- Email: [email protected]
Analysis of atmosphere channel for space-to-ground optical communications
Optics Communications 306 (2013) 42–48
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Analysis of atmosphere channel for space-to-ground optical communications Xiaorui Wang, Lei Guo n, Yejun Liu, Lincong Zhang College of Information Science and Engineering, Northeastern University, P.O. Box 365, Shenyang 110819 China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 March 2013 Received in revised form 14 May 2013 Accepted 15 May 2013 Available online 2 June 2013
Since atmosphere deeply influences the beams propagated in space-to-ground optical communications, it is important to study its effects on the beam transmission in the atmospheric channel. In this paper, we analyze the impacts of atmospheric channel with the theory of free space optical communication. First, we investigate the atmospheric attenuation of different beams through the mathematical models. Then, we analyze the effects of atmospheric turbulence on the signal transmission by the atmospheric refractive index structure constant. In addition, we simulate the atmospheric transmittance with different zenith angles, beams, heights and rainfall. The results show that the wavelength, atmospheric turbulence, zenith angle, height and rainfall have important impacts on the signal transmission of spaceto-ground optical communications. This demonstrates much theoretical significance on the wavelength selection, height settings of the optical ground station as well as angle selection of transmitter. & 2013 Elsevier B.V. All rights reserved.
Keywords: Space-to-ground optical communications Atmospheric attenuation Atmospheric refractive index structure constant Atmospheric turbulence Atmospheric transmittance
1. Introduction Space-to-ground optical communications provide secure, easily deployable and license free alternate to RF wireless communication links with higher data rate and lower bit error rate [1]. Compared with the optical fiber communication systems, the advantages of the space-to-ground optical communications are flexible installation, low-cost, etc. [2]. These features of optical wireless communication technology have led interest in this field due to the rising need for high bandwidth capacity of transmission links. People have put a lot of manpower, material and financial resources on the research of space-to-ground optical communications, and the space-to-ground optical communications have been successfully realized. Major research institutions mainly involve National Space Development Agency of Japan (NASDA), National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). In 1995, a lasercom terminal was a part of the payload for the Experimental Test Satellite (ETS)-V communication satellite. A space-to-ground link was established between the ETSV and Jet Propulsion Laboratory (JPL) ground station with the bidirectional communication duration of 8 min. In July of the same year, a 2-way link of laser communication experiment was conducted between the ETS-V satellite and Communication Research Laboratory (CRL) ground station of Tokyo, which was a milestone
n
Corresponding author. Tel./fax: +86 24 83684219. E-mail address: [email protected] (L. Guo).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.05.033
in the satellite-to-ground laser communications [3]. In March 2003, the satellite terminal SILEX payload on the ESA ARTEMIS first tested the laser communication experiments with Optical Ground Station (OGS). According to the statistics, the communication probability of successful communication was 91.3%, and the total time of communication was 78 h [4]. In 2006, the ground station to Japan's Optical Inter-orbit Communications Engineering Test Satellite (OICETS) laser communications experiment was first realized between the low-orbit satellites and ground stations [5]. In 2006, the interrupted time of optical link by clouds and rain were respectively 17% and 22% in the satellite-to-ground laser communication experiment of Japan. The interference time reached 38% by the clouds, and the energy attenuation of laser beam reached as much as 27 dB [6]. In 2008, a space-to-ground bi-directional optical communication link at 5.6 Gbps was verified between a low earth orbit satellite NFIRE and a TESAT OGS hosted at the ESA site in Tenerife (Spain) [7]. Because the performance of optical wireless communication systems strongly depends on the atmospheric conditions and the characteristics of the link, in 2009, the influences on both average capacity and outage capacity of an Optical Wireless Communication (OWC) system were studied over moderate to strong turbulence channels, and the estimation of the average capacity and outage capacity was obtained [8]. In 2010, a simplified approach was developed to model the received power dynamics of the free space optical channel. The proposed model is easy to implement and use, which involves a random number generators and a low-pass filter. This approach is only valid for the
X. Wang et al. / Optics Communications 306 (2013) 42–48
systems utilizing intensity modulation with direct detection, but the limitation is acceptable since most of the commercially available systems use this modulation format. The channel model was developed based on the statistics of received power measurements from a maritime-mobile link, a land mobile link and a satellite downlink [9]. In satellite-to-ground laser communication links, atmospheric turbulence has a strong influence on the communication quality, and the communication link is difficult to establish in strong turbulence. Thus, large aperture optical antennas are used in the communication systems. The manufacture of large aperture optical antennas not only costs a lot, but also many factors including the volume of optical antennas have to be limited. In 2011, a kind of receiving/transmitting array antenna composed of several small aperture telescopes was designed, combined with space diversity [10]. The telescope arrays have advantages of less cost, slighter gravitational effects, ease of maintenance, and redundancy in operations. In 2012, the design of the Space Power Satellites (SPSs) was investigated [11]. The fully optical SPS eliminated the need for heavy, low efficiency photovoltaic cells as the first stage, so more energy could be obtained using very small and light structures. Furthermore, the effect of the turbulent atmospheric channel was examined. Tohoku University developed a 50 kg-class international scientific microsatellite named RISESAT, which can send actual scientific data obtained by payload instruments through this optical communication link. This is the first demonstration of microsatellite-to-ground optical downlink in the world [12]. In 2012, JPL described the work on the development of an optical link budget tool for an intensity-modulated direct-detected photoncounting channel utilizing pulse-position-modulation. JPL also provided approximations to the channel capacity and the loss for the deep space optical communications [13]. The atmosphere affects the characteristics of the ground-to-satellite optical communication link, so it is important to minimize these atmospheric effects in free space optical communication. In 2012, the Pulse Position Modulation (PPM), Differential Pulse Position Modulation (DPPM) and Differential Amplitude Pulse Position Modulation (DAPPM) had been evaluated for atmospheric turbulence channel. It is observed that DPPM and DAPPM have better bandwidth and power efficient in comparison to conventional modulation schemes OOK and PPM. Accordingly, the DPPM and DAPPM can perform better in the atmospheric turbulence channel [14]. The Transformational Satellite Communications System (TSAT) program is being implemented by the US military, which will gradually establish the interconnection between the ground and five in-orbit satellites until 2016, and the space laser communication rate will reach 10–40 Gbps [15]. The space-to-ground optical communications system mainly consists of the transmitting device, the receiving device and the atmospheric channel. The atmospheric channel has the open characteristic, so it is vulnerable to the impacts of different weather conditions. Due to the space-to-ground laser link in the complex and changing natural environment, the study of the atmospheric channel is very important. When the laser is transmitted in the atmospheric channel, it is easy to interact with the particles. As a result, the atmospheric attenuation effects will be caused by the absorption and scattering. The bigger the atmospheric attenuation is, the smaller the received signal power will be. Atmospheric turbulence can cause signal phase fluctuation, atmospheric scintillation, beam bending, drift and beam broadening, which cause the increase of error rate, thereby the quality of signal transmission and the reliability of the link will decrease [16]. In addition, space-to-ground optical communications are the key to connect optical satellite network and terrestrial optical fiber communication network, and it is also the key to integrated ground-air-space. Therefore, it is significant to study the influences
43
of the atmosphere on atmospheric channel of space-to-ground optical communications. In this paper, we analyze the impacts of atmospheric channel by the mathematical models and simulation. The results show that the atmospheric attenuation and atmospheric turbulence have an important effect on the signal transmission in the space-to-ground optical communications. This demonstrates much theoretical significance on the wavelength selection, height settings of the OGS as well as angle selection of the transmitter. The paper is structured in 6 sections. First, the wavelength attenuation, refractive index structure constant and the atmospheric transmittance of different zenith angles are separately analyzed with mathematical models in Section 2, Section 3, and Section 4 respectively. Then Section 5 focuses on simulating and analyzing wavelength transmittance under different atmosphere conditions. Finally, the most significant results are concluded in Section 6.
2. The mathematical analysis of wavelength attenuation The atmosphere of earth is a mixture of different gases. The main components include N2 (78.9%) and O2 (20.95%), and the remainder consists of CO2, CO, CH4, O3, SO2, etc. Moreover, the atmosphere also contains the liquid droplets and solid particles. Because of the interactions between laser beams and atmosphere, the main atmospheric effects on the laser beams are atmospheric attenuation and atmospheric turbulence, where atmospheric attenuation is mainly caused by the atmospheric absorption and scattering. According to the reference [17], we provide the mathematical model of atmospheric attenuation as: 13 dB λ nm −q αSPEC ðdB=kmÞ≅ ð1Þ V km 550 nm whereαSPEC is the specific attenuation, λ is the light wavelength, and Vis the visibility. The value of q follows as: 8 > < 1:6 f V 4 50 km ð2Þ q ¼ 1:3 f 6 km o V o 50 kmI > : 0:585 V 1=3 f V o6 m Fig. 1 is the diagram of space-to-ground optical communications. We can see that the atmosphere mainly influences the space-to-ground optical communications. Thus, we use Eq. (1) to describe the wavelength attenuation. Eq. (1) is an empirical equation, and the visibility V and the wavelength λ are the important parameters, leading to the optical attenuation. According to Eq. (1), we can calculate the attenuation of different wavelengths with the atmospheric visibility ranging from 6 km to 23 km shown in Fig. 2.
Fig. 1. Diagram of the space-to-ground optical communications.
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X. Wang et al. / Optics Communications 306 (2013) 42–48
described by 0.78um 0.85um 0.95um 1.064um 1.3um 1.55um 10um
1.8
Attenuation in [dB/km]
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Visibility in [km]
Fig. 2. Beam attenuation diagram at different wavelengths.
When the atmospheric visibility is less than or equal to 6 km, the weather condition is poor and significantly influences the signal transmission. Thus the communication channel is not reliable. When the visibility is more than 23 km, the weather is sunny. Therefore, the atmospheric channel weakly affects the signal transmission, ranging from 6 km to 23 km, there is thick fog, cloudy, sunny weather conditions, and the weather becomes bad to fine. So we analyze the wavelength attenuation at this range of atmospheric visibility. As shown in Fig. 2, when the atmospheric visibility ranges from 6 km to 23 km, the attenuation of different wavelengths gradually decreases with the increase of the wavelength. In the case of the same visibility, the bigger the wavelength is, the smaller the attenuation is. In addition, when the atmospheric visibility is less than 6 km, the weather conditions are bad, and the beam attenuation is larger. When the weather is fine for the atmospheric visibility in excess of 23 km, the beam attenuation is small and the difference is negligible. So the wavelength selection is more flexible in the sunny weather conditions. Therefore, the light source should use the larger wavelength in the actual space-to-ground optical communications, which causes less attenuation. For example in Fig. 2, the attenuation difference between the wavelengths 0.7 μm and 10 μm is about 1.8 dB/km at the visibility of 6 km, which is important in engineering applications.
! υ 2 10−5 10 −h 0:00594 exp 27 1000 h −h −h þ A exp þ2:7 10−16 exp 1500 100
c2n ðhÞ ¼
ð3Þ
Where h is the height of the OGS, υ is the rms windspeed in meters per second (m/s) and A is a value of c2n ð0Þ at the ground in m−2/3. In the following analysis, we take v¼ 21 m/s and A ¼1.7 10−14. According to Eq. (3), we calculate the atmospheric refractive index structure constant for the attenuation at different wavelengths shown in Fig. 3. As seen from Fig. 3, the atmospheric refractive index structure constant changes significantly with the height ranging from 0 to1000 m. The atmospheric refractive index structure constant decreases sharply from 0 to 500 m, but it changes smoothly from 500 to 1000 m. When the height is larger than 2000 m, the curve of refractive index structure constant is smooth, which is close to the horizontal axis. This shows that atmospheric refractive index structure constant will gradually decrease with the increase of height. Correspondingly, the atmospheric turbulence varies progressively from strong to weak. That is to say, the influence of atmospheric turbulence on the communication channel is smaller when the height is larger than 2000 m. Therefore, in order to Atmospheric structure constant of refractive index(10-14)
2.0
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 m
Fig. 3. The refractive index structure constant with different heights.
