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EC 6015 – Radar and Navigational Aids Electronics and Communication Engineering Seventh Semester (Regulations 2013) UNIT-1 Part A 1. Define threshold detection. [ N/D-16] If the receiver output is not of sufficient amplitude to cross the threshold, only noise is said to be present. This is called threshold detection.
2. Define minimum detectable signal. [ N/D-16] The weakest signal that can just be detected by the receiver is the minimum detectable signal.
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Part B 1. What are the different ranges of frequencies that radar can operate and give their applications? (10) [N/D-16] Operational radar in the past has been at frequencies ranging from about 100 MHz to 36 GHZ, which covers more than 8 octaves. IEEE standard Radar frequency Letter Band Nomenclature Band Designation HF VHF UHF L S C X K Ka V w mm
Nominal Frequency range 3 – 30 MHz 30 – 300 MHz 300 – 1000 MHz 1-2 GHz 2-4GHz 4 - 8 GHz 12 - 18 GHz 18-27 GHz 27- 40 GHz 40-75 GHz 75-110 GHz 110-300 GHz.
Specific frequency range for radar based on ITU assignments in region 2 138 – 144 MHZ 216 – 225 MHz 420-450 MHz, 850-942 MHz 1215-1400 MHz 2300 – 2500, 2700 – 3700MHz 5250-5925 MHz 13.4 – 14.0 GHz, 15.7 – 17.7 GHz 24.05 – 24.25 GHz, 33.4 – 36 GHz 59-64 GHz 76-81 GHz, 92-100 GHz. ( 126-142, 144-149, 231-235,238-248) GHz
Applications of radar: Radar has been employed to detect targets on the ground, on the sea, in the air, in space, and even below ground. The major areas of radar applications are briefly described below Military In air defense, it performs the functions of surveillance and weapon control Surveillance includes target detection, target recognition, target tracking. A missile system employs radar methods for guidance and fusing of the weapon. Remote sensing
All radar are remote sensors. However this term is used to imply the sensing of the environment. o Weather observation: It is a regular part of TV weather reporting. o Planetary observation: mapping of Venus beneath its visually opaque clouds o Short range below ground probing. o Mapping of sea ice to route shipping in an efficient manner.
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Air traffic Control
Radar have been employed around the world to safety control air traffic in the domain of airports and en route from one airport to another as well as ground vehicular traffic. Radars specifically dedicated to observing weather in the sector of airports which are called Terminal Doppler Weather Radar (TDWR)
Low enforcement and Highway safety: The radar speed meter is used for enforcing speed limits. Radars have been considered for making vehicles safety by warning of pending collision, actuating a air bag, people behind a vehicle ( or) in the side band zone. Ship safety:
Radar is found on ships and boats for collision avoidance and to observe navigation buoys especially when the visibility is poor.
Space:
Space vehicle have used radar for landing on the moon. Large ground based radars are used for the detection and tracking of satellite and other space objects.
Air craft safety and Navigation:
In dangerous wind shear to allow the pilot to avoid hazardous conditions and radio altimeter is also used to indicate the height of an aircraft above the terrain.
Other applications:
It has been used for oil and gas exploration. Entomologists and ornithologist have applied radar to study the movement of insects and birds which cannot be easily achieved by other means.
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2. Draw the block diagram of RADAR and explain the basic building blocks. (6) [N/D-16] The operation of the pulse radar may be described with simple block diagram as shown below
Power Modulator
RF pulse Antenna
Power Amplifier
Duplexer
RF pulse Low noise RF amplifier
Waveform generator
IF pulse
IF amplifier
Mixer
Matche d filter
Video Amplifier
Demodulator
Threshold Decision
Output
Block diagram of conventional pulse radar with a superheterodyne receiver
Transmitter : The transmitter may be a power amplifier such as klystron, travelling wave tube (or) transmitter amplifier. The transmitter also is a power oscillator such as magnetron. The magnetron oscillator may be widely used for pulse radars of modest capability. Features of Power amplifier: The power amplifier is preferred when high average power is necessary, when other than simple pulse waveforms are required (or) when good performance is needed. Operation:
The radar signal is produced at low power by a waveform generator, which is then input to the power amplifier (or) power oscillator. 4
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When a power amplifier ( or) power oscillator is used, it is turned on and off by a pulse modulator to generate pulse waveform. The output of the transmitter is delivered to the antenna by a waveguide or other form of transmission line, where it is radiated into space. Antenna can be mechanically steered parabolic reflectors, mechanically steered planar arrays, (or) electrically phase arrays. On transmit the parabolic reflector focuses the energy into a narrow beam and electrically steered phase array can rapidly change the direction of the antenna beam.
Duplexer: The duplexer allows a single antenna to be used on a time shared basis for both transmitting and receiving. It is a gaseous device that produces a short circuit at the input to the receiver when the transmitter is operating, so that the high power flows to the antenna not to the receiver. On reception, the duplexer directs the echo signal to the receiver and not to the transmitter. Receiver: The receiver is always superheterodyne. The input stage can be a low noise transistor amplifier The mixer and local oscillator (LO) converts the RF signal to an intermediate frequency (IF) where it is amplified by the IF amplifier. Matched filter: It is used to maximize the output peak signal to mean noise ratio. It also maximizes the detectability of weak echo signals and attenuates unwanted signal. Demodulator: IF amplifier is followed by a crystal diode which is traditionally called the demodulator. Its purpose is to assist in extracting the signal modulation from the carrier. The combination of IF amplifier, demodulator and video amplifier act as an envelope detector to pass the pulse modulation and reject the carrier frequency. Features of IF amplifier and Video amplifier:
It is designed to provide sufficient amplification or gain to raise the level of the input signal to a magnitude where it can see on a display such as cathode ray tube.
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Threshold decision:
At the output of a receiver a decision is made whether or not a target is present. The decision is based on the magnitude of the receiver output. If the output is large enough to exceed a predetermined threshold, the decision is that a target is present. If it does not cross the threshold, only noise is assumed to be present and false alarm occur due to noise crossing threshold.
Constant false alarm rate: The threshold level has to be varied adaptively in order to maintain the false alarm rate at a constant value. This is accomplished by a constant false alarm rate. Integration: Radar usually receives many echo pulse from a target. The process of adding these pulses together to obtain greater signal to noise ratio before the detection of decision is made is called integration.
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(16) [N/D-16] 3. Explain how system losses will affect on the Radar Range. System Losses: System losses denoted Ls (a number greater than one) is inserted in the denominator of the radar equation. It is the reciprocal of efficiency (number less than one). The two terms (loss and efficiency) are sometimes used interchangeably.
Microwave plumbing losses There is always loss in the transmission line that connects the antenna to the transmitter and receiver. In addition, there can be loss in the various microwave components, such as the duplexer, receiver protector, rotary joint, directional couplers, transmission line connectors, bends in the transmission lines, and mismatch at the antenna.
Transmission line Loss The transmission line generally is used for both transmission and reception the loss to be inserted in the radar equation is twice the one way loss. Flexible waveguides and coaxial lines can have higher losses than conventional waveguides. When practical transmitter and receiver should be placed close to the antenna to keep the transmission loss small.
