Showing papers by "Norman S. Kopeika published in 1997"

Laser satellite communication network-vibration effect and possible solutions

Abstract: A number of serious consortiums develop satellite communication networks. The objective of these communication projects is to service personal communication users almost everywhere on Earth. The intersatellite links in those projects use microwave radiation as the carrier. Free-space optical communication between satellites networked together can make possible high-speed communication between different places on Earth. Some advantages of an optical communication system over a microwave communication system in free space are: (1) smaller size and weight, (2) less transmitter power, (3) larger bandwidth, and (4) higher immunity to interference. The pointing from one satellite to another is a complicated problem due to the large distance between the satellite, the narrow beam divergence angle, and vibration of the pointing system. Such vibration of the transmitted beam in the receiver plane decreases the average received signal, which increases the bit error rate. We review: (1) the present status of satellite networks, (2) developing efforts of optical satellite communication around the world, (3) performance results of vibration effects on different kinds of optical communication satellite networks, and (4) seven approaches to overcome the problems caused by transmitter pointing vibration.

Restoration of atmospherically blurred images according to weather-predicted atmospheric modulation transfer functions

Abstract: Restoration for actual atmospherically blurred images is per- formed using an atmospheric Wiener filter that corrects simultaneously for both turbulence and aerosol blur by enhancing the image spectrum primarily at those high frequencies least affected by the jitter or random- ness in a turbulence modulation transfer function (MTF). The correction is based on weather-predicted rather than measured atmospheric MTFs. Both turbulence and aerosol MTFs are predicted using meteorological parameters measured with standard weather stations at the time and location where the image was recorded. A variety of weather conditions and seasons are considered. Past results have shown good correlation between measured and predicted atmospheric MTFs. Here, the pre- dicted MTFs are implemented in actual image restoration and quantita- tive analysis of the MTF improvement is presented. Corrections are shown also for turbulence blur alone, for aerosol blur alone, and for both together. © 1997 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(97)01511-0)

Beam width and transmitter power adaptive to tracking system performance for free-space optical communication

Abstract: The basic free-space optical communication system includes at least two satellites. To communicate between them, the transmitter satellite must track the beacon of the receiver satellite and point the information optical beam in its direction. Optical tracking and pointing systems for free space suffer during tracking from high-amplitude vibration because of background radiation from interstellar objects such as the Sun, Moon, Earth, and stars in the tracking field of view or the mechanical impact from satellite internal and external sources. The vibrations of beam pointing increase the bit error rate and jam communication between the two satellites. One way to overcome this problem is to increase the satellite receiver beacon power. However, this solution requires increased power consumption and weight, both of which are disadvantageous in satellite development. Considering these facts, we derive a mathematical model of a communication system that adapts optimally the transmitter beam width and the transmitted power to the tracking system performance. Based on this model, we investigate the performance of a communication system with discrete element optical phased array transmitter telescope gain. An example for a practical communication system between a Low Earth Orbit Satellite and a Geostationary Earth Orbit Satellite is presented. From the results of this research it can be seen that a four-element adaptive transmitter telescope is sufficient to compensate for vibration amplitude doubling. The benefits of the proposed model are less required transmitter power and improved communication system performance.

Analytical method to calculate optical transfer functions for image motion and vibrations using moments

Abstract: A new method of calculating the optical transfer function (OTF) for image blur caused by arbitrary motion is introduced. Previous methods, except for a few specific types of motion, were numerical rather than analytical. This new method makes it possible to obtain analytical expressions for the OTF deriving from any kind of motion by means of the statistical moments of the motion function. Analytical OTF expressions are derived for linear, quadratic, and exponential motion and for high- and low-frequency vibration. A comparison of image degradation is presented for linear and exponential motion and high-frequency vibration of the same blur extent. The method can be implemented in real-time restoration of images blurred by arbitrary motion and in image motion degradation analysis.