0.9
3. Analysis of atmospheric refractive index structure constant 0.8 0.7 Transmittance
The atmosphere of earth is a layer of gases surrounding the planet earth. The air pressure and density decrease in the atmosphere with the increase of the height due to the influence of the gravity. The weather is constantly changing, thus the atmospheric temperature is a random variable. The atmosphere is in a state of constant motion and the communication channel is in this turbulence gases. The atmospheric turbulence is an irregular random motion relative to the atmosphere overall average movement, which randomly transforms the refractive index along the optical transmission path. The value of the refractive index structure constant represents the size of refractive index, so we use the refractive index structure constant to analyze the intensity of atmospheric turbulence. According to Ref. [18], the refractive index structure constant varies with the height of OGS in the space-to-ground optical communications link, and one of the most widely used models is the Hufnagel-Valley (HV) model
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Rad Fig. 4. Atmospheric transmittance under different zenith angles.
1.6
X. Wang et al. / Optics Communications 306 (2013) 42–48
improve the impacts of the atmospheric channel on signal transmission, the height of OGS should be above 1000 m.
4. Mathematical analysis of atmospheric transmittance In the space-to-ground optical communications, the signal transmission is mainly influenced by the atmospheric attenuation and atmospheric turbulence. The atmospheric turbulence changes randomly, and it is affected by the different weather conditions, so the impacts cannot be directly simulated and analyzed. Thus we analyze the atmospheric transmittance in order to research the effects of the signal transmission in the atmospheric channel. In the optical link, according to Ref. [19], the atmospheric transmittance T is defined as the ratio of the receiving power P r to the transmitting power P t as follows: T¼
Pr ¼ e−τ Pt
ð4Þ
45
Where τ is the optical depth. Ref. [19] also points out that beam transmission spectrum of zenith can be transformed into different observation angles by mathematical transformation. If the transmittance of zenith is given by T 0 , we can calculate the transmittance T θ with the zenith angle θ as: T θ ¼ ðT 0 Þsecθ
ð5Þ
Where secθ is the air mass. The atmospheric transmittance under different zenith angles can be obtained with the vertex transmittance 0.9 as shown in Fig. 4. The horizontal ordinate is the size of zenith angle, while the vertical ordinate represents the transmittance. The transmittance is 0.9 with the zero degrees zenith angle, and the atmospheric transmittance gradually decreases with the increase of zenith angle. The transmittance sharply declines at the zenith angle of about 1.2 rad. The transmittance curve is relatively smooth when the zenith angle between 0 and 1.2 rad. Moreover, when the
Fig. 5. Atmospheric transmittance with different beams and different heights. (a) Atmospheric transmittance of different wavelengths at the height of 1 m, (b) atmospheric transmittance of different wavelengths at the height of 2 m, and (c) atmospheric transmittance of different wavelengths at the height of 3 m.
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X. Wang et al. / Optics Communications 306 (2013) 42–48
atmospheric transmittance is large, the attenuation of the signal is small. In the actual space-to-ground optical communications, the communication performance is better for the zenith angle between 0 and 1.2 rad. If the zenith angle of the OGS is greater than 1.2 rad, the other optical ground nodes or other space nodes should be considered as the relay nodes, so as to maintain a better communication performance.
5. Simulation of atmospheric transmittance 5.1. Atmospheric transmittance of different beams In the simulation, the atmospheric visibility takes 23 km according to 1976 US standard atmosphere model. The zenith angle of OGS is set to zero degrees, and the height of the OGS has a range of 1 km to 3 km. Because the wavelength from 0.2 μm to 0.4 μm is absorbed by the atmospheric ozone layer, the wavelength of the simulation is set from 0.5 to 10 μm.
The simulation results are shown in Fig. 5. When the visibility and zenith angle are certain, the beams have a larger atmospheric transmittance with increase of the height. Along with the increase of the wavelength, the transmittance window gradually increases. In space-to-ground optical communications, we should choose a larger height in order to get a bigger transmittance and a smaller attenuation of the signal. Reference [17] points out that the commonly used wavelengths range from 0.85 μm to 1.55 μm in laser communication. In Fig. 5, it also can be seen that the atmospheric transmittance window is a little narrow near 1 μm, but it is still suitable for the communications due to the larger transmittance, which proves that the theoretical research is consistent with the practical application. 5.2. Atmospheric transmittance with the different beams and different zenith angles We assume the height of the OGS as 2 km in accordance with the analysis of section 3, the visibility is 23 km and the zenith
Fig. 6. The atmospheric transmittance with different zenith angles. (a) Atmospheric transmittance with zenith angle of 301, (b) atmospheric transmittance with zenith angle of 601, (c) atmospheric transmittance with zenith angle of 701 and (d) atmospheric transmittance with zenith angle of 891.
X. Wang et al. / Optics Communications 306 (2013) 42–48
angle is selected from 01 to 891. The results are shown in Fig. 6. We can see that the atmospheric transmittance decreases with the increase of the zenith angle. This is chiefly because the communication path is vertical with the zero degrees zenith angle. Thus, the communication distance is the shortest and the transmittance of the vertical path is the largest. The path of the space-to-ground optical communications becomes slant when the zenith angle is increasing. This means that the communication distance will increase, which results in a larger attenuation of the signal transmission. The larger the zenith angle is, the longer distance of the communication path is. So the transmittance will gradually decrease. It also can be seen from Fig. 6 that the atmospheric transmittance has a little change with the zenith angle ranging from 01 to 301, but the atmospheric transmittance varies significantly at zenith angle of 701 . In the actual space-to-ground optical communications, according to the size of the zenith angle, we need to consider the relay nodes to achieve better communication performance.