Duplexer loss: The loss due to a gas duplexer that provides the receiver from high power of the transmitter is generally different o transmission and reception.
Antenna losses: It should be accounted for in the antenna gain, shaping of the antenna pattern. for example, to provide a CSC pattern, result in a loss that is included as an additional lowering of the antenna gain rather than as a system loss.
Beam shape loss: The train of pulses returned from a target by a scanning antenna is modulated in amplitude by the shape of the antenna beam. Only one out of pulses has the maximum antenna gain G. assume a constant amplitude pulse train as determined by the maximum antenna gain, and then add a beam shape loss to the total system losses in the radar equation.
Scanning loss: When the antenna scans rapidly enough, relative to the round trip time of the echo signal, the antenna gain in the direction of the target on transmit might not be the same as that on receive. This result in an additional loss called scanning loss.
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RADOME: Loss introduced by a radome will depend on the type and the radar frequency. Air supported radomes can have lower loss; the loss with dielectric space-frame radomes can be higher.
Phased Array Losses: Some phased array radars have additional transmission line losses due to the distribution network that connects the receiver and transmitter to each of the many elements of the array.
Signal processing losses: Signal processing can introduce loss that has to be tolerated. The factors below can introduce significant loss that has to be accounted. o Nonmatched filter: There can be from 0.5 to 1.0 dB of loss due to practical rather than ideal matched filter. o Constant false alarm rate (CFAR) receiver: This loss can be more than 2.0 dB depending on the type of CFAR. o Automatic integrators: The binary moving window detector, for example, can have a theoretical loss of 1.5 to 2.0 dB. Other automatic integrators might have more or less loss. o Threshold level: A threshold is established at the output of the radar receiver to achieve some specified probability of false alarm or average alarm time. Depending on how accurately the threshold can be set and maintained, the loss might be only a small fraction of dB. o Limiting loss: Some radars might use a limiter in the radar receiver. An example is pulse compression processing to remove amplitude fluctuations in the signal, the so called Dicke-fix, an electronic counter- counter measure to reduce the effects of impulsive noise employs a hard limiter. o Straddling loss: A loss called the range straddling loss, occurs when range gates are not centered on the pulse or when, for practical reasons, they are wider than optimum. o Sampling loss: The difference between the sampled value and the maximum pulse amplitude represents a sampling loss.
Losses in Doppler processing Radar: 8
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For example, in some medium prf pulse Doppler radars, eight different waveforms each with different prf, might need to be transmitted so as to obtain at least three dwells in order to solve the range ambiguities. MTI Doppler processing also introduces loss due to the shape of the Doppler filters if the target velocity does not correspond to the maximum response of the Doppler filter. o Collapsing loss: If the radar were to integrate additional noise samples along with signal-plus noise pulses, the added noise would result in a degradation called collapsing loss. o Operator loss: Most modern high performance radars provide the detection decision automatically without intervention of a human operator. o Equipment degradation: It is not uncommon for radars operated under field conditions to have lower performance than when they left the factory. This loss of performance can be recognized and corrected by regularly testing the radar, with built in test equipment that automatically indicates when equipment deviates for specifications.
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EC 6015 – Radar and Navigational Aids Electronics and Communication Engineering Seventh Semester (Regulations 2013) UNIT-2 Part A 1. What are the methods used to reduce blind speeds? Methods to reduce blind speeds are Operate the radar at long wavelengths. Operate with a high pulse repetition frequency Operate with more than one pulse repetition frequency Operate with more than one rf frequency.
[ N/D-16]
2. What is eclipsing loss? [ N/D-16] Since pulse Doppler radar cannot receive when it is transmitting, the high duty cycle and result in loss if the echo signal arrives when a pulse is being radiated and the receiver is gated off. This is called eclipsing loss, and can be anything between zero and a large value depending on the exact location of the received echo pulse within the timing of the transmission.
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Part B 1. Explain in the detail digital MTI processing. (16) [ N/D-16] Digital MTI processing:
Most of the basic theoretical aspects of MTI filter design were formulated when the delay lines were analog devices. Sophisticated MTI Doppler filter were difficult to implement with analog method. So it was rare for MTI radar to employ more than two analog delay lines in a delay line canceler. The rapid development of digital technology beginning in the early 1970s however allowed the delays to be obtained by storing digital words in a memory for whatever length of time was required. Digital Doppler filters with many delay lines are now practical so that sophisticated filters can be readily obtained when a large number of pulses are available for processing.
Advantages of digital MTI processing: Unwanted changes in the delay times of acoustic delay lines due to temperature changes are eliminated by the accurate timing of digital method. Digital MTI is more stable and reliable than analog MTI, and requires less adjustments during operation in the field. Digital processors can be made reprogrammable. There is no problem in making the delay time in the digital memory synchronous with the radars prf, something difficult to do with acoustic delay lines. Greater dynamic range can be obtained than was possible with acoustic delay lines. Although digital processing does not have the serious weakness associated with analog delay lines. Blind phases I and Q channels: Single phase detector and filter channel are present in MTI radar. With the single phase detector and single processing channel, there is a loss when the Doppler shift signal is not sampled at the peak positive and negative values of the sine wave. Blind phase: When the phase between the Doppler signal and the sampling at the prf results in a loss, it is called a blind phase. The blind phase is different from the blind speed. Blind speed: A blind speed occurs when the sampling pulse appears at the same point in the Doppler cycle at each sampling time. Block diagram of a digital MTI Doppler signal processor:
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The block diagram of a digital MTI signal processor with I and Q channels is shown in figure. The signal from the IF amplifier is split into two channels. The phase detector in each cahnnel extract the Doppler shifted signal. In the n channel the Doppler signal is represented as Ad cos(2πfdt +Φ0) and in the Q channel it is the same except the sine replaces the cosine.
A/D converter: The signals are then digitized by the analog to digital (A/D) converter. A sample and hold circuit is needed ahead of the A/D converter for more effective digitizing. (I2 +Q2)1/2: The magnitude of the Doppler signal is obtained by taking the square root of (I2 +Q2). Sometimes, for simplicity, the sum of the magnitudes of the two channels + is taken or the greater of the two channels might be used instead. Methods for implementing A/D converter: Depending on the speed and number of the bits required. Since there are two channels ( the I and the Q), sampling in each can be at one-half of the Nyquist rate, which generally makes the implementation of the A/D converters simpler. Limitations on the improvement factor due to the A/D converter: All analog signals that lie within the same quantization step of the A/D converter are represented by the same digital word. 3
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Since the rms value of the noise accompanying the signal is usually greater than the quantization step of the A/D converter, the digital word can change slightly from pulse to pulse in a noise like manner. Thus the quantization of the analog signal result in noise or uncertainty called quantization noise, which can limit the improvement factor. The improvement factor due to quantization noise as
Iq = 20 log [2N – 1) * (dB) Where N- number of bits. This is approximately 6 dB per bit. Thus a 10 bit a/D theoretically limits the improvement factor to about 60 dB. Dynamic range: The dynamic range is the maximum signal to noise ratio that can be handled by an A/D converter without saturation. The noise level relative to the quantization step affects the dynamic range. The available dynamic range is given by
Where N- number of bits in the A/D converter, k = rms noise level divided by the quantization level. The larger k is, the less the dynamic range. A value of k is less than one means that the receiver noise at the input to the a/D converter is less than the quantization noise, which results in loss of the delectability Other limitations: There are several practical conditions that need to be considered when good MTI performance is required. Errors and reduced performance can be due to Other than a 90⁰ phase difference between the I and Q reference signals. Gain and phase imbalance in the two channels. Timing jitters in the sample and hold circuit. Nonlinearity in the A/D Range straddling loss due to the sampling not being at the peak of the output of the matched filter.