Performance limitations of free-space optical communication satellite networks due to vibrations-analog case

Abstract: Free-space optical communication between satellites networked together can make possible high-speed communication between different places on earth. The use of optical radiation as a carrier between the satellites creates very narrow beam divergence angles. Due to the narrow-beam divergence angle and the large distance between the satellites, the pointing from one satellite to another is complicated. The complication is due to vibration of the pointing system caused by two stochastic fundamental mechanisms: (1) tracking noise created by the electro-optic tracker and (2) vibrations created by internal satellite mechanical mechanisms. We derive mathematical models of signal, noise, approximate SNR, and approximate bit error rates of optical communication satellite networks as functions of the system parameters, the number of satellites, and the vibration amplitude. The optical intersatellite network model considered includes transmitter satellite, repeater satellites, and receiver satellite all networked together. These models are the basis for pointing system design of appropriate complexity and performance to make the network as simple and inexpensive as possible. An example of practical communication between Anchorage in Alaska to Johannesburg in South Africa by a free-space optical communication network composed of nine low-earth-orbit satellites is given. From the analysis it is clear that even low vibration amplitude of the satellite pointing systems decreases dramatically the network performance.

Adaptive optical transmitter and receiver for space communication through thin clouds.

Abstract: Optical space communication from satellite to ground or air to air consists of clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. Having dealt with the other effects previously, here we concentrate on pulse broadening in the time domain. We derive a mathematical model of an adaptive optical communication system with a multiscattering channel (atmospheric cloud). We use knowledge about the impulse response function of the cloud to adapt the communication parameters to the transfer function of the cloud. The communication system includes a receiver and a transmitter. We adapted the transmitter to atmospheric conditions by changing the bit error rate. One can adapt the receiver to the atmospheric condition by changing the parameters of the detector and the filter. An example for a practical communication system between a low Earth orbit satellite and a ground station cover by cloud is given. Comparison and analysis of an adaptive and semiadaptive system with cloud channels are presented. Our conclusion is that in some cases only by such adaptive methods is optical communication possible.

Performance limitations of free-space optical communication satellite networks due to vibrations: direct detection digital mode

Abstract: Free-space optical communication between satellites net- worked together can permit high data rates between different places on Earth. The use of optical radiation as a carrier between the satellites permits very narrow beam divergence. Due to the narrow divergence and the large distance between the satellites, pointing from one satellite to another is difficult. The pointing task is further complicated by vibration of the pointing system caused by tracking noise and mechanical impacts. In this work we derive mathematical performance models for digital direct detection communication satellite networks as a function of the system parameters, the number of satellites, and the vibration amplitude. The optical intersatellite network model considered includes a transmitter sat- ellite, regenerative satellites, and a receiver satellite all networked to- gether. A comparison between three communication system modulation schemes—on-off keying (OOK), pulse position modulation (PPM), and pulse polarization binary modulation (PPBM)—is presented. These mod- els are the basis for optical communication tracking- and pointing-system design of appropriate complexity and performance to make the network as simple and inexpensive as possible. From the analysis it is clear that even low vibration amplitude of one satellite pointing system decreases the network performance dramatically. © 1997 Society of Photo-Optical Instru- mentation Engineers. (S0091-3286(97)01111-2)

Method and apparatus for the restoration of images degraded by mechanical vibrations

Abstract: A method and apparatus to restore images degraded by motion or vibration as characterized by the measurement of the relative motion between the object and the imaging device, calculation of the Optical Transfer Function (OTF) from the Line Spread Function (LSF), and applying a restoration filter M according to the following rule ##EQU1## where H is the image motion OTF, C is the discrete Laplacian operator which is used to minimize the error and γ is the convergence parameter of an iterative algorithm.

Real-time two-dimensional filtering of video signals

Abstract: A method and device for real time processing of analog signals, such as video signals, representative of frames of data acquired in row order in a two dimensional field. The incoming signal is processed (for example filtered) by analog means, transposed to column order, processed again by analog means, and transposed back to row order. The transpositions are done by digitizing each frame of the signal, transposing the digital signal, and converting the transposed digital signal back to analog form. The process can be pipelined to be accomplished in real time with only a constant delay of two frames.

Restoration of images degraded by extreme mechanical vibrations

Abstract: A method of numerically calculating the optical transfer function appropriate to any type of image motion and vibration, including random, has been recently developed. This method has been verified experimentally with real vibrations, and the close agreement justifies implementation in image restoration from blur deriving from any type of image motion, including random displacement. Here, image restorations of actual physically degraded images are presented, based on a constrained least squares improvement of the original Wiener filter. Even for extreme vibrations where the blur extent is much larger than the blurred detail, restoration is quite complete. The key to restoration is the determination of the optical transfer function unique to the particular image motion and vibration causing the blur. Results are also presented for low vibration frequency motion of random blur extent, as well as for high vibration frequencies.

