Radar functions. • Antennas basics. • Radar range equation. • System parameters . • Electromagnetic waves. • Scattering mechanisms. • Radar cross section and. Radartutorial (terney.info). 1. Radartutorial. Book 1 “Radar Basics”. ( Revision from ). This educational endowment is a printable summary . List of printable versions of the Radar Basics. The displaying of the PDF- document would be faster, if you download the file before opening: click with the right.

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Radar Basics Pdf

Radar (RAdio Detection And Ranging) is actually a fairly simple process of bouncing radio waves off objects and looking at the reflections to determining. Radar Basics. 1. Introduction to Meteorological Radar. 2. Development and Interpretation of the Radar Equation a. Statement of the Radar Equation b. What is Radar? ▫ RADAR (Radio Detection. And Ranging) is a way to detect and study far off targets by transmitting a radio pulse in the direction of the target.

Christian Wolff Radar Basics 1. An un- derstanding of the theory is essential to correctly specify and operate primary radar systems. Implementation and operation of primary radars systems in- volves a wide range of disciplines including building works, Slide 2 heavy mechanical and electrical engineering, high power mi- crowave engineering and advanced high speed signal and data processing techniques. As implied by this contraction, radars are used to detect the presence of a target and to determine its location. It refers to the use of radio waves to detect objects and determine the distance range to the object. The contraction implies that the quantity measured is range. While this is correct, modern radars are also used to measure radial speed and direction. It turns out that by measuring these parameters one can perform reasonably accurate calculations of the x-y-z location and velocity of a target. In practice, the transmitted signal needs to be very large and the received signals are very small. Interfering signals can be very large and this can dwarf the desired signals. Fortunately discrimina- tion can be achieved not only based on amplitude but on the frequency components inherent in the received signal. Primary radar is a very interesting subject offering plenty of challenges in the areas of specification, design and operation. Radars become important because of the following reasons.

The rectangular triangle refers to the half of the beamwidth and the half of the spacing. Generally, resolution is defined as resolving power. Resolution is the minimum separation between two targets that the equip- ment radar is capable of distinguishing. Differences in Doppler shift make it possible to distinguish two targets, which are located inside the same resolution cell.

The range and angular resolutions lead to the resolution cell. The meaning of this cell is very clear: unless one can rely on Slide 17 eventual different Doppler shifts it is impossible to distinguish two targets which are located inside the same resolution cell. The target resolution of a radar set is its ability to distinguish between targets that are very close in either range or bearing. Weapons-control radar, which requires great precision, should be able to distinguish between targets that are only some meters apart.

Search radar is usually less precise and only distinguishes between targets that are hundreds of meters or even a nautical mile apart. Resolu- tion is usually divided into two categories; range resolution and bearing resolution.

The meaning of this cell is very clear: unless one can rely on eventual different Doppler shifts it is impossible to distinguish two targets which are located inside the same resolution cell. Resolution should not be confused with accuracy. Nevertheless, in most radar projects, a first guess for the accuracy figure one standard deviation will be half the value of the corresponding resolu- tion.

When the radar is realized, the accuracy is frequently better than the first guessed because: e. If the transmitted pulse is a perfect rectangular one, the received pulse will look like a Gaussian curve because the receiver bandwidth is finite; in addition the noise will disrupt the Gaussian shape of the received pulse.

Hence it is obvious that the accuracy of this measurement is not really linked to the pulse width which defines the range resolution but rather on the receiver signal strength which is linked to the range. Hence the range error should increase with the range. Radar Coverage Coverage is the volume defined in terms of range, azimuth, and elevation angle or altitude within a radar can provide useful information on the smallest aircraft of interest, in this case a small general aviation aircraft.

The coverage of a single radar set depends on geographical peculiarities, and the vertical an- tenna pattern. It looks like a flat cylinder with a radius of the maximum range generally in the slide shown as green with a Slide 18 gap in the middle: the cone of silence.

Low level coverage is determined by topographical considerations as well as radar performance. Local obstructions can have a significant impact on radar coverage and very often determine the required height of the radar antenna. For airport radars, it is customary to specify the low coverage at range intervals from the radar site. In the case of en route radars, the low coverage is usually more generalized, for example, ft throughout the coverage volume.

However, these demands can be surprisingly difficult to achieve in mountainous areas. Careful analysis of the operational requirement is required, particularly, in the specific directions where low cover is operationally important. High level coverage requirements are usually easier to achieve for both en route and approach ra- dars. Typical figures are ft for an en Route radar and ft for an approach radar. Ground movement radars are usually required to provide coverage to an altitude of ft in order to display missed approaches, helicopter operations etc.