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2. Explain in detail about conical scan and sequential lobing. (16) [ N/D-16] The monopulse tracker utilized multiple fixed beams to obtain the angle measurement. It is also possible to time share a single antenna beam to obtain the angle measurement in a sequential manner time sharing a single antenna beam is simpler and use less equipment than simultaneous beams, but it is not as accurate. Sequential lobbing
Two lobes are required to track in each axis, each lobe must be sequentially switched four pulses are required. The radar measures the returned signal levels. The voltages in the two switched position should be equal.
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The first U.s. army angle tracking air defense radar in the 1930s switched a single beam between squinted angular positions to obtain an angle measurement. This is called lobe switching, sequential switching or sequential lobing. Angular displacement:
The difference in amplitude between the voltages obtained in the two switched position is a measure of the angular displacement of the target from the switching axis. When the echo signals in the two beam positions are equal the target is an axis and its direction is that of the switching axis.
Conical Scan radar The beam might be switched right, up left, down and so forth. After living with this type of scanning for a while, it must have become obvious that the four horns and RF switches could be replaced by a single feed that radiated a single beam squinted off axis. The sequential feed could then be continuously rotated to obtain angle measurement in two coordinates. This is a conical scan radar.
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Conical Scan: The basic concept of conical scan (or) con-scan is shown in figure. Squint angle: The angle between the axis of rotation and the axis of the antenna beam is the squint angle. Consider a target located at position A.
Conical Scan frequency: The amplitude of the echo signal will be modulated at a frequency equal to beam rotation frequency is called the conical scan frequency. The amplitude of the modulation depends on the angular distance between the target direction and the rotation axis. 7
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Block Diagram Description: A block diagram of the angle tracking portion of conical tracking radar is shown below. The antenna is mounted so that it can be mechanically positioned in both azimuth and elevation by separate motors.
Nutuating feed: The parabolic antenna feed can be a rear- feed design for mechanical convenience, when the feed is designed to maintain the plane of polarization as it rotates about the axis, it is called a nutuating feed.
The nutuating feed is one which causes the plane of polarization to rotate. It is preferred over the rotating feed since a rotating polarization can cause the amplitude of the target echo signal to change with time even for a stationary target on axis.
A change in amplitude caused by echo signals can result in degraded angle tracking accuracy. The nutuating feed is usually more complicated, however than the rotating feed. If the antenna is small enough, it might be easier to mechanically rotate the tilted reflector rather than the feed, thus avoiding the problems of either a rotary joint or flexible RF joint for the nutuating feed.
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Receiver: The receiver is a super heterodyne except for features related to the conical scan tracking. The error signal is extracted in the video after the second detector. Range gate: Range gating eliminates noise and excludes all targets other than the desired target. The error signal from the range gate is compared with both the elevation and azimuth reference signals in the angle error detectors which are phase sensitive detectors. Phase detectors: The phase sensitive detector is a nonlinear device in which the input signal is mixed with a reference signal. The magnitude of the dc output from the angle error detector is proportional to the angle error, and its sign indicates the direction of the error. The angle error outputs are amplified and used to drive the antenna elevation and azimuth servo motors. Automatic Gain Control (AGC):
As with monopulse radar, AGC is employed in the conical scan radar. It has the purpose of maintaining constant angle error sensitivity inspite of amplitude fluctuations or change of the echo signal due to changes in range constant angle error sensitivity is required to provide stable tracking. AGC is also important for avoiding saturation by large signals which could cause the loss of the scanning modulation and the accompanying error signal. The gain of the AGC loop at the conical scan frequency should be low so that the error signal will not be suppressed by the AGC action.
Optimum squint angle: The greater the squint angle in the conical- scan tracker the greater will be slope of the error signal around boresight and the more accurate will be the angle measurement. The maximum slope occurs at a squint angle equal to 0.41 of the half power bandwidth.
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UNIT-3 Part A 1. What is CFAR loss? [ N/D-16] Constant false alarm rate (CFAR) is a property of threshold or gain control devices that maintain an approximately constant rate of false target detections when the noise, and/or clutter levels, and/or ECM ( electronic counter measures) into the detector are variable.
2. What is radio sonde?
[ N/D-16]
A radiosonde is a battery-powered telemetry instrument package carried into the atmosphere usually by a weather balloon that measures various atmospheric parameters and transmits them by radio to a ground receiver. Modern radiosondes measure or calculate the following variables: altitude, pressure, temperature, relative humidity, wind ( both wind speed and wind direction) cosmic ray readings readings at high altitude and geographical position ( latitude/ longitude). Radiosondes measuring ozone concentration are known as ozonesondes.
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Part B 1. Explain in detail about RADAR Signal Management. (16) [ N/D-16]
Signal management includes everything with the waveforms and their processing that is required for radar to do its job of detecting and locating targets and determining something about the nature. Signal management starts with the design of signal waveform and its radiation in space, the collection by the receiver of echo signals reflected from targets and other objects, the use of signal processing to extract the desired signal and reject undesired echoes, the use of data processing to extract information about the detected signals, and keeping within the resources and constraints that affect signals and their management. Component parts of Radar Signal management:
Signal Processing: This is the processing for the purpose of detecting desired echo signals and rejecting noise, and undesired echoes from clutter. It includes the following Matched filter To maximum the signal to noise ratio at the output of the radar receiver, and thus maximize delectability of echo signals.
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Detector I Integrator The means for processing in a convenient and efficient manner the number of pulse receiver from a target so as to take full advantage of the total signal energy received from a target. Clutter reduction To eliminate or reduce unwanted clutter by one or more methods of which filtering of moving targets based on the Doppler frequency shift is the most important. CFAR: CFAR is used to maintain a constant false alarm rate at the output of the threshold detector when the radar cannot eliminate unwanted echoes. Electromagnetic compatibility The elimination of interference from other radars and other electromagnetic radiations that can enter the radar receiver is referred to as electromagnetic compatibility. Electronic counter countermeasure (ECCM) In military radar, these methods employed to reduce or eliminate the effectiveness of jamming, deception, and other hostile electronic active and passive measures whose purpose is to degrade radar performance. Threshold detection The decision as to whether the output of the radar is a desired signal. Data processing: These are the processes that take place after the detection of the desired signals for the purpose of acquiring further information about the target. Target locating: In range, angle and sometimes radial velocity. Location information is not generally thought of as either signal processing or data processing. Target trajectory: The recognition of the type of target is being viewed by the radar. It might include the recognition of aircraft from birds, one type of aircraft or ship from another, recognition of various types of weather, and information about the land and sea environment. Weapon control: In military systems, the use of the radar output for the control and guidance of weapons. Waveform design: The selection of the waveform depends on what is required of the radar for detection in noise, clutter, interference, and electronic counter – counter measures as well as for the extraction of information from the radar signal. Waveform design will affect the signal and data processing.
