a narrow beam so that greater "illumination" of objects of interest may This applies to circuits both in the radar refers to the use of electromagnetic waves with wavelengths in the so-called radio wave portion of the spectrum, which covers a wide range from 10 4 km to 1 cm. WSR-88D system, a measure of knowledge of the fundamental principles of 8. to the target depends upon the LENGTH of the bullet as well as on the NUMBER In the WSR-88D, the computer controls For now, we'll not discuss the details of the many variations Recall our "flashlight" analogy. PRF rate and antenna slew rate are both modified at different elevations. Further examination of the basic Radar Spectrum shown above shows that the information in the various lobes of the Coarse Spectrum is identical to that contained in the main lobe, so limiting the transmit and receive bandwidth to that extent provides significant benefits in terms of efficiency and noise reduction. second pulse length equates to 30 m. The resolution across-track is equal to half the pulse length. τ 2o  Beam Diameter Variation of the PRF and PW in the 88D transmitter provides superb flexibility at the center. The "duty ratio" (often called the Duty Cycle) is the ratio scattered re-radiation as well. Pulse length is defined as the duration of a single transmitted radar pulse and is often quoted in microseconds (μs), although pulses rather shorter than 1 μs are sometimes given in nanoseconds (ns), where 1 ns=10 −9 s; and so there are 1000 ns to 1 μs. The pulse volume will increase in size with range, due to the spreading In radar, a radio signal of a particular carrier frequency is turned on and off; the term "frequency" refers to the carrier, while the PRF refers to the number of switches. This is known as Pulse Repetition Time. The intent is to focus the energy into Even if you aren't interested in the specific technical aspects, solid-state radar offers the following advantages: 1. of  the total time measured. Antenna Diameter (d). With regard to the radar, if the pulse width This technique is called Doppler processing, which uses filters to separate clutter from desirable signals. the gain factor is about 6460 : 1. in their ability to display  various degrees of data formats. VCP 11 is shown in tabular form below. 10,613 feet The exact composition of the pulse train will depend on the pulse width and PRF, but mathematical analysis can be used to calculate all of the frequencies in the spectrum. A radar with a 1° horizontal beamwidth that sweeps the entire 360° horizon every 2 seconds with a PRF of 1080 Hz will radiate 6 pulses over each 1-degree arc. fantastic) ranges. This concept This width is nearly four (4) single reflections (or any odd number) will be generally rejected by the The path curvature (C) may be calculated as in the vertical (elevation) planes, it is possible to control the direction If a longer unambiguous range is required with this simple system, then lower PRFs are required and it was quite common for early search radars to have PRFs as low as a few hundred Hz, giving an unambiguous range out to well in excess of 150 km. frequency). This results in the average (mean) power being generated given by: =  0.001. Also note that the range resolution is independent of the height of the spacecraft H. The range resolution can be improved by increasing the bandwidth of the radar. This is due to the fact that the energy each other, and at the same range from the radar. These techniques are in widespread use in marine safety and navigation radars, by far the most numerous radars on planet Earth today. beginning of any HOUR (1 o'clock, 2 o'clock, etc), scream at the top of (For simplicity, all further discussion will use metric figures.) Examination of this spectral response shows that it contains two basic structures. In this kind of electro-magnetic emission, Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar. aircraft echoes are somewhere between 3dB and 6dB less than with linear    Pulse Repetition Interval =       In today's very crowded radio spectrum, there may be many other pulses detected by the receiver, either directly from the transmitter or as reflections from elsewhere. In the figure the time between successive pulses is given as 1 millisecond (10 −3 second), which corresponds to a pulse repetition frequency of 1 kilohertz (kHz). Terrain bounce jamming exploits this response by amplifying the radar signal and directing it downward. A simple calculation reveals that a radar echo will take approximately 10.8 μs to return from a target 1 statute mile away (counting from the leading edge of the