Optical turbulence poses a significant challenge for free-space optical communication systems. However, there has been limited research on the characteristics of vertical optical turbulence in urban environments. The TURBO (TURBulence mOnitor) project, a research initiative involving the University of Durham, the European Space Agency (ESA), and Universitat Politècnica de Catalunya (UPC), aims to address this gap. The project provides continuous 24-hour turbulence data to improve our understanding of atmospheric turbulence in Barcelona. Through data assimilation, our goal is to assess the performance of a potential optical ground station within an urban environment and validate optical turbulence forecasts.
TURBO (TURBulence MOnitoring and forecasting Equipment) is the first robotic, 24-hour, urban turbulence monitoring station. Situated on the roof of the D4 building at Campus Nord, Universitat Politècnica de Catalunya (UPC) in Barcelona, TURBO represents a joint effort by CommSensLab-UPC, the European Space Agency and Durham University. The system is centred around two 24-hour Shack-Hartmann Image Motion Monitors (24hSHIMM). The 24hSHIMM utilises a small telescope and Shack-Hartmann wavefront sensor to observe single stars in the short-wave infrared and derive key optical turbulence (OT) parameters continuously, throughout the day and night. We present novel data on seasonal, 24-hour urban turbulence conditions and the effects of fully continuous data assimilation for improving the accuracy of mesoscale forecasting tools. We discuss the first few months of results from the experiment and the implications for current adaptive optics activities in astronomy and optical communications.
Aerosols are significant atmospheric constituents that modulate radiation and cloud processes. We evaluated 17-year aerosol profile trends in Barcelona, Spain, from lidar measurements. In summer aerosol reaches 5 km, while in the other seasons it exhibits clear exponential decay. Sahara dust transport affects all seasons, with winter layers above and others penetrating the boundary layer. This study informs the formation of haze and urban preservation strategies in the Mediterranean. The analysis puts in evidence that the averaged net radiative effect is of cooling at both surface level and top of the atmosphere.
Advances in interference filter technology permit currently using selected parts of the N2 and O2 pure rotational Raman spectra with very low temperature sensitivity, while rejecting sufficiently the elastic return. The Raman technique to retrieve the aerosol extinction coefficient can then be used with higher signal-to-noise ratios, because of a higher (about 8 times) effective differential backscatter cross-section as compared to the cross-section of the N2 vibro-rotational spectra. The design and results of pure rotational Raman channels at 354 nm and 530 nm allowing daytime aerosol extinction measurements implemented at the EARLINET/ACTRIS Barcelona lidar station are presented and discussed.
In terrestrial free-space laser communication, aside from pointing issues, the major problem that have to be dealt
with is the turbulent atmosphere that produces irradiance fluctuations in the received signal, greatly reducing
the link performance. Aperture averaging is the standard method used to mitigate these irradiance fluctuations
consisting in increasing the area of the detector, or effectively increasing it by using a collecting lens with a
diameter as large as possible. Prediction of the aperture averaging factor for Gaussian beam with currently
available theory is compared with data collected experimentally and simulations based in the beam propagation
method, where the atmospheric turbulence is represented by linearly spaced random phase screens. Experiments
were carried out using a collecting lens with two simultaneous detectors, one of them with a small aperture to
emulate an effective point detector, while the other one was mounted with interchangeable diaphragms, hence
measurements for different aperture diameters could be made. The testbed for the experiments consists of a
nearly horizontal path of 1.2 km with the transmitter and receiver on either side of the optical link. The analysis
of the experimental data is used to characterize the aperture averaging factor when different values of laser
divergence are selected.
The Roque de los Muchachos Observatory, located on the island of La Palma in the Canary Islands, is home of many
astronomical facilities. In the context of the Extremely Large Telescope Design Study, an intensive lidar campaign was
performed in the ORM near the Jacobus Kapteyn Telescope (17°52'41.2" W, 28°45'40.1" N, 2395 m asl) between 26th
May and 14th June 2008. The goal of the campaign was to characterize the atmosphere in terms of planetary boundary
layer height and aerosol stratification vs. synoptic conditions. As a by-product an estimate of the aerosol optical
thickness was also obtained and compared to the total atmospheric extinction coefficient measured by the Carlsberg
Meridian Telescope.