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Antenna: This is not just for radiating and collecting radar signals, but is the means by which angle information is obtained and by which the radar coverage is achieved. The antenna can act as a spatial filter that can affect the spectral properties of wideband signals. Resources of signal management: The radar engineer has available the following resources for pursuing the management of signals and extraction of information. Energy: Sufficiently large transmitted energy is important for detection of weak signals in noise at long range for obtaining accurate radar measurements. Bandwidth: This is the classical measure of information and is especially important for accurate range measurement and the temporal resolution of targets. Time: Time is necessary for accurate measurement of the Doppler frequency. Space: This applies to the physical aperture area required for an antenna. The lager the antenna aperture, the greater the echo energy at the receiver.
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2. Discuss in detail about Linear Beam Power Tubes. (16) [ N/D-16]
In the linear beam tubes, the electrons emitted from the cathode are formed into a long cylindrical beam that receives the full potential energy of the electric field before the beam enters the RF interaction region. Example of Linear wave tube: The klystron, traveling wave tube, klystron and extended interaction amplifier are the examples of linear beam tubes.
The last two are basically hybrid devices that combine the technology of the klystron with the RF structure of the TWT. An axial magnetic field is used in the linear- beam tubes to confine the electron beam and keep electrons from hitting the RF structure. Linear beam tubes can produce much higher power than other power sources. Klystrons are capable of more than a megawatt of average power. High power is result in part, of their larger size and high voltages. A sketch of the principal parts of klystron is shown below.
At the left is the cathode which emits a stream of electrons that is formed into a narrow cylindrical beam by the electron gun. The electron gun consists of other beam control electrode to provide a means for turing the beam on and off to generate pulses, and the anode. The multiple RF cavities, which corresponds to the LC resonant circuits of conventional lower frequency amplifiers , are at a note potential electrons are
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not intentionally collected by the anode, as in some other tubes: instead they are removed by the collector electrode after the beam has given up its RF energy to the output RF cavity. Operation: The RF input signal is applied across the interaction gap of the first cavity. Those electrons arrive at the gap when the input signal voltage is maximum experience a voltage greater than those electrons which arrive at the gap when the input is minimum Thus the electrons that see the peak of the sine wave are speeded up and those that trough are slowed down. The process whereby some electrons are speeded up and others slowed down is called velocity modulation of the electron beam. In the drift space, electrons that are speeded up during the peak of one cycle catch up with those slowed down during the previous cycle. The result is that the electrons of the velocity- modulated beam become bunched or density modulated, after travelling through the drift space. A klystron usually has one or more appropriately placed intermediate cavities to enhance the bunching of the electron beam, which increases the gain. If the interaction gap of the output cavity is placed at the point of maximum bunching, power can be extracted from the density-modulated beam. The gain of the klystron might be 15 to 20 dB per stage when synchronously tuned, so that a fourcavity klystron might provide over 50 dB gain. After the bunched electron beam delivers its RF power to the output cavity, the energy of the electron beam that remains is dissipated when the spent electrons are removed by the collector. The energy dissipated by the collector is the energy lost and reduces the efficient of the tube. If the collector is insulated from the body of the tube and a negative voltage is applied to the collector, the electrons in the spent beam will have lower kinetic energy so that less heat is produced when they impact upon the collector. This results in an increase in the efficiency of the tube. Figure shows a single stage collector, but both the klystron and TWT usually employ multiplestage depressed collectors for greater efficiency. The multiple stages are intermediate voltages, which allow catching the spent electrons at a voltage near optimum. Bandwidth of a klystron: The frequency of a klystron is determined by its resonant cavities. When all are turned to the same frequency, the gain the tube is high, but the bandwidth is narrow, usually a fraction of one percent for a tube of modest power output. This is synchronous tuning. To maximize the klystron’s efficiency the next to last cavity is tuned upward in frequency and is outside the pass band. Although the gain is reduced by about 10 dB, the improved electron bunching results in greater efficiency and is 15 to 25 percent more output power. Broad banding of a multi cavity klystron may be accomplished by stagger tuning cavities, similar to the method for broad banding a conventional multistage IF amplifier. When stagger tuned it has a 77 MHZ bandwidth and a gain of 44 dB.
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Theory shows that bandwidth of klystron can be significantly increased by increasing its power and its beam preveance.
Frequency changing to tuning:
Conventional narrow band klystron may have their frequency changed mechanically over a relatively wide frequency range. The individual cavities of a klystron be changed in frequency by having a flexible wall in the resonant cavity, by a movable cavity element in the cavity or by a sliding contact movable cavity wall.
Channel tuner mechanism: It avoids the problem of frequency tracking of the resonant cavities by pre tuning the cavities, generally at the factory. The tuning information is stored mechanically within the tuner mechanism. When a particular frequency is selected, the tuner mechanism provides the correct tuner position for each cavity to furnish the desired klystron frequency response. The tuning plungers can be actuated manually or remotely by push buttons and a servomotor. The frequency can be changed in seconds. Power: Some of the highest power radar, transmitters have used klystrons. The ability of a klystron to produce higher power than other microwave power sources is, in part, due to its geometry. The region of beam formation, RF interaction, and beam collection are separate. Efficiency: RF efficiency ( percent) = 90 – 20 * micro preveance. Reliability and life: High power transmitters employing power vacuum tubes have sometimes had the unwarranted reputation for poor reliability and short life. An S- Band klystron: It was a six-cavity tube tunable from 2.7 to 2.9 GHz, the frequency band reserved for air- traffic control radar. It had a peak power of from 0.5 to 2 MW, average power of from 0.5 to 3.5 KW, 50 dB gain, 45% efficiency, and one – dB bandwidth of 39 MHz. Travelling Wave Tubes: Like the klystron, the travelling wave tube is also a linear beam tube with the cathode, RF circuit, and collector separated from one of another. The klystron and TWT were invented at different times in different parts of the world, but they are similar to one another. there is continuous interaction of the electron beam and the RF field over the entire length of the propagating strut are of the traveling wave tube. In the klystron, on the other hand the interaction occurs only at the gaps of a relatively few resonant cavities. The chief characteristic of a TWT is that, it has wide bandwidth. The major parts of a TWT are indicated below. Both the TWT and the klystron employ the principle of velocity modulation to cause the electron beam current to be periodically bunched.
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The electron beam passes through the RF interaction circuit known as the slow wave structure or periodic delay line. Rabit-ear Oscillation: In some travelling wave tubes with coupled cavity circuits, oscillations appear for an instant during the turn on and turn-off portions of the pulse. These are called rabit-ear oscillation because of their characteristic appearance when the RF envelope of the pulse waveform is displayed visually on a CRT. Hybrid variants of the klystron: By combining the various features of the klystron and the traveling wave tube, an RF power source can be obtained which has bandwidth, efficiency, and gain flatness better than either the conventional klystron or TWT. This is achieved by replacing one or more of the klystron cavity circuits used in traveling wave tubes. There have been three variants of the klystron in which this is done; the Twystron, the extended interaction klystron, and clustered cavity klystron.
