We report on a novel technology for high-speed inter-satellites optical communication by bidirectional beam tracking. By establishing the relation between the compensation effect and the parameters of response time and overshoot situation, the stability can be well compensated simply by the control system. Thus the relation between compensation effect and maintain time can be predicted from ground tests, and the certain evaluation standard could be established to meet the requirements of system. The other critical factors, such as signal-to-noise ratio and pointing angle error, have also been considered to improve the stability. The general approach can provide us a powerful path to overcome the performance limitation of bidirectional beam tracking, which can be expected to be widely applied in Free Space optics communications in future.
© 2015 Optical Society of America
Thanks to the in-orbit tests successfully operated in recent years, the high-speed satellite optical communication is gradually moving towards engineering applications, which can open the doors to the next-generation of Free Space Optics communications [1–5]. Laser communication works in a highly confidential and immune way with large capacity, small terminal, and light weight over the traditional microwave communication technique . Nevertheless, the performance limitations due to the long communication distance and extreme optical detection conditions, which usually allow for laser communication in optical diffraction limit environments, have largely restricted the applications of optical communication between satellites [7–9].
Bidirectional optical tracking communication is an alternative strategy to compensate satellite orbit and positioning accurately with advantages of long-term stability and high laser link quality . The first bidirectional optical tracking communication experiment in the world has been successfully established between OICETS and ARTEMIS [3,10,11], which is preformed by JAXA and ESA using a laser beam in 2005. The holding time of the link is 10 mins with the communication data rate ~50Mbps (OICETS-ARTEMIS) and ~2Mbps (ARTEMIS-OICETS). The satellite to ground bidirectional optical communication links are performed between AlphaSat and Tan DEM-X at ESA site in 2013, with communication data rate up to ~1.8-2.8Gbps [1,12–14]. Because the laser beam is narrow, and the energy of receiver light is very weak, both high-performance pointing-acquisition-tracking (PAT) system and high-precision control accuracy are usually required [1,9,10]. Moreover, the pointing and tracking errors between satellites are usually coupled with each other, which make it a serious challenge to determine the compensation effect to the two terminals simultaneously. The characteristics of bidirectional tracking process and the constraint conditions of bidirectional beam stabilized tracking are critical to improve the stability of the communication links and extend the holding time of links. However, systematic study on constraint conditions of the stabilized inter-satellites bidirectional beam has rarely been reported yet.
Here for the first time we consciously devote to exploring a highly precise control system to explain the compensation effect in terms of the parameters in bidirectional beam tracking, which are easily monitored. In our study, the theoretical model of bidirectional beam stabilized tracking constraint conditions was firstly deduced, which is approximate to the actual situation. The kinetic compensation effect states were evolved into a curve relationship between parameters observed in control system, such as response time and overshoot situation. In this way, compensation effects could be exactly determined in the pre-designed control systems. Finally, the relation between compensation effect and maintain time was predicated from ground tests, and the certain evaluation standard could be established to meet the requirement of system. All of these can ensure high stability of bidirectional beam tracking communication.
2. Constraint conditions of stabilized tracking
2.1 The structure of the inter-satellites optical communication
The bidirectional beam tracking mechanism between satellites has been shown in Fig. 1. The transmitting terminal of satelliteI transmits a beacon light condition, while satelliteII is receiving. BN is the receiving antenna aperture plane. Once satelliteII has processed the information, it will transmit the signal light to satelliteI. Point A and B are the center of the receiving and transmitting terminal antenna aperture, respectively.
2.2 The control system
We use a series of control units to achieve the supposed value to satisfy parameters of system. As for satellite I, the pointing angle θ1* is the position information received from satelliteII. θ1 is the true value achieved by satelliteI’s control unit using negative feedback manner to get close to the given value. The mechanical structure turns the Charge Coupled Device (CCD) towards the spot to ensure the tracking stability. In addition, satelliteII works in the same way as satelliteI.
The dynamic process is significantly important to the tracking stability, and each part of the system plays an essential role in the long-term stability of inter-satellites optical communication. In our study, the whole system of the inter-satellites optical communication has been clearly designed as Fig. 2, along with the illustration of each functional unit in detail.
Furthermore, we could simplify the control system, as demonstrated in Fig. 3:where θ* is the true value of pointing angle, θin is the measuring value of θ*, and θout is the output pointing angle. θin cannot be equal to θ* because of the angle error due to the noise effect, and the output pointing angle is following the value of θin by a negative feedback. F(s) is the processing function including filters and proportional-integral-derivative (PID) controller. In our study, the bidirectional beam tracking system is an optical loop system. Each section completes the job of actual position following the value provided, which is a first order inertial element. When each section works together, the whole tracking communication system will evolve into a typical two-order system, which can be simplfied as the a typical two order system. We will talk it in detail in following section 3.
