Atmos. Meas. Tech., 3, 1233–1239, 2010 www.atmos-meas-tech.net/3/1233/2010/ Atmospheric doi:10.5194/amt-3-1233-2010 Measurement © Author(s) 2010. CC Attribution 3.0 License. Techniques Atmospheric influence on a laser beam observed on the OICETS – ARTEMIS communication demonstration link A. Loscher¨ ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands Received: 18 March 2010 – Published in Atmos. Meas. Tech. Discuss.: 10 May 2010 Revised: 26 July 2010 – Accepted: 20 August 2010 – Published: 14 September 2010 Abstract. In 2006 bi-directional optical inter-satellite com- the limits concerning the capacity of X-band downlink sys- munication experiments were conducted between the Japan tems. One possible solution would be the use of optical Aerospace Exploration Agency (JAXA) Optical Inter-orbit terminals, increasing the bandwidth significantly, in a di- Communications Engineering Test Satellite (OICETS) and rect Earth observation satellite-to-ground or Earth observa- the European Space Agency (ESA) multi-purpose telecom- tion satellite-to-relay-satellites-to-ground configuration. The munications and technology demonstration satellite (Ad- links themselves are potentially a source of information con- vanced Relay and Technology MISsion) ARTEMIS. On 5 cerning Earth’s atmosphere depending on the transmission April 2006, an experiment was successfully carried out by geometry. maintaining the inter-satellite link during OICETS’s setting The Optical Inter-orbit Communications Engineering Test behind the Earth limb until the signal was lost. This setup Satellite (OICETS, Japanese name KIRARI) (Jono et al., resembles an occultation observation where the influence 2006) was developed by the Japan Aerospace Exploration of Earth’s atmosphere is evident in the power fluctuations Agency (JAXA) and launched into a circular Low Earth Or- recorded at ARTEMIS’s (and OICETS’s) receiver. These bit (LEO) 23 August 2005 to an altitude of 610 km at an incli- fluctuations do not exist or are at a low level at a link path nation of 97.8◦. The main payload is a laser-based commu- above the atmosphere and steadily increase as OICETS sets nication terminal called the Laser Utilizing Communication behind the horizon until the tracking of the signal is lost. This Equipment (LUCE). It was designed to conduct inter satellite specific experiment was performed only once since atmo- laser communication experiments with the European Space spheric science was not the goal of this demonstration. Nev- Agency’s (ESA) Advanced Relay and Technology MISsion ertheless, this kind of data, if available more frequently in (ARTEMIS) as its counterpart and orbit-to-ground commu- future, can help to study atmospheric turbulence and validate nications (e.g. with the OGS, ESA’s Optical Ground Station). models. The data present here were recorded at ARTEMIS. OICETS is still in orbit and functional but the instrumenta- tion have not been used since the conclusion of the experi- ments. 1 Introduction ARTEMIS is a multi-purpose telecommunication and technology demonstration satellite. The ARTEMIS mission Free space laser links have been studied in the context began on 12 July 2001 and was subject to an unprecedented of terrestrial wireless communication to establish perfor- rescue that spanned 18 months after a malfunction of the up- mance envelopes and design guidelines (Henninger and Wil- per stage of the Ariane 5 launcher. The spacecraft control fert, 2010). During the last years the capability to employ team managed to raise the satellite, which was stranded in an abnormally low transfer orbit to its intended geostationary this technique for establishing communication links between ◦ satellites and satellite-to-ground was developed in Europe. orbit (GEO) at ∼36 000 km 21.5 East on 31 January 2003 One reason for those developments is the growing amount of using innovative procedures and onboard advanced technol- data generated by Earth Observation Satellites which reached ogy. ARTEMIS is still fully operational serving its purpose in the domains of communication, data relay and navigation. The first ever transmission of an image by laser link from Correspondence to: A. Loscher¨ satellite-to-satellite took place on 30 November 2001 with ([email protected]) the SILEX (Semiconductor Inter-satellite Link Experiment) Published by Copernicus Publications on behalf of the European Geosciences Union. 