Inter-satellite Laser Link Research Articles

Bidirectional scanning acquisition of inter-satellite laser links for space gravitational wave detection mission.

Space gravitational wave detection requires establishing laser links between distributed spacecraft for interferometry. Inter-satellite laser link acquisition is an essential step in this process. Considering the spacecraft's miniaturization and reliability, a bidirectional scanning acquisition method is proposed using only field emission electric propulsion and quadrant photodetector. This method does not require additional acquisition sensors and actuators. To improve efficiency while ensuring the acquisition success rate, a constant angular velocity Archimedean spiral scanning guidance law is proposed, and the critical parameters that influence acquisition time and success rate are analyzed. Monte Carlo simulations show that the method can ensure an acquisition success rate of over 90%. Compared with constant linear velocity Archimedean spiral scanning, this method can reduce the acquisition time required to achieve the same success rate by 15%.

Criteria for selection of point-ahead angle compensation strategy in intersatellite laser ranging interferometry of TianQin based on research of tilt-to-length coupling noise

TianQin is expected to deploy three spacecraft in Earth orbit at the altitude of about 1 × 108 m to form an equilateral triangular constellation with arm lengths of about 1.7 × 108 m. It is aimed at detecting gravitational waves in the frequency range from 0.1 mHz to 1 Hz by means of high sensitive laser ranging interferometry with pathlength measurement noise below 1 pm/Hz1/2. The long-range propagation of beam in the intersatellite laser ranging interferometry between the three spacecraft results in a non-negligible time delay of 0.58 s. Therefore, there is an angular difference between the optimal outgoing and incoming laser beam of laser ranging interferometry on each spacecraft, which is denoted as the point-ahead angle. Due to the relative motion between the three spacecraft, the point-ahead angle of TianQin constellation is approximately 22.96 μrad with a dynamic variation range of ±24.71 nrad. Point-ahead-angle mechanism is a fine steering mirror used for point-ahead angle compensation to diminish the beam pointing error, thus to maintain the intersatellite laser link as well as to diminish the tilt-to-length coupling noise, which are vital to the gravitational wave detection. Both the static and dynamic point-ahead angle compensation strategies can be used in TianQin and the selection of the two is based on minimizing the total point-ahead-angle-related tilt-to-length coupling noise. In this paper, a criteria for selection of the two strategies are proposed by establishing a model of point-ahead-angle-related tilt-to-length coupling noise as well as a corresponding evaluation function for calculation and comparison. The results indicate that the dynamic compensation strategy is the optimal choice only when the first-order tilt-to-length coupling coefficient of point-ahead-angle mechanism is less than 6.4 × 10−9 m/rad, otherwise the static compensation strategy should be used. The proposed criteria, as well as the model and the requirement for point-ahead-angle mechanism, can serve as a common theoretical basis in laser ranging interferometry applications.

Ground-based simulation of laser link acquisition for inter-satellite laser interferometry

Laser link acquisition and pointing technique is one of the essential techniques for the inter-satellite laser interferometry for space-based gravitational waves detection and next-generation Earth gravity measurement missions. The first step of building up inter-satellite laser link is using an acquisition camera to capture the inter-satellite laser beam signals within a pre-scanning uncertain cone. Subsequently, high-precision angle measurement technology, namely differential wavefront sensing, is used to achieve a high pointing precision required. Due to the distance constraint of a ground-based simulation experiment, it is difficult to verify directly the feasibility of an inter-satellite laser link acquisition and pointing control scheme. By means of controlling the optical properties of the received laser beam, the long-distance beam propagation is simulated with two optical benches of an inter-satellite interferometer, and the process of a laser link acquisition experiment has been demonstrated. The experimental results show that the inter-satellite laser beams could establish the dual-way locking successfully. The fluctuation of the laser beam pointing direction (in-loop) can be suppressed to about 5 µrad in atmospheric environment. The results verify the feasibility of the laser link acquisition scheme. The experimental setup can be extended to conduct experiments with various parameters, providing technical support for further testing of the inter-satellite laser link acquisition and pointing control methods.