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UNIT-4 Part A 1. What is kickback noise?
[ N/D-16]
It is basically the noise from the switching first stage on the input of the comparator. If the output of the first stage swings quickly in large range, it will make a glitch at the comparator inputs. The effects is worst if they are driven by high impedance (in switched cap circuit for instance). Then, you get an offset.
2. Define Navigation Navigation is the art of directing the movements of craft from one point to another along a desired path.
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3. What is the need of a Chronometer? With the help of Chronometer, the navigator was able to determine his longitude by noting the transit time of heavenly bodies.
4. What is the need of Adcock direction finders? The Adcock direction finders are designed to eliminate polarization errors by dispensing with the horizontal members.
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5. What are the four methods of navigation? Navigation by pilotage Celestial or astronomical navigation Navigation by dead –reckoning Radio navigation
6. Define astronomical navigation Celestial navigation is accomplished by measuring the angular position of celestial bodies.
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7. Define navigation by dead reckoning The position of the craft at any instant of time is calculated from the previously determined position, the speed of its motion with respect to earth along with the direction of its motion and the time elapsed.
8. What is the important source of antenna effect? The important source is the asymmetry of the loop antenna with respect to the ground.
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9. How the antenna effect is minimized? To minimize the antenna effect, the centre of the loop is earthed and its output is thereby balanced.
10. Give the disadvantage of loop direction finder. The loop is small enough to be rotated easily. This results in a small signal pickups. To facilitate manual operation, the loop is located near the receiver.
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11. What are the errors arising in direction finders? Errors due to abnormal polarization of the incoming wave Errors due to abnormal propagation Site errors Instrumental errors
12. What are the two types of radio ranges in use? Low frequency four course radio range VHF Omni directional radio range
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13. What are the sources of errors in VOR system? Ground station and aircraft equipment Site irregularities Terrain features Polarization
14. Define LORAN LORAN is Long Range Navigational Aid and is a pulse system. The ground station transmit a train of pulses with fixed time relation between them and at the receiver, these pulses are identified and the delay between them is measured on a cathode ray oscilloscope
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Part B 1. Explain in detail about Adcock Direction Finders and its advantages over loop antenna.
(8) [ N/D-16]
Adcock antenna: The Adcock antenna is an antenna array consisting of four equidistant vertical elements which can be used to transmit or receive directional radio waves. The Adcock array was invented and patented by British engineer Frank Adcock in 1919, and has been used for a variety of applications, both civilian and military, ever since Although originally conceived for receiving low frequency (LF) waves, it has also been used for transmitting, and has since been adapted for use at much higher frequencies, up to ultra high .frequency (UHF). In the early 1930s, the Adcock antenna (transmitting in the LF/MF bands) became a key feature of the newly created radio navigation system for aviation. The low frequency range (LFR) network, which consisted of hundreds of Adcock antenna arrays, defined the airways used by aircraft for instrument flying. The LFR remained as the main aerial navigation technology until it was replaced by the VOR system in the 1950s and 1960s. The Adcock antenna array has been widely used commercially, and implemented in vertical antenna heights ranging from over 130 feet (40 meters) in the LFR network, to as small as 5 inches (13 cm) in tactical direction finding applications. Radio direction finding: Frank Adcock originally used the antenna as a receiving antenna, to find the azimuthal direction a radio signal was coming from in order to find the location of the radio transmitter; a process called radio direction finding. Prior to Adcock's invention, engineers had been using loop antennas l to achieve directional sensitivity. They discovered that due to atmospheric disturbances and reflections, the detected signals included significant components of electromagnetic interference and distortions: horizontally polarized radiation contaminating the signal of interest and reducing the accuracy of the measurement. Adcock—who was serving as an Army officer in the British Expeditionary force in wartime France at the time he filed his invention—solved this problem by replacing the loop antennas with symmetrically interconnected pairs of vertical monopole or dipole antennas of equal length. This created the equivalent of square loops, but without their horizontal members, thus eliminating sensitivity to much of the horizontally polarized distortion. The same principles remain valid today, and the Adcock antenna array and its variants are still used for radio direction finding. A radio direction finder (RDF) is a device for finding the direction, or bearing, to a radio source. The act of measuring the direction is known as radio direction finding or sometimes simply direction finding (DF). Using two or more measurements from different locations, the location of an unknown transmitter can be determined; alternately, using two or more measurements of known transmitters, the location of a vehicle can be determined. RDF is widely used as a radio navigation system, especially with boats and aircraft.
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RDF systems can be used with any radio source, although the size of the receiver antennas are a function of the wavelength of the signal; very long wavelengths (low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless very useful for marine navigation as they can travel very long distances and "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres. For aerial use, where the horizon may extend to hundreds of kilometres, higher frequencies can be used, allowing the use of much smaller antennas. An automatic direction finder, often capable of being tuned to commercial AM radio transmitters, is a feature of almost all modern aircraft.
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2. Explain in detail about VOR Receiving Equipment.
(8) [ N/D-16] VOR receiving equipment: There are many different types of airborne equipment of different types of different degrees of complexity available. However all equipments have the following parts viz an antenna, a control box, a receiver, and navigation circuits. Antenna: The antenna is a v-type dipole antenna. The control box contains an ON-OFF switch, frequency selector or tuner, and an aural volume control. The volume control regulates only the intensity of the signal going into the headset or the loudspeaker. Receiver: The receiver is a conventional superhetrodyne receiver. The navigation circuits take the signals from the receiver, and measure the phase angle difference between the reference signal and the variable signal. As the phase angle difference is defined fixed amount for each radial, it is therefore possible to determine the bearing of the aircraft from the VOR beacon, and this information can be presented visually. Similarly, if the equipment can be adjusted to a desired bearing and indicate the relationship of the aircraft to the bearing and when the aircraft has to reach the bearing, it is possible to preset tracks then fly to and continue along them. The visual indicators comprise a manually operated Omni bearing selector, a deviation indicator, and a TO / FROM indicator, and these are normally combined in one instrument known as an Omni bearing indicator. The information available from the navigation circuits is presented on the deviation indicator, and a TO / FROM indicator with relation to the setting of the Omni bearing selector. Information derived from the VOR may also be presented on a radio magnetic indicator. A block diagram of the airborne equipment receiver is shown in the figure. The VOR enables a pilot to select, identify and locate a line of position from a particular VOR beacon. The information can be obtained.
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3. Explain in detail radar direction determination.
The angular determination of the target is determined by the directivity of the antenna. Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined. The accuracy of angular measurement is determined by the directivity, which is a function of the size of the antenna. Radar units usually work with very high frequencies. Reasons for this are: quasi – optically propagation of these waves. High resolution (the smaller the wavelength, the smaller the objects the radar is able to detect). Higher the frequency, smaller the antenna size at the same gain. The Electromagnetic waves almost behave like light beams and well can be calculated in accordance with optical rules. This isn't surprising either since light also only is regarded as an electromagnetic wave. The difference only consists in the frequency which is much higher at the light than at electromagnetic waves in the radar frequency range.
