Abstract. We argue for the integration of the statistical models already widely used in radar technology into lidar technology. The aim is to assess the validity or degree of confidence of an alert to be issued in view of not overloading the pilot with nuisance alerts. We present the basics of the detection theory. We give three examples of simulations illustrating the use of these statistical models either for designing lidars or for preparing lidar missions. We describe the simulator having been developed and used. We also present the idea of developing mixtures of statistical models as an approach to thresholding and object classification at mission time. Some experimental data are presented to validate both the simulator output and the use of mixtures of models for object segmentation or classification.

Snow precipitation interaction with a generic 3D lidar is modeled. The randomness and the intensity of the signal as a function of the visibility and snowflake size and density distribution are reproduced. To do so, a representative snow density distribution is modeled as a function of visibility. Taking into account the laser beam and pulse characteristics, the probability to have one or many snowflakes of a given size in the lidar sampling cell is calculated. Knowing the number and the size of the snowflakes, the magnitude of the lidar signal is calculated. Finally, a filtering algorithm based on the relative intensity of the snowflakes is discussed.

Information about the size distribution of liquid droplets in a fog can be retrieved by measuring the backscattering lidar depolarization parameter D in circular polarization. Using a polarimetric off-axis lidar, measurements at different backscattering angles are performed on fogs made of water droplets and of mineral oil. Estimation of the effective droplet size is obtained using constrained linear inversion. Mie theory is used to calculate the variation in depolarization parameters for different effective droplet sizes. The calculation is performed for various scattering angles. These calculations provide a kernel for the constrained linear inversion scheme. It is shown that the refractive index has an effect on the retrieved droplet sizes as well as the choice of scattering angles. These measurements confirm that the circular depolarization parameter measured near the backscattering angle can be modeled as a function of the forward-scattering diffraction peak. The results of the constrained linear inversion of measurements are consistent with in situ measurement of the droplet size distribution.

The Fast-Steering secondary mirror (FSM) of Giant Magellan Telescope (GMT) consists of seven 1.1m diameter segments with effective diameter of 3.2m. Each of the seven segments is held by three axial supports and a central lateral support with a vacuum system to mirror gravity compensations. The mirror assembly has a tip-tilt feature capable of guiding to attenuate telescope wind shake and mount control jitter. Both on-axis and off-axis mirror segments were optimized under various design considerations using finite element analyses and optical performance analyses. An integrated model of the mirror cell assembly was developed including sub-assembly parts consisting of axial supports, lateral support, breakaway mechanism, seismic restraints, and pressure seal. In this paper for the optimized FSM mirror and mirror cell assembly, the design considerations are addressed, and performance prediction results are discussed in detail with respect to the specifications.

The Fast-steering Secondary Mirror (FSM) of Giant Magellan Telescope (GMT) consists of seven 1.1 m diameter circular segments with an effective diameter of 3.2 m, which are conjugated 1:1 to the seven 8.4 m segments of the primary. Each FSM segment contains a tip-tilt capability for fast guiding to attenuate telescope wind shake and mount control jitter by adapting axial support actuators. Breakaway System (BAS) is installed for protecting FSM from seismic overload or other unknown shocks in the axial support. When an earthquake or other unknown shocks come in, the springs in the BAS should limit the force along the axial support axis not to damage the mirror. We tested a single BAS in the lab by changing the input force to the BAS in a resolution of 10 N and measuring the displacement of the system. In this paper, we present experimental results from changing the input force gradually. We will discuss the detailed characteristics of the BAS in this report.