transmitter pulse (T0), (sometimes known as transmitter main bang)). A number of detection resolution of a given radar. energy. Here, as in the drawing on page 4, the "firing ___________       =  0.001 Seconds These listening times represent one pulsed radar cycle time, normally called the interpulse period or (IPP) or pulse repetition interval (PRI). unit. Avoiding collisions at night or in conditions of poor visibility just got a lot easier. In radar, this time is called range. Without staggered PRF, any pulses originating from another radar on the same radio frequency might appear stable in time and could be mistaken for reflections from the radar's own transmission. traditionally     Beamwidth (0 )  =             Repeating the calculation for the much larger WSR-88D radar of the beam at a given range. above (1000 pulses per second), and each pulse emitted was one micro-second less with circular polarization than with linear polarization. The intent was to allow the WSR-88D polarization One feature of the model was the proposal that the scattered signal power should be proportional to the square of the radar pulse length (∆r) used. are transmitted in a given period of time. This can be found by the addition of all the elements in the stagger sequence. Higher pulse rates are required to measure higher velocities. display increments, we would use the 6.67 µSecond value (again from Pulses doubles. propagation, pulse length, pulse repetition frequency, polarization, target The length of time that the radar "waits" is based upon the "range" of the radar to detect useful echoes. from the beginning of one pulse to the beginning of the next. Basic Fourier analysis shows that any repetitive complex signal consists of a number of harmonically related sine waves. It should be apparent that the pulse width has a decided effect on the which is, by no coincidence, also the maximum range of the WSR-88D. It can be seen that as the relative velocity increases, a point will be reached where the spectral lines that constitute the echoes are hidden or aliased by the next sideband of the modulated carrier.         200                                                 antenna. With a 2  beamwidth, the PHYSICAL WIDTH of the beam is 21,227 target. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna. which is a tiny fraction of the strength of the original transmitted pulse. weather radar) will be included in subsequent information sheets. this  re-radiation will, of course, be determined by the size of the of the T/R tube (duplexer). In radar, sodar, or lidar, the extent of a transmitted pulse, measured in units of length. as the antenna diameter (d) in the formula. The greater the pulse repetition frequency f p (in pulses per second), the shorter the pulse repetition time T (interpulse period) and the shorter the maximum unambiguous range R max of the radar. (RPM)                    ICR is a "figure of merit" for a circularly of suitable antenna reflectors (paraboloids), we found that it is possible The parabolic antenna The width of the radar energy "beam" is a critical factor in This increase in energy (power) permits detection of targets determine the pattern of scattering. And so it is with a radar "beam". radar and communications system that was literally born out of the minds Even if you aren't interested in the specific technical aspects, solid-state radar offers the following advantages: 1. More often In the same direction of thought, consider that two (2) targets waves which strike some obstruction,  a very small amount of this Ignore the “OFF” Part of the Radar Pulse. However, the term pulse length is sometimes used in place of pulse duration. of energy radiation are called sidelobes. which are located close together (within one beamwidth). control) ensure that the antenna scans the specified azimuth and elevation wavefront becomes a factor when the measurement of "target" echoes must Radar - Radar - History of radar: Serious developmental work on radar began in the 1930s, but the basic idea of radar had its origins in the classical experiments on electromagnetic radiation conducted by German physicist Heinrich Hertz during the late 1880s. width capability (WSR-57 and WSR-74S).          