•
The quasi-optically sight is a little further than the visual view because of this one at deeper frequencies more effective diffraction. Well, the radar horizon lies far away than the visual horizon. Obstacles on the way there affect also differently. A couple of trees affect the visual sight fatally while electromagnetic waves can possibly penetrate this obstacle. An observer location in larger height is just as effectively as a higher one antenna location for the electromagnetic waves in the optics since by the bend of the earth's surface flat objects disappear very fast behind the horizon.
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Radar line of sight • •
The electromagnetic waves follow the rules of the optics in higher frequencies (>100 MHz). All radar unit systems almost without exception work in this frequency domain. Well, therefore the wave fronts also propagate to quasi-optical rules. The earth's curvature may prevent the radar seeing a target within the maximum range given by the radar range equation. Therefore results a „dead zone” for every radar system in which one targets can't be detected.
True bearing The True Bearing (referenced to true north) of a radar target is the angle between true north and a line pointed directly at the target. This angle is measured in the horizontal plane and in a clockwise direction from true north. (The bearing angle to the radar target may also be measured in a clockwise direction from the centerline of your own ship or aircraft and is referred to as the relative bearing.)
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The antennas of most radar systems are designed to radiate energy in a one-directional lobe or beam that can be moved in bearing simply by moving the antenna. As you can see in the Figure 2, the shape of the beam is such that the echo signal strength varies in amplitude as the antenna beam moves across the target. In actual practice, search radar antennas move continuously; the point of maximum echo, determined by the detection circuitry or visually by the operator, is when the beam points direct at the target. Weapons-control and guidance radar systems are positioned to the point of maximum signal return and maintained at that position either manually or by automatic tracking circuits.In order to have an exact determination of the bearing angle, a survey of the north direction is necessary. Therefore, older radar sets must expensively be surveyed either with a compass or with help of known trigonometrically points. More modern radar sets take on this task and with help of the GPS satellites determine the north direction independently.
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Transfer of bearing information •
The rapid and accurate transmission of the bearing information between the turntable with the mounted antenna and the scopes can be carried out for servo systems and counting of azimuth change pulses. Servo systems are used in older radar antennas and missile launchers and works with help of devices like synchro torque transmitters and synchro torque receivers. In newer radar units we find a system of azimuth- change pulses (ACP). In every rotation of the antenna a coder sends many pulses, these are then counted in the scopes. Newer radar units work completely without or with a partial mechanical motion. These radars employ electronic phase scanning in bearing and/or in elevation (phased- array-antenna).
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4. Explain in detail about sparkles in thermometer code and metastability in flash ADC.
(16) [ N/D-16]
Flash ADCs are made by cascading high-speed comparators. Figure 1 shows a typical flash ADC block diagram. For an N-bit converter, the circuit employs 2N-1 comparators. A resistive-divider with 2N resistors provides the reference voltage. The reference voltage for each comparator is one least significant bit (LSB) greater than the reference voltage for the comparator immediately below it. Each comparator produces a 1 when its analog input voltage is higher than the reference voltage applied to it. Otherwise, the comparator output is 0. Thus, if the analog input is between VX and VX , comparators X1 through X4 produce 1s and the remaining comparators produce 0s. The point where the code changes from ones to zeros is the point at which the input signal becomes smaller than the respective comparator reference-voltage levels. 4
5
Figure 1. Flash ADC architecture. If the analog input is between VX and VX , comparators X1 through X4 produce 1s and the remaining comparators produce 0s. 4
5
This architecture is known as thermometer code encoding. This name is used because the design is similar to a mercury thermometer, in which the mercury column always rises to the appropriate temperature and no mercury is present above that temperature. The thermometer code is then decoded to the appropriate digital code. The comparators are typically a cascade of wideband low-gain stages. They are low gain because at high
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frequencies it is difficult to obtain both wide bandwidth and high gain. The comparators are designed for low-voltage offset, so that the input offset of each comparator is smaller than an LSB of the ADC. Otherwise, the comparator's offset could falsely trip the comparator, resulting in a digital output code that is not representative of a thermometer code. A regenerative latch at each comparator output stores the result. The latch has positive feedback, so that the end state is forced to either a 1 or a 0. Given these basics, some adjustments are needed to optimize the flash converter architecture.
Sparkle Codes Normally, the comparator outputs will be a thermometer code, such as 00011111. Errors can cause an output like 00010111, meaning that there is a spurious zero in the result. This out-of-sequence 0 is called a sparkle, which is caused by imperfect input settling or comparator timing mismatch. The magnitude of the error can be quite large. Modern converters like the MAX109/MAX104 employ an input track-and-hold in front of the ADC along with an encoding technique that suppresses sparkle codes. Metastability When the digital output from a comparator is ambiguous (neither a 1 nor a 0), the output is defined as metastable. Metastability can be reduced by allowing more time for regeneration. Gray-code encoding, which allows only 1 bit in the output to change at a time, can greatly improve metastability. . Thus, the comparator outputs are first converted to gray-code encoding and then later decoded to binary, if desired. Another problem occurs when a metastable output drives two distinct circuits. It is possible for one circuit to declare the input a 1, while the other circuit thinks that it is a 0. This can create major errors. To avoid this conflict, only one circuit should sense a potentially mestatable output. Input Signal-Frequency Dependence When the input signal changes before all the comparators have completed their tasks, the ADC's performance is adversely impacted. The most serious impact is a drop-off in signal-to-noise ratio (SNR) plus distortion (SINAD) as the frequency of the analog input frequency increases. Measuring spurious-free dynamic range (SFDR) is another good way to observe converter performance. The "effective bits" achieved by the ADC is a function of input frequency; it can be improved by adding a track-and-hold (T/H) circuit in front of the ADC. The T/H circuit allows dramatic improvement, especially when input frequencies approach the Nyquist frequency, as shown in Figure 2 (taken from the MAX104 data sheet). Parts without T/H show a significant drop-off in SFDR.
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Clock Jitter SNR is degraded when there is jitter in the sampling clock. This becomes noticeable for high analog-input frequencies. To achieve accurate results, it is critical to provide the ADC with a low-jitter, sampling clock source.
5. Explain in detail DECCA Navigation system.
DECCA is a hyperbolic electronic navigation system. Salient features of DECCA Navigation system. Operates in LF band. Between 70 to 120 kHz. Uses unmodulated continuous waves. In DECCA navigation system the fix is obtained by measuring the phase difference between the signals of the two stations which is phase locked. DECCA chain consists of 4 stations, 1 master & 3 slaves. The master station is at the centre and three slaves at the corners of a triangle. This arrangement gives the three sets of hyperbolic position lines, one set corresponding to the master and each slave. Fix is obtained over a considerable area by the intersection of two hyperbolic lines.
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In DECCA system each transmitter has different frequency so the direction from each station will differentiate by the receiver. Generally harmonically related frequencies radiated by each transmitters and phase measurements done at common harmonic frequency which is obtained at the receiver by using multiplying circuits.