1234 A. Loscher:¨ Atmospheric influence on optical communication links system, which consists of the OPALE (Optical Payload for Inter-Satellite Link Experiment) terminal on ARTEMIS and the PASTEL (PASsenger TELecom) terminal on the French Earth observation satellite SPOT-4 (Systeme` Probatoire pour l’Observation de la Terre), launched on 22 March 1998, in a 832 km sun-synchronous LEO orbit at an inclination of 98.7◦ (Tolker-Nielsen and Oppenhaeuser, 2002). The link experiments between ARTEMIS and OICETS lasted in total ∼8 month. The first bi-directional inter-orbit laser communication was established on 9 December 2005, experiments where carried out until the middle of August 2006 resulting in 100 successfully established links (Jono et Figure 1 Observation geometry of the communication experiment between ARTEMIS and OICETS al., 2007). on Fig.5 April 1.2006. Observation geometry of the communication experiment Usually the link is established well above Earth’s atmo- between ARTEMIS and OICETS on 5 April 2006. The experiment started at 07:21:06 UTC and ended at 07:22:49 UTC, the resulting sphere to ensure an optimal communication environment. geometric link path altitude is shown in Fig. 2. The receiver onboard ARTEMIS lost the signal before160 its counterpart onboard OICETS and fluctuations of the received power For the special experiment conducted on 5 April 2006, the (normalized by the receiver antenna area and given here in nW/m2) are visible a bit earlier link was maintained until the experiment came forcibly to an (~ 5sec,140 which roughly corresponding to 8km) than in the data recorded at OICETS end caused by atmospheric influences. The received power (Takayama et al., 2007). The ARTEMIS120 data record starts at 07:21:36 and ends at 07:22:41. The explanation for started to fluctuate strongly with a decrease of the average re- the earlier disconnection is straightforward: although both beams propagate through the same media100 the signal originating from OPALE on ARTEMIS gets disturbed just before ceived power finally resulting in a disconnection of the link reaching the receiver on OICETS. The beam travels a long distance undisturbed through (Takayama et al., 2007). free space80 before being influenced by Earth’s atmosphere. After emerging from the disturbing volume the distance to the receiver is rather short. The opposite is true for the Altitude [km] The time of the experiment was chosen with respect to beam emitted60 by LUCE on OICETS. The distance to the disturbing volume is rather short the relative positions of ARTEMIS and OICETS so the link but after leaving Earth’s atmosphere the disturbed and deflected beam has to travel a long distance 40which in fact amplifies the power fluctuations recorded at ARTEMIS. The power could be established at an altitude where the residual at- transmitted by LUCE is 100 mW which is the modulated mean value via a telescope of mosphere could be assumed as negligible. Due to the rel- 0.26 m diameter,20 the receiving telescope of OPALE has a diameter of 0.25 m. Figure 3 shows the power detected at ARTEMIS for a 40 second period starting at 0 ative motion of the satellites (basically the propagation of 07:22:00 UTC (the respective altitude range can be seen in Fig. 2) where the red line is the OICETS along its orbital trajectory) a classical occultation mean of 0.1 sec intervals. The link is established above 100 km link altitude and thus no atmospheric influence is to be expected. The procedureTime [UTC] is as follows: Both terminals point event occurs. The laser beams start to propagate through the to each other utilizing an open-loop control based on on-board orbital models. OPALE emits a strong wide-divergence (750 μrad) laser (801 nm) beam scanning a cone of atmosphere at high altitudes reaching gradually denser re- uncertainty.Fig. 2. UponAltitude reception - time LUCE diagram emits its communication of the communication beam (847 nm, experiment 5.5 μrad) gions as OICETS gets closer to the Earth’s rim as seen from between ARTEMIS and OICETS on 5 April 2006. Start and end4 of ARTEMIS. The signal can be tracked as long as the receiver data records are marked in blue for ARTEMIS and red for OICETS. at OICETS can see the transmitter at ARTEMIS (and vice versa, the link is bi-directional), thus, until OICETS sets be- low the horizon and the beams hit the Earth’s surface. In illustrated in Fig. 1, showing ARTEMIS in its geostationary practice, one has to consider ray bending in the atmosphere orbit at ∼36 000 km whereas OICETS has a close to polar in addition to purely geometric considerations to derive the orbit at ∼610 km. At time t1 the bi-directional link acqui- exact ray path. In fact, tracking is lost far above the surface sition is established and the beams propagate undisturbed due to fluctuations in the optical power received caused by trough free space. As OICETS starts to set behind Earth’s atmospheric inhomogenities. atmosphere at t2 as seen from ARTEMIS, fluctuations of the The results presented here are based on the data recorded received power at both detectors start to increase with a de- at ARTEMIS. This experiment focuses on communication crease of the average power received until the link is lost. applications; nevertheless it can provide valuable insights for The data is recorded jointly at OPALE’s detector at 8 kHz the atmospheric science community. and LUCE (LUCE’s sampling rate is 1 kHz).