Overview of the inter-orbit and orbit-to-ground laser communication demonstration by OICETS

Overview of the inter-orbit and orbit-to-ground laser communication demonstration by OICETS Takashi Jono *a, Yoshihisa Takayama a, Koichi Shiratama b, Ichiro Mase b, Benoit Demelenne c, Zoran Sodnik d, Aneurin Bird d, Morio Toyoshima e, Hiroo Kunimori e, Dirk Giggenbach f, Nicolas Perlot f, Markus Knapek f and Katsuyoshi Arai a a Japan Aerospace Exploration Agency, Tsukuba, Ibaraki, Japan b NEC TOSHIBA Space Systems. Ltd, Fuchu, Tokyo, Japan c ESA, Station de Redu, Redu, Belgium d ESA, European Space Technology Center, Noordwjk, The Netherlands e National Institute of Information and Communications Technology, Koganei, Tokyo, Japan f German Aerospace Center, Oberpfaffenhofen, Wessling, Germany Keywords: OICETS, KIRARI, ARTEMIS, free-space laser communication ABSTRACT The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agency's geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results. 1. INTRODUCTION The Optical Inter-orbit Communication Engineering Test Satellite (OICETS, Japanese name KIRARI ) was developed by the Japan Aerospace Exploration Agency (JAXA) [1-2] and was launched into low earth sun-synchronous orbit at an altitude of 610 km and an inclination of 97.8 degree on 23 August 2005. A main objective of OICETS was to demonstrate the free-space inter-orbit laser communications by using a laser communications terminal called the Laser Utilizing Communications Equipment (LUCE). Its additional objective was the demonstration of orbit-to-ground laser communications. The inter-orbit demonstration was planned with the cooperation of the Geostationary Earth Orbit (GEO) satellite, the Advanced Data Relay Technology Mission (ARTEMIS) developed by the European Space Agency (ESA) [3-4]. Moreover, a Low Earth Orbit (LEO) to ground laser communication demonstration was planned with the cooperation of the two Optical Ground Stations (OGS) developed by the National Institute of Information and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. Functional verifications of the satellite bus system and LUCE were conducted for about 3 months after launch. During the verification campaign, the basic functions of LUCE were verified, and an acquisition and tracking sequences and optical sensors were then verified by tracking stars such as Sirius and the Mars. On 9 December 2005, the first bidirectional laser communications link between OICETS and ARTEMIS was successfully established [5]. The inter-orbit laser communication demonstration was successfully conducted over a period of six months since December 2005. We conducted more than 100 inter-orbit experiments. Acquisition sequences, tracking performance and bit error characteristics were measured and evaluated. These results show more than 90 % probability of acquisition and less than 10-6 bit error rate. The orbit-to-ground trials with NICT were performed in the end of March and May 2006. The first reception of the downlink data at the ground station was observed on 28 March at the bit error rate of 10-5 [6-8]. During the experiment period, the optical link was successfully established repeatedly. The month of June 2006 was the period for the orbit-to- * jyono.takashi@jaxa.jp