The Fast Steering Secondary Mirror (FSM) for the Giant Magellan Telescope (GMT) will have seven 1.05 m diameter circular segments and rapid tip-tilt capability to stabilize images under wind loading. In this paper, we report on the assembly, integration, and test (AIT) plan for this complex opto-mechanical system. Each fast-steering mirror segment has optical, mechanical, and electrical components that support tip-tilt capability for fine coalignment and fast guiding to attenuate wind shake and jitter. The components include polished and lightweighted mirror, lateral support, axial support assembly, seismic restraints, and mirror cell. All components will be assembled, integrated and tested to the required mechanical and optical tolerances following a concrete plan. Prior to assembly, fiducial references on all components and subassemblies will be located by three-dimensional coordinate measurement machines to assist with assembly and initial alignment. All electronics components are also installed at designed locations. We will integrate subassemblies within the required tolerances using precision tooling and jigs. Performance tests of both static and dynamic properties will be conducted in different orientations, including facing down, horizontal pointing, and intermediate angles using custom tools. In addition, the FSM must be capable of being easily and safely removed from the top-end assemble and recoated during maintenance. In this paper, we describe preliminary AIT plan including our test approach, equipment list, and test configuration for the FSM segments.

The Giant Magellan Telescope (GMT) wavefront control system will provide active optics control and optimized atmospheric turbulence correction to every instrument on the 25.4 m diameter GMT. All subsystems of the GMT wavefront control system have passed their preliminary design reviews, and several are now in the detailed design and prototyping phases. Significant progress has been made developing optimized control algorithms, and simulating observatory performance over a wide range of environmental conditions. We review the wavefront control strategy in each observing mode, and the associated image quality error budgets. We also describe recent d and prototyping progress, and our plans to complete the wavefront control system development and prepare for first light.

The Giant Magellan Telescope (GMT) will feature two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM has an effective diameter of 3.2 m and consists of seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary. Each FSM segment contains a tip-tilt capability for fast guiding to attenuate telescope wind shake and mount control jitter. This tiptilt capability thus enhances performance of the telescope in the seeing limited observation mode. The tip-tilt motion of the mirror is produced by three piezo actuators. In this paper we present a simulation model of the tip-tilt system which focuses on the piezo-actuators. The model includes hysteresis effects in the piezo elements and the position feedback control loop.

The Giant Magellan Telescope (GMT) will be equipped with two Gregorian secondary mirrors; a fast-steering secondary mirror (FSM) for seeing-limited operations and an adaptive secondary mirror (ASM) for adaptive optics observing modes. The FSM has an effective diameter of 3.2 m and is comprised of seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary mirror. Each FSM segment has a tip-tilt capability for fast guiding to attenuate telescope wind shake and jitter. The FSM is mounted on a two-stage positioning system; a macro-cell that positions the entire FSM segments as an assembly and seven hexapod actuators that position and drive the individual FSM segments. In this paper, we present a technical overview of the FSM development status. More details in each area of development will be presented in other papers by the FSM team.

The Giant Magellan Telescope (GMT) will be featured with two Gregorian secondary mirrors, an adaptive secondary mirror (ASM) and a fast-steering secondary mirror (FSM). The FSM has an effective diameter of 3.2 m and built as seven 1.1 m diameter circular segments, which are conjugated 1:1 to the seven 8.4m segments of the primary. Each FSM segment contains a tip-tilt capability for fine co-alignment of the telescope subapertures and fast guiding to attenuate telescope wind shake and mount control jitter. This tip-tilt capability thus enhances performance of the telescope in the seeing limited observation mode. As the first stage of the FSM development, Phase 0 study was conducted to develop a program plan detailing the design and manufacturing process for the seven FSM segments. The FSM development plan has been matured through an internal review by the GMTO-KASI team in May 2016 and fully assessed by an external review in June 2016. In this paper, we present the technical aspects of the FSM development plan.

