Laser Radar with Improved Software Reliability Factor

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Laser Radar with Improved Software Reliability Factor

S. Sivaram

  1. Department of Basic Sciences & Humanities, Gudlavalleru Engineering College

    ,Gudlavalleru,Krishna district.A.P

    P.Sudarshan Ray

  2. Department of I.T, KITS Warangal -15

    1. A. Chari

  3. Department of OR & SQC, Rayalaseema university Kurnool, A.P

P. Vemulamma

5. Department of Computer Science & Engineering, Vaagdevi college of engineering, Warangal

Ch. D. V. Subba Rao

4. Department of CSE, S.V.U.C.E, Tirupati-2

P. S. Rama Chandra Rao

  1. Department of Mathematics & Humanities, Vaagdevi college of engineering, Warangal

    P. Uday Kiran Ray

  2. Department of ECE, KITS Warangal-15

Abstract :

INTRODUCTION TO RADAR SYSTEMS

94 GHz by about 5 dB (on average) than horizontal polarization, which is the opposite to the experience at microwaves. (It should be cautioned that the few measurements available were taken at low grazing angle and over a limited range of wind speeds.) As with 'trees and grass, the value of (To for snow increases with frequency and grazing angle.77.78.102 Crusted snow (that which has melted and refrozen), produces higher values of (To at 35 GHz and 94 GHz than wet snow, grass, or trees. At microwaves the backscatter from rain can be explained according to the Rayleigh scattering model with the reflectivity varying as the fourth power of the frequency.

the clear-air attenuation. A fair-weather cumulus cloud has low attenuation at millimeter wavelengths. A one-way attenuation of 0.3 dB/km at 94 GHz might not be significant for short-range radar, but it can be overwhelming for long-range radar. For example, a 5-km radar will experience a two-way attenuation of 3 dB, which is tolerable in most situations. A 50-km radar, however, will have 30-dB attenuation, which is usually intolerable. Thus long-range radar is not likely at millimeter wavelengths under normal circumstances. Although the attenuation at ~nillirnetcr wavelengths is far greater than that at microwave wavelengths, the losses iticurred in propagating ~~lillimeter wavelengths through fog, haze, and smoke is less than at infrared or visible wavelengths. Anotlier example where a particular property of millimeter- wave radar has both favorable and unfavorable aspects is that of the doppler frequency shift. It was shown in Sec. 3.1 that tlic doppler frequeticy shift was proportional to the carrier (rf) frequency. This rest~lts in more accurate relative-velocity measurements with millimeter wavelengths than at lower frequencies. The large doppler shifts at millimeter wavelengths, however, can sometimes result in the echo signal being outside the receiver bandwidth, which complicates the receiver design. Also, the large doppler frequencies at millimeter wavelengths cause the blind speeds of MTI radars to appear at lower velocities than at microwaves, an undesirable property. Thus, a frustrating paradox of the millimeter-wavelength region is that some of its claimed good points are also its weaknesses. Clutter at millimeter wavelengths. In chap. 13 it was implied that knowledge of radar clutter characteristics at microwaves was less than desired. There is even less information on clutter at frequencies above 10 GHz. Details will not be given here, but the general trends will be summarized for the several types of clutter that have been measured. The a&rage clutter cross section per unit area for one set of measurements of trees and vegetation at millimeter wavelengths is given by75.102 aO(dB) = – 20 + 10 log (0125) – 15 log R (14.37) wlicrc 0 is tl~e grazing angle in degrees and R is in centimeters. Thus, a0 increases with increasing frequency. Vertical polarization produced echoes 3 to 4 dB greater than horizontal polarization. These measurements were made in Georgia during the summer and fall months and included deciduous trees and pine trees. The grazing angles were from 2 to 25". The pulse

These measurements were made in Georgia during the summer and fall months and included deciduous trees and pine trees. The grazing angles were from 2 to 25°. The pulse width was. 100 ns and the antenna beam width was in the vicinity of one degree. The value of 0-0 for sea clutter at low grazing angle and low winds has been reported 76 to