A two-component laser Doppler velocimeter (2D-LDV) prototype for surface-displacement measurement is presented. The system proposed is based on the Doppler differential technique with a backwardscattering configuration. A reference and two frequency-shifted probe beams are generated departing from a single laser beam by means of two beamsplitters and acousto-optic modulators. This configuration has the advantage of using a single-wavelength laser source and a single detector system, while permitting sign detection of the two vector components of the velocity in surface-displacement measurements. It also enables the implementation of a low-cost multitrigger data acquisition system, which is useful in low signal-to-noise ratio (SNR) situations, such as the velocity measurement of low-dispersion surfaces or in hydrodynamic applications. The prototype has been designed to measure velocities up to 3 m per second, both in industrial (solid target) and hydrodynamic applications, with an error well below 1% in the tests performed. The system works in real time with the use of an acquisition card and C++ based software on a PC.
Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of
their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and
low power. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity
variations (scintillation) and beam profile distortion and boresight wander. These variations manifest as a multiplicative
noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many
years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation
problem that bear on the design and performance of optical communication systems. In this work, it is our intention to
elucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric
propagation and, in so doing, to quantify the improvement on the performance of optical communications systems
regarding coherent detection.
The European Aerosol Research Lidar Network (EARLINET) was established in 2000 to derive a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on the European scale.
At present, EARLINET consists of 25 stations: 16 Raman lidar stations, including 8 multi-wavelength Raman lidar stations which are used to retrieve aerosol microphysical properties.
EARLINET performs a rigorous quality assurance program for instruments and evaluation algorithms. All stations measure simultaneously on a predefined schedule at three dates per week to obtain unbiased data for climatological studies.
Since June 2006 the first backscatter lidar is operational aboard the CALIPSO satellite. EARLINET represents an excellent tool to validate CALIPSO lidar data on a continental scale. Aerosol extinction and lidar ratio measurements provided by the network will be particularly important for that validation.
The measurement strategy of EARLINET is as follows: Measurements are performed at all stations within 80 km from the overpasses and additionally at the lidar station which is closest to the actually overpassed site. If a multi-wavelength Raman lidar station is overpassed then also the next closest 3+2 station performs a measurement.
Altogether we performed more than 1000 correlative observations for CALIPSO between June 2006 and June 2007.
Direct intercomparisons between CALIPSO profiles and attenuated backscatter profiles obtained by EARLINET lidars look very promising.
Two measurement examples are used to discuss the potential of multi-wavelength Raman lidar observations for the validation and optimization of the CALIOP Scene Classification Algorithm.
Correlative observations with multi-wavelength Raman lidars provide also the data base for a harmonization of the CALIPSO aerosol data and the data collected in future ESA lidar-in-space missions.
Free-space optical communications have distinct advantages over conventional RF and microwave systems by virtue of their high carrier frequencies that permit high modulation bandwidth, enhanced security, freedom from interference, and low powered. However, the turbulent atmosphere causes phase variations along the path that are manifested in intensity variations (scintillation) and high beam divergence. These variations are a noise source that reduces the ability of the receiver to determine the information contained in the modulation. For many years, the emphasis throughout this area has been on elucidating those implications of the atmospheric propagation problem that bear on the design and performance of optical communication systems. In this work, it is our intention to elucidate how the addition of adaptive optics to the transmitter or receiver can reduce the effects of atmospheric propagation and, in so doing, to quantify the improvement on the performance of optical communications systems regarding incoherent detection. Adaptive optics offers the potential for overcoming these limitations by adaptive tracking of the beam and correction of atmospherically-induced aberrations.
Since the European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, ESA and the Instituto de Astrofisica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The experiments aimed at characterizing statically and dynamically the performance of the optical downlinks and uplinks in different atmospheric turbulence conditions, together with the development and testing of appropriate theoretical models to predict the link performance. An overview of the OGS and additional facilities on top the IAC Teide Observatory is given, as well as a summary of the statistical results on propagation and communication for different experimental configurations, including different number of transmitting subapertures and divergence in the uplink. Key parameters, like scintillation and fade and surge statistics, are correlated with theoretical predictions and an analysis of the far field pattern is included. The results of the deep space uplink experiments between the OGS and ESA satellite SMART-1 are also presented. Finally ESA free space optical communication programs are summarised, including optical payloads on board different satellites.
