Verification of Ground Station Diversity for Direct Optical TTC-Downlinks from LEO Satellites by means of an Experimental Laser Source

Abstract

Most of today's satellites make use of microwave data-links for their TTC-downlinks. However, these data-links are limited in terms of achievable data-rate and spectrum availability. Free-space optical communication links might play an important role in the future, as they outperform microwave links in many relevant parameters, namely terminal-size and -mass, power consumption, and data-rate. A solution taking advantage of these attributes might be the usage of direct optical downlinks from a LEO satellite to an Optical Ground Station (OGS). The challenge of limited availability due to cloud coverage can be overcome by using Ground Station Diversity with several Optical Ground Stations placed at suitable locations around the globe. With this approach, high link availabilities can be achieved. The Optical Communication Group of DLR's Institute of Communications and Navigation is currently developing an experimental laser source for compact LEO satellites. The pointing will be accomplished by utilizing the attitude control system of the satellite bus, aligning the source towards an Optical Ground Station during a satellite pass. As no Coarse Pointing Assembly (CPA) is necessary, a system with a low mass (<1kg) and volume can be built. Thus, this concept might be a cost-effective solution for small and compact satellites with the need for a downlink with a high data-rate. At first, this paper will describe the experimental payload, a directly modulated laser diode with a relatively low data-rate, which is intended to be used on a compact-satellite. The feasibility of the CPA-less pointing concept will be shown. Data-rates 1Gbit/s are achievable with this approach and relatively small OGS-apertures 60cm, depending on the attitude accuracy of the satellite bus. Then we will give an overview about the Ground Station Diversity concept. Based on cloud coverage data, it will be pointed out that the concept results in link availabilities close to 100%. A Transportable Optical Ground Station (TOGS) with an aperture of 60cm, which is currently under development, will be also presented. Due to its low mass and easy transportability it can be used to carry out a global validation campaign.