This paper, “A solid state Doppler wind lidar system on ISSa," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
Doppler lidars measure the range resolved Line-of-Sight (LOS) wind component by extracting the Doppler shift of radiation backscattered from atmospheric aerosols and molecules.The virtual instrument was tested with an existing airborne Doppler lidar to get confidence on the simulation.The local model (LM) of the German Weather Service produced data for all the levels in the atmosphere with the same information as for the global model within a finer grid (7km).
Hardware development for remote sensing costs a lot of time and money. A virtual instrument based on software modules was developed to optimise a small visibility and cloud base height sensor. Visibility is the parameter describing the turbidity of the atmosphere. This can be done either by a mean value over a path measured by a transmissometer or for each point of the atmosphere like the backscattered intensity of a range resolved lidar measurement. A standard ceilometer detects the altitude of clouds by using the runtime of the laser pulse and the increasing intensity of the back scattered light when hitting the boundary of a cloud. This corresponds to hard target range finding, but with a more sensitive detection. The output of a standard ceilometer is in case of cloud coverage the altitude of one or more layers. Commercial cloud sensors are specified to track cloud altitude at rather large distances (100 m up to 10 km) and are therefore big and expensive. A virtual instrument was used to calculate the system parameters for a small system for heliports at hospitals and landing platforms under visual flight rules (VFR). Helicopter pilots need information about cloud altitude (base not below 500 feet) and/or the visibility conditions (visual range not lower than 600m) at the destinated landing point. Private pilots need this information too when approaching a non-commercial airport. Both values can be measured automatically with the developed small and compact prototype, at the size of a shoebox for a reasonable price.
KEYWORDS: LIDAR, Fiber optic gyroscopes, Oscilloscopes, Signal to noise ratio, Amplifiers, Sensors, Signal detection, Picosecond phenomena, Pulsed laser operation, Visibility through fog
Acommercial laser rangefinder was used to measure precipitation. The high pulse repetition rate of the rangefinder (600 Hz) together with a fast digitizer (LeCroy with Ethernet) allows to see single snow flakes or water dropletts. The characteristics of the signals were determined to distinguish between fog, rain and snowfall.
Since the presentation of the virtual backscatter lidar instrument in Firence, a lot of new modules have been added to the virtual instrument. A multiple hard target arrangement with selectable reflection parameters can be placed in a cloud or fog layer to simulate the collision avoidance problem. Further the complete atmospere with aerosols, molecules and clouds can be selected for airborne or spaceborne Doppler lidar simulations.
We present a new software toolbox based on LabVIEW, that simulates the return signal of a backscatter lidar. The beam propagates through two different media: a layer, which is enclosed by two parallel planes and a surrounding medium. A large variety of environmental and instrumental conditions can be chosen for the calculation. Multiple scattering can be taken into account. This new version M contains a variety of new modules including a hard target which can be tilted to simulate the effect of pulse extension.
Using the inverse filtering of the outgoing laser pulse and of the return signal, one can increase the range accuracy of a laser radar for a single pulse. The method was tested experimentally and in the virtual world. A virtual lidar instrument was developed for further studies. The pulse length can vary from 10 ns up to 500 ns for hard targets ranges to remain within 50-cm accuracy. For a gain switched pulse like from a CO2 Doppler lidar with 3 ms pulse duration, the hard target return can be determined with 1-m resolution. For a hard target with an aspect angle to the laser radar, one can determine this angle with sufficient accuracy. The method will be described in detail.
Hardware development for remote sensing costs a lot of time and money. A virtual instrument based on software modules can be developed faster and is flexible for changes. Such a virtual instrument can be used to optimize the sensor in advance. Adaptation of the different user aspects is possible. Finally, tests of the signal quality with existing sensors can be used to understand misalignments and internal sensor problems.
The traffic density on the roads is increasing every day. Therefore, the development of equipment contributing to higher traffic safety deserves particular attention. The perceptibility of automotive lighting and light signals under poor visibility conditions is one of the safety aspects involved. The planned system has the task to ensure that the rear lights are visible to the driver of the vehicle driving behind. Spray, however, as being whirled up by the tires at rain-covered roads also affects the visibility very strongly. Depending on moisture on the road and driving speed, this spray is dragged like a flag of more than 20m behind the vehicle. That is why a measuring principle employing spot-like scanning of only a small measuring volume would not be suitable for this purpose. Thus, a method is needed that can detect the turbidity in a long-extending measuring volume.
