Pointing, Acquisition And Tracking Research Articles

A Composite Control Method Based on Model Predictive Control and a Disturbance Observer for the Acquisition, Tracking, and Pointing System

Nonlinear disturbances, such as friction, backlash, and base vibration, are the main factors restricting the further improvement of the dynamic performance of the Acquisition, Tracking and Pointing (ATP) system. However, the modeling and compensation of the time-varying nonlinearities are still challenging. In this paper, a compound control algorithm based on model predictive control (MPC) and a Proportional Multiple-Integral State-Augmented Kalman Filter (PMISAKF) disturbance observer is proposed to improve the disturbance rejection performance. Specifically, a PMISAKF is used to estimate the system’s state and external disturbances in real-time. These observed disturbances are then treated as known external inputs at the current or future time points and incorporated into the predictive model of MPC. This allows the optimization process to take these external factors into consideration, enabling the calculation of an optimal control strategy and achieving high pointing and tracking accuracy for nonlinear time-varying disturbances. Experimental results show that the proposed method can effectively improve the system’s anti-interference ability and tracking accuracy.

Development of a Numerical Simulation Model to Support the Design of a Ship–Satellite Communication System for Autonomous Marine Navigation

In recent years, the concept of autonomous navigation systems has gained substantial significance, with the potential to change the traditional concept of autonomous navigation. The presented numerical simulation investigates the feasibility of a ship’s autonomous navigation system through a laser communication infrastructure handled by a two-degrees-of-freedom (DoFs) acquisition, tracking and pointing (ATP) system able to enable ship–satellite data transmission. The methodology introduced presents the geometrical and kinematic delineation of the model, coupled with the implemented control system, aimed at assessing the pointing accuracy. The minimum requested pointing accuracy is 100 µrad and the analysis highlights the need of using methodologies to reduce the pointing error. Two approaches are investigated to examine a possible improvement of the system, and results show that the pointing phase is influenced less by ship motions and more by errors that occur during the satellite’s positioning and the ship motion acquisition process. A trade-off in choosing parameters to improve the system’s accuracy leads to a satellite’s first targeting time of 0.25 s alongside the probability of hitting the target once every 0.0013 s. The reliability of the system is evaluated through a brief sizing of the optical electromechanical component of the system using the trade-off parameters chosen to improve the pointing phase accuracy.

On the capacity of a SIM-based cooperative NLOS UVC system with best relay selection

Ultraviolet communication (UVC) is a promising technology due to its ability to operate in non-line-of-sight (NLOS) mode thereby eliminating the pointing acquisition and tracking (PAT) requirement as needed by infrared and visible light communications. However, NLOS UVC suffers from very high attenuation and turbulence-induced fading when operated over a long distance. Due to these limitations, the existing literature on the NLOS UVC is mostly restricted to short-distance communications only. Therefore, this paper addresses these challenges by proposing an outdoor subcarrier-intensity-modulation (SIM) based multi-relay cooperative communication system employing the best relay selection and decode-and-forward (DF) relaying protocol. The turbulence-induced fading is modelled using lognormal distribution under weak atmospheric turbulence conditions. We derive novel closed-form analytical expressions of outage probability and ergodic capacity. Correctness of the derived analytical expressions is validated through numerical simulations.

Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems

The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized.

Model-Rebuilt Disturbance Observer of a Tracking System Based on Acceleration Fusion for Laser Power Transmission

When using laser energy to power long-distance and fast-moving targets, it requires a fast-response and high-precision acquisition, pointing and tracking (APT) system. A fast-steering mirror (FSM) system was used in this paper to track the solar cell array as laser power receiver. The disturbance suppression performance is a key indicator for the FSM stabilization. Generally, a fiber-optic gyroscope (FOG) is employed in the high-sampling-rate velocity loop to enhance the anti-interference ability. However, with the expansion of miniaturized applications, a relatively large, heavy, and high-power FOG is hard to be installed on the small mirror. With this case, this paper used a small-size and high-bandwidth MEMS linear accelerometer in the acceleration loop, substituting the gyroscope. However, the drift and high-frequency noise of the MEMS accelerometer in low frequency will cause APT disturbance. Therefore, an acceleration fusion method with a modified complementary filter was proposed to blend signals of the charge-coupled device and the accelerometers. The fused virtual acceleration can eliminate drift and reduce noise in low frequency and was eventually used in the model-rebuilt disturbance observer loop. At last, the measured results show that the disturbance suppression performance is improved using the presented method, and low-price and small-size MEMS accelerometer can be applied in the APT system.

