Ove Steinvall

Effects of target shape and reflection on laser radar cross sections

Laser radar cross sections have been evaluated for a number of ideal targets such as cones, spheres, paraboloids, and cylinders by use of different reflection characteristics. The time-independent cross section is the ratio of the cross section of one of these forms to that of a plate with the same maximum radius. The time-dependent laser radar cross section involves the impulse response from the object shape multiplied by the beams transverse profile and the surface bidirectional reflection distribution function. It can be clearly seen that knowledge of the combined effect of object shape and reflection characteristics is important for determining the shape and the magnitude of the laser radar return. The results of this study are of interest for many laser radar applications such as ranging, three-dimensional imaging-modeling, tracking, antisensor lasers, and target recognition.

Gated viewing for target detection and target recognition

Gated viewing using short pulse lasers and fast cameras offers many new possibilities in imaging compared with passive EO imaging. Among these we note ranging capability, large target-to-background contrast also in low visibility, good penetration capability trough obscurants and vegetation as well as through shadows in buildings, cars, etc. We also note that short wavelength laser systems have better angular resolution than long-wave infrared systems of the same aperture size. This gives an interesting potential of combined IR and laser systems for target detection and classification. Beside military applications civilian applications of gated viewing for search and rescue as well as vehicle enhanced vision and other applications are in progress. This presentation investigates the performance for gated viewing systems during different atmospheric conditions, including obscurants and gives examples of experimental data. The paper also deals with signal processing of gated viewing images for target detection. This is performed in two steps. First, image frames containing information of interest are found. In a second step those frames are investigated further to evaluate if man-made objects are present. In this step a sequence of images (video frames) are set up as a 3-D volume to incorporate spatial information. The object will then be detected using a set of quadrature filters operating on the volume.

Advanced short-wavelength infrared range-gated imaging for ground applications in monostatic and bistatic configurations

Some advanced concepts for gated viewing are presented, including spectral diversity illumination techniques, non-line-of-sight imaging, indirect scene illumination, and in particular setups in bistatic configurations. By using a multiple-wavelength illumination source target speckles could be substantially reduced, leading to an improved image quality and enhanced range accuracy. In non-line-of-sight imaging experiments we observed the scenery through the reflections in a window plane. The scene was illuminated indirectly as well by a diffuse reflection of the laser beam at different nearby objects. In this setup several targets could be spotted, which, e.g., offers the capability to look around the corner in urban situations. In the presented measuring campaigns the advantages of bistatic setups in comparison with common monostatic configurations are discussed. The appearance of shadows or local contrast enhancements as well as the mitigation of retroreflections supports the human observer in interpreting the scene. Furthermore a bistatic configuration contributes to a reduced dazzling risk and to observer convertness.

Experimental evaluation of an airborne depth-sounding lidar

An experimental evaluation of an airborne depth-sounding Lidar is described. The system, called FLASH (FOA laser airborne sounder for hydrography), is based on a scanning frequency-doubled Nd:YAG laser carried by a helicopter. An in-situ profiling instrument for measuring water parameters is also described. This system, called HOSS (hydro-optical sensor system), is also carried by a helicopter and has been used to collect data in parallel with the lidar measurements. A discussion of the lidar performance coupled to the measured water and instrumental parameters is included. Examples of measured wave forms are compared with those obtained by analytical and Monte Carlo modeling.

Range accuracy and resolution for laser radars

Laser radars offer a potential for high range accuracy and range resolution due to short pulses and high bandwidth receivers. For angular non-resolved targets (1 D profiling) the analysis of the waveform offers the possibility of target recognition due to range profiling. For 2 D and 3 D imaging the angular and range resolution are the critical parameters for target recognition while in other applications such as in lidar mapping the range accuracy plays an important role for the performance. The development of the next generation laser radars including 3 D sensing focal plane arrays (FPAs) enable a full range and intensity image to be captured in one laser shot. Moreover, gated viewing systems also give a viable solution for providing 3 D target information. This paper uses simulation to illustrate the limits for accuracy and range resolution in waveform processing due to the laser pulse shape, detector noise, target shape and reflectivity as well as turbulence.

Laser system range calculations and the Lambert W function

The knowledge of range performance versus atmospheric transmission, often given by the visibility, is critical for the design, use, and prediction of laser and passive electro-optic systems. I present a solution of the ladar-lidar equation based on Lamberts W function. This solution will reveal the dependence of the maximum range on the system and target parameters for different atmospheric attenuations and will also allow us to take the signal statistics into account by studying the influence on the threshold signal-to-noise ratio. The method is also applicable to many range calculations for passive systems where the atmospheric loss can be approximated by an exponential term.

Three-dimensional laser radar modeling

Laser radars have the unique capability to give intensity and full 3-D images of an object. Doppler lidars can give velocity and vibration characteristics of an objects. These systems have many civilian and military applications such as terrain modelling, depth sounding, object detection and classification as well as object positioning. In order to derive the signal waveform from the object one has to account for the laser pulse time characteristics, media effects such as the atmospheric attenuation and turbulence effects or scattering properties, the target shape and reflection (BRDF), speckle noise together with the receiver and background noise. Finally the type of waveform processing (peak detection, leading edge etc.) is needed to model the sensor output to be compared with observations. We have developed a computer model which models performance of a 3-D laser radar. We will give examples of signal waveforms generated from model different targets calculated by integrating the laser beam profile in space and time over the target including reflection characteristics during different speckle and turbulence conditions. The result will be of help when designing and using new laser radar systems. The importance of different type of signal processing of the waveform in order to fulfil performance goals will be shown.

Imaging Q-switched CO2 laser radar with heterodyne detection: design and evaluation

A Q-switched CO2 laser radar with heterodyne detection has been designed and evaluated. A simplified theory has been used to optimize the Q-switched laser for high-resolution ranging. The return signal statistics from diffuse, glint, and topographical targets have been investigated, and statistical distributions have been fitted to the experimental data. Detection of specific targets in laser radar images using range gating has also been studied.

Airborne laser depth sounding: system aspects and performance

In Sweden work on airborne depth sounding lidar has taken place for more than ten years. Activities include the development of a helicopter borne lidar called FLASH as well as instrumentation (HOSS) for in situ measurement of the optical water parameters. These instruments have been used in rather extensive field trials some of which are discussed in more detail. Recently we have been engaged in analysis of lidar data to determine system performance and possible ways to optimize that relation to lidar parameters and anticipated bottom depth and topography. Examples from that analysis are presented. Comparisons between laser and acoustic soundings are presented. The FLASH system has been further developed into two operational systems called Hawk Eye with Saab Instruments as the main contractor and Optech Inc. as the main subcontractor. The first of these two systems has now been delivered for helicopter installation and acceptance flights. Data from Hawk Eye is discussed.

Measured signal amplitude distributions for a coherent FM-cw CO2 laser radar.

Measurements of signal amplitude distributions with a FM-cw CO2 laser radar have been made against various targets in both imaging and staring modes. Data show good agreement with theoretical distributions. From the measurements conclusions are drawn about the atmospheric- as well as target-induced effects. Beam wandering effects are shown to be of importance in the staring mode.