Christian Werner, Pierre Flamant, Oliver Reitebuch, Friedrich Koepp, Juergen Streicher, Stephan Rahm, Engelbert Nagel, Michael Klier, H. Herrmann, Claude Loth, P. Delville, Ph. Drobinski, B. Romand, Ch. Boitel, D. Oh, M. Lopez, Mireille Meissonnier, Didier Bruneau, Alain Dabas
Lidar is a well established remote sensing method. One gets range resolved information from remote location. Ranging, cloud ceiling, aerosol layer identification are a few examples. Using a lidar from satellites gives topographic maps of the Earth's surface (Degnan 1997). For environmental purposes the aerosol smoke stack emission canbe monitored (Measures 1983, Klein und Werner 1993). The measuring principle is based onthe time measurementof a laser pulse reflected froma target. Targets canbe hard targets like the Earth's surface or diffuse targets like clouds. The backscattered signal contains the information on the density characteristics of diffuse targets or the reflection characteristics of the hard target. The range resolution depends onthe pulse characteristics transmitted. In the lidar application the range resolution Lx is proportional to the laser pulse length zt: 2 Therefore, for satellite laser ranging short pulses are used to get resolution in the order of centimeters (for example t =200 ps gives 3 cm resolution for a single event). Can one get the same resolution with a longer pulse? -is one of the questions this paper focusses on. The second question is the identification of aerosol concentrations with high range resolution.
Pierre Flamant, Christian Werner, Friedrich Koepp, Claude Loth, P. Delville, Oliver Reitebuch, Ch. Boitel, Didier Bruneau, Ph. Drobinski, R. Haering, H. Herrmann, Michael Klier, M. Lopez, Mireille Meissonnier, Engelbert Nagel, B. Romand, L. Sauvage, M. Schrecker, Juergen Streicher, Guenter Wildgruber
An airborne coherent Doppler Lidar to retrieve mesoscale wind fields has been developed in the frame of the Franco- German WIND project. The instrument is based on a pulsed CO2 laser transmitter, heterodyne detection and wedge scanner. The performance of the instrument operating on the ground and in the aircraft is reported.
Cloud ceiling determination using laser radar (lidar) is a well known application of this remote sensing technique. It is no problem to measure large distances up to some kilometers, since the particles of interest (water droplets) reflect the laser radiation pretty well, even when using very tiny light sources (eye safety criterion). This detection of white scatterers (clouds) points of course to the question whether it will be possible to measure dark particles for example smoke as well. The possible measurement range of the remote sensing smoke detectors cover medium scale observations, like corridor sensors, as well as large scale systems, like replacing fire alarm sensors in a tunnel by one single lidar system. It will be reported on the double impact using the laser radar technique: the range resolved measurement of black smoke as well as using the transmission path of the laser light as a control device.
LabVIEW (National Instruments) provides a powerful instrumentation system for simulations, including an excellent graphical presentation environment. Our Doppler Lidar simulation tool contains signal propagation and scattering in the atmosphere, a model of the heterodyne front end in the low SNR-regime and a processing unit for signal digitizing and frequency estimation. As a consequence of LabVIEW's programming language, G, this end-to-end simulator for Laser Doppler wind measurement can run on either a Windows PC, a Macintosh PowerPC or a SUN station.
Lidar technology and applications are well-established (Kirchbaumer et al. 1993). A backscatter lidar technology experiment was tested in space in 1994 (Winker et al. 1994). Scientists need global information on wind, clouds, and aerosol layers. On a space-borne platform, only a limited amount of power is available for a lidar system. Therefore, a compromise is necessary between the possibilities and the requirements.
The poor signal to noise ratio of Doppler lidar data, given by the small amount of back scattered radiation and the large distance to the measurement volume, enforces the need of some averaging technique. The major question at which state of signal processing the averaging should take place have been investigated. The major problems with incoherent averaging, spurious estimates and as a consequence a bias on the averaged estimate, can be overcome using accumulation technique before the estimate is determined. Computer simulation have shown that this technique shows the same efficiency as one would expect using coherent averaging.
