Tsun-yee Yan - Academia.edu (original) (raw)
Papers by Tsun-yee Yan
Proceedings of SPIE, Jun 9, 1992
This paper describes an extended-source spatial acquisition process based on the maximum likeliho... more This paper describes an extended-source spatial acquisition process based on the maximum likelihood criterion for interplanetary optical communications. The objective is to use the sun-lit Earth image as a receiver beacon and point the transmitter laser to the Earth-based receiver to establish a communication path. The process assumes the existence of a reference image. The uncertainties between the reference image and the received image are modeled as additive white Gaussian disturbances. It has been shown that the optimal spatial acquisition requires solving two nonlinear equations to estimate the coordinates of the transceiver from the received camera image in the transformed domain. The optimal solution can be obtained iteratively by solving two linear equations. Numerical results using a sample sun-lit Earth as a reference image demonstrate that sub-pixel resolutions can be achieved in a high disturbance environment. Spatial resolution is quantified by Cramer-Rao lower bounds.
This paper presents the system architecture proposed for the Mobile Satellite Service (MSS) by th... more This paper presents the system architecture proposed for the Mobile Satellite Service (MSS) by the National Aeronautics and Space Administration (NASA)/JPL. The demand assigned Frequency Division Multiple Access (FDMA) scheme is described, and results for the associated network access protocol developed by JPL are presented. Both the total number of users that the system can support and the system spectral efficiency are given for a variety of traffic conditions, including those postulated for the Mobile Satellite System Architectures and Multiple Access Techniques Workshop. The results are given for both first-and second-generation one-and two-satellite systems.
Proceedings of SPIE, Aug 16, 1994
Performance of deep space optical communications is impacted by atmospheric outages. By monitorin... more Performance of deep space optical communications is impacted by atmospheric outages. By monitoring intensities of known stellar objects with calibrated telescopes, atmospheric attenuation statistics can be recorded. This paper describes a program to develop an atmospheric transmission model for optical communications by making autonomous visibility measurements of known stellar objects from three locations in the southwest United States. The model will be used in link margin analyses for optical communications channels. The statistics will be updated on a quarterly basis and calibrated using image data taken over a long period of time. This calibration is expected to allow extrapolation to other ground-based locations through the use of global weather data bases.
Proceedings of SPIE, Apr 26, 1999
This paper describes an extended-source spatial acquisition and tracking scheme for planetary opt... more This paper describes an extended-source spatial acquisition and tracking scheme for planetary optical communications. This scheme uses the Sunlit Earth image as the beacon signal, which can be computed according to the current Sun-Earth-Probe angle from a pre-stored Earth image or a received snapshot taken by other Earth-orbiting satellite. Onboard the spacecraft, the reference image is correlated in the transform domain with the received image obtained from a detector array, which is assumed to have each of its pixels corrupted by an independent additive white Gaussian noise. The coordinate of the ground station is acquired and tracked, respectively, by an open-loop acquisition algorithm and a closed-loop tracking algorithm derived from the maximum likelihood criterion. As shown in the paper, the optimal spatial acquisition requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate when translation in the relative positions of onboard and ground transceivers is considered. Similar assumption of linearization leads to the closed-loop spatial tracking algorithm in which the loop feedback signals can be derived from the weighted transform-domain correlation. Numerical results using a sample Sunlit Earth image demonstrate that sub-pixel resolutions can be achieved by this scheme in a high disturbance environment.
Proceedings of SPIE, Apr 24, 1997
The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration termin... more The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions. It has been under active development over the past few years. The instrument uses a CCD array detector for both spatial acquisition and high-bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field-of-view acquisition and permits effective platform jitter compensation and point-ahead control using only one steering mirror. The use of a fiber-coupled transmitter modularizes the transmitter design and de-couples its thermal management problems from the main system optics. The reduction in design complexity can lead to a reduced system cost and an improved system reliability. This paper describes recent progress on the development of the OCD terminal.