47
5.3. Atmospheric transmittance with the different beams and different rainfall When the daily rainfall is less than 10 mm, it drizzles. When the daily rainfall of moderate rain is more than 10 mm, and the daily rainfall in excess is 24 mm will be the heavy rain. Because the communication will be interrupted in the heavy rain, we choose the daily rainfall of 2 mm, 5 mm and 12.5 mm in the simulation. The atmospheric visibility takes 23 km according to 1976 US standard atmosphere model. The zenith angle of OGS is set to zero degrees and the height of the OGS is 2 km. The simulation results are shown in Fig. 7. We can see that the rainfall has an important effect on the space-to-ground communications. The beam transmittance gradually decreases with the increase of the rainfall. The transmittance will increase with the increase of wavelength under the same conditions. When the rainfall is 2 mm, the average transmittance is about 65%. The impact of the drizzle is slight on the signal transmission.
Fig. 7. The atmospheric transmittance with different rainfall. (a) Atmospheric transmittance with the daily rainfall of 2 mm, (b) atmospheric transmittance with the daily rainfall of 5 mm and (c) atmospheric transmittance with the daily rainfall of 12.5 mm.
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X. Wang et al. / Optics Communications 306 (2013) 42–48
As the rainfall is 12.5 mm, the average transmittance is about 50%, and that means the signal attenuation is approximately 50%. Therefore, the signal attenuation will be larger in the heavy rain, and the communication may be interrupted. In the actual communication process, the effect of rainfall on the communication is a non-negligible factor. When the rainfall leads to the interruption of the communication, we should consider the relay nodes or larger wavelengths in the communication. 6. Conclusions In this paper, we have used the mathematical model to analyze the wavelength attenuation in different visibilities, the atmospheric refractive index structure constant with different heights and the atmospheric transmittance with different zenith angles. We conclude that the wavelength attenuation decreases with the increase of visibility, and the larger wavelength has the advantage of less attenuation. Along with the increase of the height of the OGS, the atmospheric refractive index structure constant will decrease, thus the atmospheric turbulence will become weak. When the vertical transmittance of the atmosphere is fixed, the transmittance gradually decreases with the increase of the observation zenith angle, and the atmospheric transmittance has a little change in the range of 0–301, but the transmittance sharply declines around the zenith angle of 701. Finally, the atmospheric transmittance is simulated under various weather conditions. The simulation results show that, the atmospheric transmittance gradually increases with the increase of the height at the certain visibility and zenith angle. When the zenith angle changes from 01 to 891, the transmittance gradually decreases at the given visibility and height. Besides, the transmittance gradually decreases with the increase of the rainfall. This study has demonstrated the theoretical significance on the wavelength selection of light source, the setting of height of OGS and the selection of zenith angle in the actual space-to-ground optical communications. In addition, it is good for space-to-ground optical communications by analyzing the influences of atmospheric channel. Moreover, this paper has important significance
on reliability and availability of space-to-ground optical communications ink.
Acknowledgment This work was supported in part by the National Natural Science Foundation of China (61172051 and 61071124), the Program for New Century Excellent Talents in University (NCET-110075 and NCET-12-0102), the Fundamental Research Funds for the Central Universities (N110204001, N120804002, N110604008 and N110804003), and the Specialized Research Fund for the Doctoral Program of Higher Education (20110042110023, 20110042120035 and 20120042120049). References [1] Plank Thomas, Proceedings of EUCAP, 2011, p. 2508. [2] H.T. Mo, L.J. Lei, Z.Y. Wang, Information communications 22 (5) (2009) 26. [3] H.L. Jang, et al., The technologies and systems of space laser communications, 1st ed., National Defense Industry Press, Beijing301. [4] Q. Fu, H.L. Jang, et al., Chinese Optical and Applied Optical Abstracts 5 (2) (2012) 116. [5] R.J. Cesarone, D.S. Abraham, S. Shambayati, Proceeding of ICSOS (2011) 410. [6] M. Toyoshima, Y. Takayama, T. Takahashi, et al., IEEE Aerospace & Electronic Systems Magazine 23 (8) (2008) 10. [7] H. Kampfner, F. Heine, D. Dallman et al., in :IEEE Aerospace Conference, 2011, p. 1. [8] G.S. Tombras, A.D. Tsigopoulos, E.A. Karagianni, et al., Journal of Communications and Networks 11 (4) (2009) 384. [9] B. Epple., IEEE/OSA Journal of Optical Communications and Networking 2 (5) (2010) 293. [10] J.Y. He, in: Proceedings of ICSOS, 2011, p. 343. [11] A. Jooshesh, T.A. Gulliver, S. Uysal, in: Proceedings of BWCCA, 2012, p. 302. [12] K. Yoshida, Y. Sakamoto, Y. Tomioka, et al., IEEE/SICE International Symposium on System Integration 29 (6) (2012) 939. [13] B. Moision, J. Wu, S. Shambayati, IEEE Aerospace Conference, 2012, p. 1. [14] S. Tiwari, V.K. Jain, S. Kar, in: Proceeding of ICOE 2012, p. 1. [15] M. Toyoshima, S. Yamakawa, T. Yamawaki et al., in: Proceedings of SPIE 5338 2004, p. 1. [16] Y.W. Liu, in: Proceedings of ICECE 2010, p. 5510. [17] B. Flecker, M. Gebhart, E. Leitgeb, et al., in: Proceedings of SPIE 2006, p. 1. [18] L.C. Andrews, Ronald.L Phillips, Optical Engineering 39 (12) (2000) 3272. [19] H. Hemmati, Deep space optical communications, 1st ed., Tsinghua University press, Beijing114.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Analysis of atmosphere channel for space-to-ground optical communications Xiaorui Wang, Lei Guo n, Yejun Liu, Lincong Zhang College of Information Science and Engineering, Northeastern University, P.O. Box 365, Shenyang 110819 China
art ic l e i nf o
a b s t r a c t
Article history: Received 29 March 2013 Received in revised form 14 May 2013 Accepted 15 May 2013 Available online 2 June 2013
Since atmosphere deeply influences the beams propagated in space-to-ground optical communications, it is important to study its effects on the beam transmission in the atmospheric channel. In this paper, we analyze the impacts of atmospheric channel with the theory of free space optical communication. First, we investigate the atmospheric attenuation of different beams through the mathematical models. Then, we analyze the effects of atmospheric turbulence on the signal transmission by the atmospheric refractive index structure constant. In addition, we simulate the atmospheric transmittance with different zenith angles, beams, heights and rainfall. The results show that the wavelength, atmospheric turbulence, zenith angle, height and rainfall have important impacts on the signal transmission of spaceto-ground optical communications. This demonstrates much theoretical significance on the wavelength selection, height settings of the optical ground station as well as angle selection of transmitter. & 2013 Elsevier B.V. All rights reserved.