LUCE-O Electrical part Optical part Flight direction (+X axis) (+Y axis) S-band antenna To the earth (+Z axis) Fig.1 In-orbit satellite configuration of OICETS. Fig. 2 Overview of the LUCE. ground demonstrations with DLR. The ground station recorded the bit error rate of 10-6 on the reception of the downlink data [9]. September was given to the trials with NICT again. The ground station and OICETS performed proper acquisition and tracking, and OICETS finally received the uplink data at the bit error rate of 10-7 on 19 September 2006. In this paper, we present descriptions of the in-orbit experiment, results of the in-orbit laser communications experiment between OICETS and ARTEMIS, and orbit-to-ground laser communication conducted with the NICT OGS and DLR OGS. 2. DESCRIPTIONS OF IN-ORBIT EXPERIMENTS OICETS is a relatively small satellite with a mass of approximately 570 kg. OICETS orbit for the experiment is a circular orbit with a height of about 610 km and an inclination of 97.8 degrees. LUCE consists of two parts; the optical and electrical parts. The optical part includes a telescope mounted on two axes gimbals. The electrical part provides functions to control the acquisition, tracking and pointing mechanisms as well as the communication electronics. Fig. 1 shows in-orbit satellite configurations, and Fig. 2 shows an overview of the LUCE s optical and electrical parts. The optical antenna has a diameter of 26 cm and is categorized as a center-feed Cassegrain mirror-type telescope. Planned in-orbit demonstration and experiments were performed by international cooperation. Entire in-orbit experiments and operation systems consist of a JAXA s OICETS satellite on low earth orbit, ESA s ARTEMIS satellite on geostationary orbit, NICT s optical ground station located in Tokyo, Japan and DLR s optical ground station located in Oberpfaffenhofen, Germany and some satellite operation facilities. Fig. 3 shows a schematic drawing of the whole system and facilities. KODEN (Kirari Optical Communication Demonstration Experiments with the NICT optical ground station) and KIODO (KIrari s Optical Downlink to Oberpfaffenhofen) are the project names of orbit-to-ground station experiment, respectively. The Laser communication scheme of OICETS is based on the ESA s SILEX system. Intensity modulation, or On-Off Keying (OOK) and direct detection system are used. The forward link from ARTEMIS to OICETS uses 2 Pulse Position Modulation (2PPM) format at 2.048 M bits/sec while the return link from OICETS to ARTEMIS uses the Non-Returnto-Zero (NRZ) format at 49.3724 Mbits/sec. The uplink, ground station to OICETS, uses the same format and bit rate as the one used for the forward link scheme while the downlink from OICETS to a ground station uses the same format and bit rate as the one used for the return link scheme. Wavelength, transmitted power and other characteristics of the laser communications terminals are listed in Table 1.

Laser terminal, satellite orbit or station location Wavelength Polarization Telescope diameter Table 1 Characteristics of the laser communications terminals. ARTEMIS DLR OGS 21.5 deg. East, Oberpfaffenhofen, geosynchronous Germany earth orbit OICETS height of 600 km, low earth orbit Communication: 847nm Beacon:801nm Communication: 819nm Beacon:808nm NICT OGS Koganei, Tokyo, Japan Beacon:808nm Communication: 815nm LHCP LHCP Random LHCP or random 0.26m 0.25m 0.40m Transmisson:1.5m Receiving:0.20m or 1.5m Stars Ka-band link Telemetry/command ARTEMIS Operation Control Center (Italy) ARTEMIS Ka-band link Mission data ESA Redu station (Belgium) 2Mbps Optical link 50 Mbps OICETS Alignment calibration 50 Mbps Optical link 2Mbps S-band link Telemetry /command 50 Mbps /mission data DLR optical ground station (Germany) NICT optical ground station (Japan) Mission plan 2 Mbps In-orbit test equipment 50 Mbps ARTEMIS mission data ARTEMIS mission control facility JAXA Tracking stations Mission data Real time data Orbit prediction data JAXA Tsukuba space center (Japan) OICETS operation control facility OICETS mission control facility Flight dynamics facility Mission data Orbit prediction data 3. RESULTS OF THE OICETS-ARTEMIS INTER-ORBIT EXPERIMENT 3.1 Over view of the inter-orbit experiment The experiment campaign consists of three phases as described below. - Commissioning phase: To establish inter-orbit laser communication link establishment, to verify the modulation function and the error counting function, and to confirm interoperability between JAXA and ESA space network operation systems. - Experiment phase: To evaluate the beam pointing characteristics and acquisition and tracking characteristics under various special conditions. - Routine phase: To demonstrate and evaluate an operational link in a condition of normal setup of both satellites. Fig. 3 In-orbit experiment and operation system.