Proceedings of SPIE, Apr 26, 1999
One of the NASA technology development program at the Jet Propulsion Laboratory aims to increase ... more One of the NASA technology development program at the Jet Propulsion Laboratory aims to increase the information return capability while reducing the size of the spacecraft via laser communications. The deep space optical transceiver developed under this program employs puke position modulation (PPM) for both uplink and downlink transmissions. An integral part of the transceiver is the development of signal acquisition and tracking subsystem. This paper describes the baseline design of the electronic assembly within the transceiver and modifications that are necessary for deep space communications. A two phase breadboard activity will be described to reduce technological risks associated with the development. 1. INTRODUCTION Laser communication is an enabling technology applicable to future NASA near Earth and deep space missions that desire higher communications capacity than currently available by RF technologies. One of the NASA technology development program at the Jet Propulsion Laboratory (JPL) aims to increase the information return capability by at least an order cf magnitude while reducing the size of the spacecraft. The technicd merit of laser communications is derived from the fact that it offers a much higher collimated signal than conventional microwave. This super-collimated beam can result in a terminal design with greatly reduced size, mass and power requirements. Furthermore, laser communication systems are not susceptible to radio frequency interference and are not subject to frequency or bandwidth regulation. Currently, data return from deep space missions is accomplished by direct space-to-ground link operating at S-. X-, or Ka-band. Planned deep space missions could demand return of vast quantities of scientific data either by direct link or through relays such as Mars missions. As more and more missions begin to operate at high downlink data rates to the DSN, resource and spectrum allocation and radio frequency interference (FWI) could become difficult to manage. Laser communications become a viable alternative to provide such a vast return. Link Range Data rate Coded BER Transmit power Transmit losses Transmitter gain Pointing losses Space loss Atmospheric losses Receiver gain Receiver optics losses Received signal Background signal level Required singnal level Allowances and Adjustments Lick Margin 8.98E+O8 km 6.00 AU 6.00E+01 kbps PPM
Proceedings of SPIE, Aug 6, 1993
The concept o f mobile communications via a geosynchronous satellite has been proposed in recent ... more The concept o f mobile communications via a geosynchronous satellite has been proposed in recent years. Twelve companies have filed applications with the Federal Communications Commission to offer nationwide Mobile Satellite Services (MSS). Chis paper addresses several technical issues related to MSS from the network design point o f view. MSAT-X, which stands for the Mobile Satellite Experiment, is the task assigned to the Jet Propulsion Laboratory by the United States National Aeronautics and Space Administration to develop low-cost, spectral-efficient, satellite-based mobile communications technologies for 1990's. A1 though the discussions and recommendations presented in this paper are 1 imi ted to MSAT-X, technologles developed under MSAT-X may be o f the utmost importance to the first generation MSS. with the National Aeronautics and Space Administration.
Telecommunications and Data Acquisition Progress Report, Sep 13, 1998
A generalized maximum a posteriori (MAP) symbol synchronizer for arbitrary nonoverlapping pulse s... more A generalized maximum a posteriori (MAP) symbol synchronizer for arbitrary nonoverlapping pulse shape and data-transition density is derived, and a simplified realization is presented. Acquisition performance at low to very low symbol signalto-noise ratios (SNRs) is investigated and compared against the conventional datatransition tracking loop (DTTL). Simulation results show that the MAP symbol synchronizer can operate at very low symbol SNRs where the DTTL fails. Furthermore, the new symbol synchronizer reduces the initial acquisition time by at least one order of magnitude as compared with the DTTL. It also has been shown that the new symbol synchronizer is suitable for minimum-shift-keying (MSK) signaling waveforms.
IEEE Transactions on Information Theory, May 1, 1980
An optimal linear mean-square estimation algorithm is derived under the constraint that the algor... more An optimal linear mean-square estimation algorithm is derived under the constraint that the algorithm be multiplication-free. A classical linear estimation problem with block length N generally requires N2 multiplications. For many on-line signal processing shuations a large number of multiplications is objectionable. This class of estimation problems includes the classical linear filtering of a random signal in random noise, as well as the linear equaliition of digital data over a dispersive channel with additive noise. Here we consider the linear estimation problem on a binary computer where the estimation parameters are constrahted to be powers of two and thus all multiplications are replaced by shifts. Then the optimal constrained linear estimation problem resembles an integer-programming problem except that the allowable discrete points are nonhttegers. The branch-and-bound principle is nsed to convert this mfnimixation problem to a series of convex programming problems. An algorithm is given for the solution as well as numerical results for filtering and data equaliiation. These examples show that the multiplication-free con-Manuscript
A method for optimal adaptive setting of pulse-positionmodulation pulse detection thresholds, whi... more A method for optimal adaptive setting of pulse-positionmodulation pulse detection thresholds, which minimizes the total probability of error for the dynamically fading optical free space channel, is presented. The threshold's adaptive setting, in response to varying channel conditions, results in orders of magnitude improvement in probability of error, as compared to use of a fixed threshold. The adaptive threshold system itself is based on a robust channel identification system that uses average signal strengths to estimate the degree of fade and total attenuation in the channel, and a radial basis function network for estimating pulse spreads, all with excellent accuracy.
An essential requirement in the design and implementation of optical receivers for deep space com... more An essential requirement in the design and implementation of optical receivers for deep space communications is the development of a low-complexity, fast, and intelligent frame and symbol synchronization technique. In this paper we present the results of a sub-sequence pattern matching technique that detects and isolates the data frame boundaries based on partially received frames. This is in contrast to the traditional correlation based methods which are not as computationally efficient, and require full reception of the acquisition sequence prior to synchronization. This technique allows fast convergence and detection of the boundaries with as low as 2-6 bytes of received synchronization word. In addition, a confirmation method is proposed which allows tuning of the receiver detection module for a given probability of false boundary detection.
In this study we will show that the research on Nversion high-reliability software structures can... more In this study we will show that the research on Nversion high-reliability software structures can be extended to neural networks architecture. In addition, we will explore the possibility of applying this structure to a spacecraft tracking problem. One such system is the Automated Spacecraft Monitoring System (ASMS), a beacon-monitoring or detection system. Four neural networks, each trained for various operating environments, are implemented in an Nversion structure. The results of the networks are combined to form a composite outcome. The combined outcome is used as part of a hypothesis testing procedure to distinguish between the presence or absence of the beacon signal. The results show that any of a number of composite outcomes outperforms the use of any single neural network. Further, the simple average of network results provides the composite outcome with best performance.
Performance of deep space optical communications is impacted by atmospheric outages. By monitorin... more Performance of deep space optical communications is impacted by atmospheric outages. By monitoring known stellar objects with calibrated telescopes, atmospheric attenuation statistics can be recorded. This paper describes a program to develop an atmospheric transmission model for optical communications by making automous visibility measurements of known stellar objects from three locations in the southwestern US. The data will be correlated and infused with weather data to establish the atmospheric transmission model. This model will be sent in link margin analysis for optical communications channels. The statistics will be updated on a quarterly basis and calibrated using data taken over a long period of time. This calibration is expected to allow extrapolation to other ground-based locations through use of global weather databases.
Free-space laser communication is a promising technology to meet the demand for cost-effective wi... more Free-space laser communication is a promising technology to meet the demand for cost-effective wideband support for future NASA missions. JPL has begun construction at the Table Mountain Facility in California of a 1-meter telescope system, known as the Optical Communication Telescope Laboratory (OCTL) and capable of tracking spacecraft from low Earth orbit to deep space, for development and validation of vital optical communication technologies. One of them is accurate pointing for both up (ground-to-space) and down (space-to-ground) links. This article describes an acquisition and tracking scheme for downlink optical telemetry that is based on a correlation-type technique developed for extended sources covering several elements of the detector array. The purpose of developing such a scheme is to enable simultaneous beam tracking and data detection using a small number of optical sensors. The open-loop acquisition derived from the maximum-likelihood criterion involves a transform-domain correlation between the received laser image and the reference image derived from the known intensity profile of the transmitting laser. The optimal acquisition algorithm requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate of the transmitting laser when a rotation-invariant movement is considered. A maximumlikelihood-based closed-loop tracking algorithm is developed as well, in which the loop feedback signals are formulated as weighted transform-domain correlations between the received laser image and the previously estimated reference image. This scheme is expected to be able to achieve sub-pixel resolutions in a high-disturbance environment.
