SOTA:Small Optical Transponder for Micro-Satellite

Yoshisada Koyama, Morio Toyoshima, Yoshihisa Takayama and Hideki Takenaka Space Communication Systems Laboratory

National Institute of Information and Communications

Technology (NICT), Koganei, Japan koyama.yoshisada@nict.go.jp

Koichi Shiratama Space Engineering Division

NEC TOSHIBA Space Systems, Ltd. Ichiro Mase and Osamu Kawamoto

Space Systems Division NEC Corporationd

Abstract— NICT initiated R & D activities of Small Optical TrAnsponder (SOTA) for micro-satellites to demonstrate attractive features of optical technology. Development of the SOTA started based on the bread board model (BBM). Design review of BBM revealed the necessity of engineering model (EM) for critical technologies of 2-axes gimbal and receiving optics. Evaluation of the EM carried out and final design of the SOTA was fixed. This paper describes design progress of the SOTA.

Keywords-component; space laser communication; satellite communication, laser communication terminal

I. INTRODUCTION

Recently, development of Micro or Mini satellites is very active throughout the world because it requires less developmental resources such as low cost and short development period compared with traditional large satellites. Such satellites use a RF link for mission data down link which limit data transfer capability. Optical communication technology is very attractive for such down link application because of its large capacity without troublesome frequency allocation problem.

After successful laser communication demonstration between space and optical ground stations in NICT[1] and other international stations using Laser Utilizing Communications Equipment (LUCE) onboard Optical Inter-orbit Communications Engineering Test Satellite (OICETS) [2,3,4], NICT initiated R & D activities of Small Optical TrAnsponder (SOTA) for micro-satellites to demonstrate attractive features of optical technology in the frame of the Space Optical Communication Research Advanced Technology Satellite (SOCRATES) project. The objectives of this project are;

On orbit acquisition, tracking and communication performance verification of the small optical terminal

To acquire propagation data through the atmosphere at various wavelength

Communication quality measurement To measure the effect of coding on communication quality Basic quantum key distribution experiment Experiments with international OGSs Command operation by optical link(Future) Link experiment with aircraft (Future) Link experiment with satellite (Future)

II. SOTA BREAD BOARD MODEL (BBM)

The development of SOTA started based on the proposed bread board model from industry team. The model consisted of an optical part (SOTA-OPT) and a controller (SOTA-CONT). In the optical part, two quadrant detectors for acquisition (A-QD) and for tracking (T-QD), four laser transmitters and a 45mm diameter receiving optics with a fine pointing quadrant detector (FQD), Fine Pointing Mechanism (FPM) and an Avalanche Photo Diode (APD) were installed on the optical bench. [5] The optical bench was mounted on an Elevation (El) actuator and Azimuth (Az) rotational table. These tables were commercial off-the-shelf (COTS) devices which needed to modify for space application. The main design parameters are listed in the table 1. Fig 1 shows the configuration of the BBM.

TABLE 1 Major design parameters of the BBM

Required resources Mass Power

5.3 kg 22.8 Watt

Acquisition and Tracking Angular Range Az: >±10deg, El: >±10deg

Communication Link range Wavelength Data Rate

1000km TX 1: 975nm TX 2 & 3 : 800nm band TX 4 : 1550nm RX: 1064nm 10Mbps

SOTA-OPT SOTA Electronics

Figure 1 Configuration of the SOTA BBM

2011 International Conference on Space Optical Systems and Applications

978-1-4244-9685-3/11/$26.00 ©2011 IEEE 97

The photograph of the SOTA BBM is shown in Figure 2.

(a) (b) Figure 2 The SOTA BBM (a) Optical part, 145W x 161D x 166H, (b) Electronics, 128W x 160D x 89H (mm.)

After review and discussion of the BBM design, it was pointed out that the following technologies should be verified by engineering model.

A. Elevation actuator and Az rotational table

The COTS devices used in the SOTA BBM had limitations in operating temperature and angular range. Especially, in planning on orbit experiments, limited angular range reduces the experiment opportunity. It was concluded that angular range of the actuators should be enlarge to increase experimental opportunity. Then, development of a new 2-axes gimbal engineering model was decided.

