a Power Amplifier. The Power Amplifier would typically be a klystron, Traveling Wave Tube (TWT) or solid state.

54. Mixer / Exciter The first stage of cascaded mixers
Mixer / Exciter The first stage of cascaded mixers. The function of this mixer stage is to modulate a prospective intermediate frequency (IF) with the transmitting signals waveforms. The I- (in-phase) and Q- (quadrature) signals from the Waveform Generator are defined signals for comparing with the backscatter in the receivers synchronous detector.

55. Waveform-Generator The Waveform-Generator generates the transmitting pulse in low- power. It generates the transmitting signal on an IF- frequency. It permits generating predefined waveforms by driving the amplitudes and phase shifts of carried microwave signals. These signals may have a complex structure for a pulse compression.

56. Phase Sensitive Detector The IF-signal is passed to a phase sensitive detector which converts the signal to base band, while faithfully retaining the full phase and quadrature information (I & Q- processing) of the Doppler signal.

57. Radar Signal Processor The signal processor is that part of the system which separates targets from clutter on the basis of Doppler content and amplitude characteristics. It generates plots and tracks from the video signals of the receiver.

58. Radarscope / Monitor The indicator presents to the observer a continuous, easily understandable, graphic picture of the position of radar targets. In recently radars the indicator would be a computerdisplay.

59. Blind Speed At the comparison of the echoes between two or more pulse periods the fall can appear, that the airplane flies with exactly this one radial speed, some a phase shifting of correct 360° causes. In accordance with the periodicity of the sine function this fall can appear even at all integral multiples of ±n · 360°.

60. The value of phase shifting is zero in these falls too
The value of phase shifting is zero in these falls too. The target isn't recognized as a moving target therefore. It flies with a so called blind speed. The blind speed is dependent on the transmitted frequency and on the pulse repetition frequency of the radar unit.

62. Blind speed in connection with the Doppler shift

63. If the Doppler frequency produced by a moving target is exactly the same as the PRF (fD = PRF) then “sampling” occurs at the same point on each Doppler cycle. It is as if the target were stationary. The same effect occurs if fD is an integer multiple of PRF. Hence targets with certain radial velocities tend to be invisible to an MTI pulse radar.

64. The blind speed is a radial speed of the airplane at which the phase shifting of the echo-signal has the value ±n · 360° between two pulse periods. With blind speeds moving targets are suppressed by a MTI system like ground clutters.

67. Doppler-Filter Practical moving target detectors on a fully-coherent radar equipped with monopulse processing involve a Doppler filter bank. The I and Q output from the phase-sensitive analogue/digital converter (ADC) are fed to a filter bank.

68. Doppler-Filter Block diagram of an MTI- Systems in a monopulse radar with a digital receiver

69. The output of the filters is followed by frequency-domain weighting to reduce filter sidelobe levels, and the magnitude of the output in each spectral band is computed. The whole MTI-wiring is carried out for three identically channels (Σ, ΔAz and ΔEl)

70. All Doppler filters shown in the figure are similar
All Doppler filters shown in the figure are similar. The filter works with help of the Puls-Pair-Method: a fixed target suppression happens by the phase comparison of the echoes received by at least two pulse periods. Doppler Filters reduce the influence of the Blind speeds too.

71. Doppler Filterbank Division of the Doppler frequency domain into N separate bands offers a very flexible approach towards discriminating against fixed and moving clutter. If moving clutter (such as that from weather or birds) appears with a non-zero mean Doppler shift, the thresholds at the outputs of the various Doppler filters may be raised accordingly.

72. Radar Time-Line with three short- range- pulse periods for MTI- processing.

73. Doppler Filter as Low Pass The basic principle incorporated in all MTI systems is that Doppler filter bank enables targets and clutter to be separated in the frequency domain. The output of these filters has separate thresholds which optimize detection for the relevant part of the frequency band concerned.

74. Ground clutter is shown mainly in the Zero Doppler filter
Ground clutter is shown mainly in the Zero Doppler filter. The Zero Doppler Filter may be designed as a digital low pass filter. It detects targets with a Doppler frequency corresponding to a radial speed less than 20 knots. The next filter detects targets with a Doppler frequency corresponding to a radial speed of less than 40 knots, like rain clutter or chaff.

75. The knot is a unit of speed equal to one nautical mile per hour, which is equal to exactly 1.852 km/h and approximately 1.151 mph. The abbreviation kn is preferred by members of the International Hydrographic Organization (IHO), which includes all major seafaring nations. However, the abbreviations kt (singular) and kts (plural) are also widely used. The knot is a non-SI unit accepted for use with the SI. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels one minute of geographic latitude in one hour.