Keywords: Space-to-ground optical communications Atmospheric attenuation Atmospheric refractive index structure constant Atmospheric turbulence Atmospheric transmittance
1. Introduction Space-to-ground optical communications provide secure, easily deployable and license free alternate to RF wireless communication links with higher data rate and lower bit error rate [1]. Compared with the optical fiber communication systems, the advantages of the space-to-ground optical communications are flexible installation, low-cost, etc. [2]. These features of optical wireless communication technology have led interest in this field due to the rising need for high bandwidth capacity of transmission links. People have put a lot of manpower, material and financial resources on the research of space-to-ground optical communications, and the space-to-ground optical communications have been successfully realized. Major research institutions mainly involve National Space Development Agency of Japan (NASDA), National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). In 1995, a lasercom terminal was a part of the payload for the Experimental Test Satellite (ETS)-V communication satellite. A space-to-ground link was established between the ETSV and Jet Propulsion Laboratory (JPL) ground station with the bidirectional communication duration of 8 min. In July of the same year, a 2-way link of laser communication experiment was conducted between the ETS-V satellite and Communication Research Laboratory (CRL) ground station of Tokyo, which was a milestone
n
Corresponding author. Tel./fax: +86 24 83684219. E-mail address: [email protected] (L. Guo).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.05.033
in the satellite-to-ground laser communications [3]. In March 2003, the satellite terminal SILEX payload on the ESA ARTEMIS first tested the laser communication experiments with Optical Ground Station (OGS). According to the statistics, the communication probability of successful communication was 91.3%, and the total time of communication was 78 h [4]. In 2006, the ground station to Japan's Optical Inter-orbit Communications Engineering Test Satellite (OICETS) laser communications experiment was first realized between the low-orbit satellites and ground stations [5]. In 2006, the interrupted time of optical link by clouds and rain were respectively 17% and 22% in the satellite-to-ground laser communication experiment of Japan. The interference time reached 38% by the clouds, and the energy attenuation of laser beam reached as much as 27 dB [6]. In 2008, a space-to-ground bi-directional optical communication link at 5.6 Gbps was verified between a low earth orbit satellite NFIRE and a TESAT OGS hosted at the ESA site in Tenerife (Spain) [7]. Because the performance of optical wireless communication systems strongly depends on the atmospheric conditions and the characteristics of the link, in 2009, the influences on both average capacity and outage capacity of an Optical Wireless Communication (OWC) system were studied over moderate to strong turbulence channels, and the estimation of the average capacity and outage capacity was obtained [8]. In 2010, a simplified approach was developed to model the received power dynamics of the free space optical channel. The proposed model is easy to implement and use, which involves a random number generators and a low-pass filter. This approach is only valid for the
X. Wang et al. / Optics Communications 306 (2013) 42–48
systems utilizing intensity modulation with direct detection, but the limitation is acceptable since most of the commercially available systems use this modulation format. The channel model was developed based on the statistics of received power measurements from a maritime-mobile link, a land mobile link and a satellite downlink [9]. In satellite-to-ground laser communication links, atmospheric turbulence has a strong influence on the communication quality, and the communication link is difficult to establish in strong turbulence. Thus, large aperture optical antennas are used in the communication systems. The manufacture of large aperture optical antennas not only costs a lot, but also many factors including the volume of optical antennas have to be limited. In 2011, a kind of receiving/transmitting array antenna composed of several small aperture telescopes was designed, combined with space diversity [10]. The telescope arrays have advantages of less cost, slighter gravitational effects, ease of maintenance, and redundancy in operations. In 2012, the design of the Space Power Satellites (SPSs) was investigated [11]. The fully optical SPS eliminated the need for heavy, low efficiency photovoltaic cells as the first stage, so more energy could be obtained using very small and light structures. Furthermore, the effect of the turbulent atmospheric channel was examined. Tohoku University developed a 50 kg-class international scientific microsatellite named RISESAT, which can send actual scientific data obtained by payload instruments through this optical communication link. This is the first demonstration of microsatellite-to-ground optical downlink in the world [12]. In 2012, JPL described the work on the development of an optical link budget tool for an intensity-modulated direct-detected photoncounting channel utilizing pulse-position-modulation. JPL also provided approximations to the channel capacity and the loss for the deep space optical communications [13]. The atmosphere affects the characteristics of the ground-to-satellite optical communication link, so it is important to minimize these atmospheric effects in free space optical communication. In 2012, the Pulse Position Modulation (PPM), Differential Pulse Position Modulation (DPPM) and Differential Amplitude Pulse Position Modulation (DAPPM) had been evaluated for atmospheric turbulence channel. It is observed that DPPM and