B. Receiving optics with a fine pointing sensor, a FPM, an APD and TX4 fiber

The receiving optics is very small and uses many tiny optical elements in it. On the other hand, required alignment tolerance is very tight. Therefore an engineering model of the receiving optics is necessary to confirm its manufacturability. Finally, complete content and organizational editing before formatting. Please take note of the following items when proofreading spelling and grammar:

III. SOTA ENGINEERING MODEL (EM)

A new 2-axes gimbal EM

and a receiving optics EM were designed and manufactured with some related electronics circuits for evaluation. Figure 3 shows the engineering model of 2-axes gimbal with a receiving optic on it. The size of the gimbals is 207Wx178Dx 280H.

A. 2-Axes Gimbal EM

Design parameter

The main parameters of the 2-axes gimbal are listed in TABLE 2. The angular range of ±10gedree in the BBM was expanded to ±45degree. Micro-stepping motors with harmonic drives® which reduction ratio of 100 were used in both Azimuth and Elevation axes.

TABLE 2 2-axes gimbal Parameter

Micro-stepping drive performance

The 2-axes gimbal is controlled by pulse count without additional angular encoder. In this case backrush property is very important for precise control of the gimbal. Figure 4 shows measured drive angle vs. cumulative command pulse count. The backrush of 20arc-sec. was identified. This property was modeled and embedded in the simulation model of gimbal control for precise analysis.

Figure 4 Drive angle vs. command pulse property

Mechanical characterization

Mechanical properties of the 2-axes gimbal such as fundamental frequency and quiescence during vibration are very important for onboard equipment. The 2-axes gimbal EM was evaluated by vibration test with dummy harness. Figure 5 shows the test configuration of vibration test.

Figure 5 Vibration test configuration

Actuator Step drive motor with Harmonic Drive (HD)®Reduction Ratio of HD® : 1:100

Angular Range Az: ±45deg. EL : ±45deg. Drive Micro-Step Drive Angular Rate max. 3deg/s @2667pps Resolution 0.001125 deg/ pulse Size 207Wx178Dx280H

Z

Y X

98

Five acceleration sensors were mounted to monitor the structural response. Three of them were attached orthogonally to a cube on the optical bench. Figure 6 shows the response of X axis sensor, Y axis sensor and Z axis sensor during modal survey. The graph shows overwrite of each sensor response at X, Y and Z axis test in one graph. The analyzed fundamental frequency was 95Hz. But the graph shows 50Hz. After vibration test, measurements of static stiffness and precise dimension of mechanical parts were carried out. The reason of deterioration of the fundamental frequency was concluded that the clearance between some important parts were slightly large. Therefore more tight dimensional control of the important parts is required in SOTA-PFM manufacturing.

Figure 6 Measured response during modal survey.

After modal survey, random vibration of over-all acceleration level of 18.6Grms for Z axis and around 7Grms for X and Y axis were applied. During these test no significant rotational movement was observed which meant additional lunch lock mechanism was not necessary.

B. Receiving optics EM

Design and trial manufacture of the receiving optics EM were carried out. Figure 7 shows an engineering model of the receiving optics with a FPM, a fine QD (F-QD), an APD and TX4 fiber input. A beam expander of 45mm diameter aperture was placed in front of FPM.

Figure 7 Receiving optics EM

TABLE 3 shows target requirements and measured value after trial manufacture.

TABLE 3

Items Requirements Measured Beam Expander

Diameter Magnification

45 mm

x 10

45 mm X 10

Alignment Error Fine QD

APD TX 4 fiber

< 125 urad < 125 urad < 125 urad

7.5 urad 15 urad 19 urad

Beam divergence TX 4

150 urad (FWHM)

170 urad (FWHM)

The measured alignment error of sensors and fiber were less than 20 μrad which were well within requirements. Measured beam divergence angle of the TX4 fiber was 170 μrad in FWHM.

The discussion on the interface conditions between the SOTA and a micro satellite was started during EM phase. Most critical item is allocated mounting area size for the SOTA-OPT. To satisfy this requirement, it is necessary to reduce the width and depth of the SOTA-OPT. Then modification of 2-axis gimbal width and shorten the receiving optics length were took place during proto-flight model design phase.

IV. SOTA PROTO-FLIGHT MODEL (PFM)

A. Link Budget Progress The Link budget was also reviewed in parallel with EM

activities. TABLE 4 shows the link budget of the BBM phase. It requires high output power of ground station lasers, because of very small aperture diameter of 1.8 cm for both Acquisition QD (A-QD) and Tracking QD (T-QD) and very large background level estimation.