Efficient MIMO Configuration for Bi-Directional Vertical FSO Link with Multiple Beam Induced Pointing Error

We proposed the statistical misalignment model and the power-efficient configuration of transceivers for bi-directional multi-input and multi-output (MIMO) based vertical free space optical (FSO) links. Spatial diversity based MIMO FSO systems could be used to mitigate atmospheric fading issues. However, the increased number of channels can cause additional pointing error in pointing, acquisition and tracking (PAT) systems. The statistical misalignment model for detecting misalignment error is derived from the multiple transceivers. For the bi-directional characteristics of non-terrestrial back-haul networks, transmission performance is down-leveled to the worse in the asymmetric MIMO configuration of transceivers. The symmetric structure can mitigate the effect of increased pointing error to improve transmission performance. The proposed technique can be applied to the design of power-efficient FSO systems for non-terrestrial wireless back-haul networks.

A Reinforcement-Learning-Based Beam Adaptation for Underwater Optical Wireless Communications

Underwater optical wireless communications (UOWCs) have recently appeared as an attractive solution for many applications, such as remote controlling and sensing due to its advantages, such as high transmission rate, ultrawide bandwidth, and low latency. However, due to the harsh underwater conditions, UOWC faces challenges, such as water absorption, scattering, and pointing-acquisition-and-tracking (PAT) problems. This is mainly due to the dynamicity existing underwater. Consequently, this leads to packet loss and hence deteriorates the reliability and link quality of such networks. Such a problem can affect the degree of connectivity and end-to-end (E2E) performance of the communication system. The existing solutions in the literature are based on predefined models, assuming full knowledge of the environment. However, such models do not optimally treat the dynamicity existing underwater. This article proposes novel beam adaptation methods based on reinforcement learning (RL) for point-to-point UOWC. The first method aims to optimize the light beamwidth; the second method focuses on adapting the beam orientation, whereas the last one optimizes both the light’s beamwidth and beam orientation. Our proposed RL-based solutions yield optimal positioning and beamwidth of the light source and improve the considered communication link’s success rate. They also guarantee better link quality in terms of signal-to-noise ratio (SNR) compared to the uncertainty disk static method for four different underwater environments, including pure seawater, clean ocean, coastal ocean, and turbid harbor.

Design and simulation of the ATP system considering the advanced targeting angle in quantum positioning system

A compensation implementation scheme of the advanced targeting process based on the fine tracking system is proposed in this paper. Based on the working process of the quantum positioning system (QPS) and its acquisition, tracking and pointing (ATP) system, the advanced targeting subsystem of the ATP system is designed. Based on six orbital parameters of the quantum satellite Mozi, the advanced targeting azimuth angle and pitch angle are transformed into the dynamic tracking center of the fine tracking system in the ATP system. The deviation of the advanced targeting process is analyzed. In the Simulink, the simulation experiment of the ATP system considering the deviation compensation of the advanced targeting is carried out, and the results are analyzed.

Artificial vision algorithm for earth station location from CubeSats in narrow beam optical communications

The use of CubeSats promises several advantages within a low economical budget margin for different missions. Nowadays, data transfer through optical channels (lasercom) is considered a potential technology for satellite wireless communications, providing several advantages in terms of data transfer rate with lower energy consumption. However, a main drawback is the location of the Optical Ground Station (OGS). Therefore, an artificial vision algorithm for the processing of images captured by the camera of a CubeSat is proposed in this work, in order to carry out the identification and recognition of an OGS by means of a beacon signal, to aid the Pointing, Acquisition and Tracking (PAT) onboard subsystem for the communication’s stage.