It is generally admitted that the relative location of an aerosol between an observation device and the observed scene will have an influence on the detected image quality. These effects are usually classified under the label “shower curtain effect” (SCE). The usual formulation describing it is as follows: an observer standing away from a shower curtain can detect the presence of a person standing just behind it whereas the opposite is not true. Starting from a discussion of experimental results which seemed to invalidate the SCE, we show that it is not the only mechanism at work and that thorough analysis of the measurement setup is required before reaching such conclusion. We base our discussion on four cases, two of them of the passive detection type, the two others being of the active type. We also show that the ratio of scattered to unscattered light at the detector is of utmost importance. We show this by further developing our model [10] of the point spread function (PSF) of the receiver. This model allows the discussion of the SCE in the frequency domain in terms of the cuton and cutoff frequencies of the receiver. In the end, we show that the apparent paradoxical results we had found cannot actually be placed under the “shower curtain effect” denomination because: 1-) the amount of unscattered light captured is higher than the amount of scattered light, and 2-) the receiver cuton frequency is much higher than the aerosol cutoff frequency rendering most mechanisms of the shower curtain effect ineffective.

Scanners with one pair of Risley prisms are robust and precise and they can be operated continuously. In this paper, we present a new scanner based on the use of two pairs of Risley prisms. The concept was driven by the need to add flexibility to Risley prism scanners used for lidar 3D mapping applications, while maintaining compactness and robustness. The first pair covers a FOV narrower than the second pair. The second pair is used to position the first Risley pair scan pattern anywhere within its own, larger, FOV. Doing so, it becomes possible, without additional scanner components, to increase the sampling point density at a specific location, to increase the sampling uniformity of the scanned area, and, while in motion, to maintain the sampling of a specific area of interest.

Operation under degraded visual environment (DVE) presents important strategic advantages. 3D mapping has been performed under DVE and good quality images have been obtained through DVE with active imaging systems. In these applications, the presence of fog clouds degrades the quality of the remotely sensed signal or even renders the operation totally impossible. In view of making the active imaging method more robust against dense fog, the use of polarimetry is herein studied. Spherical particles typical of fog do not depolarize incident polarized light in the backscattering (180°) direction. So, in principle, there should be less dazzling caused by aerosols for active imaging systems operating using the secondary polarization. However, strong depolarization still occurs at angles close to 180°. The greater the ratio of size to wavelength, the closer to 180° will the depolarization occur. When the cloud optical depth is small, the major scattering events seen by an active camera are the single backscattering events. However, when the optical depth of the cloud is higher than 1, multiple scattering becomes more important and causes depolarization due to the backscattering around 180°. The physics of this process will be discussed. Experimental results supporting the analysis will be presented. Those experimental results were obtained under controlled environment using the DRDC-Valcartier aerosol chamber. The experimental method herein proposed is based upon the use of ICCD range gated cameras wherein gate width and gate location may be varied on the fly. The optimal conditions for the use of these devices in view of obtaining the best image contrast are experimentally studied and reported in this paper.

We present a method which estimates the normalized point-source sensitivity (PSSN) of a segmented telescope when only information from a single segment surface is known. The estimation principle is based on a statistical approach with an assumption that all segment surfaces have the same power spectral density (PSD) as the given segment surface. As presented in this paper, the PSSN based on this statistical approach represents a worst-case scenario among statistical random realizations of telescopes when all segment surfaces have the same PSD. Therefore, this method, which we call the vendor table, is expected to be useful for individual segment specification such as the segment polishing specification. The specification based on the vendor table can be directly related to a science metric such as PSSN and provides the mirror vendors significant flexibility by specifying a single overall PSSN value for them to meet. We build a vendor table for the Thirty Meter Telescope (TMT) and test it using multiple mirror samples from various mirror vendors to prove its practical utility. Accordingly, TMT has a plan to adopt this vendor table for its M1 segment final mirror polishing requirement.

