ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreemenet for building the Optical Ground Station (OGS), in the IAC Teide Observatory, in order to perform In Orbit Testing (IOT) of Optical Data Relay payloads onboard communication satellites, the first being ARTEMIS. During its recent launch, ARTEMIS was put into a degraded orbit due to a malfunction on the launcher's upper stage. ESA rapidly adopted a recovery strategy aimed to take the satellite to its nominal geostationary position. After completion of the first manoeuvres, ARTEMIS was successfully positioned in a circular parking orbit, at about 31,000 kilometers, and turned into full operation. In this orbit, its optical payload has been tested with the OGS, before establishing the link with SPOT IV. New tracking algorithms were developed at OGS control system in order to correct for ARTEMIS new orbit. The OGS has established a bi-directional link to ARTEMIS, behaving, seen from ARTEMIS, as a LEO terminal. Preliminary results are presented on the space-to- ground bi-directional link, including pointing acquisition and tracking (PAT) performance, received beam characterization and BER measurements.

The German Aerospace Center (DLR) is currently performing an Optical Free-space Data Transmission Experiment along with the European Aeronautic Defence and Space Company (EADS), Germany and Contraves Space AG, Switzerland. The scope of this experiment is to verify the tracking capabilities of the OPTEL 02, a space-qualified optical terminal built by Contraves, and to demonstrate optical high data rate transmission through the atmosphere. Two laser diode transmitters at 980 nm, each mounted on a theodolite for static pointing and laterally separated by about 4 meters, were placed on a mountain top in the German Alps at a height of 1620 m. Either pseudo-noise pattern or video data was transmitted. The OPTEL 02, performing acquisiton and tracking, is situated at the DLR site in Oberpfaffenhofen near Munich at 620 m. An APD receiver front-end is connected to the OPTEL 02 in order to receive data up to 270 Mbps. The optical path length between Wallberg and Oberpfaffenhofen is 61 km (about 38 miles). Either bit error rate measurements or video transmission can be performed. This paper presents the experimental setup used, link budget calculations for the particular scenario (including beam divergence, refraction, damping and scintillations), and finally the preliminary results of the experiment.

This paper discusses test methods and results of fSONA Communications Corporation's SONAbeam 155-M, SONAbeam 622-M and SONAbeam155-S systems - free space-optical data communication systems that use 1550 nm lasers. Presented are results of environmental qualification tests and field performance tests over link ranges of 450 meters and 5 kilometers. The SONAbeam 155-M, SONAbeam 622-M and 155-S are three representatives of a family of products that work in the range of 34 - 1250 Mbps. Very robust performance is the emphasis in the design of these systems.

The European Space Agency (ESA) has undertaken the development of Optical Data Relay payloads, aimed at establishing free space optical communication links between satellites. The first of such systems put into orbit is the SILEX project, in which an experimental link between a GEO satellite (ARTEMIS) and a LEO satellite (SPOT IV) will be used to relay earth observation data. In order to perform In Orbit Testing (IOT) of these and future optical communications systems, ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreement for the building of the Optical Ground Station (OGS) in the IAC Teide Observatory, which consists basically of a 1-meter telescope and the suitable instrumentation for establishing and testing bi-directional optical links with satellites. The presence of the atmosphere in the data path posses particular problems, with an impact on the instrumentation design. The transmission, reception and measurement functions, along with the overall control of the instruments, are performed at OGS by the Focal Plane Control Electronics (FPCE). The design and performance of this instrumentation is presented, emphasizing the Pointing, Acquisition and Tracking, the Tuneable Laser and the Master Control.