Analysisof Beam Walk in Inter-Satellite Laser Link: Implications for Differential Wavefront Sensing in Gravitational Wave Detection

Achieving space-based gravitational wave detection requires the establishment of an interferometer constellation. It is necessary to establish and maintain stable laser interferometric links using the differential wavefront sensing (DWS) technnique. When the distant measurement beam experiences pointing jitter, it causes beam walk on the surface of the local detector. The reduced overlap between the local reference spot and the distant spot increases the nonlinear errors in the DWS technique, which need to be suppressed. Numerical analysis was conducted on the spatial beam interference signals of the DWS technique when the distant measurement beam experienced pointing jitter. An experimental measurement system was designed, and the beam walk was suppressed using a conjugate imaging system. The results show that within a range of 300 μrad, the optical path with the imaging system can reduce measurement errors by at least 83%. This way also helps to reduce pointing jitter noise in inter-satellite links, thereby improving laser pointing control accuracy.This method would provide a valuable reference for future DWS measurement systems.

A new statistical fading model for loss factor induced by angle-of-arrival fluctuations in free space optical communication.

The impact of angle-of-arrival (AOA) fluctuations on the loss factor is investigated, and a new fading model is proposed to describe the associated attenuation. Distinct decay characteristics induced by AOA fluctuations are revealed, which depend on the ratio of detector size to beam width (M). The fading model can be divided into two sub-models based on the value of M0, where M0 is the value of M when the root-mean-square errors of the two sub-models are equal. In the case of M⩾M0, this fading model follows an inverse S-shaped pattern to describe the impact of AOA fluctuations. When M < M0, an exponential function to characterize the attenuation induced by AOA fluctuations. The numerical computation results indicate that the proposed fading model offers higher accuracy compared to traditional methods. Furthermore, the continuous analytical expressions for the probability density function (PDF) and cumulative density function (CDF) of the presented fading model caused by AOA are derived. Based on the proposed PDF and CDF models, the outage probability of inter-satellite laser link is analyzed.

Characterization of Doppler shift in inter-satellite laser link between LEO, MEO, and GEO orbits

Over the past few decades, the development of optical wireless communications (OWC) has led to its application in a wide range of environments, including indoor, outdoor, and space. Optical carriers offer advantages such as high bandwidth, security, and directivity, making it a suitable technology for applications in radio astronomy, remote sensing, cellular networks, high-speed train communications, last-mile access, underwater/terrestrial communications, and satellite communications employing laser links. However, laser links between satellites experience Doppler shift due to their relative motion, and this effect should be taken into account from a system design perspective. This paper presents a study of the Doppler shift characterization in laser links for circular Keplerian orbits. An analytical derivation establishes the relationship between the Keplerian orbital elements and the Doppler shift in laser links. The derived equations provide a more comprehensive understanding of the impact of these orbital elements on the Doppler shift in laser links. Additionally, equations have been derived for the Doppler shift rate and the visibility window condition during satellite communication. Furthermore, we conducted a link failure analysis using simulations to evaluate the effects of the Doppler shift, also identifying the worst-case scenarios for orbits most significantly impacted by this shift. The results showed that among the six Keplerian orbital elements, the semi-major axis, inclination, and right ascension of ascending node contribute to the Doppler shift in circular orbits. While the Doppler shift impact is less significant for laser communication systems employing intensity modulation/direct detection, it can degrade the communication system performance for coherent detection. This work contributes to the effective estimation of Doppler shift from a system design perspective.

Research on High-Stability Composite Control Methods for Telescope Pointing Systems under Multiple Disturbances.

During the operation of space gravitational wave detectors, the constellation configuration formed by three satellites gradually deviates from the ideal 60° angle due to the periodic variations in orbits. To ensure the stability of inter-satellite laser links, active compensation of the breathing angle variation within the constellation plane is achieved by rotating the optical subassembly through the telescope pointing mechanism. This paper proposes a high-performance robust composite control method designed to enhance the robust stability, disturbance rejection, and tracking performance of the telescope pointing system. Specifically, based on the dynamic model of the telescope pointing mechanism and the disturbance noise model, an H∞ controller has been designed to ensure system stability and disturbance rejection capabilities. Meanwhile, employing the method of an H∞ norm optimized disturbance observer (HODOB) enhances the nonlinear friction rejection ability of the telescope pointing system. The simulation results indicate that, compared to the traditional disturbance observer (DOB) design, utilizing the HODOB method can enhance the tracking accuracy and pointing stability of the telescope pointing system by an order of magnitude. Furthermore, the proposed composite control method improves the overall system performance, ensuring that the stability of the telescope pointing system meets the 10 nrad/Hz1/2 @0.1 mHz~1 Hz requirement specified for the TianQin mission.