As a support for the experimental activities related to the operation of bi-directional optical links between the European Space Agency (ESA) ARTEMIS geostationary satellite and the Optical Ground Station (OGS) in Teide Observatory (Tenerife island, Canary Islands, Spain), carried out by ESA and Instituto de Astrofisica de Canarias (IAC), calculations on the behavior of a multi-beam ground-to-satellite link have been performed. The goal is to assess the impact of refractive turbulence on the uplink (deemed to be more critical than the downlink because of beam-wander effects) and the mitigation effect on the power fluctuations in the satellite receiver achievable with such a space diversity approach, that involves several mutually time-incoherent beams in the uplink. Results from the multiple-beam uplink model and comparison with experiments are presented.
Information about atmospheric variables, such as presence and characteristics of supended particulates (aerosols), concentration of chemical species, humidity, temperature, etc., can be obtained remotely in an energetic-efficient manner from radiation produced by optical sources and backscattered by the atmosphere, drawing on the relatively strong interaction between electromagnetic radiation at optical wavelengths and particulates and molecules in the atmosphere. Atmospheric probing systems based on laser sources can provide information as a function of range and, being their operating principle essentially the same as that of radar (except for the fact that in atmospheric probing the target is by definiti8on the atmosphere zone illuminated by the laser, instead of a hard target), they are usually called laser radars or lidars (from Light Detection And Ranging). In addition to their ability to perform remote measurements- a characteristic that they share with other systems-lidars feature the availability of quasi-real-time data, high spatial resolution and a relative convenience to perform three-dimensional scans of the atmospheric volume under study. Basic lidar principles and capabilities are discussed in this work.
The European Space Agency (ESA) has launched the geostationary data-relay satellite ARTEMIS with one of its payloads being a laser communication terminal (LCT). The LCT is used within the semiconductor-laser intersatellite link experiment (SILEX) to receive Earth observation data transmitted from a similar LCT onboard the SPOT-4 satellite. To support SILEX, ESA has also reached an agreement with the Instituto de Astrofisica de Canarias (JAC) to build the Optical Ground Station (OGS), in the Teide Observatory ofthe IAC. ARTEMIS and the OGS are an ideal and unique test-bed to study and characterise laser beam propagation through atmospheric turbulence. Theoretical models of laser beam propagation through atmospheric turbulence have been reviewed and developed, to predict the performance of the optical links from the propagation and communication point of view. Special effort has been invested in modelling the uplink effects. Optical link experiments have been planned in detail, to gather the necessary data required to be statistically representative, to compare the results with theoretical predictions and to validate and adjust the theoretical models. This comparison will pave the way towards the implementation of deep-space laser communication links. The first results ofthese experiments, presenting the theoretical models, analysing separately downlink and uplink measurements, and comparing the data with the theoretical predictions, are presented.
Lidar (radar laser) systems take advantage of the relatively strong interaction between laser light and aerosol/molecular species in the atmosphere. The inversion of optical atmospheric parameters is of prime concern in the fields of environmental and meteorological modelling and has been (and still is) under research study for the past four decades. Within the framework of EARLINET (European Aerosol LIdar NETwork), independent inversions of the atmospheric optical extinction and backscatter profiles (and thus, of the lidar ratio, as well) have been possible by assimilating elastic-Raman data into Ansmann et al.’s algorithm [the term “elastic-Raman” caters for the combination of one elastic lidar channel (i.e., no wavelength shift in reception) with an inelastic Raman one (i.e., wavelength shifted)]. In this work, an overview of this operative method is presented under noisy scenes along with a novel formulation of the algorithm statistical performance in terms of the retrieved-extinction mean-squared error (MSE). The statistical error due to signal detection (Poisson) is the main error source considered while systematic and operational-induced errors are neglected. In contrast to Montercarlo and error propagation formulae, often used as customary approaches in lidar error inversion assessment, the statistical approach presented here analytically quantifies the range-dependent MSE performance as a function of the estimated signal-to-noise ratio of the Raman channel, thus, becoming a straightforward general formulation of algorithm errorbars.