An optical wireless system useful for short metropolitan distance connections is proposed. Differently from other apparata described in literature or commercially available, our solution is completely transparent to third window telecommunication channel thanks to standard single mode optical fiber interfaces, without any electro-optic conversion. Such a system guarantees the high performances required to optical communication networks (BER, SNR and so on) without link fault and bit broadening penalties. System characterization and BER performances at 10Gbit/s, also in presence of 1.55micrometers multi-wavelength signal, are presented for connections up to 200m building-to-building roof distance. The implemented transmitter and receiver devices are compact in dimensions, very low cost and can find application not only in metropolitan network links in case of digging impediments due to time, license and cost constraint, but also in disaster recovery and local extraordinary high-bandwidth demand.

For free-space propagation, often laser sources with Gaussian intensity profile are utilized. A Cassegrainian telescope's central obstruction can block a significant amount of the transmitted beam energy. The previously known approaches to solve the problem include: use of an axicon optical element and or sub-aperture illumination of the telescope primary mirror. Two new approaches have been identified to eliminate all secondary mirror and baffle vignetting. These approaches include a beam-slicer and a beam-splitter/combiner optics. The new approaches eliminate the precise alignment required for axicon devices and are simple to fabricate. The beam-splitter/prism combiner approach results in a clean far field pattern and has transmission divergence that is limited to the diffraction limit for each of the sub-aperture transmission beams.

New experimental results are presented for the far-infrared p-Ge laser that enhance its prospects for application to secure satellite and short-range terrestrial free-space communications on a THz carrier. An optical means of gain modulation has been discovered that may potentially permit far-IR pulse generation via active mode-locking with low drive power. A compact high-field permanent-magnet assembly is demonstrated for applying the magnetic field required for laser operation without need of liquid helium. Compact light-weight laser-excitation electronics have been designed to run off a low voltage direct current supply.

A new method for improving centroid accuracy, thereby pointing accuracy, is proposed. Accurate centroid estimation is critical for free-space optical communications where the number of photons from the reference optical sources such as stars or an uplink beacon is limited. It is known that the centroid accuracy is proportional to the SNR. Presence of various noise sources during the exposure of CCD can lead to significant degradation of the centroid estimation. The noise sources include CCD read noise, background light, stray light, and CCD processing electronics. One of the most widely used methods to reduce the effects of the noise and background bias is the thresholding method, which subtracts a fixed threshold from the centroid window before centroid computation. The approach presented here, instead, utilizes the spot model to derive the signal boundary that is used to truncate the noise outside the signal boundary. This process effectively reduces both the bias and the noise. The effectiveness of the proposed method is demonstrated through simulations.

A variety of avalanche photodiodes (APD's) were tested with pulse position modulated (PPM) Q-switched laser pulses incident on the detector, with varying amounts of attenuation. The detector output was recorded and post- processed in order to determine the signal and noise slot statistics, as well as, to estimate bit-error-rates (BER). The probability distribution functions predicted by a Webb+Gaussian model were compared to the measured slot statistics, as were theoretical BER curves. Allowing noise equivalent temperature to be a free fitting parameter yielded good fits between measurements and theory. All the measurements used 256-ary PPM and 10 - 25 ns slot widths, with a Q-switched Nd:YVO4 laser modulated at 50 K - 100 K pulses per second. A 3 mm diameter, silicon (Si) APD with 80% quantum efficiency (QE) at 532 nm displayed a sensitivity deteriorated to 18 photons/bit in the presence of 100 photons per 25 ns slot of background light. A 0.8 mm diameter near infrared (NIR) enhanced Si APD with QE of 0.38 displayed sensitivities of 23 - 32 photons/bit for a BER of 10^-2 at 1064 nm in the absence of background light. Backgrounds of 400 photons per 25 ns slot degraded the sensitivity to approximately 58 photons/slot. Finally a 3 mm diameter NIR enhanced Si APD yielded a sensitivity of approximately 100 photons/bit 1064 nm for BER of 10^-2 with no background present.