The commissioning phase was started on 5 December 2005. We carried out the commissioning phase for 2 weeks, the experiment phase for 2months and the routine phase for 5 months. All of the experiments were completed on 10 August 2005. Table 2 shows a summary of the OICETS-ARTEMIS inter-orbit experiments. One hundred experiments were succeeded in acquisition and tracking during the experiment campaign. Table 2 Overview of the OICETS-ARTEMIS experiment. Phase Configuration Succeeded Failed Commissioning Normal 7 0 Normal 64 2 OICETS Beam pointing (for calibration of pointing bias) 9 1 ARTEMIS Beam pointing (for OICETS optical receiver characteristics) 2 0 Experiment and routine LUCE operable minimum(15mw) received irradiance for tracking 2 0 Tracking and communication characteristics under the interference moon and sun light 3 0 Evaluation of the atmospheric effects on the tracking performance 3 2 Repeater communication mode 2 0 OICETS no calibration 8 1 Total 100 6 3.2 Beam pointing experiment This experiment means a calibration of pointing bias of OICETS transmitting laser beam. Methods of the experiment were as follows: - ARTEMIS and OICETS established the optical communications link. - OICETS changed the beam direction by adding offsets to the point-ahead angle. The added angle sequence followed a spiral pattern. - The received power on ARTEMIS were measured and post-processed in order to recover the OICETS transmitted intensity. - A beam profile of OICETS was processed from the adding angle, and a bias pointing error was estimated. We conducted this experiment 9 times. Fig. 4 shows a beam profile which was the first data measured on 19 December Fig. 4 Transmitted laser Beam profile of OICETS measured on 19th December 2005. Fig. 5 Transmitted laser Beam profile of OICETS measured on 26th January 2006 after bias error calibration.

2005. The pointing bias error was estimated of about 3 micro radians from the experiment result. Fig. 5 shows a beam profile after the calibration of pointing bias. It measured on 26 January 2006, and added calibration angels were 2.5 micro radians in X-axis and 1.8 micro radians in Y-axis, respectively. 3.3 Tracking performance An acquisition sequence design is very important to succeed in acquiring incoming laser beam. The acquisition sequence between OICETS and ARTEMIS was started by the ARTEMIS beacon beam scanning. The fine pointing sensor error of X and Y axes and the fine pointing sensor level (received power) of OICETS are shown in Fig. 6. These data are obtained from an in-orbit experiment performed on 9 February 2006. The figure indicates that the fine pointing mechanisms worked properly. There was a sharp increase in the fine pointing sensor level after the initial acquisition. This indicates that ARTEMIS started to transmit the communication beam. Overall duration of acquisition sequence with both terminals was about 33 seconds in this experiment. The fine pointing error means OICETS tacking error. In this case, three sigma value of the tracking error during whole tracking was at about 0.3 micro radians. Fig. 7 shows three sigma value of the tracking error of each in 80 executed experiments during the routine phase. These tracking errors were less than 0.4 micro radians. 3.4 Communication performance OICETS transmitted a Pseudo-random Noise (PN) code data stream to ARTEMIS by using the optical link. The data stream received by ARTEMIS was converted to electrical signal and transmitted through downlink to an ESA s ground station in Redu, Belgium, via a Ka-band radio frequency feeder link of ARTEMIS. The bit error of the return link was measured in the Redu station. For the forward link, the PN code data stream was transmitted in the Redu station The data stream passed through the Ka-band feeder link, ARTEMIS and the optical link. OICETS was equipped with a bit 1.0 0.9 0.8 3σtracking error (μ rad ) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1-Apr-06 1-May-06 31-May-06 30-Jun-06 30-Jul-06 29-Aug-06 Date OICETS tracks communication beam Fig. 7 Three sigma value of the tracking error of each in 80 execution experiment in routine phase OICETS tracks beacon beam from ARTEMIS Fig. 6 The fine pointing sensor error of X, Y axis and the fine pointing sensor level in initial acquisition and tracking phase. error counter to measure the bit error of the forward link. The Bit Error Rate (BER) performances in 80 executed experiments during the routine phase are shown in Fig. 8. Both optical links achieved the bit error rate at less than 10-6.