Cao et al.1 published a paper where differentiating bioaerosols (pollens) appeared feasible when linear depolarization ratio signature at multiple wavelengths could be obtained. The measurements were performed at 4 wavelengths. The bioaerosols were disseminated in a controlled environment and the discrimination analysis was based on Mahalanobis distances. Poor discrimination was obtained for single wavelength measurement while acceptable and good discrimination was reported for two and three wavelengths. This innovative work has raised the following question: to which extent does the addition of circular polarization signature to the existing linear polarization increase the overall discrimination capability? In order to answer that question, the measurements of Cao et al. were repeated for linear and circular depolarization ratios. We demonstrate experimentally that the linear and circular depolarization ratios are related to each other via a known simple theoretical mathematical expression in the case of randomly oriented particles. Hence, by measuring one, you obtain the other and consequently there is no additional information that is gained by doing measurements with the two polarization states. This suggests that there is no need for full Mueller matrix measurement systems for detection and discrimination of bioaerosols.

Ladar technology has long since established its advantages as a reliable method for automated Terrain Mapping. One still remaining important problem of this methodology though happens at data processing time. Ladars generate huge amounts of data referred to as 'point clouds'. The very first task in data processing consists of segmenting the terrain image between ground and non-ground data points. The standard processing methods all rely on some slope analysis technique. At the present moment, all these techniques still require interactive evaluation and manual editing of the results. In this work, Ladar polarization is used to discriminate between solid targets by using their polarization signatures. The addition of this feature, over and above range and intensity, could greatly help in the process wherein ground and nonground points are to be separated. Linear and circular polarizations measurements were performed on different specimens in various conditions and at various wavelengths. The results presented herein are a validation of the fact that typical solid targets show a response to the Ladar sensor which conforms to the behavior predicted by the most recent polarimetric BDRF theories. Hence, their polarization signature is expected to be repeatable. The results presented herein also show that, to the extent that more than one wavelength is used, solid targets can be discriminated against each other by the use of their polarization signatures.

Strong experimental data supporting the theoretical relationship between linear and circular depolarizations for randomly oriented particles are presented. The analysis of the data leads to the first experimental validation of the theoretical representation of the scattering Mueller matrix as having indeed only one free parameter for randomly oriented particles, which is the depolarization parameter, d. Consequently, there is no added information on the nature of the aerosols when the four Stokes parameters are measured for randomly oriented aerosols, as opposed to measuring only the linear polarization or only the circular polarization related parameters. This conclusion has a direct impact on the analysis of the level of complexity of the systems that are required to analyze aerosols based on their depolarization signatures. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3657505]

Lidar bioaerosols discrimination based on depolarization signature is studied. The measurements were performed over 25 pollens and 2 dusts under controlled environment at a distance of 100 m, at wavelengths of 355 nm, 532 nm, 1064 nm and 1570 nm, and both linear and circular polarizations were used. It is found that discrimination of bioaerosols using single wavelength linear depolarization ratios is difficult because most of them are quite alike. However, two or more wavelengths measurements make it possible to discriminate different bioaerosols against others, especially when a depolarization ratio cumulative distribution is available.

Measurements of the depolarization ratio of water droplets were performed to study the relationship between layer integrated depolarization and layer integrated backscattered light for linear and circular polarization illumination. Since those particles are spherical, the depolarization of the signal is attributed to multiple scattering effects. The experimental data reported in this article support Hu relationship between the single scattering fraction A(s) A and the linear accumulated depolarization ratio. For circular polarization, a modified Hu relationship is established and it is shown that the use of the accumulated depolarization parameter instead of the accumulated depolarization ratio allows harmonization of the linear and circular polarization measurements into a simple mathematical expression.

Recently, an empirical relationship between the layer integrated backscattered light and the layer accumulated depolarization ratio has been established for linear polarization for the case of water droplet clouds. This is a powerful relation, allowing calibration of space lidar and correction of the lidar signal for multiple scattering effects. The relationship is strongly based on Monte Carlo simulations with some experimental evidence. We support the empirical relationship with strong experimental data and then show experimentally and via second order scattering theoretical calculations that a modified relationship can be obtained for circular polarization. Also, we demonstrate that other empirical relationships exist between the layer accumulated linear and circular depolarization ratios and the layer integrated backscattered light for submicrometer particles and nonspherical particles.