The fundamental performance limits and channel capacity of optical communications systems operating over the free space channel will be examined using quantum detection theory. The performance of the optimum quantum receiver for on-off keying (OOK) and optical binary phase shift keying (BPSK) is first examined as a pure state (no noise) problem. The classical capacity of the binary symmetric channel for these two modulation schemes is then evaluated for the optimum quantum receiver by making use of the concept of quantum measurement states. The performance of M-ary pulse position modulation, which requires a product state representation, is evaluated along with the performance of certain 'dense signal sets.' Performance comparisons with classical techniques shows over 5 dB improvement in some cases when quantum detection is employed. As a further application of the quantum detection theory, the capacity of the binary channel with on-off keyed modulation and quantum detection is evaluated, and shown to exceed the capacity obtained with classical photon counting.

Free-space quantum key distribution (QKD), more popularly know as quantum cryptography, uses single-photon free-space optical communications to distribute the secret keys required for secure communications. At Los Alamos National Laboratory we have demonstrated a fully automated system that is capable of operations at any time of day over a horizontal range of several kilometers. This has proven the technology is capable of operation from a spacecraft to the ground, opening up the possibility of QKD between any group of users anywhere on Earth. This system, the prototyping of a new system for use on a spacecraft, and the techniques required for world-wide quantum key distribution will be described. The operational parameters and performance of a system designed to operate between low earth orbit (LEO) and the ground will also be discussed.

We investigate methods of coding for a channel subject to a large dead-time constraint, i.e., a constraint on the minimum spacing between transmitted pulses, with the deep-space optical channel as the motivating example. Several constrained codes designed to satisfy the dead-time constraint are considered and compared on the basis of throughput, complexity, and decoded error-rate. The performance of an iteratively decoded serial concatenation of a modulation code with an outer code is evaluated and shown to provide significant gains over Reed-Solomon concatenated with Pulse Position Modulation.

Cloud opacity is one of the main atmospheric physical phenomena that can jeopardize the successful completion of an optical link between a spacecraft and a ground station. Hence, the site location chosen for a telescope used for optical communications must rely on knowledge of weather and cloud cover statistics for the geographical area where the telescope itself is located. In this work, the effects of cloud cover on an optical link are statistically described, considering ten observation sites at locations in the southwestern United States, From California to Texas. The data used for the preparation of this work are surface observation data provided by the National Climatic Data Center (NCDC). NCDC provides hourly information on the cloud coverage of an observation site. Using proper algorithms, these data give a statistical description of link blockage over the ten selected observations sites. Statistics averaged over a number of years for each observation site are presented. Cloud coverage statistics for two and three site diversity are also given for a ground network of optical telescopes. Finally, it is shown quantitatively how the use of two or three telescopes can improve the probability of completion of an optical link and how to select the right locations for a ground network of telescopes in the southwestern United States.

The performance of a free-space optical (FSO) communication system is investigated when communication is established via a short-range, turbulent optical channel. The system under investigation utilizes on-off-keying (OOK) modulation combined with direct-detection to establish a duplex communication link. It is further assumed that the optical beam obeys a Gaussian profile. The received signal is detected using a p-i-n diode which is followed by a trans-impedance amplifier (TIA), limiting amplifier, and a clock/data recovery subsystem. Furthermore, it is assumed that optical front-end provides a relatively large aperture so that the impact of turbulence is somewhat mitigated and that the channel/system parameters result in a weak turbulent condition. The performance of the proposed system for a bit error rate of 10^-9 in the absence of forward error correction (FEC) is assessed in terms of probability of fade (PF), average number of fades per second (FPS), mean fade duration (MFD), mean-guard-to-mean-burst (MGMB) ratio, and mean time between fades (MTBF).