Inter-satellite optical wireless communication (IsOWC) systems challenges and applications: a comprehensive review

Abstract Satellite communication, in over half a century have attained remarkable breakthrough in high speed transmission. These innovations, on the other hand, have come in tandem with significant performance improvements in other IT and telecommunications systems. As a result, these significant benefits are not as visible to the broader public as they would be as if this burst of performance had occurred in isolation. In nutshell, space satellite communication using optical technology provide powerful alternative to radio frequency (RF) and make communication reliable and cost effective. Satellite communications systems allow a whole new race of civilian and defence operations in surveillance, telecommunications, object tracking and space exploration. Since independent satellites are restricted by volume, space and strength, small satellites production in bulk clusters may be effective for a range of wildfire monitoring, science missions, mapping in gravity and water management exploration, among others. The development of communications satellites would provide for a more comprehensive understanding of the near-earth environment as well as a more efficient and cost-effective way of reaching space, thanks to the utilisation of multi-satellite systems. As a result, when satellites are designing, IsOWC is an utmost point to be considered. The numerous researches being performed in the satellite communications for introducing multi communications based on different orbits are described in this study. IsOWC systems over radio frequency based communication are preferred due to large bandwidth, high speed, absence of electromagnetic interference (EMI), lower transmission losses and improved security. We also present a detailed listing of challenges in Is-OWC such as Doppler shift, point ahead angle, satellite vibration and tracking, acquisition tracking and pointing (ATP) and background noise sources.

Survey on acquisition, tracking and pointing (ATP) systems and beam profile correction techniques in FSO communication systems

Abstract Free space optical (FSO) communication has shown promising advantages among other wireless schemes. It provides a very high data rate, freedom from licensing, low cost of deployment and requires low power. FSO is a technology that has undergone rapid development over the last several years. These communication systems are a line-of-sight technology in which information is transmitted through the atmosphere on modulated laser beams or light-emitting diode (LED) beams. When FSO technology was first introduced, it was seen as an attractive option to bridge the “last mile bottleneck” that is present in many of today’s optical fiber-based networks. As compared to existing radio frequency (RF) based wireless systems, this technology possesses multiple advantages, such as high bandwidth, license-free band use, long operational range, spatial reusability, security and immunity to electromagnetic interference. The narrow and directional characteristics of a laser beam employed in FSO enable spatial reuse and make it hard to eavesdrop, thus raising the level of security. The use of light as carrier in this technology provides immunity to electromagnetic interference. Despite its many advantages, this technology is susceptible to some weather conditions, such as fog, rain, sleet and snow, and to misalignment of transmitter–receiver terminals. Atmospheric conditions will impair the propagation of an optical signal because the propagation of light may undergo absorption and scattering. Pointing error caused by misalignment of the transceivers is another major challenge in FSO communication system. The pointing error may result in degradation or even total loss of the received signal. This error may arise because of transceiver sway, platform vibration, the motion of mobile stations, errors or uncertainties in the tracking system. Another type of pointing error is beam wandering caused by the in-homogeneity of large-scale eddies in the atmosphere (i.e., atmospheric turbulence), where the transmitted beam may deviate from its intended path. This survey paper focuses on the systems involved in the alignment of the transmitter and the receiver so that the maximum amount of power is collected by the receiver and the analysis of beam profile in various atmospheric conditions and their mitigation methods.

Approach of recognition and precision location for the beacon in satellite optical communications

Satellite optical communications have high communication speed and good confidentiality, which can establish high-speed and large-capacity communication links between satellites and the ground or other satellites. The requirements for the PAT (pointing, acquisition and tracking) system of satellite optical communication terminals are very high. In satellite optical communications, the biggest influence on the beacon’s recognition and tracking is the background light on CCD (Charge Coupled Device). Therefore, denoising work is required during the recognition or tracking process. This paper uses corner detection method to extract the beacon frame by frame. Mark the spot, and use the optical flow method to calculate the optical flow displacement based on the two consecutive frames. In addition, calculation accuracy of the beacon’s centroid is compared with that of the tradition method. The feasibility is verified by simulation experiment, the tracking process is completed, and the noise can be precisely eliminated.