TABLE 4 Link budget at the BBM phase

For proto model, the link budget was updated as shown in TABLE 5. The required power of the OGS was reduced more realistic value, because the aperture diameter for A-QD and receiving optics were increased to 2.3 cm and 5 cm respectively.

For the downlink, additional ground station of 20 cm in aperture dia. is added. This station is mobile one with unique

Item unitUP Link Down Link

A-QD T-QD F-QD RX TX1 TX4Data Rate bps 10.0M 10.0M 10.0MWavelength nm 1064 975 1543TX power dBm +70 +40 +22.4 +16.0Beam

Divergence urad 100 367 174

Atom. Loss dB 10 17.6 15.7RX Ant. Dia. cm 1.8 1.8 4.5 4.5 30 30RX Level dBm -18.4 -48.4 -43.4 -43.4 -62.4 -61.0Margin dB 1.6 1.6 6.6 6.9 1.5 4.9

99

triaxial telescope. The data rate is reduced from 10 Mbps to 1 Mbps when the mobile ground station is used, because of its

small aperture diameter of 20 cm.

TABLE 5 Simplified link budget of the Proto-flight model

B. Proto-flight model design

Major modifications of the SOTA PFM design from BBM and EM design are; The width of 2-axis gimbal is reduced to 177mm. Beam expender in the receiving optics is replaced to a

Cassegrain telescope which is very small and light weight. And the diameter of telescope is increased to 5cm.

Acquisition QD and tracking QD are replaced into one detector as T-QD with increased aperture diameter of 2.3cm.

The SOTA has four transmitters (TX1~4) which operate at

980nm (TX1), 800nm band (TX2 and 3) and 1550nm (TX4). TX1 and TX4 transmit an optical signal which is selectable from data sources of PN, Image data and telemetry data of the SOTA at data rate of 10Mbps or 1Mbps. TX2 and TX 3 are used for basic quantum measurements for satellite quantum key distribution. The light from these transmitters are linearly polarized.

The up-link signal is asynchronous NRZ which carrier

frequency is 40 kHz and signal bandwidth is 1 kHz. The wavelength of up-link is 1064nm. The optical signal of up-link is received by the Receiving optics of 5cm in diameter and focused on a multimode fiber which is connected to a PIN Photo diode in the SOTA-CONT.

In TABLE 6, performance summary of the SOTA PFM is listed. Acquisition, tracking and pointing function are realized by a 2-axes gimbal, FPM and related sensors. Angular range is expanded to ±50degree both Az and El axis. Estimated fundamental frequency is more than 60Hz. Micro-stepping motors are used to drive 2-axes gimbal with minimum angular resolution of 0.001125deg/pulse.

Power consumption of the SOTA depends on operational

modes. Minimum power consumption is expected as 28.1W with operating transmitter is TX1 only. When TX1 and a

receiver are operating simultaneously, the required power reaches 39.5W.

Total weight of the SOTA is estimated as 5.7kg. The size

of SOTA-OPT is 177.5W x 130D x 264H (mm). By selecting the direction of the SOTA FOV, this size will be acceptable for the satellite bus.

TABLE 6, performance summary of the SOTA PFM

Acquisition, Tracking and Pointing Functions 2-Axes Gimbal

Angular Range Az: ±50deg. EL : ±50deg. Resolution 0.001125deg/pulse

Angular rate 3deg/s @2667pps Resonant freq. >60Hz FPM Angular Range : ±0.7deg Sensors Detector FOV sensitivity T-QD InGaAs QD ±40 mrad -60 dBm F-QD InGaAs QD ±2 mrad -60 dBm

Receiving optics Diameter : 50mm Magnification : x10 Communication Functions TX data: PN/Data/TLM with

or without LDGM RX : Asynchronous NRZ

TransmittersWave-length

Peak Power

Data Rate

Beam Div.