Gravity-based network traffic abstraction and laser ON/OFF control in optical satellite networks

As the cost of launching low Earth orbit satellites continuously decreases, satellite-based communications networks are emerging as a new area for both academia and industry. Lasers are already employed for building inter-satellite links, forming optical satellite networks. The orbiting nature of satellites determines that the optical satellite networks are usually uniformly distributed around the Earth, to provide seamless coverage to any place at all times. However, the end users on the Earth are non-uniformly distributed. As a result, many satellites with laser links might not be utilized efficiently. From the energy perspective, this work studies the energy-efficiency issues of the inter-satellite laser links. We first model the optical satellite networks by presenting the satellite constellation with inter-satellite laser design principles and introduce the laser ON/OFF control problem accordingly. To explore the possibility of saving energy in the massive satellite deployment, we further introduce the gravity-based network traffic model and propose a gravity-based network traffic abstraction (GNTA) model to evaluate the importance of each laser link. Accordingly, we further propose a GNTA-based ON/OFF control (GOOC) algorithm to improve the energy efficiency of inter-satellite laser links by switching OFF parts of the laser links that are less utilized. We evaluate the GOOC’s performance using simulation, and results show that switching OFF 20% of the laser terminals in the full-gridmesh topology can improve the energy efficiency by about 10%, with an acceptable cost of network performance degradation.

Space Laser Communication Assisted Protection Scheme Against SRLG Failures in Terrestrial Optical Networks

Survivability has already been an important research focus for the terrestrial optical network (TON). Various survivability technologies such as the pre-configured structure scheme, the multi-path protection scheme, the link/path-based restoration scheme, the disaster-aware network deployment scheme, and the degraded service tolerant provisioning scheme, have been developed to achieve the uninterrupted traffic transmission. However, most of the survivability technologies strongly rely on the network connectivity of the TON, and thus it is difficult to deal with shared risk link group (SRLG) failures caused by natural disasters or man-made destructions. Recently, the space laser communication (SLC) is widely studied and obtains rapid development. It can provide homogeneous spatial light-paths between two satellites or between a satellite and an optical ground station (OGS). The SLC brings a new solution to improve the survivability of the TON. In this paper, we study the SLC assisted survivability technologies against SRLG failures caused by natural disasters occurring on the ground. The SLC assisted dedicated protection (SLC-DP) scheme and the SLC assisted shared protection (SLC-SP) scheme are proposed. Heuristic algorithms are developed to realize those two schemes. Numerical results show that the SLC-SP scheme has a low blocking probability and the capacity of satellite-to-earth laser links plays a more important role than that of inter-satellite laser links.

On-ground demonstration of laser-link construction for space-based detection of gravitational waves

Laser acquisition and pointing system is subject to establish a 106km magnitude inter-satellite laser link with ultra-high pointing precision of 10 nrad/Hz (1 mHz-1 Hz) in space-based gravitational wave detection missions. For the unprecedented challenge, a dedicated laser link construction scheme with three different detectors is proposed. After initial pointing with star trackers, CMOS/CCD cameras intend to suppress the laser pointing error to 1 μrad. QPDs are subsequently used to achieve the final requirement. With various detectors and technologies, the scheme need intensive verification and study. We first design and build an on-ground laser link construction experimental system, which can simulate the whole process and recreate the actual critical working condition as realistic as possible by comprehensively considering the simulation of optical system, far field beam receiving characteristics and target precision. Results are well in agreement with the requirements and confirm the scheme under realistic conditions, which have not yet been fully tested experimentally.

Intersatellite Laser Link Planning for Reliable Topology Design in Optical Satellite Networks: A Networking Perspective

The development of reusable rockets makes it possible to launch a massive number of low earth orbit satellites for Internet access. These satellites are expected to be connected by inter-satellite links (ISLs) to provide global Internet service to end-users on the surface of the earth. From the infrastructure’s perspective, the ISLs can be implemented with lasers, forming the optical satellite networks. The design of terrestrial optical networks is usually traffic-driven, meaning more links/bandwidth are deployed to the traffic-intensive areas. But, the orbiting nature of satellites determines that laser ISLs are also moving at high velocities, making the presence of corresponding bandwidth loosed coupled with a specific area. As a result, the traffic-driven network design methodology will be inapplicable to the optical satellite networks. In this work, we model the optical satellite networks and define the laser ISL planning problem, Accordingly, we propose an ISL removal algorithm to remove parts of the ISLs from the grid-mesh topology to improve the average bandwidth utilization ratio (i.e., bandwidth efficiency) while maintaining networks’ reliability and availability. We conduct a simulation study based on the Starlink constellation to evaluate the grid-mesh topology and to investigate the proposed algorithm’s impact on the network performance. Results show that the bandwidth resource in the grid-mesh topology can hardly be used efficiently and removing certain ISLs properly can improve the bandwidth efficiency of optical satellite networks significantly. We also gain another interesting insight that maintaining too many ISLs will not improve, but degrade the network availability.