In this way, when the input signal is speedy, we can obtain the compensation effect η, expressed as:
2.3 Measurement of pointing angle error
Here we present the center of mass coordinate of laser spot at . According to the center of mass method , the center of laser spot can be expressed as:16], denotes the pointing angle error, and is the divergence angle of beacon beam. Using Eq. (6) and Eq. (7), the relationship between and is given by
From Eq. (8), we can see that the CCD angle measurement error is remarkable when the is constant and SNR0 is 5, and the angle error is 1 μrad accounted for 16.67% of the measurement angle error. In addition, when the pointing angle beam is half of the divergence angle, the measurement angle error of CCD accounted for 59.64%, and when SNR0 is 70 and the measurement angle error is under 10%.
2.4 The effects of pointing angle error
In the bidirectional beam tracking process, the terminals are simultaneously tracking beacon beam from the opposite terminal, which are affected by the pointing accuracy of the pointing, acquisition and tracking (PAT) system. The maximum mean square of the pointing error angle is given as follows Eq. (10), which can be simply monitored by the pre-designed control system.Eq. (13), it can be seen that, in bidirectional optical communication, tracking variance constraint is relative to the SNR0, CCD measuring error and compensation effect. Methods of eliminating noise are more ideal, and SNR of imaging plate is high. So the pointing angle error has little effect on angle measuring accuracy. If the measuring error is ignored, the compensation effect is the main factor of the stability of system.
Figure 4 gives the relationships between σ2 and θb, as well as η and SNR0. We can see that both θb and compensation effect η have obvious influence on σ2. When the θb is a fixed value, the compensation effect η is better, the greater σ2 is allowed.
3. Analysis of the optical system
In order to determine the compensation effect η by parameters that can be easily monitored, we use the knowledge from classical control theory to describe it. In general, two order control system is the most basic system, and many advanced control systems can be simplified to this situation under certain conditions. Based on the discussion in the section 2.2, we use a typical 2-nd order system (ωn^2)/(s(s + 2ξωn)) to represent the open loop function F(s), as illustrated in Fig. 5.
After the analysis of the typical two order control system, we can get the overshoot Mp and response time ts (Δ = 5%), which can be expressed as the equations belowEq. (10), and it can also be described by the parameters in the optical systemFig. 6. We can see the compensation effect η is easily determined by overshoot Mp and response time ts in the control system. It provides us a general method to make it possible to predicte system standards and requirements.
4. Simulation experiment on the ground
In order to study the long-term stability of link quality, we have carried out the ground tests for the bidirectional beam tracking in inter-satellite optical communications. The tracking system mainly contains three parts: two laser communication terminals, a dynamic link simulator and two computers, as shown in Fig. 7. In the experiment, we study the special tracking system and optimize the inherent natures, such as the moment of inertia of the terminal, the maximum torque of the motors, and the performance of the controller and so on. And then we use the optimized system to carry out the 10 times ground simulation tests, orbiting tests as well as software simulations.
The maintain time of the optical link has been measured with operation time of 1.5h, and the relative speed is from 0.1mrad/s to 0.4mrad/s. Figure 8 shows the results of maintain time of link from ground tests under various constraint conditions. We can find that, when η is located in the range of 0.3~0.6, the temporarily stable states of link can be achieved. When η is smaller than 0.3, the compensation is poor, the system is hard to ensure the stabilized tracking quality. When η is beyond the 0.6, it means that the optical link of system has a good compensation effect and can be kept in an extremely stabilized state.
In addition, compensation effect η can be ensured by the maintain time, which is also closely related to the response time and the overshoot from Eq. (16), which allows us to simply monitor the compensation effect by indexes of the control system. In order to get a stable link state, we can manage to get an evaluation standard of the response time and the overshoot to meet the system requires. The exciting method can provide us a powerful path to deal with performance limitation of inter-satellite bidirectional beam tracking due to vibrations and atmospheric turbulence, as well as the tracking and pointing error.
We have successfully explored a novel bidirectional beam tracking technique in inter-satellite optical communication. The constraint conditions of the steady tracking process were deduced. A minimal control system was also established for the first time, which allow for monitoring the compensation effect simply by the response time and overshoot situation. We have also established a certain evaluation standard by measuring the indexes of control system designed. Moreover, the relation between compensation effect and maintain time was predicted by ground tests, software simulation and orbiting test results. Thus, the compensation effect η can be determined to meet the maintain time the system required. The general approach can provide us a powerful path to overcome performance limitation of inter-satellite bidirectional beam tracking due to vibrations and atmospheric turbulence, as well as the tracking and pointing error.