.. but not increase with frequency much above X band," but also 77 the trend of increasing reflectivity with decreasing wavelength also holds for sea clutter although the effect is not nearly as pronounced as for land clutter." However, both references state that for higher sea states (the maximum wind speed was 14 knots) the value of 0-0 at 94 GHz is less than that at X hand. Another observation is that vertical polarization seems to produce lower values

of 0-0 at 94 GHz by about 5 dB (on average) than horizontal polarization, which is the opposite to the experience at microwaves. (It should be cautioned that the few meas~lrements available were taken at low grazing angle and over a limited range of wind speeds.) As with trees and grass, the value of a0 for snow increases with frequency and grazing angle.77-78,102 Crusted snow (that which has melted and refrozen), produces higher valt~es of o0 at 35 GHz and 94 GHz than wet snow, grass, or trees. At microwaves the backscatter from rain can be explained according to the Rayleigh scattering model with the reflectivity varying as the fourth power of the frequency (Sec. 13.7). In the millimeter-wave region, the drop sizes of the raindrops span both the Rayleigh and the Mie scattering regions so that a difference in behavior can be expected. Experimental measurements indicated that, on the average, the radar backscatter at 9.4 and 35 GHz far appeared to follow the Rayleigh scattering Deviations from Rayleigh were found at 70 and 95 GHz, and the backscatter at 95 GHz was observed to be less than at 70 GHz. The back scatter at. millimeter waves was found to depend on the drop signal distribution of the pricipitation.102 Even though the rain rates were the same, the backscatter from rain containing drops 2 to 6 mm in diameter was approximately 10 dB greater than rain containing drops 1 mm in diameter or less. The amplitude fluctuations of the rain backscatter were approximated by a log-normal probability distribution over the range from 9.4 to 94 GHz and at rain rates from 1 mm/h to 60 mm/h. The variance (and standard deviation) of the log-normal distribution was independent of rain rate and decreased with increasing frequency. The de correlation times at 95 GHz varied from about 3.4 ms at 5 mm/h rain rate to 1.4 ms at 100 mm/h. At X band the de correlation times were longer and varied from 14 ms at 5 mm/h to 5.4 ms at 100 mm/h. Utility of millimeter waves. Experimentation with millimeter wavelength radiation and microwave radiation both date back to the end of the nineteenth century. The first microwave experiments were those of Hertz in 1886. Experiments at 6-mm wavelength occurred in 1895.80 In the early 1920s, millimeter-wave research was reported in the United States, Germany, and Russia, with wavelengths as short as 0.22 mm.81 Although many important applications of the microwave region have been developed, there has been almost no comparable activity in the millimeter-wave region. There are many possible reasons for this lack of activity, including lack of adequate millimeter-wave components, small antenna sizes, and difficult MTI. Rut the most restricting of all has been the relatively large attenuations experienced when propagating through the clear atmosphere, as well as the added attenuation during rain. The large attenuations are likely to limit millimeter radar to short-range applications where the total attenuations are tolerable or to applications where the atmosphere is absent, such as in space or at very high altitudes. Millimeter wavelengths might also find application when the propagation path does not traverse a large part of the earth's atmosphere, as when a ground- based radar directs its energy at or near the zenith. (At 94 GHz the two-way loss In transiting the entire atmosphere is

about 1.7 dB at zenith, 3.5 dB at 60" from the zenith, and

10 dB at 80" from the zenith.)82 Sub millimeter wavelengths.83s84 The advantages of high resolution, wide bandwidth, and small antenna aperture are even more prevalent in the sub millimeter proportion of the electromagnetic spectrum than at millimeters. The attenuation in the clear atmosphere, however, is far worse than at millimeters and varies from about 10 dB/km at l- mm wavelength to more than 100 dB/km at 0.1-mm wavelength.

Laser radar with improved reliability

The large attenuations experienced in clear air in the millimeter (60 GHz) and submillimeter regions are not present in the infrared and visible regions of the spectrum

564 INTRODUCTION TO RADAR SYSTEMS

94 GHz by about 5 dB (on average) than horizontal polarization, which is the opposite to the experience at microwaves. (It should be cautioned that the few measurements available were taken at low grazing angle and over a limited range of wind speeds.) As with 'trees and grass, the value of (To for snow increases with frequency and grazing angle.77.78.102 Crusted snow (that which has melted and refrozen), produces higher values of (To at 35 GHz and 94 GHz than wet snow, grass, or trees. At microwaves the backscatter from rain can be explained according to the Rayleigh scattering model with the reflectivity varying as the fourth power of the frequency (Sec. 13.7).

In the millimeter-wave region, the drop sizes of the raindrops span both the Rayleigh and the Mie scattering regions so that a difference in behavior can be expected. Experimental measurements indicated that, on the average, the radar backscatter at 9.4 and 35 G Hz appeared to follow the Rayleigh scattering law.79.102 Deviations from Rayleigh were found at 70 and 95 GHz. and the backscatter at 95 GHz was observed to be less than at 70 GHz. The backscatter the millimeter waves was found to depend on the drop size distribution of the precipitation. 102 Even though the rain rates were the same, the backscatter from rain containing drops 2 to 6 mm in diameter was approximately 10 dB greater than rain containing drops 1 mm in diameter or less.