1.E-02 1.E-03 1.E-04 Return Link Forward Link 1.E-05 BER 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 06/04/01 06/05/01 06/05/31 06/06/30 06/07/30 06/08/2 Date Fig. 8 Bit Error Rate (BER) performances of each in 80 execution experiment in routine phase. 4. ORBIT-TO-GROUND EXPERIMENT 4.1 Description of the orbit-to-ground experiment The OICETS satellite system design was based for the inter-orbit communication. The LUCE optical part attached to the anti-earth side of the satellite body to point toward the geostationary earth orbit satellite ARTEMIS. An attitude control system of the OICETS has two modes. The normal mode is a three-stabilized attitude control mode. The other is the Inertia Reference Mode (IRM) which is an inverted attitude configuration. In the IRM mode, its attitude is fixed at an inertia space, and the LUCE optical part is, thus, able to point toward a ground station. Fig. 9 shows an in-orbit configuration in the IRM mode for the orbit-to-ground experiment. There were some critical matters for performing the experiment with ground station compared to the experiment with ARTEMIS. The visible time from the ground station in the experiment lasted for only about 3 to 10 minutes. Relative distance was changed from 600km to 1500km during a pass. Transmitting beam divergence angle of LUCE is only about 5 micro radians. Therefore, when distance is about 1000km, LUCE has to keep pointing to a ground station within the limits of 5m spot approximately. Tracking or pointing angular velocity of LUCE is twice to 3 times as fast as the experiment with ARTEMIS. Moreover, atmospheric turbulence between the satellite and a ground station affects the laser links. 4.2 Results of the KODEN experiment KODEN is orbit-to-ground experiment with the NICT optical ground station located in Tokyo, Japan. We tried the experiment 18 times. The experiment carried out in the midnight. The success rate with respect to acquisition and tracking of each terminal was about 61%. However, the link establishments were succeeded every trial for sure under the clear-sky condition. Table 3 shows a summary of the KODEN experiment campaign. Fig. 10 and Fig 11 shows the coarse pointing sensor error, fine pointing sensor error and fine pointing sensor received power level of OICETS on 5 September 2006. Fig 10 plots the error angle during tracking for about 230 seconds after the initial acquisition. The fine pointing sensor level was saturated in the initial tracking since the NICT ground station

transmitted a beacon and communication beam in the initial acquisition and tracking phase during 50 seconds in this experiment case. We tried the bidirectional communication in the KODEN experiment. The PN data streams were used for the bit error count measurement. Fig. 12 and 13 shows the BER of the down and uplink respectively on 19th September 2006. Each BER data was calculated by the second. In Fig 12, the BER exceeding 1E-02 means that the optical receiver did not synchronize the modulated beam. The BER were changed suddenly because the laser power output from the ground station was changed in order to find out optimal emission power for a proper bit eye pattern. Trial Number Table 3 Summary of the KODEN experiment campaign. Experiment Date in 2006 Weather condition Link establishment (acquisition and tracking each terminal) 1st March 21st Clear sky Success 2nd March 2nd Cloudy No link 3rd March 28th Clear sky Success 4th March 30th Partly cloudy Success 5th May 9th Rainy No link 6th May 11th Partly cloudy Success 7th May 16th Cloudy Success 8th May 18th Rainy No link 9th May 23rd Partly cloudy Success 10th May 25th Partly cloudy Success 11th September 5th Clear sky Success 12th September 7th Partly cloudy Success 13th September 12th Rainy No link 14th September 14th Rainy No link 15th September 19th Clear sky Success 16th September 21st Clear sky Success 17th September 26th Rainy No link 18th September 28th Rainy No link 4.3 Results of the KIODO experiment The KIODO experiment was conducted with the DLR optical ground station located in Oberpfaffenhofen, Germany. The experiment was conducted 8 times in June 2006. The experiments were carried out in midnight. Table 4 shows a summary of the KIODO experiment campaign. The link establishments were succeeded under the clear-sky condition. Fig. 14 shows the fine pointing sensor error and fine pointing sensor received power level of OICETS on 14 June 2006. The downlink BER characteristics were measured in the experiment campaign. We attained the BER of 10-6 as the best result during the experiments. Trial Number Table4 Summary of the KIODO experiment campaign. Experiment Date in 2006 Weather condition Link establishment (acquisition and tracking each terminal) 1st June 7th Clear sky Success 2nd June 9th Clear sky Success 3rd June 14th Clear sky Success 4th June 15th Clear sky Success 5th June 21st Cloudy No link 6th June 23rd Cloudy No link 7th June 28th Partly cloudy Success 8th June 30th Cloudy No link

Fig. 9 In-orbit configuration in the IRM mode for the orbit-toground experiment. Fig. 10 The coarse pointing sensor error of the OICETS on 5th September 2006 in KODEN experiment. Fine pointing sensor error X [μrad] Fine pointing sensor error Y [μrad] Fine pointing sensor level [dbm] 3 2 1 0-1 -2-3 0 50 100 150 200 250 300 Time[sec] 3 2 1 0-1 -2-3 0 50 100 150 200 250 300 Time[sec] -55-60 -65-70 -75-80 -85 0 50 100 150 200 250 300 Time [sec] Fig. 11 The fine pointing sensor error and the fine pointing sensor received power level of the OICETS on 5th September 2006 in KODEN experiment.

