Over the past decade free-space optical communication has evolved to being deployed in operational space systems such as the European Data Relay Satellite system. Therefore, we expect more satellites to be equipped with laser communication terminals in the years to come. Whereas current applications of the technology focus on inter-satellite transmissions, laser-based space-to-ground connectivity has not gotten beyond its initial stages. This seems to be attributed to a chicken-and-egg problem: space systems engineers shy away from equipping spacecraft with direct-to-Earth optical terminals as an appropriate ground segment is missing; on the other hand, commercial satellite service providers are afraid to invest in an optical ground segment, as there are hardly any potential customers on the horizon. The European and German Space Operations Centers would like to overcome this dilemma. We have, therefore, together with industrial partners initiated a ground segment for space-to-ground optical communication in the first place. With a few upcoming in-orbit demonstrations of optical terminals we nourish hope to push the technology another step forward and to prove its maturity level to the space sector. Here, we give an outline of the developmental efforts at the German side. In particular, we survey several ground locations appropriate to support laser communications and detail the basic design of an optical ground station. Finally, we look ahead at possible future adaptations of this initial arrangement.

The demand for increasing bandwidth for the downlink on small satellites (< 500 kg) is unbroken. Driven by new sensor capabilities for small and cube satellite applications with higher spatial and hyperspectral resolution and a rising demand of the end users for these kind of data.

This development has shifted the bottleneck towards the downlink. Optical downlink technology can solve these issues and can be an alternative to the today usually used radio frequency downlinks.
At first the optical downlink technology can deliver much higher data rates already today and has even more promising prospects with up to several 100Gbps in the next years. Furthermore optical links are, due to the physical properties of a laser beam, not ITU regulated and thus there is no need to apply for frequencies bands. Another important aspect is security. Optical links are mature, interference-free and robust against eavesdropping and in addition the medium for upcoming technologies like quantum key distribution.

Thus the market for optical direct-to-earth laser communication is growing fast. Tesat has a long heritage in 1064 nm wavelength optical communication technology for data relay and long-range applications. Now Tesat expands its laser communication portfolio towards optical direct-to-earth solutions for shorter distances on cube and small satellite platforms. Therefore Tesat has become the industrialization partner of the 1550 nm wavelength Optical Space Infrared Downlink System (OSIRIS) technology, developed at Institute of Communications and Navigation of the German Aerospace Center (DLR). The technology will follow the upcoming CCSDS standardization for optical direct-to-earth links (O3K). Additionally the technology features an optical uplink functionality with up to 1 Mbps for telemetry and telecommand (TMTC) and automatic repeat request (ARQ).

The first outcome of this cooperation is the 100 Mbps laser communication transmitter CubeLCT which is especially designed for operation on cube satellites. With a volume of about 10x10x3 cm³ (about 0,3 cube satellite units), an electrical power of only 8 watt and a mass of about 360 gramm this can be considered as smallest commercial available laser transmitter on the market. While for the CubeL smallest size, weight and power (SWAP) was the objective for the development for the second member of this product family performance was in the focus. TOSIRIS has a channel rate of 10 Gbps from low-earth-orbit to ground (1800km distance), an integrated mass memory because most satellite bus interfaces cannot support the high data rate and a hemispherical coarse point assembly to point independent from the spacecraft orientation towards the ground.

The in-orbit verification of the miniaturized optical downlink technology in the CubeLCT is done on the mission called PIXL (Photo Imager Xross Laser). It is a 3U earth observation CubeSat from Gomspace. The first TOSIRIS mission is scheduled for 2020 onboard of the international space station (ISS).

