微脉冲激光雷达同能见度仪水平能见度测量对比实验与分析

Visibility Comparisons over the Horizontal Path by Lidar and VisibilityMeterSun zhaobin Guo jinjia Liu zhishen Ma sen Liu zhen Laboratory of Ocean Remote Sensing of Ministry of Education of China, Ocean University of China,Qingdao, 266003, ChinaAbstract: As a new tool to detect atmosphere, 1idar can derive the value of visibility by directly collecting the scattered laser from the atmosphere. It makes lidar become a main instrument for measuring visibility in the future．Simultaneous measurements of atmosphere visibility by Micro-pulsed lidar and visibility meter NQ-1 have been performed to validate the feasibility of MPL systems designed by Ocean Remote Sensing Laboratory ,Ocean University Of China， from October 21 2005 to November 21 2005 in ShiLaoRen Tour Garden in Qingdao . This paper summarizes the characters of the lidar system and the comparison results.Key words：visibility ，Micro-pulsed lidar(MPL),Visibility meter, extinction coefficient, aerosol1、Introduction:For obvious reasons, visibility is a crucial parameter for aeronautical, seafaring, expressway and military operations. Atmospheric visibility can be reduced by fog, snow, dust, sand, smog or any combination of them, and it is a part of normal atmospheric phenomena. According to the American Meteorological Society’s Glossary, daytime atmospheric visibility is defined as the greatest distance from an observer where a prominent dark object against the sky at the horizon can be seen and identified by an unaided eye. At night, a known, preferably unfocused, moderately intense light source is placed as the visibility target of recognition for the measurement of maximum visible distance. In most domestic and overseas meteorological departments, the method to measure visibility is by eyeballing which lacks of criterion and objectivity. In recent years, many instrumental approaches for measuring visibility have been developed such as using forward scatter meters ,backward scatter meters an d some prototype systems that use a camera and automatically analyze an image by recognizing (or not recognizing) predefined marks. Visibility meter is the general term for instruments used to make direct measurements of visual range in the atmosphere or of the physical characteristics that determine the visual range of the atmosphere. Visibility meters may be classified according to the quantities that they measure. Telephotometers and transmissometers measure the transmissivity or, alternatively, the extinction coefficient of the atmosphere. Nephelometers measure the scattering function of the particles suspended in the atmosphere. A third category of visibility meters makes use of an artificial “haze” of variable density that is used to obscure a marker at a fixed distance from the visibility meter. Most of these visibility meters have been commercially used in some meteorological departments and airports. The principle of the visibility meter we used is that it evaluates visibility by directly measuring transmission coefficient of the atmosphere between two fixed points. A new preferred instrumental method is using lidar. The principle for visibility measurement of lidar is to retrieve visibility by measuring the extinction coefficient of the atmosphere which is more accurate and objective. It is a new method in atmosphere detection .The limitations of traditional lidar are that itcosts a lot and it is expensive for maintenance and needs professional technicians to operate. Thus Micro-pulsed lidar, a cheaper, less complex lidar, was designed to satisfy this requirement. Actually, MPL has come to market since 1994 produced by Science and Engineering Services Inc. (SESI) USA. In this experiment, the MPL used was designed independently by Ocean Remote Sensing Laboratory, Ocean University Of China and the visibility meter was designed by Shanghai ChangWang Meteorotech Ltd. In this paper, we mainly discuss the lidar method and the visibility comparisons with the NQ-1 visibility meter.2、System description and methodologyThe micro-pulsed Lidar, an eye safe, compact, solid state lidar for profiling atmospheric cloud and aerosol scattering, has been demonstrated in Fig.1 .The main specifications of the MPL are shown in Table.1.Fig.1 The diagram of MPLTable.1 the diameters of the MPLTransmitter Laser: Nd: YAG ,SHG 532nmOutput wavelength 532nm Output energy perpulse 6µJ Repetition rate 2KHzReceiverCassegrain/203mmField of view （full angle ）:0.5mradDetector PMT Detection mode Photon countingVertical resolution 37.5m Detection range50m-30KmV olume 0.8m 3Weight 25Kg The derivation of the extinction coefficient from lidar data using the slope method is the lidar equation (J.D.Campbel and D.L.Hlavka,1998)(1)22()[(()[()])()]()()b ap c P