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DECCA Reception:
.
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Range and Accuracy
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UNIT-5 Part A 1. Define Over sampling Ratio.
[N/D-16]
In most cases 10-bit resolution is sufficient, but in some cases higher accuracy is desired. Special signal processing techniques can be used to improve the resolution of the measurement. By using a method called 'Oversampling and Decimation' higher resolution might be achieved, without using an external ADC.
2. Draw the block diagram of Delta Sigma Modulator injected with noise.
[N/D-16]
Block diagram of Delta – sigma Modulator
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3. What is meant by Localizer? The localizer operates in the VHF band (108-110 MHZ) and consists of a transmitter with an antenna system. The radiation of which has two lobes, one with a predominant modulation of 90 Hz and other with 150 Hz.
4. What are the types of Radar present in the Ground controlled approach systems? Surveillance radar element Precision approach radar
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5. What are the disadvantages of ILS? Provides a single approach path along the extended centre line of the runway. It is site sensitive and subject to distortion and bending of the approach path due to site irregularities.
6. What are the basic elements of a MLS system? Azimuth beam equipment Elevation beam equipment Distance measuring equipment
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7. What is meant by Doppler navigation? It employs the Doppler Effect to determine the velocity of the craft in a frame of coordinates fixed with respect to the aircraft.
8. Define Frequency Trackers The frequency tracker locates the centre of the noise like Doppler spectrum and gives the output the pure signal of this frequency.
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9. Define Inertial Navigation Inertial navigation is a system of dead reckoning navigation in which the instruments in the craft determines its accelerations and by successive integration, obtain its velocity and displacement.
10. What are the features of Navigation over earth? The system of coordinated should be fixed with reference to earth. The coordinate system most convenient for use is latitude and longitude. Avery large gravitational fields is present at the surface of the earth.
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11. What are the components of inertial navigation systems? Accelerometers Gyros and stabilized platforms
12. Define DECTRA DECTRA is a Decca tracking and ranging. This is a long range hyperbolic navigational system working at a frequency of about 70 KHz. The system is designed to provide navigation information over a long route, particularly along the sea.
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13. Define CONSOL It is a rotating beacon operating in the LF/MF band which employs a system of three antennas producing a multi lobed pattern which is switched to produce a number of equi signals as in the radio range.
14. Define CONSOLAN It is same as CONSOL except that a two antenna system is used instead of three antennas.
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15. What are Marker Beacons? These are Radio beacons which are intended to mark some salient points.
16. Define SHORAN Short Navigation System is a secondary radar system in which fix is obtained by the craft, which carriers the interrogator, by simultaneously interrogating two ground beacons.
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Part B
1. Explain in detail about decimation filtering with necessary block diagrams. (16) [ N/D-16]
Decimation
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2. With the help of necessary diagrams, explain delta sigma DAC.
(16)
[N/D-16]
Delta Sigma DAC
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3. Explain in detail about Ground Controlled Approach System (GCA).
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GCA is a high precision radar system sited near the airport runway. With the help of this system controller on the ground can bring the aircraft into approach zone & then guide it along the path. The system consists of two radars one called surveillance radar element (SRE) & other called as precision approach radar (PAR). SRE is a search radar with a PPI display. As the SRE is not an essential part of the approach system. The following data relating to an early version of SRE may however be noted. Surveillance Radar Element
Precision Approach Radar: •
This precision radar has a maximum range of about 15-20 km & scans the approach zone both in azimuth & elevation. The precise performance & display details depend to some extent on the manufacturer of the equipment. Radar has to scan a 20° azimuth sector & a 7° elevation sector to meet the operational requirements. For the accuracy we have two separate antennas are used one for azimuth & other for elevation scanning. By setting the power we can control the angle of scanning.
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Also the position of the PAR w.r.t runway is shown in the figure below.
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PAR precision depends upon precise determination of the beam position. The PAR uses the single radar transmitter which is connected alternately to the two antennas so two scans are interlaced.
Features of Position approach radar
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For large coverage large antenna is used (13 ft * 1.625 ft). Two types of PAR used fixed and movable. For movable PAR antenna should be lighter than the fixed one. The data obtained by the PAR are displayed on two CRTs one displaying range & elevation angle & other displaying range & azimuth angle. The accuracy of PAR is such that at a distance of 1 mile it is possible to detect deviations of glide-slope as little as 8m.
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4. Explain in detail pulsed Doppler system.
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Doppler Effect Doppler radar directs a beam of electromagnetic waves towards the earth. Some of the energy reradiated by the earth towards the aircraft is received & compared when the aircraft has a component of the velocity in the direction of the beam, the difference frequency called Doppler shift is nearly proportional to the velocity component. This is Doppler Effect.
Beam Configuration: •
Consider an aircraft flying over the earth, transmitting EM waves in a narrow beam making an angle Ф with the horizontal
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If the aircraft is in level flight & the beam is directed in the vertical plane containing the forward velocity V of the craft the component of the velocity in the direction of the beam is V cosФ & the Doppler shift is
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But this is only one component of the shift is obtain in general three component is needed Some of the configurations is shown in figure below
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Track Stabilization
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Components of DNS
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Pulsed Doppler radar may be one of the two types Incoherent type Coherent type In incoherent operation the phase of radiation will change from pulse to pulse to obtain Doppler shift the pulses received from two opposite beams, which arrives at same time, are compared Incoherent Pulsed Doppler system
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•
• • • •
AFC of the local oscillator is necessary at these frequencies therefore a sample of transmitted signal is taken from a directional coupler & applied to the AFC circuit Generally four beam configuration is used Pulsed magnetron is used as the transmitter & this is switched to the beam pairs(1-3, 2-4) sequentially A duplexer is used to permit common antenna for transmission & reception Received signals is applied to a superheterodyne receiver, the output of this is Doppler frequency signal
Coherent Pulsed Doppler system • • •
• • • •
Compare to incoherent in Coherent Doppler radar system employs a continuous wave oscillator & a pulsed power amplifier Relatively low frequency generated by a quartz-crystal oscillator & stepped up by a chain of multipliers using step-recovery diodes or varactors The local oscillator frequency is generated by heterodyning the oscillations at the transmission frequency with an oscillator at the intermediate frequency
The output of mixer is centred at the IF The mixer & IF amplifier are followed by a coherent detector to which the other input is a reference frequency voltage Reference frequency is obtained by mixing the IF with an offset oscillator output & taking the difference frequency output By setting the offset oscillator both positive & negative Doppler shifts are obtained.