ESA is planning to perform the Atmospheric Dynamic Mission from the International Space Station. The Space Station is a multi purpose platform, many experiments will be carried out during the same time. Therefore, the Doppler lidar instrument ALADIN, the main sensor to get wind information in the troposphere is not on an ideal platform. There is no full coverage and there is limited observation time caused by other orientation of the space station. To answer the question of the usefulness of a Doppler lidar on the space station the so-called targeted observation was mentioned. Is it possible to get result for the weather forecast improvement if one focuses the observation in sensitive areas of the globe. Both sensor specialists and numerical weather prediction scientists work together to answer this question in the Aladin Impact Study. First results will be presented.
The use of coherent Doppler lidar for measuring atmospheric wind fields attracts considerable interest. A number of computer programs have been developed to simulate atmospheric return signals and extract wind speed information from it. We present a new software toolbox based on LabVIEW, differing from previous ones not only by a brilliant graphical user interface but also by stochastic return signal simulation, sophisticated heterodyne front end modelling and the choice among several frequency estimators.
A LIDAR is not only used for lIght detection and ranging, but nowadays it is one ofthe most powerful instruments for analysing the atmosphere. A lidarconsists of a transmitter and a receiver. The transmitter is sending a fairly collimated pulse or modulated beam of monochromatic laser light of a certain frequency into the atmosphere where this light is absorbed or scattered by the particles (molecules, water droplets, ice crystals, dust) of the atmosphere. The highly sensitive receiver records light which is reflected by the atmosphere within acertainfield of view(FOV) and with or without a certain state of polarisation. The transmitterand the receiver may be mounted atthe same place (then we calithe lidar monostatic) or they may be located in different places (then we call the lidar bistatic). Furthermore, the FOV may be directed along the same or different direction asthe (FOV of the) transmitted beam. Very often the FOVs of the transmitter and the receiver are coaxial cones; sometimes the receiver has several FOVs (e.g. a narrow central FOV and a ring shaped outer FOV) thus enabling recording of light of several states of polarisation (e.g. parallel and perpendicular to the direction of polarisation of the transmitted beam) simultaneously. Furthermore, some lidars are capable of transmitting and receiving light of severalfrequencies almost simultaneously. In addition to that some lidars provide a possibility of scanning the atmosphere. lidars may be ground-based, airborne or space-based. The primary goal of Iidar sensing is to acquire meteorologically important parameters of the atmosphere from the lidar signal. To be able to obtain such parameters, it is necessary to describe the lidar signal physically sufficiently precisely by a mathematical formula and to solve this formulaforthe interesting parameters. In this report the application for visibility detection is the object.
More than 10 years of operation of a cw-Doppler lidar for boundary layer winds together with the hardware development of a compact laser Doppler anemometer for ballistic wind measurements and the development of an airborne sensor, concluded in the proposed design of an operational system. The different problems with lasers (10 micron systems vs. 2 micron systems, pulsed or cw), detectors and atmospheric stability conditions will be presented. The hardware of the compact laser Doppler anemometer and the airborne sensor ADOLAR will be analyzed with respect of a proposed automatic operational system. Criteria of the proposed system are the laser, the detection and signal processing algorithm and the reliability of the system in a harsh environment like a battlefield. For the laser the first criteria is the all- weather capability. A cw laser system is affected by a low overcast. Without additional information on the cloud base height the results may be misinterpreted. A pulsed system gives both, wind profile and cloud base height. All other criteria for the selection of an optimal sensor will be presented.
The atmospheric laser Doppler instrument ALADIN will provide information about the wind velocity using the motion of small particles and the resulting Doppler shift between the transmitted and received laser frequency. The accuracy of the frequency estimation depends mainly on the signal-to-noise ratio (SNR) of the received signal. An improvement of the signal quality i.e. SNR can be done by reducing the bandwidth and by averaging over some shots. The bandwidth reduction can be performed either analogue or digital. The analogue technique includes a lot of risks and error sources, the digital demodulation and filtering shows only one critical component, the analogue to digital converter. The shot averaging can also be performed by two different techniques: coherent or incoherent accumulation. Both, the bandwidth reduction as well as the averaging techniques are presented.