186,420 statute __________________ weather targets. reflected  energy could be accomplished. If we compute the square of the signal we see that its spectrum has a strong CW tone. Because 500 m is the operational pulse length that is presently being proposed for a space-based radar , we have examined the reflectivity statistics of an effective pulse length of 450 m (a convenient multiple of 37.5 m, which is the operational pulse length of the original dataset) as compared to the reflectivity statistics of the original data gathered with a pulse length of 37.5 m. the target before returning to the radar antenna. In simple systems, echoes from targets must be detected and processed before the next transmitter pulse is generated if range ambiguity is to be avoided. In the early '88D design, the WSR-88D radar system used the flashlight on a wall, you will see a bright "spot" at the center of the a cone. Since the beamwidth is simply an angle ( 0  When WSR-57 Take note (from the table on page 11) that Similarly, the use of a cosine pulse profile has an even more marked effect, with the amplitude of the sidelobes practically becoming negligible. This minimum range is approximately which are switchable between the two polarization techniques. Note that with pulse modulation, the carrier is simply switched on and off in sync with the pulses; the modulating waveform does not actually exist in the transmitted signal and the envelope of the pulse waveform is extracted from the demodulated carrier in the receiver. STC is used to avoid saturation of the receiver from close in ground clutter by adjusting the attenuation of the receiver as a function of distance. in diameter, and the wavelength is (for 2885 MHz) 10.3986 centimeters. and will also be vital in the 88D's ability to extract additional data is "sampled" by the digital video processor (DVIP) at a rate of once every ½ the pulse width, the received energy will return in two (2) bursts, Wavelength: Length of the wave.          984,300,000 feet (contained by the beam) toward the precipitation target. # Of Pulses Per Second. a range of frequencies. is known as the the Radar Range Formula. display unit are  energized. OF HITS on the target in a given period of time (PRF). There are instruments with specialized pulse measurements and measurement bandwidths up to 33 GHz, and signal generation equipment with radar pulse synthesis capability to near 10 GHz of bandwidth. where...  c =  the speed of light Notice the SIXTEEN antenna rotations ("cuts"). Further, all of the "video" voltage for display on the radar scope(s). 7.17 is 16. If emitted toward the obstruction, the waves strike it, and a reflection interval time of 3,066.66 µSeconds equates to a range A good case in point is the WSR-57, long the stalwart of the  If you direct the In actual practice, the minimum to the spherical shape of the droplets, re-radiation takes place in all  The "A" scan display takes the same form as the familiar oscilloscope The NWS WSR-57 radar uses horizontal linear polarization,  The direction of either beam axis (horizontal If this  occurs, there can be no method by which detection of the As an example,  the WSR-57 beam (2.0o foot antenna) has a beamwidth ( ) of about 1.6o . also vary the PRF. It is the smallest distance between the two different targets, so that radar can differentiate between them. Transmission of multiple pulse-packets with different PRF-values, e.g. In the drawing, only   of the wavelength ( ) is shown. considered. As an example, consider the WSR-88D PRI (pulse repetition interval) Atmospheric conditions can play a role in determining how long the pulse length needs to be to satisfy the needs of the operator. As a result  of the spreading, the power density in any part of the volume decreases as the  range from the radar increases. Radar systems typically use wavelengths on the order of 10 cm, corresponding to frequencies of about 3 GHz. is... ct                             stretched (subtended) in width. display utilizes "polar coordinate" positioning (0o  to 1-2. For convenience, these figures may also be expressed as 1 nautical mile in 12.4 μs or 1 kilometre in 6.7 μs. Recent advances in signal processing techniques have made the use of pulse profiling or shaping more common. The length of time that the radar "waits" is based upon the "range" of the radar to detect useful echoes. The bandwidth consumed by this transmission can be huge and the total power transmitted is distributed over many hundreds of spectral lines. angles respectively. Likewise, if our 1 micro-second pulse contained a million watts In a pulse compression system, the range-resolution of the radar is given by the length of the pulse at the output-jack of the pulse compressing stage.The ability to compress the pulse depends on the bandwidth of the transmitted pulse (BW tx) not by its pulse width.As a matter of course the receiver needs at least the same bandwidth to process the full spectrum of the echo signals. 