r n r DTC n r n n r r O r CE r T β=∗−−=Where:n(r): the measured signal return in photo electron counts per second at range rOc: the overlap correction as a function of range caused by field of view conflicts in the transmitter-receiver systemβ(r): the backscattering cross section due to all types of atmospheric scattering C: a dimensional system calibration constant E: the transmitted laser pulse energy T: atmospheric transmittancen b :background contribution from ambient light n ap(r): the afterpulse correction for detector run onDTC: the detector offset deadtime correction as a function of raw count rate. Knowing that T eτ−=a nd r τσ=, where τi s the optical thickness through the layer and τi sthe extinction cross section, we can rewrite Eq. (1) as:2()()()r c P r O r CE r e σβ−= (2)When there is a complete overlapping between the transmitter and the receiver fields of view, the convergence factor is equal to 1, the overlap ceases to exist and the correction factor O c (r)becomes 1.0. For the section of this function where O c = 1.0, taking the natural log of both sides of Eq. (4) yields:ln ()ln[()]2P r CE r r βσ=− (3)Because this layer is assumed homogeneous , CE βb ecomes constant. Plotting ln[P] versus r takes the form of y = mx +b with -2σa s the slope m.From Koschmieder lawexp()w k V εσ=− (4)ε: the mean brightness contrast threshold of normal eyesV k :the horizontal visibility σw ：the extinction coefficientwe can obtain the horizontal visibility V kln k wV εσ=−（5）WMO recommend thatεequals 0.02,while ICAO recommend 0.05. In this experiment we adopt εas 0.02.3.912k wV σ=−(7)NQ-I visibility meter is a instrument which adopts advanced photo-electronic andcommunication technique to evaluate visibility based on the principle of forward scattering. The main parameters for NQ-I are as following. Detailed information for this equipment may refer to Shanghai ChangWang Meteorotech Ltd.Fig.2 NQ-1 Visibility meterTable.2 the parameters of NQ-1 visibility meterDetectionrange6—16000m(or 6—50000m adjustable)Detectionaccuracy±10% (when ≤60m error ±6m) Power supply AC 220V ±10% communication RS-232/RS-485/GPRS/CDMA(selectable) SuitableTemperature-35~55 ℃Remarks Resistance to salt and fog erosion, suitable for using in coastal area3、The result of comparisonWe compare the visibility retrieved from MPL and visibility meter, the result is shown in Fig.3 and Fig.4.Fig.3 Lidar/Visibility meter comparisons over the horizontal pathFrom the figure above, we can find a good correlation between Lidar and the visibility meter when the visibility is at a range of 1-5km. we obtained the root mean square deviation is 0.3473km and the root mean square relative deviation is 14.54%. These data we obtained were collected between 19:00 to 20:00 on October 27, 2005, a thin foggy day. At this time, the influence of background was not as strong as in daytime and a thin fog avoided multiple scattering.Fi.g.4 Lidar/Visibility meter comparisons over the horizontal pathFig.4 indicates the whole data we obtained from October 21 2005 to November 21 2005.During this period, we experienced fine, cloud, foggy, rainy days especially low temperature at the end of these days. Although there is a bigger deviation at the end of this experience, we can find a good trend in these data.4、Summary and conclusionsSome 550 data samples of lidar/visibility meter comparisons have been obtained under different situations from October 21,2005 to November 21,2005 .All the data obtained were measured at the horizontal path. From the results above, we may find our measurements show a good correlation between the two instruments at the horizontal path .However, the results of the whole experiments shows no such satisfaction. The cause for this phenomenon we interpreted to be the results of temperature difference. At the last days of this experiment, there was a great temperature fluctuation that the temperature may fall below zero on the top of mountain at night, it may be 10 at midnoon. Because the adjustable arms are made from aluminum, which has a high expanding coefficience, the optical path may change and the overlap of the system is changed at the same time. The result is that we can’t have valid data all the time. We will try to solve this problem in the following work and find other ways to improve the stability of this lidar system..Reference1、Research and Experiments for Visibility by Micro-pulse Lidar (MPL) Jinjia Guo 20042、Lidar-Transmissometer Visibility Comparisons Over Slant and Horizontal Paths J.L GaumetAND A.Petitpa 19823、Operational Cloud Boundary Detection and Analysis from Micropulse Lidar Data J. R.Campbell and D. L. Hlavka etc.4、Mobile lidar for visibility measurement XIE Chen—bo etc. HIGH POW ER LASER AND PAR F1C1 E BEAM S Vo1．17。