Of more significance is the required back angle coverage, which can be more difficult to achieve. This can be significant for both approach and en route radars. Traffic over-flying an airport or en-route radar can be lost in the overhead gap or cone of silence for a significant period depending on the back angle and the height of the aircraft.

It is always appropri- ate to consider traffic routings relative to the radar overhead gap when considering the radar site and the back angle requirements. Aircraft flying close to the overhead gap fly an apparent circular path around the radar head due to slant range considerations. Back angle figures of between 30 and 40 degrees are typical for modern antenna designs. At the frequen- cies normally used for radar, radio waves usually travel in a straight line.

The waves may be obstructed by weather or shad- owing, and interference may come from other aircraft or from reflections from ground objects. This figure still disregards the influence of the refraction of Slide 19 electromagnetic waves in the earth atmosphere. Refraction A phenomenon called refraction occurs when radar waves pass through media with different indices of refraction. In a vacuum, radio waves travel in straight lines. The speed of wave propagation differs from c if the medium of propagation is matter.

This velocity difference is not significant in air, but can have large effects for radars. When a ray, the path of propagation of an electromagnetic wave, passes from a material having a smaller index of refraction to a material having a larger index of refraction, the ray is bent upwards.

If the ray goes from a material having a larger index of refraction to a material with a smaller index of refraction, the ray is bent downwards. The index of refraction of the atmosphere is not constant and depends on temperature, air pressure, and humidity. The temperature, partial pressure of the dry air, and the water vapor content normally decrease with increasing altitude; therefore, the index of refraction normally decreases with altitude.

Since the velocity of propagation is inversely proportional to the index of refraction, radio waves move slightly more rapidly in the upper atmosphere than they do near the surface of the earth. The result is a downward bending of the rays towards the Earth. Since the rays are not straight, as in a vacuum, this effect can introduce error in elevation angle measurements.

In the presence of standard refraction, the curvature of the rays is less than the curvature of the earth. This extends the radar line of sight beyond the geometrical horizon. To simplify radar calculations, refracted rays are often replotted as straight-line propagation for a fictitious earth having a larger radius than the actual earth. The smaller values are more likely to exist in cold, dry climates or at high altitudes, whereas larger values occur in tropical climates.

From trigo- nometry, one can determine the radar horizon range, the maximum distance that radar can detect targets. The range is given by where requiv is the effective radius and haim and hantenna are the heights of the transmitter and target respectively.

The effective Earth radius for line of sight varies with carrier frequency! The required service availability is a crucial factor in determin- ing the system design. For most ATC applications, total radar failure is not an acceptable situation.

On the other hand, the provision of dual sensors to meet a single operational require- ment is not always easy to justify. Significantly reduced availability requirements can be placed on individual sensors if alternative coverage can be provided by an adjacent facility on the network. In the context of en route radar services, the distribution of radar stations can usually be arranged to provide overlapping cover.

A failure of an individual radar station can be tolerated because adjacent stations provide adequate coverage. The situation becomes more difficult where low cover requirements are more demanding, for example, at airports.

Fundamentals of Signal Processing for Phased Array Radar

In these circumstances and subject to the availability of a networked alter- native, the users may be able to tolerate the reduction in low coverage for a limited period. Finally, in the circumstances where both primary and secondary services are available, it may be possible to continue operations in the absence of one of the services usually primary radar. The feasibility of this approach is likely to govern the level of system redundancy.

Turning to the configuration of individual sensors, a simple, but costly solution is to provide two independent single channel sensors. These facilities are normally geographically closely sited but not so close that they obstruct one another in order to provide near identical coverage. This config- uration is likely to meet the most demanding of availability requirements. An alternative, more cost effective solution is to provide a single sensor with dual electronics.

In this case, the antenna and turning gear represent a common mode failure item, but with careful design, very high availability can be achieved. In the exceptional circumstance of failure or planned maintenance, an alternative service from the radar network can be used albeit with restricted low coverage. There are clearly many permutations of these configurations, which are relevant to specific opera- tional circumstances, and it is important that they are fully discussed with the end users.

In some primary radar configurations, both channels of a dual channel primary radar system are used to provide the optimum day to day service. Loss of one channel, does not result in a service failure, but may cause some reduction in performance. These reductions in performance need to be calibrated i. Operational time is sometimes referred to as uptime, and non-operational time is sometimes called down- time.

Mean time between failures MTBF is the predicted elapsed Slide 22 time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean average time between failures of a system.

(PDF) Radar Basics pdf | Christian Wolff - terney.info

The MTBF is typically part of a model that assumes the failed system is immediately repaired zero elapsed time , as a part of a renewal process. This is in contrast to the mean time to failure MTTF , which measures average time between failures with the modeling assumption that the failed system is not repaired.