Polari-zation

TX 1 980nm 540mW 10Mbps 0.5mrad N/A TX 4 1543nm 80mW 10Mbps 0.2mrad LHCP

Receiver Wavelength Sensitivity Data Rate InGaAs PD 1064nm -60dBm 1kbps

Power TX 1 TX1+RX TX2,3&4 TX2,3&4+RX

28.1W 39.5W 32.5W 37.3W

Weight SOTA-OPT SOTA-CONT SOTA-WHN Total

2.8kg 2.5kg 0.4kg 5.7kg

Dimension SOTA-OPT SOTA-CONT

177.5x117x278 146x160x110.5 Figure 8 shows block diagram of the SOTA-PFM.

TX1

Acq. QD

TX4

800nmband

1064nm

980nm

1543nm

1064nm

2-Axis Gimbal

FPM CONT & RX

CPU

GIM & FPM DRV

TX 1,TX 4 LD & DRVR

PS 1

PS 2

TX 2,TX 3 LD & DRVRTX2

TX3

�Figure 8 The block diagram of the SOTA-PFM

Figure 9 and Figure 10 show the photograph of SOTA-

OPT and SOTA-CONT proto-flight model.

Item unit UP Link Downlink to

20cm OGS Downlink to 1.5m OGS

A-QD F-QD RX TX1 TX4 TX1 TX4Data Rate bps - - 1k 1M 1M 10M 10MWavelength nm 1064 975 1543 975 1543TX power dBm +37 +24.3 +16.0 +24.3 +16.0Beam Divergence urad 200 367 174 367 174Atom. Loss dB 13 17.6 15.7 17.6 15.7RX Ant. Dia. cm 2.3 5 5 20 150RX Level dBm -58.8 -56.1 -56.1 -63.9 -65.9 -56.9 -58.9Margin dB 1.2 3.9 3.9 0.9 2.4 1.4 2.9

100

Figure 9 Photograph of the SOTA-OPT PFM

Figure 10 The SOTA-CONT FPM

V. FUTURE WORKS

Integration of the SOTA-PFM has been completed. In the next phase, the following activities are under planning.

Overall performance test under ambient condition. Environmental verification test. Integrated vibration test mounted on the Micro Satellite

structural and thermal model (STM) to confirm and verify the interface between the SOTA and the satellite.

VI. CONCLUSIONS

Design progress of the SOTA is presented. The SOTA-PFM design has completed taking BBM and EM results and necessary modifications. The SOTA is very small and light weight with attractive performances for on-orbit demonstration. We hope the effectiveness of optical down link from micro satellite will be demonstrated very near future.

ACKNOWLEDGMENT

The authors would like to express sincere applications to all the members who are involved in the SOCRATES project.

REFERENCES

[1] M. Toyoshima, T. Takahashi, K. Suzuki, S. Kimura, K. Takizawa, T.

Kuri, W. Klaus, M. Toyoda, H. Kunimori, T. Jono, Y. Takayama, and K. Arai, “Results from Phase-1, Phase-2 and Phase-3 Kirari Optical Communication Demonstration Experiments with the NICT optical ground station (KODEN),” 24th International Communications Satellite Systems Conference of AIAA, AIAA-2007-3228, Korea, April 13 (2007).

[2] A. Alonso, Zoran Sodnik, J. Perdigues,” Preliminary Analysis of OICETS to OGS Downlink”, Proceedings of International Workshop on Ground-to-OICETS Laser Communications Experiments 2010 (GOLCE2010),p142, ISSN 2185-1484, Tenerife, Spain, May (2010)

[3] K. E. Wilson, J. Kovalik, A. Biswas, M. Wright, W. T. Roberts,” OCTL Planning, Tests and Operations for the Bi-directional Link to the OICETS”,p153, Proceedings of International Workshop on Ground-to-OICETS Laser Communications Experiments 2010 (GOLCE2010),p153, ISSN 2185-1484, Tenerife, Spain, May (2010)

[4] F. Moll,, “KIODO 2009: Trials and Analysis,” Proceedings of International Workshop on Ground-to-OICETS Laser Communications Experiments 2010 (GOLCE2010),p161, ISSN 2185-1484, Tenerife, Spain, May (2010)

[5] M. Toyoshima, H. Takenaka, Y. Shoji, Y. Takayama, Y. Koyama, and M. Akioka, "Small Optical Transponder for Small Satellites," International Symposium on Communication Systems, Networks and Digital Signal Processing, 2nd Colloquium in Optical Wireless Communications at the IEEE International Conference (CSNDSP10), OWC-10, Northumbria University, United Kingdom, July 21-23 (2010).

101

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具