Space Technology Conference and Exposition, Aug 22, 1999
This paper describes a generic extendedimage spatial acquisition and tracking technique developed... more This paper describes a generic extendedimage spatial acquisition and tracking technique developed to enable highly accurate and stable pointing to a moving target through an imaging device. It is intended to be a fin+pointing scheme complimentary to the imager's general control subsystem. that is able to provide a "coarse" pointing capability such that the target remains within the image& field of view. This correlation-type scheme compares the received image obtained from the detector array with a priorly established reference in the transform domain to estimate the target's movement. The received image is assumed to be an extended-image covering more than one element of the detector array and have each of its pixels corrupted by an independent additive white Gaussian noise. The coordinate of the target area is acquired and tracked, respectively, by an open-loop acquisition algorithm and a closed-loop tracking algorithm derived from the maximum likelihood criterion. The resulting movement estimate is used to direct the imager to closely follow the moving target. This technique has many potential applications, including free-space optical communications and astronomy where accurate and stabilized optical pointing is essential. 'One sstrkomy unit & roughly 149.6 x lo6 kilometers.
The novel network testbed simulators are described: the request protocol simulator and the link p... more The novel network testbed simulators are described: the request protocol simulator and the link protocol simulator. These testbeds are useful for evaluating delay-throughput performances of demand-assisted multiple-access (DAMA) protocols in mobile fading environments. The request protocol testbed simulates the multiple-access scheme for making connection requests in the DAMA protocol. The link protocol testbed simulates the link-layer protocol for data transmissions.
Proceedings of SPIE, Jun 9, 1992
This paper describes an extended-source spatial acquisition process based on the maximum likeliho... more This paper describes an extended-source spatial acquisition process based on the maximum likelihood criterion for interplanetary optical communications. The objective is to use the sun-lit Earth image as a receiver beacon and point the transmitter laser to the Earth-based receiver to establish a communication path. The process assumes the existence of a reference image. The uncertainties between the reference image and the received image are modeled as additive white Gaussian disturbances. It has been shown that the optimal spatial acquisition requires solving two nonlinear equations to estimate the coordinates of the transceiver from the received camera image in the transformed domain. The optimal solution can be obtained iteratively by solving two linear equations. Numerical results using a sample sun-lit Earth as a reference image demonstrate that sub-pixel resolutions can be achieved in a high disturbance environment. Spatial resolution is quantified by Cramer-Rao lower bounds.
This paper presents the system architecture proposed for the Mobile Satellite Service (MSS) by th... more This paper presents the system architecture proposed for the Mobile Satellite Service (MSS) by the National Aeronautics and Space Administration (NASA)/JPL. The demand assigned Frequency Division Multiple Access (FDMA) scheme is described, and results for the associated network access protocol developed by JPL are presented. Both the total number of users that the system can support and the system spectral efficiency are given for a variety of traffic conditions, including those postulated for the Mobile Satellite System Architectures and Multiple Access Techniques Workshop. The results are given for both first-and second-generation one-and two-satellite systems.
Proceedings of SPIE, Aug 16, 1994
Performance of deep space optical communications is impacted by atmospheric outages. By monitorin... more Performance of deep space optical communications is impacted by atmospheric outages. By monitoring intensities of known stellar objects with calibrated telescopes, atmospheric attenuation statistics can be recorded. This paper describes a program to develop an atmospheric transmission model for optical communications by making autonomous visibility measurements of known stellar objects from three locations in the southwest United States. The model will be used in link margin analyses for optical communications channels. The statistics will be updated on a quarterly basis and calibrated using image data taken over a long period of time. This calibration is expected to allow extrapolation to other ground-based locations through the use of global weather data bases.