An optical ground-to-ground direct-detection transmission experiment over 61 km is being performed by the German Aerospace Center (DLR) in cooperation with the European Aeronautic Defence and Space Company (EADS) and Contraves Space AG, Switzerland. Transmission direction is from the mountain Wallberg in the German Alps down to Oberpfaffenhofen (west of Munich). This beam path suffers strongly from optical turbulence especially at the near-ground part along the last kilometers before the receiver. This causes a very demanding situation regarding received-power scintillations. Transmit power from one data source is 1W at 980 nm. Of special interest is the effect of secondary transmitter apertures with 4m lateral offset to the first. Under strong turbulence conditions this provides statistically independent speckle patterns at the receiver thus improving system performance dramatically. This paper presents measurements at the transmission channel, with emphasise on statistical parameters of the scintillations and angle-of-arrival variations with one and two transmitter sources.

We studied terminal architectures and configurations for optical cross-links within microsatellite swarms and assessed the applicability of available technologies. Typical applications for microsatellite swarms are phased array telescopes, interferometric missions, and space-based radar. Key drivers for an optical terminal are well-developed technology and ruggedness. The terminal should do without automatic tracking or fine pointing, coarse pointing should be simple. As an example we cover a scenario where four microsatellites form a planar, square formation of 1 km side length, where the data rate is 100 kbit/s, and where an active double-pass lidar between each of the satellites provides a ranging accuracy of better than 10 m. The terminal transmit power is some 160 mW at a wavelength of 980 nm, the receive apertures have a diameter of 5 mm, and the size, weight, and power requirement of one terminal is estimated to be 60X80X70 mm³, 900 g, and 5 W, respectively.

Communication links with multi-giga-bits per sec (Gbps) data-rates depicting both LEO-GEO and GEO-to-Ground optical communications were characterized in the laboratory. A 5.4 Gbps link, with a capability of 7.5 Gbps, was demonstrated in the laboratory. The breadboard utilized a 13 cm diameter telescope as the transmit aperture that simulates the LEO terminal. The receiver is a 30-cm telescope that simulates the GEO terminal. The objective of the laboratory breadboard development is to validate the link analysis and to demonstrate a multi-gigabit link utilizing off-the-shelf or minimally modified commercially available components (optics and opto-electronics) and subsystems. For a bit-error-rate of 1E-7, the measured required received signal is within 1 to 2 dB of that predicted by the link analysis.

We have adopted the phase modulation technique from the digital Syncbit communication system, and developed an analog transmission system which allows to transmit a set of analog channels between standard bent - pipe satellites. This new analog coherent modulation communication system provides excellent receiver sensitivity which in turn allows to keep the optical output power of the transmitter to a minimum. As a result power consumption from the host spacecraft -- a valuable resource -- can be kept to a minimum. The analog communication system design is such, that it uses all the electro-optical building blocks of the digital communication system. Transmitter laser, modulator, booster amplifier and optical receiver frontend are nearly identical for the analog and the digital terminals. The subsystem is contained in two standard electronics module frames and fits into the modular electronics unit of the terminal.

Pulse position modulation (PPM) provides a means of using high peak power lasers for transmitting communications signals from planetary spacecraft to earth-based receiving stations. Large aperture (approximately 10 m diameter) telescopes will be used to collect and focus the laser communications signal originating from a deep space transmitter on to a PPM receiver. Large area (1 - 3 mm diameter) sensitive detectors, preceded by appropriate narrow (0.1 - 0.2 nm) optical band-pass filters and followed by low-noise, high-gain, amplifiers will serve as the PPM receiver front end. A digital assembly will form the backbone of the receiver. The PPM receiver will achieve and maintain slot synchronization based on sub slot sums generated by a field programmable-gated array (FPGA). Spacecraft dynamics and timing issues between the ground- based receiver and the transmitter on board the spacecraft must be taken into account. In the present report, requirements and design of a prototype PPM receiver being developed over the next year will be elaborated. The design is driven by the need to demonstrate and validate PPM reception using a variety of detectors under simulated conditions representative of those to be encountered in a deep space optical communications link.