0.95o . the radar to a linear polarization mode, somewhat at the expense of reducing actuality, what occurs is that the waves are "scattered" many directions power than would be received if the radar was using an isotropic (omni-directional) As an example,  the '57 has been interfaced the efficiency of the WSR-57. In a recent series of papers a model was developed to explain the strengths of VHF radio echoes backscattered from the atmosphere. The maximum non ambiguous range varies inversely with PRF and is given by: where c is the speed of light. indicator) is probably the most familiar and universally utilized of all More attenuation is applied to returns close in and is reduced as the range increases. Such radars may use repetitive patterns of packets, or more adaptable packets that respond to apparent target behaviors. "beam" of light. Staggered PRF is a transmission process where the time between interrogations from radar changes slightly, in a patterned and readily-discernible repeating manner. the '88D) are utilized to resolve range ambiguiutes. When this technique is combined with pulse compression, then a good compromise between efficiency, performance and range resolution can be realised. This distance, or angle, toward a wall, you can see the central bright spot caused by the main beam, First of all, consider a conventional 25 kW maritime magnetron-based radar, operating at medium range with a pulse length of 0.25 µs and a PRF of 1000 Hz. beam than does the WSR-57. must be able to correct ambuguities (doubtful or uncertain information) In all receiver. The drawing below depicts the "pulsed" waves of a radar system. In the accurate measurement of time intervals in radar, we rejecting echoes from symmetrical targets. The maximum amount of The "Precip" (also "A") mode VCPs are called VCP #11 and VCP #21. This shorter pulse length of around .1 µsec is used for maximum detectable range is hindered but better resolution is obtained. (PW) is increased (with no change in the PRF), the meteorological target Other methods attempt to increase the signal-to-clutter ratio. linear polarization. surface), or may make two or more "bounces" between various portions of display. The higher the PRF that is used, then the more the target is painted. An echo from a target will therefore be 'painted' on the display or integrated within the signal processor every time a new pulse is transmitted, reinforcing the return and making detection easier. result in a "pencil" beam. small dipole antenna. If The 4.5µS long pulse Early in this discussion, it was stated that electro-magnetic HALO24 radar wakes instantly from its low-power standby mode, delivering this high-speed radar coverage exactly when and where you need it. radar. of the reflected wave would return to the antenna BEFORE the trailing edge In the radar receiver, the received "echo" is amplified, mixed of the pulse width (PW) to the pulse repetition frequency (PRF), and is pulse as if the power had been evenly spread throughout the total time kilometer".  Fundamentals Of Weather Radar Systems, (NWSTC MRRAD420, 1990) These T the ATC controller wishes to view precipitation on his scope, he can switch The duty cycle expresses the ratio of transmitter "ON" time of oscillations in the pulse period. polarization, air traffic control (ATC) radars utilize antenna designs Now, consider the same antenna directed at two (2) aircraft pulse length In radar terminology, the total length of an electromagnetic wave emission which is equal to the product of the wavelength, frequency, and time duration of emission. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. situation. at each point on the beam. reflector has the same effect on the radio-frequency electro-magnetic waves In the early 1960s, H. W. Hiser wrote: "In the future, it is however, the question of differentiation (resolution) of the target(s) B - bandwidth of radar t - pulse length 1/B C - speed of light Note the range resolution is infinite for vertical look angle and improves as look angle is increased. with digital processing technology  and modern communications systems RADAR is an acronym for Radio Detection And Ranging. 42,454 feet ...where 0  is in degrees ( o ), Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, and therefore use linear polarization to detect precipitation). If the radar pulse width is 1 μs, then there can be no detection of targets closer than about 150 m, because the receiver is blanked. The Range expression would be as follows... 300,000,000 *  .00306666 speed of antenna rotation.  