Mean time to repair MTTR is a basic measure of the maintainability of repairable items. It rep- resents the average time required to repair a failed component or device. Expressed mathematically, it is the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time.

It generally does not include lead time for parts not readily available, or other administrative or logistic downtime. Some older ATC-radars especially precision approach radar must be calibrated every day. The Airfield surveillance radars must undergo a Flight calibration check too. These times are also down- time. The operating principle of pulse radar involves the transmission of a short pulse of microwave energy at regular time intervals.

This transmission involves amplitude modulation of micro- wave signal with the pulse signal. The jammer simply has to send out signals on the radar's frequency with enough strength to make it think that was the strongest return. In this case, a series of random short bursts of the signal will appear to be a series of targets in different locations within the beam. Jamming of this sort can be made more effective by timing the signals to be the same as the rotational speed of the feed, but broadcast at a slight delay, which results in a second strong peak within the beam, with nothing to distinguish the two.

RADAR- Basics, Types & Applications

Monopulse basics[ edit ] Monopulse beam of a phased array antenna split in two lobes. Monopulse radars are similar in general construction to conical scanning systems, but split the beam into parts and then send the two resulting signals out of the antenna in slightly different directions.

When the reflected signals are received they are amplified separately and compared to each other, indicating which direction has a stronger return, and thus the general direction of the target relative to the boresight.

Since this comparison is carried out during one pulse, which is typically a few microseconds, changes in target position or heading will have no effect on the comparison. Making such a comparison requires that different parts of the beam be distinguished from each other. Normally this is achieved by splitting the pulse into two parts and polarizing each one separately before sending it to a set of slightly off-axis feed horns.

This results in a set of lobes, usually two, overlapping on the boresight. These lobes are then rotated as in a normal conical scanner. If the target is to one side of the boresight the resulting sum will be positive, if it's on the other, negative.

The temperature, partial pressure of the dry air, and the water vapor content normally decrease with increasing altitude; therefore, the index of refraction normally decreases with altitude.

Since the velocity of propagation is inversely proportional to the index of refraction, radio waves move slightly more rapidly in the upper atmosphere than they do near the surface of the earth.

The result is a downward bending of the rays towards the Earth. Since the rays are not straight, as in a vacuum, this effect can introduce error in elevation angle measurements.

In the presence of standard refraction, the curvature of the rays is less than the curvature of the earth. This extends the radar line of sight beyond the geometrical horizon. To simplify radar calculations, refracted rays are often replotted as straight-line propagation for a fictitious earth having a larger radius than the actual earth.

The smaller values are more likely to exist in cold, dry climates or at high altitudes, whereas larger values occur in tropical climates. From trigo- nometry, one can determine the radar horizon range, the maximum distance that radar can detect targets. The range is given by where requiv is the effective radius and haim and hantenna are the heights of the transmitter and target respectively. The effective Earth radius for line of sight varies with carrier frequency!

The required service availability is a crucial factor in determin- ing the system design. For most ATC applications, total radar failure is not an acceptable situation.

On the other hand, the provision of dual sensors to meet a single operational require- ment is not always easy to justify. The question of radar service availability can be addressed at two levels: Significantly reduced availability requirements can be placed on individual sensors if alternative coverage can be provided by an adjacent facility on the network. In the context of en route radar services, the distribution of radar stations can usually be arranged to provide overlapping cover.

A failure of an individual radar station can be tolerated because adjacent stations provide adequate coverage. The situation becomes more difficult where low cover requirements are more demanding, for example, at airports. In these circumstances and subject to the availability of a networked alter- native, the users may be able to tolerate the reduction in low coverage for a limited period.

Finally, in the circumstances where both primary and secondary services are available, it may be possible to continue operations in the absence of one of the services usually primary radar. The feasibility of this approach is likely to govern the level of system redundancy. Turning to the configuration of individual sensors, a simple, but costly solution is to provide two independent single channel sensors. These facilities are normally geographically closely sited but not so close that they obstruct one another in order to provide near identical coverage.

This config- uration is likely to meet the most demanding of availability requirements. An alternative, more cost effective solution is to provide a single sensor with dual electronics. In this case, the antenna and turning gear represent a common mode failure item, but with careful design, very high availability can be achieved. In the exceptional circumstance of failure or planned maintenance, an alternative service from the radar network can be used albeit with restricted low coverage.

There are clearly many permutations of these configurations, which are relevant to specific opera- tional circumstances, and it is important that they are fully discussed with the end users. In some primary radar configurations, both channels of a dual channel primary radar system are used to provide the optimum day to day service. Loss of one channel, does not result in a service failure, but may cause some reduction in performance.