Free-Space Terabit Optical Interconnects

The continuous growth of Internet data traffic is progressively increasing the pressure over wireless access technologies. To overcome the imminent bandwidth bottleneck, free-space optics (FSO) is currently deemed as a key breakthrough towards next-generation ultra-high-capacity wireless links. As this technology matures, it is starting to find its place not only in future mobile access networks, but also in applications such as datacenter interconnections, satellite communications and high-frequency trading. To meet the ever-increasing demand for high-capacity links, future-proof FSO systems should seek maximum compatibility with state-of-the-art optical fiber systems, which nowadays can already provide data rates per channel in the Terabit/s range. However, the ultra-wideband potential of FSO systems comes at the expense of very tight requirements in terms of optical beam alignment, so that reliable end-to-end communications can be ensured. Following this research challenge, in this work we demonstrate a dynamic single-wavelength 1 Tbps FSO link with high pointing error tolerance, boosted by adaptive probabilistic constellation shaping (PCS) and an active gimbal-based acquisition, tracking and pointing (ATP) mechanism. Our results demonstrate successful indoor FSO transmission over 3 m with enhanced resilience towards pointing errors, enabling to adapt the data rate in the range of 0.8–1 Tbps.

Mechanical design and determination of bandwidth for a two-axis inertial reference unit

The inertial reference unit (IRU) serves as a master reference for acquisition, tracking and pointing (ATP) system due to its ability to maintain stability with respect to inertial frame. However, the bandwidth of IRU system is usually limited by the dynamics of mounted inertial sensors and structural resonances. In this paper, magnetohydrodynamics (MHD) angular rate sensor (ARS) with extremely low noise at high frequencies (>3Hz) was combined with a conventional gyro serving as inertial sensing unit of the developed IRU. This sensing unit was verified experimentally to exhibit a nearly perfect transfer function with “unity” gain and “zero” phase error. A central hinge that allows only rotation about the actuating axes was used as the supporting structure. A structural parameter design approach based on analytical stiffness model was developed for the central hinge. The relative errors between theoretical stiffness values, finite element analysis (FEA) simulations and experimental data were found to be within ±13%. A double-T network notch filter with optimal parameters was implemented to suppress the low-frequency mechanical resonance. With the notch filter in the forward path, the open-loop dynamic model of the IRU was measured with the results indicating a completely damped resonant peak and an identification error less than ±0.5 dB in magnitude and ±5° in phase. The developed IRU was ultimately certified to achieve more than ±6 mrad as a drive range and more than 120 Hz as a closed-loop bandwidth under a digital controller designed with frequency shaping technique.

Demonstration and verification experiment in deep space optical communications

In deep space optical communications, pointing, acquisition and tracking (PAT) system need to locate the position of the beacon, precisely. The way of employing the natural celestial body as the beacon come to draw attention, in contrast to regarding the laser as the beacon which has a series of problems of limited power, serious attenuation, high cost and difficult design realization. In this paper, the primary goal is to study the problem of precise positioning for the beacon by taking the natural celestial body as the beacon. Given the long distance, with small target and large uncertain region in deep space optical communications, the location of the beacon can be realized by the combination of rough and precise positioning based on the image feature of the beacon. Firstly, it performs the rough positioning according to the image characteristic information of the beacon’s edge, then in the uncertain region, the processing scope has been narrowed according the results of the rough positioning, finally the beacon is precisely located based on its minutiae feature by the way of feature point matching to achieve the position of the beacon. The impact due to the autorotation of celestial body on the image transform has been also discussed and analyzed with the mapping relationship of coordinate and geometric transform. The feasibility of the algorithm is verified by a long distance far-field Earth–Moon experiment. The design of system implementation on embedded hardware system can provide a reference for the real-time positioning system in deep space optical communications.