Proceedings of SPIE, Apr 26, 1999
This paper describes an extended-source spatial acquisition and tracking scheme for planetary opt... more This paper describes an extended-source spatial acquisition and tracking scheme for planetary optical communications. This scheme uses the Sunlit Earth image as the beacon signal, which can be computed according to the current Sun-Earth-Probe angle from a pre-stored Earth image or a received snapshot taken by other Earth-orbiting satellite. Onboard the spacecraft, the reference image is correlated in the transform domain with the received image obtained from a detector array, which is assumed to have each of its pixels corrupted by an independent additive white Gaussian noise. The coordinate of the ground station is acquired and tracked, respectively, by an open-loop acquisition algorithm and a closed-loop tracking algorithm derived from the maximum likelihood criterion. As shown in the paper, the optimal spatial acquisition requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate when translation in the relative positions of onboard and ground transceivers is considered. Similar assumption of linearization leads to the closed-loop spatial tracking algorithm in which the loop feedback signals can be derived from the weighted transform-domain correlation. Numerical results using a sample Sunlit Earth image demonstrate that sub-pixel resolutions can be achieved by this scheme in a high disturbance environment.
Proceedings of SPIE, Apr 24, 1997
The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration termin... more The Optical Communications Demonstrator (OCD) is a laboratory-based lasercom demonstration terminal designed to validate several key technologies, including beacon acquisition, high bandwidth tracking, precision beam pointing, and point-ahead compensation functions. It has been under active development over the past few years. The instrument uses a CCD array detector for both spatial acquisition and high-bandwidth tracking, and a fiber coupled laser transmitter. The array detector tracking concept provides wide field-of-view acquisition and permits effective platform jitter compensation and point-ahead control using only one steering mirror. The use of a fiber-coupled transmitter modularizes the transmitter design and de-couples its thermal management problems from the main system optics. The reduction in design complexity can lead to a reduced system cost and an improved system reliability. This paper describes recent progress on the development of the OCD terminal.
Proceedings of SPIE, Apr 26, 1999
One of the NASA technology development program at the Jet Propulsion Laboratory aims to increase ... more One of the NASA technology development program at the Jet Propulsion Laboratory aims to increase the information return capability while reducing the size of the spacecraft via laser communications. The deep space optical transceiver developed under this program employs puke position modulation (PPM) for both uplink and downlink transmissions. An integral part of the transceiver is the development of signal acquisition and tracking subsystem. This paper describes the baseline design of the electronic assembly within the transceiver and modifications that are necessary for deep space communications. A two phase breadboard activity will be described to reduce technological risks associated with the development. 1. INTRODUCTION Laser communication is an enabling technology applicable to future NASA near Earth and deep space missions that desire higher communications capacity than currently available by RF technologies. One of the NASA technology development program at the Jet Propulsion Laboratory (JPL) aims to increase the information return capability by at least an order cf magnitude while reducing the size of the spacecraft. The technicd merit of laser communications is derived from the fact that it offers a much higher collimated signal than conventional microwave. This super-collimated beam can result in a terminal design with greatly reduced size, mass and power requirements. Furthermore, laser communication systems are not susceptible to radio frequency interference and are not subject to frequency or bandwidth regulation. Currently, data return from deep space missions is accomplished by direct space-to-ground link operating at S-. X-, or Ka-band. Planned deep space missions could demand return of vast quantities of scientific data either by direct link or through relays such as Mars missions. As more and more missions begin to operate at high downlink data rates to the DSN, resource and spectrum allocation and radio frequency interference (FWI) could become difficult to manage. Laser communications become a viable alternative to provide such a vast return. Link Range Data rate Coded BER Transmit power Transmit losses Transmitter gain Pointing losses Space loss Atmospheric losses Receiver gain Receiver optics losses Received signal Background signal level Required singnal level Allowances and Adjustments Lick Margin 8.98E+O8 km 6.00 AU 6.00E+01 kbps PPM
Proceedings of SPIE, Aug 6, 1993
The concept o f mobile communications via a geosynchronous satellite has been proposed in recent ... more The concept o f mobile communications via a geosynchronous satellite has been proposed in recent years. Twelve companies have filed applications with the Federal Communications Commission to offer nationwide Mobile Satellite Services (MSS). Chis paper addresses several technical issues related to MSS from the network design point o f view. MSAT-X, which stands for the Mobile Satellite Experiment, is the task assigned to the Jet Propulsion Laboratory by the United States National Aeronautics and Space Administration to develop low-cost, spectral-efficient, satellite-based mobile communications technologies for 1990's. A1 though the discussions and recommendations presented in this paper are 1 imi ted to MSAT-X, technologles developed under MSAT-X may be o f the utmost importance to the first generation MSS. with the National Aeronautics and Space Administration.