Performance limitations of free-space optical communication satellite networks due to vibrations: direct-detection digital mode

Abstract: Free space optical communication between satellites networked together can permit high data rates between different places on earth. The use of optical radiation as a carrier between the satellites permits very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellites the pointing from one satellite to another is complicated. The pointing task is further complicated due to vibration of the pointing system caused by tracking noise and mechanical impacts. In this work we derive mathematical performance models for digital direct detection communication satellite networks as a function of the system parameters, the number of satellites, and the vibration amplitude. The optical inter- satellite network model considered includes a transmitter satellite, regenerative satellites, and a receiver satellite all networked together. A comparison between three communication system modulation schemes on-off keying, pulse position modulation, and pulse polarization binary modulation is presented. These models are the basis for optical communication tracking and pointing system design of appropriate complexity and performance in order to make the network as simple and inexpensive as possible. From the analysis it is clear that even low vibration amplitude of one satellite pointing system decreases dramatically the network performance.

General restoration filter for vibrated image restoration

Abstract: Low temporal frequency vibrations involve random image degradation depending on the instant of exposure. Exact restoration requires calculating a specific filter unique to each vibrated image. In order to calculate the restoration filter for each image the specific degradation function is needed. Therefore, the instant of exposure has to be measured or estimated by some other means. In this work a general restoration filter for a single vibrated image is developed. The filter is independent of the instant of exposure. Assuming that receiving each image from a vibrated image ensemble is equally likely, the filter is designed to give best performance averaged over the ensemble. Even though when applying the new filter to any vibrated image the restoration achieved is slightly poorer than with an exact filter based on the specific motion function, the new filter has the advantage of simplicity. Since the instant of exposure is not needed a new appropriate filter does not have to be calculated for each exposure, the proposed filter is more practical and suitable for real time restoration.

Adaptive suboptimum detection of optical PPM signal with detection matrix and centroid tracking

Abstract: In some applications of optical communication systems, such as satellite optical communication and atmospheric optical communication, the optical beam wanders on the detector surface due to vibration and turbulence effects, respectively. The wandering of the beam degrades the communication system performance. In this research, we derive a mathematical model of an optical communication system with a detection matrix to improve the system performance for direct detection pulse position modulation (PPM) We include a centroid tracker in the communication system model. The centroid tracker tracks the center of the beam. Using the position of beam center and an apriori model of beam spreading we estimate the optical power on each pixel (element) in the detection matrix. Based on knowledge of the amplitudes of signal and noise in each pixel, we tune adaptively and separately the gain of each individual pixel in the detection matrix for communication signals. Tuning the gain is based on the mathematical model derived in this research. This model is defined as suboptimal due to some approximations in the development and is a suboptimum solution to the optimization problem of n multiplied by m free variables, where U,mare the dimensions of the detection matrix. Comparison is made between the adaptive suboptimum model and the standard model. From the mathematical analysis and the results of the comparison it is clear that this model improves significantly communication system performance.

Experimental comparison of three target acquisition models

Abstract: The role of the atmosphere in target acquisition modeling is investigated experimentally. Three models are compared to experimen- tal results measured on the Golan Heights, Israel. Concepts considered are atmospheric attenuation versus atmospheric blur, and contrast- limited (blur-limited) versus noise-limited imaging. Results indicate that the role of the atmosphere in target acquisition is blur rather than attenu- ation and that for ranges of the order of a few kilometers, modern sen- sors are limited by atmospheric blur rather than by noise. A significant portion of the atmospheric blur derives from small angle forward scatter- ing by aerosols, which actually increases measured temperature differ- ences for ranges up to a few kilometers. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)00907-6)

Aerosol modulation transfer function: an overview

Abstract: The aerosol modulation transfer function (MTF) describes blurring deriving from light scatter caused by aerosols. Little scintillations or image dancing are involved. When overall atmospheric point spread function (PSF) is analyzed for its turbulence component deriving from angle-of-arrival fluctuations or scintillations, a significant portion of the PSF is left over. This is the aerosol component. This overview describes the basic physical mechanisms for aerosol MTF and its wavelength, weather, and time exposure dependences, as well as a comparison to turbulence MiT.