Analysis and correction of geometrical error-induced pointing errors of a space laser communication APT system

The geometrical error caused pointing error is an inevitable problem in space satellite laser communication terminals which can affect the pointing accuracy of the APT (acquisition pointing and tracking) system greatly, in order to facilitate the assembling of the APT system and improve the performance of the laser communication system, the geometrical error sensitivity about the APT pointing accuracy is analyzed based on multi-body kinematics method in this paper, the error transformation matrix is derived and the geometrical error is analyzed, the simulation results provide some pointing error distribution regulars which are conductive to assembling. Based on the above research, the geometrical error correction experiment is performed and the pointing accuracy of the APT system is tested, the expectation value of the pointing error can reach 29.9 µrad which is greatly improved. This research can provide technical references for the design and analysis of space laser communication terminals.

Optical path pointing error and coaxiality analysis of APT system of space laser communication terminal

Precision beam pointing is the key indicator for APT (acquisition, pointing and tracking) system in space laser communication. The laser travels inside the optical system and the pointing vector will be affected by an assembly error of the axis and reflectors. In this paper, the model of the optical path pointing error and coaxiality error induced by the assembly error are established; the error distribution is given and a quantitative analysis is performed. The results show that the magnitude of pointing error is affected by the axis assembling error greatly but its distribution is susceptible to the reflector assembly error. Finally, the correction of coaxiality is performed and tested. The experimental results show that the coaxiality error can be greatly improved and the mean value of the coaxiality error of a beacon path and a signal path are 14 and 9.6 μrad, respectively, which meets the requirements. This work can provide guidance for design and assembly of the APT and contribute to the improvement of its pointing performance.

High Accuracy and Multi-Target Acquisition, Pointing and Tracking under Satellite Micro-Vibrations

The high precision multi-target acquisition, pointing and tracking (APT) technology under satellite micro-vibrations is urgently required with the increasing demand on space science missions. This paper thus presents one APT system simultaneously exhibiting the high accuracy and multi-target laser link functions. The performance of the APT system is investigated through verification simulation considering the influence of the carrier satellite. Furthermore, the prototype of the APT is developed, which consists of one target motion simulation unit and one acquisition, pointing and tracking unit. The prototype system can simulate multiple targets with the angle motion accuracy better than 0.081μrad, while the pointing and tracking accuracy of the prototype is better than 0.207μrad under the lab environment vibrations. The proposed ground experimental prototype system is greatly beneficial to enhance the ability to construct multiple space laser links over a long distance.

Design and analysis of performance of FSO communication system based on partially coherent beams

In this paper, we investigate that the effects of weather, exponentiated Weibull turbulence and generalized pointing errors based on Beckmann distribution on partially coherent beams and establish a joint channel statistical model, then derive new expressions of the performance parameters of free space optical communication system based on On–Off keying (OOK) modulation by using Meijer’s-G function and Fox-H function, such as average bit error rate (BER), average channel capacity and outage probability. For an atmospheric laser communication system with 1550 nm wavelength and 1 km terrestrial level link, the performance parameters of the system are simulated and analyzed. The results show that with the increase of signal to noise ratio (SNR), the compensation effect for strong turbulence becomes more obvious; on the premise of sacrificing less channel capacity, reducing the spatial coherence length or increasing the plane spot size of the transmitter can significantly improve the BER and outage probability performance of the system; with the increase of receiver aperture diameter, the aperture average effect is more obvious; increasing the receiver aperture diameter can significantly improve the overall performance of the system; in the design of APT (Acquisition,Pointing and Tracking) subsystem, when a certain boresight error is allowed, the random jitter of transmitter is reduced as much as possible.

A survey on quantum positioning system

ABSTRACT The quantum positioning system (QPS) takes advantage of the entanglement property of the quantum system itself, and it is possible to breakthrough the limitation of the positioning accuracy due to signal bandwidth and power limitation in classical radio-based global positioning system (GPS), and improve the positioning performance. In this paper, a comprehensive survey of the research status of QPS and its several key components are presented, including the quantum entanglement states preparation, the quantum satellite technique, the optical signal acquisition, tracking and pointing (ATP) system, and the quantum positioning and clock synchronization technique. We also discuss the current challenges and future research directions at the end of the paper.