The amplitude fluctuations of the rain backscatter were approximated by a log-normal probability distribution over the range from 9.4 to 94 G Hz and at rain rates from 1 mm/h to 60 mm/h. The variance (and standard deviation) of the log-normal distribution was independent of rain rate and decreased with increasing frequency. The de correlation times at 95 GHz varied from about 3.4 ms at 5 mm/h rain rate to 1.4 ms at 100 mm/h. At X band the de correlation times were longer and varied from 14 ms at 5 mm/h to 5.4 ms at 100 mm/h. Utility of millimeter waves. Experimentation with millimeter wavelength radiation and microwave radiation both date back to the end of the nineteenth century. The first microwave experiments were those of Hertz in 1886. Experiments at 6-mm wavelength

occurred in 1895.80 In the early 1920s, millimeter-wave research was reported in the United States. Germany, and Russia, with wavelengths as short as 0.22 mm. 81 Although many important applications of the microwave region have been developed, there has been almost no comparable activity in the millimeter-wave region. There are many possible reasons for this lack of activity, including lack of adequate millimeter-wave components, small antenna sizes, and difficult MTI. But the most restricting of all has been the relatively large attenuations experienced when propagating through the clear atmosphere, as well as the added attenuation during rain. The large attenuations are likely to limit millimeter radar to short-range applications where the total attenuations are tolerable or to applications where the atmosphere is absent, such as in space or at very high altitudes. Millimeter wavelengths might also find application when the propagation path does not traverse a large part of the earth's atmosphere, as when a ground-based radar directs its energy at or near the zenith. (At 94 GHz the two-way loss in transiting the entire atmosphere is about 1.7 dB at zenith,

3.5 dB at 60° from the zenith, and 10 dB at 80° from the zenith.)82 Sub millimeter wavelengths.83 . 84 The advantages of high resolution, wide bandwidth, and small antenna aperture are even more prevalent in the sub millimeter proportion of the electromagnetic spectrum than at millimeters. The attenuation in the clear atmosphere, however, is far worse than at millimeters and varies from about 10 dB/km at I-mm wavelength to more than 1000 dB/km at OJ-mm wavelength. Laser radar. The large attenuations experienced in clear air in the millimeter (60 G Hz) and sub millimeter regions are not present in the infrared and visible regions of the spectrum (Fig. 14.15). (The atttenuato in rain. Iiowcver. is still large.) Thus laser radars operating in the infrared and visible rcgiolis achieve the ;advantages of high angular resolution, wide bandwidth, and doppler frequency sensitivity without the accompanying disadvantage of high attenuation as experienced ill the sub millimeter region. because of its snr is all physical aperture a laser radar is riot suited for most surveillance apl~lications. It is, however, well suited for precision rneasureriletit and target iniaging. The rfesigri of a laser radar follows the same general principles as otlier radars. but witli some exceptions. R5.93.94.105 111 ndcfition to using a trnnsriiitter unlike those found at niicrowave frequencies, the laser radar cxl~ihits otlicr difScrcr~ccs that n~irst be accounted for wlicn considering radar desigli. For cxnmidc. tile receiver sensitivity is riot determined by thermal noise as at microwaves, hut by qili~~itt~~ii eISect~. 'I'lic tio~sc I)OWCI. per iltiit batidwidtli at most laser frequencies is given by \vlicrc /I = Planck's colistant = 6.626 x 10-" J .s, and./'= frequency. The familiar expression for thermal noise power at rtiicrowave frequencies (No = kT, where k = Boltzmann's constant arid 7' = absolute temperature) does not apply when kT Q It is the coarseness of the laser signal itself, due to its quantized nature, tliat ultimately sets tlie limit to sensitivity as given by I:q. (14.3X). 13y settirig k7;, = lrfl an "eqi~ivaletit noise temperature" can be obtained for a laser receiver. At the CO, laser wavelength of 10.6 pm, T,