Free-space optical links for high-speed network communications between buildings must consider the detrimental environmental effects of terminal base disturbances. Terminal base motion results in tracking and pointing losses, which cause link outages if the base motion is sufficiently large (beyond terminal field of regard) or fast (too fast for the tracking system to reject). Thus it is important to characterize this environmental effect for design and test of optical terminals. Base motion is highly dependent on the installation environment of a specific link making general statements difficult. We have characterized terminal base disturbance levels through a combination of vibration measurements in numerous buildings, data gathered from operating links, and review of building stiffness and wind statistics in various cities. This paper presents a summary of our results.

To meet the micro-radian pointing accuracy required for deep space optical downlink telemetry, an algorithm has been proposed which uses the Earth as a beacon for locating the ground-based terminal. To enhance the resolution of the Earth beacon, a sequence of images is captured as the Earth image is moved across a CCD array in sub-pixel increments. Using this high resolution data, the algorithm then locates the ground-based terminal. The algorithm relies on two parameters for its solution; this paper develops techniques for estimating these parameters. To improve accuracy, a low pass filter is used in a pre-processing step to reduce the thermal noise introduced by the CCD array. The optimal cutoff frequency of the low pass filter is estimated using cross validation. The other parameter to be determined is a threshold used to extricate the Earth image from the background of the image. For this, a Monte Carlo simulation is used to determine the threshold that minimizes the mean squared error between the estimate and true locations of the ground-based terminal. Simulations indicate that the parameters can be determined very accurately using these methods. Results are presented.

The average bit error rate (BER) of optical communication systems is considered in the presence of random angular pointing jitter. The received power and the BER in the absence of jitter are reviewed and then the average BER is obtained in the presence of circularly symmetric, normally distributed jitter by using the probability density function of the optical signal. By minimizing the power penalty for average BER, the optimum ratio of the divergence angle of the laser beam to the random angular jitter at the desired BER is obtained. The derived approximation is used to determine the beam divergence angle easily against the random pointing jitter. As an example, the optimum link budget for both optical tracking and communication channels is designed in the presence of fluctuated optical signal due to random pointing jitter.

Meteorological visibility data are the most commonly used data to estimate terrestrial Free Space Optics (FSO) availability in a given city. Visibility data can be used to estimate transmission efficiency at desired IR wavelengths using a semi-empirical equation, and are often archived over many years allowing the calculation of long-term averages of availability. However, these data are taken at near-surface levels (historically within a few meters of the surface) and are therefore only appropriate for estimating FSO availability near the surface. Examination of long term cloud observations, including percent frequency of cloud ceilings occurring at various heights above the ground, show the importance of including low clouds into the consideration of FSO availability for any situation above about 30-m above ground level (AGL). In most locations, low clouds occurring very near the surface are relatively common -- more so than surface-based fog (which is measured in terms of visibility). Thus, FSO availability will decrease with height, sometimes dramatically, in most cities. Cloud data is also archived over long periods of record and can thus be used to calculate long-term averages of availability.

