For a prediction of the rate of climate change during the 21st century, there is an urgent need to better understand the global carbon cycle, in particular the processes that control the carbon flows between the various reservoirs, and their interactions with the climate system. Atmospheric carbon dioxide (CO2) represents the main atmospheric phase of this biogeochemical cycle. Due to human activities, the concentration of this most important of the Earth’s greenhouse gases has grown from a pre-industrial average atmospheric mole fraction of about 280 parts per million volume (ppm) to 390.5 ppm in 2011 which is an increase of 40%. CO2 contributes to ~63% to the overall global radiative forcing.
A new generation of Scanning Laser Doppler Vibrometer (SLDV) has been realized; based on experience and results of a former proof-of-concept design and a number of field tests. This new device SLDV III comprises a number of technical improvements in the transmitter and receiver section as well as in the evaluation of the recorded vibration signals. The subsequent paper summarizes the main features of this instrument.
The combination of a powerful acoustic transmitter with high resolution laser spectroscopy has led to a promising approach for the detection of buried landmines, especially for those with no or only minor metal content. This paper summarizes current R&D work on this new technology including a brief sensor overview and a more detailed description of our data processing methods. The SLDV sensor picks up tiny vibrations of the soil surface in the order of some μm/s in a rectangular grid of measuring points. We use a multi-threshold algorithm for the segmentation of mine cues and reduce false alarms by analyzing the stability of object size, contrast and shape in the frequency domain. In addition to the amplitude of soil vibration the phase is investigated as a secondary information channel to optimize the classification procedure.
Acoustic landmine detection (ALD) is a technique for the detection of buried landmines including non-metal mines. An important issue in ALD is the acoustic excitation of the soil. Laser excitation is promising for complete standoff detection using lasers for excitation and monitoring. Acoustic excitation is a more common technique that gives good results but requires an acoustic source close to the measured area. In a field test in 2002 both techniques were compared side by side. A number of buried landmines were measured using both types of excitation. Various types of landmines were used, both anti-tank and anti-personnel, which were buried at various depths in different soil types with varying humidity. Two Laser Doppler Vibrometer (LDV) systems of two different wavelengths for the different approaches were used, one based on a He-Ne laser at 0.633 μm with acoustic excitation and one on an erbium fiber laser at 1.54 μm in the case of laser excitation. The acoustic excitation gives a good contrast between the buried mine and the surrounding soil at certain frequencies. Laser excitation gives a pulse response that is more difficult to interpret but is potentially a faster technique. In both cases buried mines could be detected.
Like every measurement device, lidar systems need the possibility to have their results certified according to the criteria of present-day quality assurance. To this aim guidelines are needed. KRdL, the German Commission on Air Pollution Prevention, has been quite active in this field and issued several guidelines on lidar systems and measurements; a few more are actually in preparation. The present paper gives a brief account of the intended purpose, of the procedure followed to get the guidelines written, discussed, approved, and issued, of some of the differences to guidelines for in-situ measurement systems, and of lidar-specific problems and how these problems have been solved. As an example, parts of the guideline "Ground-based remote sensing of visual range - Visual range LIDAR" are presented, emphasizing the difficulties involved when visibility definitions are extended for remote sensing instruments covering a large measuring volume.
The guideline series VDI 3786 'Environmental meteorology; Meteorological measurements' is organized into several parts. The present guideline VDI 3786 Part 14 describes the determination of the three-dimensional wind vector using Doppler LIDAR ('LIght Detection and Ranging' or 'Light Identificaiton, Detection and Ranging'). The guideline refers to guideline VDI 3786 Part 2 with regard to the definition of the measurement variable wind and goes back to the guideline VDI 3786 Part 1 in considering the averaging time. Use is also made of the guideline VDI 3786 Part 8. Safety problems are not treated; reference may be made here to relevant Standards [VBG 93, DIN EN 60825-1]. Wind profiles in the atmospheric boundary layer yield a very important contribution also to the investigation of atmospheric exchange processes. The wind field in the atmospheric boundary layer is highly variable in spatial and temporal scales. For a few applications a more frequeent wind sensing is necessary, i.e. (1) on airports located in low level jet areas, (2) near chemical plants to get information of the transport of toxic gases from leakages, (3) for metrology in general to improve the weather forecast, (4) for environment protection purposes like dispersion studies. The following statements are valid for visibility measurements [visual range LIDAR (VDI 3786 Part 15)]: (1) Lidar can provide the same information of the visibility as conventional sensors, but in addition lidar will provide range resolved measurements. (2) It is possible to shrink a lidar down to the size of binoculars. (3) It is possible to measure local visibility with an eye- safe (class 1) lidar. (4) Layers can be detected up to 250 m distance in approximately 2 s even with a small size instrument.
Methods which are in discussion to enter a VDI guideline will be presented. Examples of application in local scale area selected. The VDI 'Richtlinie VDI 3786 Umweltmeteorologie,' is divided in many parts. Part 15 shows the remote sensing methods for visibility measurements, part 14 describes the wind profile measurements. Wind profiles in the atmospheric boundary layer yield a very important contribution also to the investigation of atmospheric exchange processes. The wind field in the atmospheric boundary layer is highly variable in spatial and temporal scales. For a few applications a more frequent wind sensing is necessary, i.e. (1) on airports located in low level jet areas, (2) near chemical plants to get information of the transport of toxic gases from leakages,(3) for meteorology in general to improve the weather forecast, (4) for environment protection purposes like dispersion studies. The following statements are valid for visibility measurements: (1) Lidar can provide the same information of the visibility as conventional sensors, but in addition lidar will provide range resolved measurements. (2) It is possible to shrink a lidar down to the size of binoculars. (3) It is possible to measure local visibility with an eye-safe (class 1) lidar. (4) Layers can be detected up to 250 m distance in approximately 2 s even with a small size instrument.
The sensitivity of a CO2-lidar system using the DAS-technique is based on the smallness of two different errors: the DAS measurement error and the error in the absorption coefficient. The first one is given by the system stability and the influence of the atmosphere on the return signal of the lidar, meanwhile the spectral characteristic of the laser radiation and the accuracy of the measurement set up are responsible for the error in the detection limits. The combination of both errors is responsible for the total measurement accuracy. This is important for the comparison of the measurement results with those of a FTIR-spectrometer. Due to a difference in the measurement principle, the spectrometer offers the advantage of a wide, continuous coverage of the spectrum (2-15 micrometers - depending on the used detector), but the usable range is restricted to a maximum distance of about 500m. This tunable lidar- system however can be used for measurement distances of up to 5 km, but is limited on a nubmer of discrete wavelengths.
The design, assembly and testing of the laboratory breadboard model of a CO2 laser free-space transceiver terminal with full scale optics have been successfully developed at comparatively low cost. The transceiver has a 140 Mbit/s communication subsystem and a spatial pointing, acquisition, and tracking subsystem. The most important test results are presented and their relevance for other free-space communication systems using different laser technologies is pointed out. A comparison to specifications for intersatellite and interorbit link applications is used to show the basic feasibility of optical space communications and to point out areas which need technological improvements.
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