On the other hand the decision to use optical downlinks is not only influenced by the performance of the space segment, but also by the infrastructure on the ground. So far, optical ground infrastructure has been far behind the development of the space segment. Of course there are a lot of scientific avtivities, e.g. related to the ground station availability taken into account the global cloud coverage, and also a few scientific optical ground stations are available. But for the missions these are difficult to use or to access. Thus, Tesat has started an optical ground network initiative with three main paths. First the development of the OGS600T, which is especially designed for remote operation at a satellite operator site, second the industrialization of the related ground station modem to enable upgrade of existing optical infrastructure, and third the cooperation with one of the leading RF ground network services provider to guarantee easy access for the missions.

The abstract shall give an overview about the status of the first mission using CubeL (PIXL), the development status of the TOSIRIS laser communicaiton terminal and the evolution state of the optical ground network.

New Space technologies being developed for global satellite internet constellations lead to a significantly improved performance / cost ratio also for space laser communications terminals. Tailoring these space lasercom technologies for space-ground-links and combining those with existing adaptive optics ground station (AOGS) technologies could enable a new generation of optical downlink systems. Those would offer highly cost-efficient space laser communications terminals and provide orders of magnitude growth potential on optical downlink data rates for payload telemetry data transmission in Direct-to-Earth (DTE) links. The present paper outlines key characteristics of a new system concept with LEO DTE AOGS and compares its benefits to existing concepts.

System concepts for optical DTE transmission have been discussed earlier, but so far all without using adaptive optics. A common approach is increasing downlink bandwidth and avoiding radio-frequency crowding issues. Often, such concepts rely on RF telemetry transmission to support a high service level and at the same time they augment the overall payload data return volume by exchanging a redundant RF transmitter in space with a light weight, high data rate laser terminal.

Once using adaptive optics on ground, at the same level of optical transmit power in space, a considerable increase on payload data return gets available by the ability to use receivers of higher sensitivity and higher bandwidth. On-board data handling capabilities determine how to best exploit such high rate optical downlinks. One way is to increase the percentage of data transmitted per pass by higher data rates in excess of 10 Gbps. Another exploitation is to significantly decrease the necessary contact time for a transmitted payload data volume, inherently reducing the impact of link interrupts by clouds, as for instance in scattered sky conditions.

An AOGS acts as an optical antenna that provides a data interface in form of a single mode fiber. This brings along some new enabling features, because an optical antenna with adaptive optics is fully transparent to any type of optical receiver and it has no limits on growth in data rate; the maximum optical downlink data rate is then determined by the laser terminal in space. The complementary, fiber-coupled receiver modem on ground can be located remotely, far away from the optical antenna. Such a receiver modem is selected accordingly, to fit with respect to modulation and data rate to the space terminal. A standard optical switch could be implemented, to select between different receive modems that all get their input from the same optical antenna.

Cloud coverage mitigation by site diversity requires multiple AOGS to be installed at globally distributed locations and to be linked with a data distribution network. Similar to the New Space technologies evolution for constellations, aiming at a very high number of units, the topic of cloud mitigation by a site diversity network also leads to demands for a high performance/cost ratio per single AOGS. The added complexity and cost of adaptive optics in this context is more than balanced with the fact that receive modems of higher sensitivities can be used. The other way around, a higher sensitivity allows for reducing the optical antenna diameter, leading to smaller motors and shelter dimensions, which again leads in a non-linear progressive way to cost savings.

This paper describes key performance characteristics of existing AOGS technology that is already in operation since more than 5 years. Recent developments and on-going additional AOGS implementations are discussed that depict optical antennae for space-ground links from near-Earth.

For a given LEO altitude and selected zenith angles, the effective receive power at single mode fiber output is calculated from received irradiance at telescope entrance. Phase screen propagation analysis is applied to model atmospheric channel impact. Receive power distribution statistics are derived. For identical radiant intensity of a space laser terminal, a comparative optical downlink analysis shows possible performance increases, once using different receiver technologies on ground.

The paper concludes with a summary, describing the overall downlink system concept for a small and a large optical antenna, both equipped with adaptive optics. For both types, state-of-the-art hardware implementations are shown, together with corresponding key performance characteristics.