One important challenge to implementation of efficient free- space optical (FSO) systems is optical signal scintillation and fade caused by atmospheric turbulence and optical aberration in output beam shaping devices and windows. A new method for mitigation of these harmful effects to delivery of optical radiation to remote subscriber terminals, such as aberration and refraction index non-uniformity in a free- space path, has been developed and tested in field experiments. A known approach to damping optical signal scintillation caused by turbulence in a free-space path was based on forming several substantially parallel optical beams modulated by the same transmit signal and overlapping such beams on a receive optical aperture. The beams transmitted through different free-space paths with uncorrelated optical inhomogenity have different, uncorrelated, transverse distribution of light intensity. Their overlapping provides for averaging out the light intensity non-uniformity and efficient suppression of the signal scintillation. The existing approach to mitigation of optical aberration in atmosphere requires targeting several beam shaping telescopes at a subscriber. This is not always practical. For example, in point-multipoint FSO systems servicing multiple subscribers it is advisable to allocate one telescope per subscriber to achieve highest compactness and cost effectiveness of a system. Also the existing method has limitations in solving a problem of window glass optical inhomogenity and aberration in the telescope itself. A new method for optical aberration mitigation is based on using an extended light source with sufficiently large emitting surface and properly selected width of output radiation angular spectrum coupled to the telescope targeted at a subscriber terminal. The method has been implemented in a point-multipoint base terminal having multiple output beams that could be independently targeted at different subscriber terminals. Results of the trial are presented in this paper. The extended source with given light emitting surface diameter d and angular spectrum width (Theta) may be implemented with an optical fiber having core diameter d and numerical aperture NA equals sin((Theta) /2) installed in optical path between a light source with compact light emitting surface, such as a semiconductor laser, and the telescope. Exit end of such fiber coupled to the telescope acts as an extended light source with angular size (alpha) determined by the fiber core diameter and a focal length of the telescope via a formula (alpha) equals d/f. It has been proven in our field experiments, that by using the source with properly selected angular size and angular spectrum width the following results may be achieved with single telescope targeted at a subscriber terminal: (a) damping of optical signal scintillation at a remote photo-detector (the signal standard deviation has been decreased by several times for wide range of scintillation indexes); and (b) elimination of the signal fade caused by aberration in the telescope and output window (in our experiments the extended source provided 5 to 30 times increase in average signal power at the photodetector for a variety of window glass samples used in residential construction).

With recent advances and interest in Free-Space Optics (FSO) for commercial deployments, more attention has been placed on FSO weather effects and the availability of global weather databases. The Meteorological Visual Range (Visibility) is considered one of the main weather parameters necessary to estimate FSO attenuation due to haze, fog and low clouds. Proper understanding of visibility measurements conducted throughout the years is essential. Unfortunately, such information is missing from most of the databases, leaving FSO players no choice but to use the standard visibility equation based on 2% contrast and other assumptions on the source luminance and its background. Another challenge is that visibility is measured using the visual wavelength of 550 nm. Extrapolating the measured attenuations to longer infrared wavelengths is not trivial and involves extensive experimentations. Scattering of electromagnetic waves by spherical droplets of different sizes is considered to simulate FSO scattering effects. This paper serves as an introduction to a series of publications regarding simulation of FSO atmospheric propagation. This first part focuses on attenuation due to rainfall. Additional weather parameters, such as rainfall rate, temperature and relative humidity are considered to effectively build the rain model. Comparison with already published experimental measurement is performed to validate the model. The scattering cross section due to rain is derived from the density of different raindrop sizes and the raindrops fall velocity is derived from the overall rainfall rate. Absorption due the presence of water vapor is computed using the temperature and relative humidity measurements.

The link availability of a free space communication system is affected by weather conditions such as rain, snow, and most notably fog. For carrier class services, the link availability of 99.999% is highly desired. In this paper, we will report the evaluation of atmospheric attenuation variation depending on the weather condition in metropolitan Tokyo area to obtain some network design parameters for desired link availability. For the evaluation we constructed an experimental communication network with a free space communication system. The network consists of nine free space communication devices installed at locations deferent in height. The link distance between these devices are not identical. We will show that we can estimate the attenuation of atmosphere of local links by using those communication devices. We also installed two present weather detectors (PWD) to measure attenuation in terms of visibility at two locations we installed free space communication devices. These PWD units can detect the amount and type of a precipitation allowing us to compare the attenuation caused by various weather factors. We have observed that attenuation value is considerably high starting from 150m (mean sea level) resulting in higher link outage rate compare to other low-height links.