Telecommunications and Data Acquisition Progress Report, Sep 13, 1998
A generalized maximum a posteriori (MAP) symbol synchronizer for arbitrary nonoverlapping pulse s... more A generalized maximum a posteriori (MAP) symbol synchronizer for arbitrary nonoverlapping pulse shape and data-transition density is derived, and a simplified realization is presented. Acquisition performance at low to very low symbol signalto-noise ratios (SNRs) is investigated and compared against the conventional datatransition tracking loop (DTTL). Simulation results show that the MAP symbol synchronizer can operate at very low symbol SNRs where the DTTL fails. Furthermore, the new symbol synchronizer reduces the initial acquisition time by at least one order of magnitude as compared with the DTTL. It also has been shown that the new symbol synchronizer is suitable for minimum-shift-keying (MSK) signaling waveforms.
IEEE Transactions on Information Theory, May 1, 1980
An optimal linear mean-square estimation algorithm is derived under the constraint that the algor... more An optimal linear mean-square estimation algorithm is derived under the constraint that the algorithm be multiplication-free. A classical linear estimation problem with block length N generally requires N2 multiplications. For many on-line signal processing shuations a large number of multiplications is objectionable. This class of estimation problems includes the classical linear filtering of a random signal in random noise, as well as the linear equaliition of digital data over a dispersive channel with additive noise. Here we consider the linear estimation problem on a binary computer where the estimation parameters are constrahted to be powers of two and thus all multiplications are replaced by shifts. Then the optimal constrained linear estimation problem resembles an integer-programming problem except that the allowable discrete points are nonhttegers. The branch-and-bound principle is nsed to convert this mfnimixation problem to a series of convex programming problems. An algorithm is given for the solution as well as numerical results for filtering and data equaliiation. These examples show that the multiplication-free con-Manuscript
A method for optimal adaptive setting of pulse-positionmodulation pulse detection thresholds, whi... more A method for optimal adaptive setting of pulse-positionmodulation pulse detection thresholds, which minimizes the total probability of error for the dynamically fading optical free space channel, is presented. The threshold's adaptive setting, in response to varying channel conditions, results in orders of magnitude improvement in probability of error, as compared to use of a fixed threshold. The adaptive threshold system itself is based on a robust channel identification system that uses average signal strengths to estimate the degree of fade and total attenuation in the channel, and a radial basis function network for estimating pulse spreads, all with excellent accuracy.
An essential requirement in the design and implementation of optical receivers for deep space com... more An essential requirement in the design and implementation of optical receivers for deep space communications is the development of a low-complexity, fast, and intelligent frame and symbol synchronization technique. In this paper we present the results of a sub-sequence pattern matching technique that detects and isolates the data frame boundaries based on partially received frames. This is in contrast to the traditional correlation based methods which are not as computationally efficient, and require full reception of the acquisition sequence prior to synchronization. This technique allows fast convergence and detection of the boundaries with as low as 2-6 bytes of received synchronization word. In addition, a confirmation method is proposed which allows tuning of the receiver detection module for a given probability of false boundary detection.