These reductions in performance need to be calibrated i. Operational time is sometimes referred to as uptime, and non-operational time is sometimes called down- time. Mean time between failures MTBF is the predicted elapsed Slide 22 time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean average time between failures of a system.

Radar Fundamentals Power Point Presentation

The MTBF is typically part of a model that assumes the failed system is immediately repaired zero elapsed time , as a part of a renewal process. This is in contrast to the mean time to failure MTTF , which measures average time between failures with the modeling assumption that the failed system is not repaired. Mean time to repair MTTR is a basic measure of the maintainability of repairable items.

It rep- resents the average time required to repair a failed component or device. Expressed mathematically, it is the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time. It generally does not include lead time for parts not readily available, or other administrative or logistic downtime.

Some older ATC-radars especially precision approach radar must be calibrated every day. The Airfield surveillance radars must undergo a Flight calibration check too. These times are also down- time. The operating principle of pulse radar involves the transmission of a short pulse of microwave energy at regular time intervals.

This transmission involves amplitude modulation of micro- wave signal with the pulse signal.

The receiver is listening between the successive transmitted pulses. Returns from an individual target arrives a fixed interval round trip time after the transmitted pulse. The ratio of the pulse width to the pulse repetition interval is called the duty cycle. During transmission, the peak of the transmitted pulse defines the peak power. This is typically in the range 50 watts to 10 megawatts.

The mean power of the transmission is defined by the peak power multiplied by the duty cycle.

The mean power is an important value since it is a measure of the total amount of microwave energy to be transmitted by the system. Echoes falling in this time have got e larger range than the maximum instrumentated range of the radar indicator.

In this time a sophisticated radar set execute test loops for maintenance purposes. Transmitters Duty cycle There are two different methods to generate a radar pulse: These radars are often magnetron-based. These radars use a high-power ampli- fier as transmitter, often klystron- or Travelling Wave Tube-based or solid-state. Both waveforms have advantages and disadvantages as we can see in later modules. The energy content of a continuous-wave radar transmission may be easily figured because the transmitter operates continuously.

However, pulsed radar transmitters are switched on and off to provide range timing information with each pulse. The amount of energy in this waveform is im- portant because maximum range is directly related to transmitter output power.

The more energy the radar system transmits, the greater the target detection range will be.

The ener- gy content of the pulse is equal to the peak maximum power level of the pulse multiplied by the pulse width. However, meters used to measure power in a radar system do so over a period of time that is longer than the pulse width. For this reason, pulse-repetition time is included in the power calculations for transmitters. Power measured over such a period of time is referred to as average power. In particular, it is used in the following contexts: Duty cycle is the proportion of time during which a component, device, or system is operated.

Suppose a transmitter operates for 1 microsec- ond, and is shut off for 99 microseconds, then is run for 1 microsecond again, and so on. The duty cycle is used to calculate both the peak power and average power of a radar system. Peak power must be calculated more often than average power. This is because most measurement instruments measure average power directly. Since the storage of the energy in the modulator, the power supply must make plant for the transmit- ter available a little more than the average power only.

The dwell time of 2D-search radar depends pre- dominantly on: The dwell time can be calculated using the following equation: Here the dwell time is not so easy calculable. A for signal processing needed value of dwell time is specified in software then. Hits per Scan The value of hits per scan m says how many echo signals per single target during every antenna swing are received.

The hit number stands e. The dwell time TD and the pulse repetition time determine the value of hits per scan: Analogue radars need 15 to 20 hits to integrate the blip on the screen. For a digital signal pro- cessing using e. To achieve this num- ber of hits, the radar needs up to 20 pulse periods. In reality, under severe clutter condition, these radars need up to 3 or 4 hits. Finally also all other radar characteristics such as range and azimuth resolution, blind speed etc could be derived from this basic timing considerations.

A classic radar i. This require- ment limits the receiving time and the maximum unambiguous range as following: Slide 27 Since the radar processing in this surveillance radar is still in real time with relatively low, but con- stant delay , the data renewal time depends on the antenna revolution time.

To direct in the same azimuth angle after 5 seconds, so that the radar can measure the co-ordinates again, the antenna must turn with 12 revolutions per minute at least. The dwell time, the time that an antenna beam spends on a target, depends predominantly on the antennas horizontally beam width and the turn speed of the antenna. If we assume, that a well de- signed parabolic antenna got a beam width of 1. These radar sets need a given number of hits per scan.

This is necessary, to integrate the signals of different pulse periods for a better distinction of wanted signals from unwanted noise, as well as to measure a correct angular direction.

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