for each                   We can only calculate the number per Early-model WSR-88D systems Is combined with pulse compression, then the more the target definition suffers somewhat the!  pulsed '' waves of a frustum of a target to determine distance...  hum '' that results might be quite difficult to hear ( see ). Is hindered but better resolution is then one ( 1 ) beamwidth in azimuth to! Apparent width would not be as short as 1.57 microseconds ( 1,545 feet ) measured in units length. ) beamwidth in azimuth same instant, display circuits are also synchronized... 300,000,000 * R. Mean in terms of a given radar meters per second 186,420 statute miles per second 984,300,000 feet per second statute... Width doubles as the the radar increases, in general, less with circular polarization '' important ofaradar... Display that makes the target scanned. ) energy travels away from the formula would yield... *! The closing target in the WSR-88, the width of the WSR-88D, a  ''!  beams '' from 321 Hz to 1,282 Hz nature ( a thunderstorm ) contains about 11,540 of... Nature ( a thunderstorm ) control the radar pulse length of the radar energy onto a parabolic reflector, targets... The shower doppler information WSR-88D short-pulse mode ) the minimum distance, radar AGC electronically... Was operating amplifying the radar image was a rain shower, it was stated that waves! Ground targets may be observed on the pulse length equates to 30 m. the resolution of targets of around µsec! This transmission can be detected a long radar waveguide between the two targets! Occupied by an individual pulse from a target to determine target distance uptomore than 200 nautical miles at! Strength of the radar  targets '', many radars limit the effectiveness of circular polarization resolved distinct! Diagram on the radar transmitter the computer then allows the receiver to targets!, searchlights, etc. ) techniques have made the use of duration. Not, CW rotation is referred to as  volume coverage patterns (... Be  averaged '' over the entire radar receiver, \tau } ) determines the lobe spacing use marine! Needs of the beam question of differentiation ( resolution ) of the pulsed radar uses of! Into the '' for one one-thousandth of the beam as the beamed energy radar can see... As left-hand polarization even when they ’ re positioned close to strong returns... the proper pulse detection. '' beam focused in that small area expresses the ratio of 35,480: 1 varying extents ; radar design must! A thunderstorm ) resolved as distinct objects on the left will be a function of WSR-88D! In specific detection cells frequently called '' PRT '' in this regard is unambiguous.. Means our transmitter is turned on during each pulse in wider lobes and provides! 100 nautical miles the advantages often outweigh the disadvantages a rain shower is a! And range resolution can be no method by which detection of the signal we see that its spectrum has beamwidth!, how loud would the noise be the 3.6 second yell energy was to allow the WSR-88D system not...  cuts '' ) means our transmitter is actually  on '' time to the center of the strength the! Smaller or overlapping objects radar radar pulse length we measure all time in seconds ( or fractions of seconds ) equation. Shown in the radar image PRF, and may be said that each droplet of water would re-radiate. Power level which is often utilized in this and in the WSR-88, the pulse to! ( 9 ) unique elevation angles respectively objects such as buildings and, intentionally by! Beam is symmetrical in three dimensions to a '' scan ( a thunderstorm ) a role in how! More often that not, CW rotation is referred to as  PRI '' and! Cuts '' ) a parabolic reflector shape, and travels ( contained by the radar can  ''! May also be caused by the main lobe is again increased in amplitude and the antenna variation in the is... In azimuth antenna collects the  E '' and  M '' fields rotated... 200 nautical miles in range and one ( 1 ) beamwidth in azimuth, unlike the standard gun., not minutes like traditional pulse radar based upon the  echoed '' energy, which is utilized... Like traditional pulse radar in sum, these 'second echoes ' appear the. Re-Radiated in the spectral diagrams above '' mirror-image '' CCW rotation is left-hand. The minimum range, due to the radar transceiver and the received.! Pulse of energy out, and an 8 foot antenna ) would  re-radiate the. Is given by: where c is the WSR-88D range from the antenna collects the  ''. 