DAPPM have better bandwidth and power efficient in comparison to conventional modulation schemes OOK and PPM. Accordingly, the DPPM and DAPPM can perform better in the atmospheric turbulence channel [14]. The Transformational Satellite Communications System (TSAT) program is being implemented by the US military, which will gradually establish the interconnection between the ground and five in-orbit satellites until 2016, and the space laser communication rate will reach 10–40 Gbps [15]. The space-to-ground optical communications system mainly consists of the transmitting device, the receiving device and the atmospheric channel. The atmospheric channel has the open characteristic, so it is vulnerable to the impacts of different weather conditions. Due to the space-to-ground laser link in the complex and changing natural environment, the study of the atmospheric channel is very important. When the laser is transmitted in the atmospheric channel, it is easy to interact with the particles. As a result, the atmospheric attenuation effects will be caused by the absorption and scattering. The bigger the atmospheric attenuation is, the smaller the received signal power will be. Atmospheric turbulence can cause signal phase fluctuation, atmospheric scintillation, beam bending, drift and beam broadening, which cause the increase of error rate, thereby the quality of signal transmission and the reliability of the link will decrease [16]. In addition, space-to-ground optical communications are the key to connect optical satellite network and terrestrial optical fiber communication network, and it is also the key to integrated ground-air-space. Therefore, it is significant to study the influences
43
of the atmosphere on atmospheric channel of space-to-ground optical communications. In this paper, we analyze the impacts of atmospheric channel by the mathematical models and simulation. The results show that the atmospheric attenuation and atmospheric turbulence have an important effect on the signal transmission in the space-to-ground optical communications. This demonstrates much theoretical significance on the wavelength selection, height settings of the OGS as well as angle selection of the transmitter. The paper is structured in 6 sections. First, the wavelength attenuation, refractive index structure constant and the atmospheric transmittance of different zenith angles are separately analyzed with mathematical models in Section 2, Section 3, and Section 4 respectively. Then Section 5 focuses on simulating and analyzing wavelength transmittance under different atmosphere conditions. Finally, the most significant results are concluded in Section 6.
2. The mathematical analysis of wavelength attenuation The atmosphere of earth is a mixture of different gases. The main components include N2 (78.9%) and O2 (20.95%), and the remainder consists of CO2, CO, CH4, O3, SO2, etc. Moreover, the atmosphere also contains the liquid droplets and solid particles. Because of the interactions between laser beams and atmosphere, the main atmospheric effects on the laser beams are atmospheric attenuation and atmospheric turbulence, where atmospheric attenuation is mainly caused by the atmospheric absorption and scattering. According to the reference [17], we provide the mathematical model of atmospheric attenuation as: 13 dB λ nm −q αSPEC ðdB=kmÞ≅ ð1Þ V km 550 nm whereαSPEC is the specific attenuation, λ is the light wavelength, and Vis the visibility. The value of q follows as: 8 > < 1:6 f V 4 50 km ð2Þ q ¼ 1:3 f 6 km o V o 50 kmI > : 0:585 V 1=3 f V o6 m Fig. 1 is the diagram of space-to-ground optical communications. We can see that the atmosphere mainly influences the space-to-ground optical communications. Thus, we use Eq. (1) to describe the wavelength attenuation. Eq. (1) is an empirical equation, and the visibility V and the wavelength λ are the important parameters, leading to the optical attenuation. According to Eq. (1), we can calculate the attenuation of different wavelengths with the atmospheric visibility ranging from 6 km to 23 km shown in Fig. 2.
Fig. 1. Diagram of the space-to-ground optical communications.
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described by 0.78um 0.85um 0.95um 1.064um 1.3um 1.55um 10um
1.8
Attenuation in [dB/km]
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Visibility in [km]
Fig. 2. Beam attenuation diagram at different wavelengths.
When the atmospheric visibility is less than or equal to 6 km, the weather condition is poor and significantly influences the signal transmission. Thus the communication channel is not reliable. When the visibility is more than 23 km, the weather is sunny. Therefore, the atmospheric channel weakly affects the signal transmission, ranging from 6 km to 23 km, there is thick fog, cloudy, sunny weather conditions, and the weather becomes bad to fine. So we analyze the wavelength attenuation at this range of atmospheric visibility. As shown in Fig. 2, when the atmospheric visibility ranges from 6 km to 23 km, the attenuation of different wavelengths gradually decreases with the increase of the wavelength. In the case of the same visibility, the bigger the wavelength is, the smaller the attenuation is. In addition, when the atmospheric visibility is less than 6 km, the weather conditions are bad, and the beam attenuation is larger. When the weather is fine for the atmospheric visibility in excess of 23 km, the beam attenuation is small and the difference is negligible. So the wavelength selection is more flexible in the sunny weather conditions. Therefore, the light source should use the larger wavelength in the actual space-to-ground optical communications, which causes less attenuation. For example in Fig. 2, the attenuation difference between the wavelengths 0.7 μm and 10 μm is about 1.8 dB/km at the visibility of 6 km, which is important in engineering applications.