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• • • • •
• • • • • •
This system is capable to detect sense of the velocity as well as the vertical velocity The only disadvantage of the coherent pulsed system is its greater complexity Another problem occurs at lower altitudes is that the transmitted pulse is received back before the next pulse is transmitted Solved by setting PRF Continuous Wave Doppler Radar Separate transmitting & receiving antennas are required for preventing the transmitter output from entering the receiver
The Doppler difference frequency is obtained by direct heterodyning of the transmitted & received signals This is equivalent to having an IF of zero & is called homodyne reception The difference signal is amplified in an audio amplifier & applied to frequency tracker In homodyne operation the sense of the velocity can not be obtained Suffers from reflection from nearby objects, turbulent air, precipitation Generally used fixed antenna FM-CW Doppler Radar
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•
Uses common antenna for Tx & Rx
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The received signal is mixed with sample of transmitted signal in balanced mixer & desired side band is filtered & applied to coherent mixer
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The output of filter is mixed with the nth harmonic of the FM oscillator in the
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coherent mixer output of which will be difference frequency
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After amplification the signal is fed to frequency tracker
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Sometimes uses separate antenna for Tx & Rx
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The sense information may be obtained
Frequency Trackers •
Locates the centre of the noise-like Doppler spectrum & gives pure signal of frequency
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Various configurations but most of them employ a tracking oscillator
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In this the spectrum is compared with local oscillator frequency & error signal is generated
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According to the error signal oscillator is driven & correct frequency tuned Other is two filter tracker
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In this arrangement single filter is used but the oscillator frequency is switched by a square wave & takes on alternately two values which are separated by a spectrum width
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•
Oscillator output is mixed with the Doppler signal & pass through low pass filter & envelope detector
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The output of the filter is square wave & applied to the phase detector where two signals compared & error signal is generated Accuracy of DNS
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The overall accuracy depends upon ground speed measurement & heading accuracy
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Computational errors if Analog computers are used
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0.25% may be achieved if negligible computational error.
5. Explain in detail Instrumentation landing system.
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• • • • • • • • • • • • •
Basics of Landing When visibility is good, whether in the day or at night, this operation is carried out by visual observation of the ground & landing lights. The landing is then performed under ‘Visual Flight Rules’ (VFR) conditions. Usually this is taken to indicate a horizontal visibility of 5 km or more & vertical visibility of 300 m. when these conditions are not satisfied, the landing is under ‘Instrument Flight Rules’ (IFR) conditions. Special arrangements are provided at airports to enable the aircraft to execute landings under bad visibility. So we have to provide information about its exact position in relation to desired path & horizontal & vertical positions. Two types: ILS (Instrument Landing System) MLS (Microwave Landing System) Ground Controlled Approach (GCA) GCA does not required any special navigational equipment only a communication set is needed in the aircraft. Instrument Landing System The instrument landing system (ILS) comprises the units localizers, glide path (or glide slope) & marker beacons. The localizer defines a vertical equi – signal plane which passes over the centre line of the runway & the glide-slope. Three marker beacons are also installed at certain specified distances from the end of runway. Localizer: The localizer operates in the VHF band (108- 110 MHz) & consists of a transmitter with an antenna system. The radiation from the antenna system has two lobes one with a modulation of 90Hz & other with a modulation of 150Hz.
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• •
The two signals are equal hence both the lobes are symmetrical to the runway. The antenna array by means of which this pattern is obtained consists seven or eight elements.
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Loop antenna is used in localizer and this entire antenna system is placed at the centre line of the runway and about a 300 m from the end of the runway. Total of 7 or 8 loops divided in formation of 3-1-3 or 3-2-3 the centre loop fed with carrier of 90Hz & 150Hz modulated wave. While one side loop is fed with side-band of 90Hz & 150Hz.
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• • •
•
Glide-Slope System The principle of operation of the glide-slope currently in use, called null-type glide-slope is very similar to the localizer. This system operates in the band 339.3 -335 MHz band employs two antennas having a polar diagram as shown in figure. Here the larger lobe represents the radiation from the lower antenna & transmit the carrier & smaller lobes represents the radiation from the top antenna & having only side-band frequencies.
If the aircraft flies along the null, it receives the signal of lower antenna only & the two modulations are equal, giving an equi-signal course. The glide-slope equipment & antenna have to be sited away from the runway so that they do not constitute a hazard. The modifier array is used here for error correction so the pilot can easily make out the correct course.
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Receiving Equipment: •
The receiver is typically a crystal controlled multi-channel receiver. Separate receivers are required for the localizer & the glide-slope because they operate in widely different bands. Receivers having very efficient automatic gain control this keep the output of the receiver constant when the input varies from 20μV to 100000μV. So the meter indication is perfect & ensure the correct path finding. It is important, both in the localizer & the glide-slope, that the courses are maintained correctly & the modulation levels preserved. To achieve this we have to detect the signal strength for this dipole antennas are fixed at certain specified points on & off course. Now the received signal is processed to find the modulation components & monitor the course alignment, width & clearance. If the certain condition is not satisfied then it enables the alarm circuit.
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The output of receiver is applied to two filters which separate the 90Hz & 150Hz signals, each of which is rectified by a bridge rectifier. The outputs of rectifier are connected so as to give the difference between the rectified voltages & this is applied to the indicator coil. R1 is used to compensate for different losses in the two rectifiers & filters. Voltage across R3 is applied to a coil which operates the ‘flag alarm’. Thermistor is used for temperature compensation. The indicator shown in figure consists of a meter with two centre-zero movements. Horizontal needle indicates deviation from the glide-path & the vertical needle indicates deviation from the localizer. FSD of the meter typically set at 150μA.
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Course Sharpness & Width •
The sharpness & width of the course are dependent on the relative depths of modulation of the 90Hz & 150Hz signals.
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The total signal modulation is defined by the relation : M=(A+B)/C where M is total signal modulation, A & B are the amplitudes of the 150Hz & 90Hz signals respectively & C is the carrier amplitude. The difference in the depth of modulation (ddm) of the two signals is given by (A-B)/C. Now the meter indicates when current is pass through the coil. If equi-signal course followed by the aircraft then indication is null. The sharpness & width of the course are dependent on the relative depths of modulation of the 90Hz & 150Hz signals. The total signal modulation is defined by the relation : M=(A+B)/C where M is total signal modulation, A & B are the amplitudes of the 150Hz & 90Hz signals respectively & C is the carrier amplitude. The difference in the depth of modulation (ddm) of the two signals is given by (A-B)/C. Now the meter indicates when current is pass through the coil. If equi-signal course followed by the aircraft then indication is null. Site effects in the ILS
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The localizer & glide-slope courses are affected by the nature of the site on which they are installed. The terrain type introduce error in equi-signal course. For the type of terrain if we restrict the radiation it would difficult for aircraft to achieve the right path. The power of radiation & site conditions must take into account while designing the system so capture effect can be avoided. Marker Beacons
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The ILS employs three marker beacons. It gives an indication in the aircraft when it passes over them. All of them operate at 75 MHz & work with an antenna which gives a fan-shaped beam which is typically +/- 40° wide along the approach path & +/- 80° perpendicular to it. The outer marker(OM) is placed at 7km from the touchdown point of the runway. The radiation is modulated at 400Hz giving two dashes/sec. The second one called as middle marker (MM) is placed where the glide path is 200 ft which is generally about 1km from the touchdown point. The modulation is at 1300Hz with one dash every 2/3 sec. The last inner marker which is not used at all airports is placed where the glide-path is 100 ft above the ground. It is modulated at 3000Hz & transmits 6 dots/sec. In the aircraft a single receiver tuned to 75MHz is employed. The output is available as an audio signal & also actuates three lamps one for each marker beacon.
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