In this study we will show that the research on Nversion high-reliability software structures can... more In this study we will show that the research on Nversion high-reliability software structures can be extended to neural networks architecture. In addition, we will explore the possibility of applying this structure to a spacecraft tracking problem. One such system is the Automated Spacecraft Monitoring System (ASMS), a beacon-monitoring or detection system. Four neural networks, each trained for various operating environments, are implemented in an Nversion structure. The results of the networks are combined to form a composite outcome. The combined outcome is used as part of a hypothesis testing procedure to distinguish between the presence or absence of the beacon signal. The results show that any of a number of composite outcomes outperforms the use of any single neural network. Further, the simple average of network results provides the composite outcome with best performance.
Performance of deep space optical communications is impacted by atmospheric outages. By monitorin... more Performance of deep space optical communications is impacted by atmospheric outages. By monitoring known stellar objects with calibrated telescopes, atmospheric attenuation statistics can be recorded. This paper describes a program to develop an atmospheric transmission model for optical communications by making automous visibility measurements of known stellar objects from three locations in the southwestern US. The data will be correlated and infused with weather data to establish the atmospheric transmission model. This model will be sent in link margin analysis for optical communications channels. The statistics will be updated on a quarterly basis and calibrated using data taken over a long period of time. This calibration is expected to allow extrapolation to other ground-based locations through use of global weather databases.
Free-space laser communication is a promising technology to meet the demand for cost-effective wi... more Free-space laser communication is a promising technology to meet the demand for cost-effective wideband support for future NASA missions. JPL has begun construction at the Table Mountain Facility in California of a 1-meter telescope system, known as the Optical Communication Telescope Laboratory (OCTL) and capable of tracking spacecraft from low Earth orbit to deep space, for development and validation of vital optical communication technologies. One of them is accurate pointing for both up (ground-to-space) and down (space-to-ground) links. This article describes an acquisition and tracking scheme for downlink optical telemetry that is based on a correlation-type technique developed for extended sources covering several elements of the detector array. The purpose of developing such a scheme is to enable simultaneous beam tracking and data detection using a small number of optical sensors. The open-loop acquisition derived from the maximum-likelihood criterion involves a transform-domain correlation between the received laser image and the reference image derived from the known intensity profile of the transmitting laser. The optimal acquisition algorithm requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate of the transmitting laser when a rotation-invariant movement is considered. A maximumlikelihood-based closed-loop tracking algorithm is developed as well, in which the loop feedback signals are formulated as weighted transform-domain correlations between the received laser image and the previously estimated reference image. This scheme is expected to be able to achieve sub-pixel resolutions in a high-disturbance environment.
Space Technology Conference and Exposition, Aug 22, 1999
This paper describes a generic extendedimage spatial acquisition and tracking technique developed... more This paper describes a generic extendedimage spatial acquisition and tracking technique developed to enable highly accurate and stable pointing to a moving target through an imaging device. It is intended to be a fin+pointing scheme complimentary to the imager's general control subsystem. that is able to provide a "coarse" pointing capability such that the target remains within the image& field of view. This correlation-type scheme compares the received image obtained from the detector array with a priorly established reference in the transform domain to estimate the target's movement. The received image is assumed to be an extended-image covering more than one element of the detector array and have each of its pixels corrupted by an independent additive white Gaussian noise. The coordinate of the target area is acquired and tracked, respectively, by an open-loop acquisition algorithm and a closed-loop tracking algorithm derived from the maximum likelihood criterion. The resulting movement estimate is used to direct the imager to closely follow the moving target. This technique has many potential applications, including free-space optical communications and astronomy where accurate and stabilized optical pointing is essential. 'One sstrkomy unit & roughly 149.6 x lo6 kilometers.
The novel network testbed simulators are described: the request protocol simulator and the link p... more The novel network testbed simulators are described: the request protocol simulator and the link protocol simulator. These testbeds are useful for evaluating delay-throughput performances of demand-assisted multiple-access (DAMA) protocols in mobile fading environments. The request protocol testbed simulates the multiple-access scheme for making connection requests in the DAMA protocol. The link protocol testbed simulates the link-layer protocol for data transmissions.