Performance limitations of free space optical communication satellite networks due to vibrations: heterodyne detection

Abstract: Free space optical communication between satellites networked together can permit high data rates between different places on earth. In order to establish optical communication between two satellites, the line of sight of their optics must be aligned during the entire communication time. Due to the large distance between the satellites and the required accuracy, the pointing from one satellite to another is complicated. The problem is further complicated due to vibrations of the pointing system caused by two fundamental stochastic mechanisms: 1) tracking noise created by the electro-optic tracker and 2) vibrations deriving from mechanical mechanisms. Vibration of the transmitter beam in the receiver plane causes a decrease in the received optical power. Vibrations of receiver telescope relative to the received beam decrease the heterodyne mixing efficiency. These two factors increase the bit error rate (BER) of the network. In this work we derive simple mathematical models of the network BER versus the system parameters, and the transmitter and receiver vibration statistics. Considering this simple model, we propose four methods to decrease the effects of the vibration on the network performance and to converge to desired performance requirements. An example of a practical optical heterodyne free space satellite optical communication network is presented. From this research it is clear that even low amplitude vibration of the satellite pointing systems decreases dramatically network performance.

Restoration of motion-blurred images

Abstract: This paper deals with the problem of restoration of images blurred by relative motion between the camera and the object of interest. This problem is common when the imaging system is in moving vehicles or held by human hands, and in root vision. For correct restoration of the degraded image it is useful to know the point spread function (PSF) of the blurring system. In this paper we propose a straightforward method to restore motion blurred images given only the blurred image itself. The method first identifies the PSF of the blur, and then use it to restore the blurred image. The blur identification here is based on the concept that image characteristics along the direction of motion are effected mostly by the blur and are different from the characteristics in other directions. By filtering the blurred image we emphasize are presented for both synthetic and real motion blur.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Free-space optical communication satellite networks-vibration effects and possible solutions

Abstract: A number of serious consortiums develop satellite communication networks The objective of these communication projects is to service personal communication users almost everywhere on earth The inter satellite links (ISL) in those project use microwave radiation as the carrier Free space optical communication between satellites networked together can make possible high speed communication between different places on earth The advantages of an optical communication system instead of a microwave communication system in free space are: a) smaller size and weight, b) less transmitter power, c) larger bandwidth, d) higher immunity to interference, and e) smaller transmitter beam divergence The use of optical radiation as a carrier between the satellites creates very narrow beam divergence angles Due to the narrow beam divergence angle and the large distance between the satellites the pointing from one satellite to another is complicated The problem is more complicated due to vibration of the pointing system caused by two stochastic fundamental mechanisms 1) tracking noise created by the electrooptic tracker and 2) vibrations created by internal and external mechanical mechanisms The vibrations displace the transmitted beam in the receiver plane Such movement of the transmitted beam in the receiver plane decreases the average received signal which decreases increases the bit error rate In this paper we review: 1) the present status of satellite networks 2) developing efforts of optical satellite communication around the world, 3) performance results of vibration effects on different kinds of optical communication satellite networks and 4) seven approaches to overcome the problems caused by transmitter pointing vibration

Evaluation of the PSF from motion-blurred images

Abstract: This paper deals with the problem of restoration of images blurred by relative motion between the camera and the object of interest. This problem is common when the imaging system is in moving vehicles or held by human hands, and in robot vision. For correct restoration of the degraded image we need to know the point spread function (PSF) of the blurring system. In this paper we propose a method to identify important parameters with which to characterize the PSF of the blur, given only the blurred image itself. The identification method here is based on the concept that image characteristics along the direction of motion are different than the characteristics in other directions. Depending on the PSF shape, the homogeneity and the smoothness of the blurred image in the motion direction are higher than in other directions. By filtering the blurred image we emphasize the PSF characteristics at the expense of the image characteristics. The method proposed here identifies the direction and the extent of the PSF of the blur and finally identifies the modulation transfer function (MTF) of the blurring system.