This work was supported by excellent Satellite Optical Communications team in Harbin Institute of Technology.
References and links
1. X. Li, S. Yu, J. Ma, and L. Tan, “Analytical expression and optimization of spatial acquisition for intersatellite optical communications,” Opt. Express 19(3), 2381–2390 (2011). [CrossRef] [PubMed]
2. I. I. Kim, B. Riley, N. M. Wong, M. Mitchell, W. Brown, H. Hakakha, P. Adhikari, and E. J. Korevaar, “Lessons learned from the STRV-2 satellite-to-ground lasercom experiment,” Proc. SPIE 4272, 1–15 (2001). [CrossRef]
3. R. A. Fields, D. A. Kozlowski, H. T. Yura, L. R. Wong, J. M. Wicker, C. T. Lunde, M. Gregory, B. K. Wandernoth, F. F. Heine, and J. J. Luna, “5.625 Gbps bidirectional laser communications measurements between the NFIRE satellite and an optical ground station,” Proc. SPIE 8184, 81840D (2011). [CrossRef]
4. Y. Fujiwara, M. Mokuno, T. Jono, T. Yamawaki, K. Arai, M. Toyoshima, H. Kunimori, Z. Sodnik, A. Bird, and B. Demelenne, “Optical inter-orbit communications engineering test satellite (OICETS),” Acta Astronaut. 61(1-6), 163–175 (2007). [CrossRef]
5. I. S. Ansari, F. Yilmaz, and M.-S. Alouini, “Performance Analysis of Free-Space Optical Links Over Malaga (M) Turbulence Channels with Pointing Errors,” IEEE T Wirel, Commun. 99, 1 (2015).
6. S. Y. Yu, Z. T. Ma, F. Wu, J. Ma, and L. Y. Tan, “Overview and trend of steady tracking in free-space optical communication links,” Proc. SPIE 9521, 95210N (2015).
7. V. W. S. Chan, “Optical space communications,” IEEE J. Sel. Top. Quantum Electron. 6(6), 959–975 (2000). [CrossRef]
9. S. Yu, Z. Ma, J. Ma, F. Wu, and L. Tan, “Far-field correlation of bidirectional tracking beams due to wave-front deformation in inter-satellites optical communication links,” Opt. Express 23(6), 7263–7272 (2015). [CrossRef] [PubMed]
10. N. Tanzillo, B. Dunbar, and S. Lee, “Development of a lasercom testbed for the pointing, acquisition, and tracking subsystem of satellite-to-satellite laser communications link,” Proc. SPIE 6877, 687704 (2008). [CrossRef]
11. S. Seel, H. Kämpfner, F. Heine, D. Dallmann, and G. Muhlnikel, M.M. Gregory, K. Reinhardt, J. Saucke, U. Muckherjee, B. Sterr, R. Wandernoth, Meyer, and R. Czichy, “Space to ground bidirectional optical communication link at 5.6 Gbps and EDRS connectivity outlook,”in Proceedings of the Aerospace Conference (IEEE, 2011), pp. 1–7.
12. S. Y. Yu, J. Ma, and L. Y. Tan, “Methods of improving acquisition probability of scanning in intersatellite optical communication,” J. Optoelectronics Laser, Networking 16(12), 57–62 (2004).
13. I. I. Kim, B. Riley, N. M. Wong, M. Mitchell, W. Brown, H. Hakakha, P. Adhikari, and E. J. Korevaar, “Lessons learned from the STRV-2 satellite-to-ground lasercom experiment,” Proc. SPIE 4272, 1–15 (2001). [CrossRef]
14. T. Tolker-Nielsen and G. Oppenhaeuser, “In Orbit test result of an Operational Optical Intersatellite Link between ARTEMIS and SPOT4, SILEX,” Proc. SPIE 4635, 1–15 (2002). [CrossRef]
15. C. Hindman and L. Toberton, “Beaconless satellite laser acquisition – modeling and feasibility,” in Proceedings of the MILCOM 2004-IEEE Military Communications Conference (2004), pp. 41–47.
16. I. S. Ansari, M.-S. Alouini, and J. Cheng, “Ergodic Capacity Analysis of Free-Space Optical Links With Nonzero Boresight Pointing Errors,” IEEE T Wirel, Commun. 14(8), 4248–4264 (2015).
17. U. Sterr, M. Gregory, and F. Heine, “Beaconless acquisition for ISL and SGL, summary of 3 years operation in space and on ground,” in Proceedings of the 2011-IEEE International Conference on Space Optical Systems and Applications (2011) pp. 38–43. [CrossRef]