The wind field and turbulenceof the Atmospheric Boundary Layer are important parameters for various application fields, like meteorology, atmospheric physics, environmental protection, wind-energy utilization, air-traffic control, and-so-on. Their high variability in spatial and temporal scales necessitates a fast remotesensing method. From the available techniques the Doppler Lidar has proved to be the most promising candidate. Therefore, DLR has combined its well established Laser Doppler Anemometer (LDA) and its laser-based Ceilometer with a Sonic Anemometer.
The concept of a lidar is the optical synonym of the well known radar. It describes an optical remote sensing technique for the detection of the atmospheric parameters like the optical density, etc. The name lidar originates, analogous to radar, from the first application of this technology, the determination of the distance to large solid bodies (laser range finder). The evolution of the lidar technique reaches from the measurement of the distance to diffuse targets, for example clouds (cloud ceilometer), to the determination of the range resolved turbidity of the atmosphere (visibility sensor). The modern lidar systems have been reduced so far, that they can be built into a van or even a car. The problem of the eye-safety of the laser radiation has been solved by using low power laser diodes, but with a high repetition rate. The first results with a modified laser range finder, manufactured by Jenoptik GmbH, will be reported. This device, at the size of binoculars, promises to be a first approach for a handheld visibility sensor, measuring local turbidities, like fog banks.
Claude Loth, Alain Dabas, Pierre Flamant, D. Oh, J. Delume, B. Romand, Jacques Pelon, Didier Bruneau, J.-L. Zarader, Christian Werner, Friedrich Koepp, H. Herrmann, Michael Klier, W. Nagel, Stephan Rahm, Juergen Streicher, J. Wildgruber
WIND is a joint project between France (CNRS-CNES-Meteo France) and Germany (DLR) to develop an airborne wind Doppler lidar for meteorological applications. The instrument specifications are derived from the measurement objectives as well as the state-of-the-art in technology. Presently an operational airborne wind lidar can be designed around the CO2 laser technology, heterodyne detection, and a conical scanning of the lidar line-of-sight to sample the atmospheric wind field. The 10-micron spectral domain is suitable for long range measurements for it corresponds to an atmospheric window and an adequate backscatter coefficient in the troposphere. The first flights are scheduled early 1995 on board the Falcon 20 operated by DLR.
Lidar is the short form of light detection and ranging. The first application of a lidar system was, as in the radar technique, the determination of the distance to large-sized particles (target recognition). Nowadays, it is of more interest to measure the structure of the atmosphere in far distances (remote sensing) to get, for example, information about the mass concentration of the industrial pollution or the visibility conditions in dense fog. In this case the action and reaction of the laser light with the particles is made by very small and different scatterers (molecules, atoms, or aerosols) and, therefore, extremely complex. A simulation program that helps to determine the visibility with a lidar has been developed to present the effects of the components of the system (laser, transmitter, receiver) as well as the parameters of the atmosphere (inhomogeneities, fog, clouds) in a convenient way. A change in any parameter is taken into account instantaneously, so this program can be called an almost real time simulator. A computer with a graphic user interface was chosen to realize this as simply as possible: The Commodore Amiga. The simulation is written in `C' to get the best performance for the calculations.
Lidar systems are widely used in remote-sensing measurements relating to the study of atmospheric physics and its application to environmental protection. Large optical depth values give rise to multiplescattering effects that should be corrected for many lidar applications- in atmospheric gaseous constituent concentration measurements using the differential absorption method and for optical communications. On the other hand, these effects can be used to extract information about the scatterer. In both cases, the single-scattering events need to be separated from those caused by multiple scattering. A lidar simulation program is explained. Experimental methods are described that separate the multiple-scattering effects and use it for the determination of cloud microphysical parameters.
Backscatter lidar systems are widely applied for remote measurements relating to atmospheric physics and environmental protection. One application is the observation of the dispersion of aerosol plumes close to the surface. Plumes from smoke stacks, industrial complexes and power stations are included in the problem. To evaluate various dispersion theories, many artificial plumes were observed with backscatter lidars installed in a van.
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