Relative effects of blur and noise on target acquisition: the advisability of image restoration

Abstract: Any image acquired by optical, electro-optical or electronic means is likely to be degraded by the environment. The resolution of the acquired image depends on the total MTF (Modulation Transfer Function) of the system and the additive noise. Image restoration techniques can improve image resolution significantly; however, as the noise increases, improvements via image processing become more limited because image restoration increases the noise level of the image. The purpose of this research is to check and characterize the MTF and noise level influences on target acquisition probability by a human observer, i.e., checking the worthwhileness of the restoration. The immediate quantity that was measured is not the probability of detection, but rather the number of targets of different sizes and degradation recognized in each scene. Conditions when restoration is advisable are determined. Further research will include real-world target recognition probability.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Imaging through the atmosphere from satellites: restoration of images based on atmospheric MTF

Abstract: The recently developed atmospheric Wiener filter, which corrects for turbulence and aerosol blur and path radiance simultaneously, is implemented in digital restoration of AVHR imagery over the five wavelength bands of the satellite instrumentation. Restoration is most impressive for higher optical depth situations which cause more blur, with improvement in regard to both smallness of size of resolvable detail and contrast. Turbulence modulation transfer function (MTF) is calculated from meteorological data. Aerosol MTF is consistent with optical depth, measured with a sum-photometer. The product of the two yields atmospheric MTF which is implemented in the atmospheric Wiener filter. Turbulence blur, aerosol blur, and path radiance contrast loss are all corrected simultaneously, as if there were no intervening atmosphere. Image restorations with accompanying atmospheric MTF curves are presented. However, restoration results using a simple inverse atmospheric MTF filter were quite similar. This indicates the satellite images were characterized by very low noise and that turbulence jitter was very limited which, in turn, indicates that the turbulence MTFs integrated upwards over the path length wee not significant when compared to aerosol MTFs. Restorations are shown for various wavelength bands and are quite apparent even under clear weather conditions.

Vibrated image restoration from two consecutive images

Abstract: Vibrations are often a major cause of image degradation in airborne ad terrestrial reconnaissance, astronomy, robotics, machine vision and computer vision systems. In the case of low-frequency vibration (the exposure time is shorter than the vibration period time) the camera motion during the exposure is random, depending on the vibration parameters (amplitude and frequency) and on the instant of the exposure time. Therefore, the random motion causes a random image blur. Knowledge of the exact motion is practical for calculation of motion optical transfer function (OTF) which can be used with common image restoration algorithms. The motion can be either measured directly using motion sensors or estimated from a sequence of images. Here, a vibration motion estimation method from a minimal sequence of two images is presented. Motion estimation from only two consecutive images is possible due to the use of information hidden in the image blur. The process is carried out in the frequency domain using a new analytical motion OTF calculation method.

Image restoration for target detection: will it help?

Abstract: The classical method for determining target acquisition probabilities has always focused on the maximum spatial frequency (frmax) discernible in the image. On the other hand, it is known that the atmosphere degrades all the spatial frequencies as determined by the atmospheric modulation transfer function (MTF). The question arises: do the 'other' frequencies below frmax affect the target acquisition probability. We will present two experimental approaches to this question. In the first, we consider different atmospheric MTFs with the same value of frmax but with different MTF shapes. In the second, we consider a novel Wiener filter which restores all the frequencies to their value prior to the atmospheric blur. Laboratory measurements of observer response time when performing target acquisition will be presented for these case. The results will allow us to check the degree that the entire MTF should enter the target acquisition model, rather than frmax only.

Real-time two-dimensional electronic image filtering and live TV restoration

Abstract: We describe novel architecture for a real-time image restoration system of live TV signals. No DSP is involved. The spatial filtering is obtained from two electronic analog filters, one for the raster lines and one for the columns. The very fast response of analog filters is the key for truly real-time video frame rate performance. The digital part of the system serves the purpose of pipe-lined parallel data conversion and flow, but not that of image processing at all. Despite the lack of DSP, this architecture exhibits some very important advantages. It does not need any computational source, it is very fast, and it is much cheaper. Also our 'parallel analog computer' can be easily incorporated in any complex system with video signal data as a simple 'plug-in' between the camera and monitor. An important aspect is that the system carries lower digitalization noise than DSP, thus yielding better SNR characteristics at a lower price. The system is not bound to nay specific kind of spatial frequency filtering and can be electronically tuned to obtain exact performance parameters. Because of these advantages, this architecture is promising for a wide variety of system such as supermarket multicamera security, military and aerospace vision systems, and medical diagnostics.