! υ 2 10−5 10 −h 0:00594 exp 27 1000 h −h −h þ A exp þ2:7 10−16 exp 1500 100
c2n ðhÞ ¼
ð3Þ
Where h is the height of the OGS, υ is the rms windspeed in meters per second (m/s) and A is a value of c2n ð0Þ at the ground in m−2/3. In the following analysis, we take v¼ 21 m/s and A ¼1.7 10−14. According to Eq. (3), we calculate the atmospheric refractive index structure constant for the attenuation at different wavelengths shown in Fig. 3. As seen from Fig. 3, the atmospheric refractive index structure constant changes significantly with the height ranging from 0 to1000 m. The atmospheric refractive index structure constant decreases sharply from 0 to 500 m, but it changes smoothly from 500 to 1000 m. When the height is larger than 2000 m, the curve of refractive index structure constant is smooth, which is close to the horizontal axis. This shows that atmospheric refractive index structure constant will gradually decrease with the increase of height. Correspondingly, the atmospheric turbulence varies progressively from strong to weak. That is to say, the influence of atmospheric turbulence on the communication channel is smaller when the height is larger than 2000 m. Therefore, in order to Atmospheric structure constant of refractive index(10-14)
2.0
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 m
Fig. 3. The refractive index structure constant with different heights.
0.9
3. Analysis of atmospheric refractive index structure constant 0.8 0.7 Transmittance
The atmosphere of earth is a layer of gases surrounding the planet earth. The air pressure and density decrease in the atmosphere with the increase of the height due to the influence of the gravity. The weather is constantly changing, thus the atmospheric temperature is a random variable. The atmosphere is in a state of constant motion and the communication channel is in this turbulence gases. The atmospheric turbulence is an irregular random motion relative to the atmosphere overall average movement, which randomly transforms the refractive index along the optical transmission path. The value of the refractive index structure constant represents the size of refractive index, so we use the refractive index structure constant to analyze the intensity of atmospheric turbulence. According to Ref. [18], the refractive index structure constant varies with the height of OGS in the space-to-ground optical communications link, and one of the most widely used models is the Hufnagel-Valley (HV) model
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Rad Fig. 4. Atmospheric transmittance under different zenith angles.
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improve the impacts of the atmospheric channel on signal transmission, the height of OGS should be above 1000 m.
4. Mathematical analysis of atmospheric transmittance In the space-to-ground optical communications, the signal transmission is mainly influenced by the atmospheric attenuation and atmospheric turbulence. The atmospheric turbulence changes randomly, and it is affected by the different weather conditions, so the impacts cannot be directly simulated and analyzed. Thus we analyze the atmospheric transmittance in order to research the effects of the signal transmission in the atmospheric channel. In the optical link, according to Ref. [19], the atmospheric transmittance T is defined as the ratio of the receiving power P r to the transmitting power P t as follows: T¼
Pr ¼ e−τ Pt
ð4Þ
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Where τ is the optical depth. Ref. [19] also points out that beam transmission spectrum of zenith can be transformed into different observation angles by mathematical transformation. If the transmittance of zenith is given by T 0 , we can calculate the transmittance T θ with the zenith angle θ as: T θ ¼ ðT 0 Þsecθ
ð5Þ
Where secθ is the air mass. The atmospheric transmittance under different zenith angles can be obtained with the vertex transmittance 0.9 as shown in Fig. 4. The horizontal ordinate is the size of zenith angle, while the vertical ordinate represents the transmittance. The transmittance is 0.9 with the zero degrees zenith angle, and the atmospheric transmittance gradually decreases with the increase of zenith angle. The transmittance sharply declines at the zenith angle of about 1.2 rad. The transmittance curve is relatively smooth when the zenith angle between 0 and 1.2 rad. Moreover, when the
Fig. 5. Atmospheric transmittance with different beams and different heights. (a) Atmospheric transmittance of different wavelengths at the height of 1 m, (b) atmospheric transmittance of different wavelengths at the height of 2 m, and (c) atmospheric transmittance of different wavelengths at the height of 3 m.
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atmospheric transmittance is large, the attenuation of the signal is small. In the actual space-to-ground optical communications, the communication performance is better for the zenith angle between 0 and 1.2 rad. If the zenith angle of the OGS is greater than 1.2 rad, the other optical ground nodes or other space nodes should be considered as the relay nodes, so as to maintain a better communication performance.
5. Simulation of atmospheric transmittance 5.1. Atmospheric transmittance of different beams In the simulation, the atmospheric visibility takes 23 km according to 1976 US standard atmosphere model. The zenith angle of OGS is set to zero degrees, and the height of the OGS has a range of 1 km to 3 km. Because the wavelength from 0.2 μm to 0.4 μm is absorbed by the atmospheric ozone layer, the wavelength of the simulation is set from 0.5 to 10 μm.
The simulation results are shown in Fig. 5. When the visibility and zenith angle are certain, the beams have a larger atmospheric transmittance with increase of the height. Along with the increase of the wavelength, the transmittance window gradually increases. In space-to-ground optical communications, we should choose a larger height in order to get a bigger transmittance and a smaller attenuation of the signal. Reference [17] points out that the commonly used wavelengths range from 0.85 μm to 1.55 μm in laser communication. In Fig. 5, it also can be seen that the atmospheric transmittance window is a little narrow near 1 μm, but it is still suitable for the communications due to the larger transmittance, which proves that the theoretical research is consistent with the practical application. 5.2. Atmospheric transmittance with the different beams and different zenith angles We assume the height of the OGS as 2 km in accordance with the analysis of section 3, the visibility is 23 km and the zenith
Fig. 6. The atmospheric transmittance with different zenith angles. (a) Atmospheric transmittance with zenith angle of 301, (b) atmospheric transmittance with zenith angle of 601, (c) atmospheric transmittance with zenith angle of 701 and (d) atmospheric transmittance with zenith angle of 891.
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angle is selected from 01 to 891. The results are shown in Fig. 6. We can see that the atmospheric transmittance decreases with the increase of the zenith angle. This is chiefly because the communication path is vertical with the zero degrees zenith angle. Thus, the communication distance is the shortest and the transmittance of the vertical path is the largest. The path of the space-to-ground optical communications becomes slant when the zenith angle is increasing. This means that the communication distance will increase, which results in a larger attenuation of the signal transmission. The larger the zenith angle is, the longer distance of the communication path is. So the transmittance will gradually decrease. It also can be seen from Fig. 6 that the atmospheric transmittance has a little change with the zenith angle ranging from 01 to 301, but the atmospheric transmittance varies significantly at zenith angle of 701 . In the actual space-to-ground optical communications, according to the size of the zenith angle, we need to consider the relay nodes to achieve better communication performance.