1.E+00 1.E+00 1.E-01 1.E-02 1.E-01 1.E-03 BER 1.E-04 BER 1.E-02 1.E-05 1.E-06 1.E-03 1.E-07 1.E-08 1.E-04 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 Time [sec] Time [sec] Fig. 12 BER of the up link on 19th September 2006. Fig. 13 BER of the dawn link on 19th September 2006. 3 Fine pointing sensor error X [μrad] 2 1 0-1 -2-3 0 20 40 60 80 100 120 140 160 180 200 Time[sec] 3 Fine pointing sensor error Y [μrad] 2 1 0-1 -2 Fine pointing sensor level [dbm] -3 0 20 40 60 80 100 120 140 160 180 200 Time[sec] -55-60 -65-70 -75-80 -85 0 20 40 60 80 100 120 140 160 180 200 Time [sec] Fig. 14 The fine pointing sensor error and the fine pointing sensor received power level of the OICETS on 4th June 2006 in KIODO experiment.

5. CONCLUSIONS Serried in-orbit experiment results were described in this paper. The results of inter-orbit experiment between the GEO satellite ARTEMIS and the LEO satellite OICETS demonstrated that inter-orbit laser communication link had a quality for operational use. LEO-to-ground laser communication links were demonstrated by the experiment between the NICT OGS, DLR OGS and the OICETS. Moreover, some fundamental data of scintillation were obtained by the LEO-toground experiments. ACKNOWLEDGEMENT The in-orbit experiments were supported by many people through international cooperation. The authors wish to express our gratitude to the OICETS operation team at JAXA, the ARTEMIS operation team at ESA, the KODEN operation team at NICT and the KIODO operation team at DLR for their support in developing, preparing, and executing the experiments. REFERENCES 1. K. Nakagawa, et al., Preliminary design of Laser Utilizing Communications Experiment (LUCE) installed on Optical Inter-Orbit Communications Engineering Test Satellite (OICETS), Proceedings of the SPIE 2381, 14-25 (1995). 2. T. Jono, et al., Acquisition, tracking and pointing system of OICETS for free space laser communications, Proceedings of the SPIE 3692, 41-50 (1999). 3. M. Faup, et al., Experience Gained in the Frame of Silex Program Development and Future Trends, AIAA 16th International Communications Satellite Systems Conference 1996, 782-783 (1996). 4. T. T. Nielsen,et al., In Orbit test result of an Operational Intersatellite Link between ARTEMIS and SPOT4, Proceedings of the SPIE Vol. 4635, 1-15 (2002). 5. T. Jono, Y. Takayama, N. Kura, K. Ohinata, Y. Koyama, K. Shiratama, Z. Sodnik, B. Demelenne, A. Bird and K. Arai, OICETS on-orbit laser communication experiments, Proc. SPIE, 6105, 610503, 1-11 (2006). 6. M. Toyoshima, K. Takizawa, T. Kuri, W. Klaus, M. Toyoda, H. Kunimori, T. Jono, Y. Takayama, M. Mokuno and K. Arai, Results of ground-to-space optical communications experiments using a low earth orbit satellite, IEEE LEOS Annual Meeting, MH2, 80-81 (2006). 7. M. Toyoshima, K. Takizawa, T. Kuri, W. Klaus, M. Toyoda, H. Kunimori, T. Jono, Y. Takayama, N. Kura, K. Ohinata, K. Arai and K. Shiratama, Ground-to-OICETS laser communication experiments, Proc. of SPIE, 6304B, 1-8 (2006). 8. Y. Takayama, T. Jono, M. Toyoshima, H. Kunimori,, D. Giggenbach, N. Perlot, M. Knapek, K. Shiratama, J. Abe, K. Arai, Tracking and pointing characteristics of OICETS optical terminal in communication demonstrations with ground stations, Free-Space Laser Communication Technologies XIX, Proc. of SPIE, 6457A (2007). 9. N. Perlot, M. Knapek, D. Giggenbach, J. Horwath, M. Brechtelsbauer, Y. Takayama, T. Jono, Results of the Optical Downlink Experiment KIODO from OICETS Satellite to Optical Ground Station Oberpfaffenhofen (OGS- OP), Free-Space Laser Communication Technologies XIX, Proc. of SPIE, 6457A (2007).