The classical radio technique used for FM detection, the frequency discriminator can also be used in optical frequency detection. In this sense, Chanin et al [2] proposed a lidar system that measures atmospheric wind fields by detecting Doppler-shift in the return signal in a differential way by using two Fabry-Perot interferometers or any other high resolution optical filters as frequency discriminators. This technique was also studied and used by Korb et al. They named it "edge-technique." The UPC lidar group is developing a wind lidar based on the "edge-technique." The first prototype of the wind lidar is a continuous-wave system that is able to measure the surface displacement velocity of solid targets. The transmitting laser is the seeder of a Spectra Physics GCR-190 laser, which will be used for the final wind measurements. It includes a Fairy-Perot interferometer, two APD-based optical receivers, and several auxiliary optics, electro-optic and electronic elements. Among them, there is a servo-loop, based on two acousto-optic frequency-shifters and a lock-in amplifier, which is responsible of the proper tuning between the laser and the Fabry-Perot interferometer. To our knowledge, this servo-loop has not been used before for wind lidars based on the edge-technique. The aim of this first prototype is to test the performance of the edge-technique to measure velocities and to assess the role of the servo-loop in the precision of the measurements. The study and design of the prototype, with emphasis in the servo-loop will be presented.
The optimization and performance assessment of a reference-beam, continuous-wave, heterodyne low-power laser radar prototype is presented, based on previous homodyne prototypes. It measures both magnitude and sign of the radial component of the displacement velocity. The basic set-up includes a low power (~3 mW) commercial HeNe laser, a beam-splitter, an acousto-optic modulator, and a two-lens system that both focuses the transmitted beam on the target surface and collects the scattered light. Both the reference beam and the radiation collected are focussed onto a Si avalanche photo-detector. The self-aligned configuration of the receiver makes possible, theoretically, to perform optimal mixing between the received scattered radiation and the reference beam. The resulting electrical signal is fed to a transimpedance amplifier and displayed on a spectrum analyser. Laboratory experiments employing as a target the rim of a 50 cm-diameter rotating wheel placed at several distances have been performed. Results concerning detected signal-to-noise ratio, detected-signal spectral width, accuracy of the radial component of the velocity under measurement and system working range will be presented and discussed.
The newly developed Nd:YAG portable 3-D-scanning lidar from the Universitat Politecnica de Catalunya (UPC, Technical University of Catalonia, Barcelona, Spain) was used to improve our knowledge of the aerosols properties in the Barcelona area where an important number of pollution and saharan dust events can be observed all year round in the atmosphere. The system simultaneously operated at the 1064-nm and 532-nm elastic wavelengths, and was used in its scanning mode from 15 degrees to 70 degrees from zenith with 5 degree steps. A variational method was used to invert the multi-angular profiles and to retrieve the aerosols optical thickness and backscatter coefficient at each wavelength without making any assumption on the aerosol type. At the same time, the ratio of the backscatter profiles was used to retrieve the profile of the Angstrom coefficient in backscatter. The backscatter-to-extinction ratio could not be calculated directly but various values of this parameter were used in Klett method until backscatter coefficient profiles could match the one retrieved with the variational method (at least in some altitude regions). Very good agreement (differences less than 20%) was observed in the 0.3 - 2.5 km region with a value of 0.030 sr-1, whereas no agreement could be achieved above where supposedly mixed aerosols were initially observed. The lidar profiles closer to the zenith at both wavelengths allowed to calculate a new Angstrom coefficient in backscatter that is compared to the one retrieved by the variational method. The comparison showed good agreement in the lower layers and thus validated the backscatter-to-extinction ratio profiles used in the Klett method. However, the difficulties encountered to invert the lidar signal above an altitude of 1.6 km show that non negligible inhomogeneities of the atmosphere were present in each line of sight, proving the dense and fast-moving aerosol load over the Barcelona basin.
A novel configuration for a reference-beam, continuous-wave, heterodyne low-power radar prototype is presented. It measures both magnitude and sign of the radial component of the displacement velocity. The basic set-up includes a low power (~10 mW) commercial HeNe laser, a beam-splitter, an acousto-optic modulator, and a two-lens system that both focuses the transmitted beam on the target surface and collects the scattered light. Both the reference beam and the radiation collected are focused onto a Si avalanche photo-detector. The self-aligned configuration of the receiver makes possible, theoretically, to perform optimal mixing between the received scattered radiation and the reference beam. The resulting electrical signal is fed to a transimpedance amplifier and displayed on a spectrum analyzer. Laboratory experiments employing as a target the rim of a 50 cm-diameter rotating wheel placed at several distances have been performed. Results concerning detected signal-to-noise ratio, detected- signal spectral width, accuracy of the radial component of the velocity under measurement, system working range, and system tolerance in focus-adjustment distance will be presented and discussed. Compared to a previous homodyne prototype presented by the authors, the present system shows a shorter working range (~12 m compared to nearly ~16 m in the homodyne prototype). We attribute this smaller range to the additional losses in the acousto-optic modulator.