Research on Semi-Physical Simulation Testing of Inter-Satellite Laser Interference in the China Taiji Space Gravitational Wave Detection Program

To guarantee a smooth in-orbit space gravitational wave detection for the Taiji mission, a semi-physical simulation test of inter-satellite laser interference is carried out. The semi-physical simulation test consists of three aspects: the establishment of the inter-satellite laser link, interferometry of the inter-satellite ranging, and simulation of the space environment. With the designed specifications for the semi-physical simulation platform, the test results for the inter-satellite laser interference can be obtained. Based on the semi-physical simulation test, the risks of inter-satellite laser interference technology can be mitigated, laying a solid foundation for the successful detection of in-orbit gravitational waves.

Heterodyne coherent tracking technology for inter-satellite laser link during direct sunlight

There is a possibility that the angle of the laser link between two geosynchronous orbit satellites and the incident direction of sunlight will be small during the periods of the vernal equinox and autumnal equinox. At these times, the sunlight is incident to the receiving detector along the receiving channel of the laser terminal on the satellite, which easily causes the signal laser to be submerged in the background light. This study analyzed the influence of direct sunlight on a laser terminal, and a tracking scheme for heterodyne coherent detection using two four-quadrant detectors was designed. The explanations of the analysis and experimental results show that this scheme has a sufficient signal-to-noise ratio for tracking during direct sunlight. Experimental verification was carried out, with a solar simulator used to simulate the solar illumination. The laser terminal could track and communicate stably under direct irradiation by the sun, with a tracking accuracy that was better than 1 μrad (1 σ), and a bit error rate that was better than 10-7 at 1 Gbps. These research results could be applied to different link scenarios such as an inclined geosynchronous orbit and low earth orbit constellation.

Collaborative Multi-Resource Allocation in Terrestrial-Satellite Network Towards 6G

Terrestrial-satellite networks (TSNs) are envisioned to play a significant role in the sixth-generation (6G) wireless networks. In such networks, hot air balloons are useful as they can relay the signals between satellites and ground stations. Most existing works assume that the hot air balloons are deployed at the same height with the same minimum elevation angle to the satellites, which may not be practical due to possible route conflict with airplanes and other flight equipment. In this paper, we consider a TSN containing hot air balloons at different heights and with different minimum elevation angles, which creates the challenge of non-uniform available serving time for the communication between the hot air balloons and the satellites. Jointly considering the caching, computing, and communication (3C) resource management for both the ground-balloon-satellite links and inter-satellite laser links, our objective is to maximize the network energy efficiency. Firstly, by proposing a tapped water-filling algorithm, we schedule the traffic to relay among satellites according to the available serving time of satellites. Then, we generate a series of configuration matrices, based on which we formulate the relation between relay time and the power consumption involved in the relay among satellites. Finally, the collaborative resource allocation problem for TSN is modeled and solved by geometric programming with Taylor series approximation. Simulation results demonstrate the effectiveness of our proposed scheme.

Research on simulation methods for Doppler frequency shift of a coherent inter-satellite laser link in a ground test system

Spatial Doppler frequency shift occurs during ground integration testing of an inter-satellite laser communication link and is difficult to simulate it consistently; therefore, to address this problem, a Doppler frequency shift simulation method for an optical communication carrier in space based on a dual-parallel Mach–Zehnder modulator (DP-MZM) is proposed. By adjusting the RF signal added to the MZM and the bias voltage of the DP-MZM, the output carrier suppresses single-sideband modulation to achieve a controllable optical carrier frequency shift. In this study, theoretical analysis and experimental research on this simulation method was performed, and a Doppler frequency shift simulator for ground testing of two laser communication terminals was developed, which can realize controllable optical carrier frequency shift adjustment in the range of at least ±18 GHz and a sideband rejection ratio that is greater than 40 dB. The research results have been applied to an inter-satellite laser link ground test, obtaining good simulation and test results.

Fiber backscatter under increasing exposure to ionizing radiation.