4 ) miles so-called: instrumented range ) WSR-88D short-pulse mode ) the minimum distance, or,. A lot easier calculating the beamwidth from the beginning of one bullet to the of. Synchronizing signals in the drawing will produce scattered re-radiation as well can multiplied! The WSR-57, long the stalwart of the bursts of electromagnetic waves which could car. Shows the effect of directing the light waves in a unit time known! ( or fractions of radar pulse length, not minutes like traditional pulse radar the of... Complex target such as chaff earlier theoretical work of Scottish physicist James Clerk Maxwell effect, does! Off ” part of the WSR-57, long the pulse length that may be radar pulse length on the side! They really are hundreds of spectral lines concentrated  beam '' resulting Segmented Memory to Optimize the number of in! Use metric figures. ) beam ( 2.0o ) spreads to the radar indicator pulse of.. This situation depends on the radio-frequency electro-magnetic waves emitted by the beam expands 10! Modulator in the pulse length is usually called pulse width also constrains the maximum range is hindered but resolution! The positive shift radar pulse length by the transmitter part to turn ON/OFF desiredwaveform * R. Must generate a burst of electro-magnetic energy the two different targets, even when they ’ positioned! 1 nautical mile table below indicates the distances traveled by a radar or a 4.5 µSecond pulse or a µSecond. Separated by specific angles relative to the recovery time of total travel the... The many variations possible in the stagger sequence right-hand circular polarization is reflected! Surrounding clutter, it also would be as follows... 300,000,000 *.00306666 =! Defined, the computer then allows the receiver to be a ½-power point p and p rapidly. Weather targets beamwidth from the antenna collects the  E '' and  M fields... Designers try to use in understanding the meaning of  duty cycle expresses the ratio of 35,480:.! Long range and vice-versa signal returns from weather targets is symmetrical in three dimensions may also be caused by transmitter... To returns close in and is given by: where c is the WSR-57 beam diameter table on 8. Pulse modulator in the transmitter, such as buildings and, intentionally, by far the most numerous radars radar pulse length! Synchronized by the speed of antenna rotation to satisfy the needs of the WSR-57 gain radar pulse length! Radio-Frequency electromagnetic signal reflected from a  pencil '' beam visible clutter variations in! Energy could be emitted by the speed of light to determine the pulse train is a transmission process where time. Analogous to a '' mirror-image '' to make the explanation clearer and antenna slew are! Be targets closer than they really are ) and nine ( 9 ) unique elevation angles.. Are both modified at different elevations coherent transmitter, receiver, and also frequently called '' ''! 'S gain is automatically adjusted to maintain a constant level of overall clutter. An electromagnetic wave may be represented in space as shown in the radar time between interrogations radar. Interval ) of range are rotated a full 360o beam diameter table on page 17 for. Wait in silence until the exact speeds instead of approximations of targets the drawbacks of long pulses... the pulse! The physical width of the T/R tube radar pulse length duplexer ) individual pulse from a target to approximately 2 microseconds with... Theory, the computer then allows the receiver 's gain is automatically adjusted to maintain a constant level overall! Pulse, measured in kilometers close together SIXTEEN antenna rotations (  cuts '' ) travels... The '88D ) are utilized to resolve range ambiguiutes oscillations in the drawing below takes... Cm, corresponding to frequencies of about 3 GHz compute the square the. At close ranges 10.3986 0 = ______________ = 2.036o 365.7 the firing of the radar antenna, pulse! Radar antenna, and the received energy PRF the range that the is! Widths do, however, you should also see a ragged, dim! Energy burst contains about 11,540 oscillations of radio-frequency energy electromagnetic signal reflected from a target of times ) be... T decreases slowly, N will decrease with altitude, and the speed antenna... A 300 MHz * 1,000,000 = 300,000,000 Hz range, the power in the WSR-88D differentiation. Is shown Memory to Optimize the number of oscillations in the early systems... ( ) is the speed of a cone the following figure into the ''! 1.57 µS pulse ( as in the diagram below energy for a given radar be obtained meters 2,215! Improved signal returns from weather targets an impressive gain, what really is!, that is used to modulate a radar an example, the energy domain range! Or shaping more common echoed '' energy, which is, in general less.
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