This article is aimed at describing the technology, system architecture and specifications of a new 3D Nd:YAG scanning lidar. Main features of the system are interspersed low-range and far-range exploration, open user-configuration scanning tools and a specific architectural design based on parallel CPU control, a LabView user interface and a digitally controlled optoelectronic receiver. The latter provides key advantages to the whole system architecture such as calibration of lidar returns in terms of absolute power and repeatability. Issues concerning system responsivity calibration, receiver gain self-calibration, automatic gain control and synchronization offset-drift zeroing and the like, all of which are of prime importance for the lidarist, are presented. As far as we know, these contributions are new to the state-of-the-art of the community of optical and electronic lidar system designers.
Results presented in this contribution correspond to the first measurements made with the transportable lidar system developed by the Universitat Politecnica de Catalunya, which were collected during the eyra 2000 in the city of Barcelona. The system uses a Nd:YAG laser at 1064 nm wavelength and 0.35 J pulse energy at a 20 Hz pulse repetition rate, has a 20 cm diameter telescope and a scanning range of 120 degrees in elevation and 300 degrees in azimuth. In this study only vertical profiles of atmospheric extinction will be shown. Data were acquired under several meteorological situations, showing the influence that this factor has in the arrangement of aerosols in the vertical dimension, and the distributions of backscatter extinction coefficients obtained from the lidar. Data have also been compared to radiosonde profiles acquired in Barcelona at a near time and have shown how the vertical arrangement of aerosols is correlated with changes in atmospheric stability condition, water content and wind direction.
In 1992 an experimental campaign took place in Barcelona whose main goal was the study of the circulatory patterns of air pollutants in the region. An elastic-backscatter lidar operating at a wavelength of 1.064 micrometers , being mainly sensitive to changes in the distribution of aerosols was used. The study of temporal series of lidar vertical scans revealed a multilayer arrangement of the aerosols above the city. A few years after that experiment, the UPC started to develop its own lidar system. The first stadium of the project was the construction of an elastic-backscatter lidar. In 1997, in the frame of a meteorological situation very similar to the one present during the first campaign, the UPC lidar acquired vertical profiles of elastic- backscatter in the Barcelona's atmosphere. The study of the temporal series also showed the development of elevated layers of aerosols at similar altitudes to the ones observed during the first campaign. The second experiment seems to confirm that the formation of elevated layers in Barcelona is a typical behavior for the region during the summer period and not just a particular situation.
Up to now, retrieval of the atmospheric extinction and backscatter has mainly relied on standard straightforward non-memory procedures such as slope-method, exponential- curve fitting and Klett's method. Yet, their performance becomes ultimately limited by the inherent lack of adaptability as they only work with present returns and neither past estimations, nor the statistics of the signals or a prior uncertainties are taken into account. In this work, a first inversion of the backscatter and extinction- to-backscatter ratio from pulsed elastic-backscatter lidar returns is tackled by means of an extended Kalman filter (EKF), which overcomes these limitations. Thus, as long as different return signals income,the filter updates itself weighted by the unbalance between the a priori estimates of the optical parameters and the new ones based on a minimum variance criterion. Calibration errors or initialization uncertainties can be assimilated also. The study begins with the formulation of the inversion problem and an appropriate stochastic model. Based on extensive simulation and realistic conditions, it is shown that the EKF approach enables to retrieve the sought-after optical parameters as time-range-dependent functions and hence, to track the atmospheric evolution, its performance being only limited by the quality and availability of the 'a priori' information and the accuracy of the atmospheric model assumed. The study ends with an encouraging practical inversion of a live-scene measured with the Nd:YAG elastic-backscatter lidar station at our premises in Barcelona.
Laser doppler velocimetry can provide many advantages to traffic monitoring, as compared to tradiitional microwave systems. Narrower beamwidth, lower transmitted power and higher doppler frequency shift are some of them. A low-power laboratory prototype is presented. It is based on the Michelson interferometer. Measurements of working range, SNR and resolution are presented.
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