76. Frequency response characteristics of Doppler Filters as Low Pass

77. The frequency response characteristics of a typical Doppler filter bank for the case N = 8.

78. Pseudo-coherent Radar A requirement for any Doppler radar is coherence; that is, some definite phase relationship must exist between the transmitted frequency and the reference frequency, which is used to detect the Doppler shift of the receiver signal. Moving objects are detected by the phase difference between the target signal and background clutter and noise components. Phase detection of this type relies on coherence between the transmitter frequency and the receiver reference frequency.

79. If the transmitter output stage is a self oscillating device, the pulse to pulse phase is random on transmission. In coherent detection, a stable CW reference oscillator signal, which is locked in phase with the transmitter during each transmitted pulse, is mixed with the echo signal to produce a beat or difference signal. Since the reference oscillator and the transmitter are locked in phase, the echoes are effectively compared with the transmitter in frequency and phase. This phase reference must be maintained from the transmitted pulse to the return pulse picked up by the receiver. Pseudo-coherent Radar sets are sometimes called: „coherent-on-receive”.

80. The principle of a pseudo-coherent radar.

81. Synchronizer The synchronizer supplies the synchronizing signals that time the transmitted pulses, the indicator, and other associated circuits. Modulator The oscillator tube of the transmitter is keyed by a high-power dc pulse of energy generated by this separate unit called the modulator. Tx-Tube The Tx-Tube is a self-oscillating tube generating high-power microwaves.

82. Duplexer The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver. Antenna The Antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.

83. Mixer stage The function of the mixer stage is to convert the received RFf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 op to 74 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.

84. IF-Amplifier After conversion to the intermediate frequency, the signal is amplified in several IF-amplifier stages. Most of the gain of the receiver is developed in the IF-amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF-stages. Mixer stage 2 The directional coupler provides a sample of the transmitter output on every pulse. This signal adjusts the STALO frequency via the AFC but more importantly, it adjusts the phase of the COHO, locking it to the phase reference from the non-coherent transmitter.

85. Automatic Frequency Control (AFC) As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (AFC) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal.

86. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF-frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF-signal. The output of the discriminator provides a control voltage to maintain the local oscillator at the correct frequency.

87. Stable Local Oscillator (StaLO) As the receiver is normally a super heterodyne, a stable local oscillator known as the StaLO down converts the signal to intermediate frequency. Most radar receivers use a 30 up to 74 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.

88. The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power. The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 up to 74 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.

89. Phase- sensitive detector The IF-signal is passed to a phase sensitive detector (PSD) which converts the signal to base band, while faithfully retaining the full phase and quadrature information of the Doppler signal. This means, the phase-sensitive detector produces a video signal. The amplitude of the video signal is determined by the phase difference between the COHO reference signal and the IF echo signals. This phase difference is the same as that between the actual transmitted pulse and its echo. The resultant video signal may be either positive or negative.

90. Signal processor The signal processor is that part of the system which separates targets from clutter on the basis of Doppler content and amplitude characteristics

91. Directional Coupler The directional coupler provides a sample of the transmitter output on every pulse. This signal adjusts the STALO frequency via the AFC but more importantly, it adjusts the phase of the COHO, locking it to the phase reference from the non-coherent transmitter (e.g. Magnetron). The phase synchronization of the COHO by means of a sample of the magnetron output is mandatory because there is no phase correlation between two successive RF pulses of the magnetron.

92. Coherent oscillator The Coherent Oscillator (COHO) provides a low-power continuous RF-energy. It enables the down conversion process into the phase sensitive detector, whilst maintaining an accurate phase reference. The COHO lock pulse is originated by the transmitted pulse. It is used to synchronize the COHO to a fixed phase relationship with the transmitted frequency at each transmitted pulse. The COHO takes over the phase of the transmitter tube and provides it to the receiver part of the system. This is the reason why the pseudo-coherent radar is also called “coherent on receive”.

93. Indicator The indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets.

94. Disadvantages of the pseudo-coherent radar The pseudo-coherent radar is a retired one today, but some older (or low-cost) radar sets are still operational. The disadvantages of the pseudo-coherent radar can be summarized as follows: • The phase locking process is not as accurate as a fully coherent system, which reduces the MTI Improvement factor. • This technique cannot be applied to frequency agile radar. Frequency change in a magnetron relies on the mechanical tuning of a cavity and it is essentially a narrow band device.

95. • It is not flexible and cannot easily accommodate changes in the PRF, pulse width or other parameters of the transmitted signal. Such changes are straightforward in fully coherent radar because they can be performed at low level. It is also impossible to perform FM modulation (which is mandatory for a pulse compression radar) with this type of system. • Second times around echoes are returns from large fixed clutter areas located a long distance from the radar. Because they originate from a large distance, such echoes are returned after a second magnetron pulse has been transmitted. However, they pertain to the first pulse transmitted by the magnetron. Such echoes are range ambiguous but, in addition, second time around clutter will not cancel. This is due to the fact that the phase locking of the COHO applies only to the last transmitted pulse.

96. END