In this paper, image transmission using a mid-wave IR (MWIR) optical transceiver based free-space data link under low visibility conditions is presented. The all-solid-state MWIR transceiver primarily consisted of a passively Q-switched, short-pulsed Nd:YAG laser pumping a periodically poled lithium niobate (PPLN) based optical parametric oscillator and a Dember effect detector. The MILES transceiver generates pulse position waveforms. The optical data link consisting of transmitter drive electronics, pulse conditioning electronics and a computer generating pulses compatible with the 2400-baud rate RS232 receiver was utilized. Data formatting and RS232 transmission and reception were achieved using a computer. Data formatting transformed an arbitrary image file format compatible with the basic operation of pump laser. Images were transmitted at a date rate of 2400 kbits/sec with 16 bits/pixel. Test images consisting of 50X40 pixels and 100X80 pixels were transmitted through free-space filled with light fog up to 120 ft. Besides optical parametric oscillators, the proposed concept can be extended to optical parametric amplifiers, Raman lasers and other nonlinear optical devices to achieve multi-functionality.

The availability of free-space laser communications systems critically depends upon estimating the accuracy of atmospheric attenuation in different weather conditions. Fog and extreme rain are the primary types of weather that can affect shorter wavelength laser communication links. Traditionally, designers and users of free-space laser communication equipment assumed that light suffers from less atmospheric attenuation at longer infrared wavelengths such as 1.55 and 3.5 microns compare to 0.85 microns. Thought, as some experimental studies show, it is not safe to assume that the performance of detection systems in IR is always better in fogs than at visual wavelengths. In dense stages in long persisting fogs the attenuation at longer wavelengths often becomes large, or even larger, than that in the visible. To investigate performance of laser systems in the dense fog, we ran extensive MODTRAN calculations of atmospheric transmission for the different types of dense fog at visible-IR range of wavelengths. Our results show that an atmospheric attenuation in such fogs slightly increases with wavelengths in the range 0.5 - 4 micrometers and then decreases with the longer wavelengths. This is in contrast with widely used parameterization that states decreasing of the fog attenuation beyond visible. To investigate this further, we used concentrations and size distributions data collected in dense persisting fogs at different altitudes, and performed Mie scattering calculations. We determined fog extinction in a range of altitudes (10 - 100m) and at four wavelengths used in free-space communication: 0.85, 1.55, 3.5, and 10.6 microns. We found that atmospheric attenuation in dense fog sharply increases with altitude, slightly increases at 3.5 microns compared to 0.85 microns and then decreases at 10.6 microns.

The Semi conductor Inter satellite Link EXperiment, SILEX, consists of two terminals, one terminal embarked on the French LEO observation satellite SPOT4 and one terminal embarked on ESA's GEO telecommunication satellite ARTEMIS. The objective of SILEX is to perform optical communication experiments in orbit and on an operational basis transmit SPOT4 Earth observation data to ARTEMIS, which will relay the data to ground via its Ka band feeder link. SPOT4 was successfully launched on 22nd March 1998. The ARTEMIS launch on 12th July 2001 left ARTEMIS in an orbit with too low apogee, necessitating orbit raising to a circular parking orbit, altitude 31000 km, using a large fraction of the chemical propellant on board. The remaining 5000 km to GEO stationary orbit will be achieved using the low thrust innovative electric propulsion system necessitating specific attitude control software. The final orbit raising will last about 6 months and the expected lifetime of ARTEMIS after station acquisition is 5 years. While waiting for the establishment of the new attitude control software and the beginning of the final orbit raising maneuvers a test program has been undertaken to characterize the performances of the SILEX system. Testing was performed every fifth day when ARTEMIS was visible over Europe. The test program involves Optical Ground Station acquisition and tracking, inter-satellite link acquisition and tracking, bit error rate measurements and transmission of Earth observation data. The paper reports on results of the in orbit testing, giving comparisons with predictions. The conclusion of the test program is that the SILEX system has excellent performances qualifying the system for operational use by SPOTIMAGE in parallel with a detailed technological experimentation program involving the two SILEX terminals, ESA's optical ground station on Tenerife, and also NASDA's OICETS, once ARTEMIS has acquired its final orbital position.