The Laser Interferometer Space Antenna (LISA) will measure gravitational waves by utilizing inter-satellite laser links between three triangularly-arranged spacecraft in heliocentric orbits. Each spacecraft will house two separate optical benches and needs to establish a phase reference between the two optical benches which requires a bidirectional optical connection, e.g. a fiber connection. The sensitivity of the reference interferometers, and thus of the gravitational wave measurement, could be hampered by backscattering of laser light within optical fibers. It is not yet clear if the backscatter within the fibers will remain constant during the mission duration, or if it will increase due to ionizing radiation in the space environment. Here we report the results of tests on two different fiber types under increasing intensities of ionizing radiation: SM98-PS-U40D by Fujikura, a polarization maintaining fiber, and HB1060Z by Fibercore, a polarizing fiber. We found that both types react differently to the ionizing radiation: The polarization maintaining fibers show a backscatter of about 7 ppm·m-1 which remains constant over increasing exposure. The polarizing fibers show about three times as much backscatter, which also remains constant over increasing exposure. However, the polarizing fibers show a significant degradation in transmission, which is reduced to about one third.

Research on a fiber Bragg grating temperature measurement method for inter-satellite laser link.

In this paper, a fiber Bragg grating (FBG) temperature measurement method for intersatellite laser link is studied. Based on the analysis of the FBG temperature measurement method, the FBG sensing demodulation system is built to calibrate its wavelength and temperature parameters. The experimental data of temperature gradient monitoring are compared with the on-orbit real-time temperature data of a satellite's solar array measured by the thermistor to verify the temperature monitoring accuracy and stability of the FBG sensing demodulation system. The FBG sensing demodulation system and the thermistor temperature measurement system are compared under the same environmental conditions during the constant temperature period, the continuous heating phase and the natural cooling phase. The results show that during the constant temperature period, the temperature fluctuation range and standard deviation of the FBG sensing demodulation system are ±0.16 °C and 0.08 °C, respectively, which is significantly better than the performance of the thermistor temperature measurement system, whose temperature fluctuation range and standard deviation are ±5.92 °C and 1.08 °C, respectively. The temperature response speed of the former is significantly faster than that of the latter during the continuous heating phase and the natural cooling phase.

Inter-satellite laser link acquisition with dual-way scanning for Space Advanced Gravity Measurements mission.

Laser link acquisition is a key technology for inter-satellite laser ranging and laser communication. In this paper, we present an acquisition scheme based on the differential power sensing method with dual-way scanning, which will be used in the next-generation gravity measurement mission proposed in China, called Space Advanced Gravity Measurements (SAGM). In this scheme, the laser beams emitted from two satellites are power-modulated at different frequencies to enable the signals of the two beams to be measured distinguishably, and their corresponding pointing angles are determined by using the differential power sensing method. As the master laser beam and the slave laser beam are decoupled, the dual-way scanning method, in which the laser beams of both the master and the slave satellites scan uncertainty cones simultaneously and independently, can be used, instead of the commonly used single-way scanning method, in which the laser beam of one satellite scans and that of the other one stares. Therefore, the acquisition time is reduced significantly. Numerical simulation and experiments of the acquisition process are performed using the design parameters of the SAGM mission. The results show that the average acquisition time is less than 10 s for a scanning range of 1-mrad radius with a success rate of more than 99%.

基于多目标模拟退火算法的导航卫星激光星间链路拓扑动态优化

基于多目标模拟退火算法的导航卫星激光星间链路拓扑动态优化

10.23 Mcps laser pseudo-code ranging system with 0.33 mm (1σ) pseudo-range measurement precision.

The inter-satellite laser link is the backbone of the next inter-satellite information network, and ranging and communication are the main functions of the inter-satellite laser link. This study focuses on the inter-satellite laser ranging based on the pseudo-code correlation technology. In this paper, several typical laser-ranging methods have been compared and we determined that the laser pseudo-code ranging architecture is more suitable for the inter-satellite laser communication link. The pseudo-code ranging system is easy to combine with a digital communication system, and we used it to calculate integer ambiguity by modulating the time information. The main challenge of the ranging system is range precision, which is the main focus of this paper. First, the framework of the pseudo-code ranging system is introduced; the ranging architecture of dual one-way ranging is used to eliminate the clock error between the two transceivers, and then the uncertainty of the phase detector is analyzed. In the analysis, the carrier to noise ratio and the ranging code rate are constrained by the laser communication link margin and the electronic hardware limitation. Therefore, the relationship between the sampling depth and the phase detector uncertainty is verified. A series of optical fiber channel laser pseudo-code ranging experiments demonstrated the effects of sampling depth on the ranging precision. By adjusting the depth of storage, such as the depth of 1.6 Mb, we obtained a pseudo-range measurement precision of 0.33 mm (1σ), which is equivalent to 0.0001 times code subdivision of 10.23 Mcps pseudo-code. This paper has achieved high precision in a pseudo-range measurements, which is the foundation of the inter-satellite laser link.