= 1360 K and at 1.0 pm wavelength 7;. = 14.400 K, which it indicates that laser radar receivers are generally of greater effective tcriil?eri~ti~~.c (or noise figure) than low- noise microwave receivers. Tlic laser radar can employ the equivalent of either the microwave video receiver or the si~pcrlicterodync receiver. Tlie fo~.riier is called an incoherent (envelope) receiver or direct pliotodetcction. – Plic Inttcr is called a coherent (heterodyne) receiver or photomixer. Wlieri tlie background noise is low and for short-pulse modulation the laser envelope detector can operate :IS a qt~arit~~rn-litnitcd device and give essentially the same detectitlity (inverse of the noise equivalent power) as heterodyne detector^.^^ (This is unlike microwaves where the video detector is far less sensitive.) Tlie heterodyne, or photomixing, receiver can have a narrow passbarid arid significantly reduce tlie effects of background noise. Its sensitivity also car1 approach that of an ideal quantum detector. The photomixing receiver is more complicated than the direct photodetection receiver and it requires a stable transmitter and local oscillator. Wlieri tlie target is in motion relative to the radar a large doppler frequency shift occurs which can place the echo signal outside tlie receiver passband. For example, with a relative velocity of 5 m/s (10 kt) thedoppler frequency shift at 1-pm wavelength is 10 MHz. A rapidly tuning laser local osciflator or a large bank of IF filters are necessary to compensate for tlie /arge doppler freqilericy sliift. When the target is in tile far field of the laser antenna, and if the antenna beam is larger tliari tlic target, tlie i~sual form of tlie simple radar equation of Sec.

1.2 applies. However. the beatnwidtli rather tliari the antenna gain is usually measured at laser frequencies so that (;, = nZ/Oi [nay be silbstitrlted into the radar equation. Tlie minimum detectable signal for quarituni-lirnitcd detcctiorl isR5 where 11, = required nuniber of signal photoelectrons, q = quantum efficiency of the detector, and r = observation time. Witli tliese substitutions, and with 2B

= l/r (assuming a video

Tlie minimum detectable signal for quarituni-lirnitcd detcctiorl isR5 where 11, = required nuniber of signal photoelectrons, q = quantum efficiency of the detector, and r = observation time. Witli tliese substitutions, and with 2B

= l/r (assuming a video receiver) the radar equation can be written OTHER RADAR TOPICS 565 (Fig. 14.15). (Thc

attcnuation in rain. however. is still large.) Thus laser radars operating in the infrarcd and visihlc regions achicvc- thc advalltagcs ofhigh angular resolution, wide bandwidth,

. and doppler frequency sensitivity without the accompanying disadvantage of high attenuation as expericnccd in the submillimcter rcgion. Bccausc of its small physical apcrture a laser radar is not suited for most surveillance applications. It is, however, well suited for precision measurement and target imaging..The design of a laser radar follows the same general principles as other radars. hut with some cxceptions.RS.93.94.I05 In addition to using a transmitter unlike those found at microwave frequencies, the laser rada I' exlJi bits ot her d irlercnces that Illust be accounted for when considering radar design.

For example. the receiver sensitivity is not determined by thermal noise as at microwaves, but by quant um effects. The noise power per unit bandwidth at most laser frequencies is given by No = hI' (14.3R) (14.39) \vhere II = Planck's constant = 6.626 x 10- J-l J .s, and f = frequency. The familiar expression for thermal noise power at microwave frequencies (No = kT, where k = Boltzmann's constant and T = absolute temperature) does not apply when kT hI It is the coarseness of the laser signal itself. due to its quantized nature, that ultimately sets the limit to sensitivity as given by Fq. (14.3R). By settling kJ:. = Ill: an "equivalent noise temperature" can be obtained for a laser receiver. At the CO 2 laser wavelength of 10.6 11m, I;. = 1360 K and at 1.0 11m wavelength 7;. = 14.400 K. which indicates that laser radar receivers are generally of greater effective temperature (or noise figure) than low-noise microwave receivers. The laser radar can employ the equivalent of either the microwave video receiver or the super heterodyne receiver. The former is called an incoherent (envelope) receiver or direct photo detection. The latter is called a coherent (heterodyne) receiver or photomixer. When the back ground noise is low and for short-pulse modulation the laser envelope detector can operate as a quantum-limited device and give essentially the same detect ability (inverse of the noise equivalent power) as heterodyne detectors. 8s (This is unlike microwaves where the video detector is far less sensitive.) The heterodyne, or photomixing, receiver can have a narrow passband and significantly reduce the effects of background noise. Its sensitivity also can approach that of an ideal quantum detector. The photomixing receiver is more complicated than the direct photodetection receiver and it requires a stable transmitter and local oscillator. When the target is in motion relative to the radar a large doppler frequency shift occurs which can place the echo signal outside the receiver passband. For example, with a relative velocity of 5 mls (10 kHz) the doppler frequency shift at I-Jim wavelength is 10 MHz. A rapidly tuning laser local oscillator or a large hank of IF filters are necessary to compensate for the farge doppler frequency shift.

Conclusions :

When the target is in the far field of the laser antenna, and if the antenna beam is larger than the target. the usual form of the simple radar equation of Sec. 1.2 applies. However. the beam width rather than the antenna gain is usually measured at laser frequencies so that advanced radar InSAR can also be developed.

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