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5.3. Atmospheric transmittance with the different beams and different rainfall When the daily rainfall is less than 10 mm, it drizzles. When the daily rainfall of moderate rain is more than 10 mm, and the daily rainfall in excess is 24 mm will be the heavy rain. Because the communication will be interrupted in the heavy rain, we choose the daily rainfall of 2 mm, 5 mm and 12.5 mm in the simulation. The atmospheric visibility takes 23 km according to 1976 US standard atmosphere model. The zenith angle of OGS is set to zero degrees and the height of the OGS is 2 km. The simulation results are shown in Fig. 7. We can see that the rainfall has an important effect on the space-to-ground communications. The beam transmittance gradually decreases with the increase of the rainfall. The transmittance will increase with the increase of wavelength under the same conditions. When the rainfall is 2 mm, the average transmittance is about 65%. The impact of the drizzle is slight on the signal transmission.
Fig. 7. The atmospheric transmittance with different rainfall. (a) Atmospheric transmittance with the daily rainfall of 2 mm, (b) atmospheric transmittance with the daily rainfall of 5 mm and (c) atmospheric transmittance with the daily rainfall of 12.5 mm.
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As the rainfall is 12.5 mm, the average transmittance is about 50%, and that means the signal attenuation is approximately 50%. Therefore, the signal attenuation will be larger in the heavy rain, and the communication may be interrupted. In the actual communication process, the effect of rainfall on the communication is a non-negligible factor. When the rainfall leads to the interruption of the communication, we should consider the relay nodes or larger wavelengths in the communication. 6. Conclusions In this paper, we have used the mathematical model to analyze the wavelength attenuation in different visibilities, the atmospheric refractive index structure constant with different heights and the atmospheric transmittance with different zenith angles. We conclude that the wavelength attenuation decreases with the increase of visibility, and the larger wavelength has the advantage of less attenuation. Along with the increase of the height of the OGS, the atmospheric refractive index structure constant will decrease, thus the atmospheric turbulence will become weak. When the vertical transmittance of the atmosphere is fixed, the transmittance gradually decreases with the increase of the observation zenith angle, and the atmospheric transmittance has a little change in the range of 0–301, but the transmittance sharply declines around the zenith angle of 701. Finally, the atmospheric transmittance is simulated under various weather conditions. The simulation results show that, the atmospheric transmittance gradually increases with the increase of the height at the certain visibility and zenith angle. When the zenith angle changes from 01 to 891, the transmittance gradually decreases at the given visibility and height. Besides, the transmittance gradually decreases with the increase of the rainfall. This study has demonstrated the theoretical significance on the wavelength selection of light source, the setting of height of OGS and the selection of zenith angle in the actual space-to-ground optical communications. In addition, it is good for space-to-ground optical communications by analyzing the influences of atmospheric channel. Moreover, this paper has important significance
on reliability and availability of space-to-ground optical communications ink.
Acknowledgment This work was supported in part by the National Natural Science Foundation of China (61172051 and 61071124), the Program for New Century Excellent Talents in University (NCET-110075 and NCET-12-0102), the Fundamental Research Funds for the Central Universities (N110204001, N120804002, N110604008 and N110804003), and the Specialized Research Fund for the Doctoral Program of Higher Education (20110042110023, 20110042120035 and 20120042120049). References [1] Plank Thomas, Proceedings of EUCAP, 2011, p. 2508. [2] H.T. Mo, L.J. Lei, Z.Y. Wang, Information communications 22 (5) (2009) 26. [3] H.L. Jang, et al., The technologies and systems of space laser communications, 1st ed., National Defense Industry Press, Beijing301. [4] Q. Fu, H.L. Jang, et al., Chinese Optical and Applied Optical Abstracts 5 (2) (2012) 116. [5] R.J. Cesarone, D.S. Abraham, S. Shambayati, Proceeding of ICSOS (2011) 410. [6] M. Toyoshima, Y. Takayama, T. Takahashi, et al., IEEE Aerospace & Electronic Systems Magazine 23 (8) (2008) 10. [7] H. Kampfner, F. Heine, D. Dallman et al., in :IEEE Aerospace Conference, 2011, p. 1. [8] G.S. Tombras, A.D. Tsigopoulos, E.A. Karagianni, et al., Journal of Communications and Networks 11 (4) (2009) 384. [9] B. Epple., IEEE/OSA Journal of Optical Communications and Networking 2 (5) (2010) 293. [10] J.Y. He, in: Proceedings of ICSOS, 2011, p. 343. [11] A. Jooshesh, T.A. Gulliver, S. Uysal, in: Proceedings of BWCCA, 2012, p. 302. [12] K. Yoshida, Y. Sakamoto, Y. Tomioka, et al., IEEE/SICE International Symposium on System Integration 29 (6) (2012) 939. [13] B. Moision, J. Wu, S. Shambayati, IEEE Aerospace Conference, 2012, p. 1. [14] S. Tiwari, V.K. Jain, S. Kar, in: Proceeding of ICOE 2012, p. 1. [15] M. Toyoshima, S. Yamakawa, T. Yamawaki et al., in: Proceedings of SPIE 5338 2004, p. 1. [16] Y.W. Liu, in: Proceedings of ICECE 2010, p. 5510. [17] B. Flecker, M. Gebhart, E. Leitgeb, et al., in: Proceedings of SPIE 2006, p. 1. [18] L.C. Andrews, Ronald.L Phillips, Optical Engineering 39 (12) (2000) 3272. [19] H. Hemmati, Deep space optical communications, 1st ed., Tsinghua University press, Beijing114.