Optical Data Downlinks from Earth Observation Platforms

The increasing resolution of earth observation sensors will require much higher data rates for the data downlink in future than is feasible with conventional RF-technology. This applies for earth observation satellites as well as for aeronautic observation platforms, such as aircraft or stratospheric high altitude platforms. The most promising solution for this data downlink bottleneck is the application of optical free space transmission technologies. DLR has built diverse atmospheric flight terminals and performed several trials of optical downlinks from space (together with partnering organizations) as well as from atmospheric carriers in recent years. Here we present and compare results of such communication system trials

Wide-Field-of-Regard Pointing, Acquisition and Tracking-System for small Laser Communication Terminals

Pointing, Acquisition and Tracking (PAT)-systems are one of the central parts of a Laser Communication Terminal. In Laser Communication Terminals a staggered PAT-system consisting of Coarse Pointing Assembly (CPA) and Fine Pointing Assembly (FPA) is common. The CPA provides a Wide-Field-of-Regard (FoR) and the FPA a precise and fast alignment of the beam to the communication partner. The disadvantage is the mechanical complexity. This paper presents the development challenges of a Wide-Field-of-Regard PAT-system for small Laser Communication Terminals by combining and miniaturizing the functionality of the CPA and FPA to a single actuator. To get a small and lightweight Wide-Field-of-Regard PAT-system, the number of moving parts has to be minimized. Therefore, the system consists of an optical setup that provides a large Field-of-View (FoV) and one single mirror actuator for beam steering – thus, CPA and FPA can be combined in just one moving component. The actuator is controlled by a Digital Signal Processor (DSP). The tracking sensor is a 4-Quadrants-Detector (4QD). During the acquisition mode, the actuator moves in a predefined scan pattern until a signal can be detected on the 4QD. Consequently, the system changes to tracking mode. Our design approach succeeded in a FoR of more than 45° without a traditional CPA. The scan time for the complete FoR is less than 1 second. The block diagram and measurements taken with the prototype in the laboratory will be presented in this paper

Optical Satellite Downlinks to Optical Ground Stations and High-Altitude Platforms

Earth-observation (EO) satellite missions using highresolution optical or radar sensors are producing an immense amount of data which needs to be send down to earth. The fraction of satellite operational time in future missions is therefore clearly limited by the downlink-capability. The current X-Band architecture is facing its technological limitations in terms of data rate while causing increased demand on antenna sizes and transmit power. This bottleneck can be overcome by direct optical downlinks from EO-satellites to the ground with multi-gigabit data rates. According optical satellite terminals will be extremely small and light-weight and will require few transmit power, but one drawback is the link blockage by thick clouds. This can be overcome either by ground station diversity and careful site selection or by using optical terminals onboard high altitude platforms which serve as relays-stations for the satellite. Here we present feasibility and expected performance of these two optical scenarios and propose according space and ground station architectures

Numerical simulations of beam propagation through optical turbulence for high-altitude platform crosslinks

In this paper we introduce a simulation method for modelling clear-air atmospheric turbulence effects for long horizontal stratospheric paths. Divergence angles of several hundred microradians in combination with link distances up to 800 km require to adapt the appropriate resolution of the transverse optical field along the path. For this purpose, we implemented a propagation method in Cartesian coordinates. We choose two reference scenarios for high-altitude platform crosslinks and discuss the influence of simulation parameters to the derived results. Finally a method for computation of temporal IM/DD-time signals form simulated intensity matrices is presented

Optical Satellite Downlinks to Optical Ground Stations and High-Altitude Platforms

We have calculated the practical advantage of optical downlinks from earth observation (EO) satellites over conventional RF-downlinks in different downlink scenarios, taking into account blockage by cloud cover. The usability of the EO-sensor could be boosted by nearly a factor of forty with a future StORe-System (HAP-Relays) or by a factor four with simple direct downlinks to optical ground stations without RF-backup (and an OGS in a non-optimum location). This performance is offered by low-power transmit terminals with very small apertures in the range of few centimeters and according low mass. It seems favorable to establish a global OGS-network (and later a StORe-Network) based on compatible technology for downlinks from LEO as well as GEO and later possibly for links from deep-space probes

Aircraft to Ground Unidirectional Laser-Comm. Terminal for High Resolution Sensors

Real-time monitoring allows new possibilities in applications like disaster management or traffic observation and guidance. The German Aerospace Center is currently developing an aircraft based observation system. Among other sensors a high resolution camera platform together with an optical downlink terminal is an integral part of the system. The optical terminal was tested in the first stage of expansion in November and December 2008. At distances up to 85 km the achieved mean tracking offset with pure CPA tracking was 266 μrad. Initial communication tests have been successfully performed up to a distance of 40 km

HALE Platforms - A Feasibility Study

The report describes results of a feasibility study concerning solar powered High Altitude Long Endurance (HALE) Unmanned Aerial Vehicles (UAV). The high altitude concept was examined in order to allow the vehicles to operate above commercial air traffic.  The study also produced possible solutions for different flight altitudes and payload requirements.  It has been prepared in a cooperation of several DLR institutes, reflecting their specific know-how.  A range of feasible solutions for fixed-wing aircraft as well as airships under different requirements (altitude, payload, velocity) have been examined. Conceptual design and analysis tools were applied to size plaforms

Measurements of the Beam-Wave Fluctuations over a 142-km Atmospheric Path

An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESA's OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. Wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during the entire campaign with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (10-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory

Results of the Optical Downlink Experiment KIODO from OICETS Satellite to Optical Ground Station Oberpfaffenhofen (OGS-OP)

Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials