65 th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by the International Astronautical Federation. All rights reserved. IAC-14- B2,3,8,x26093 Page 1 of 9 IAC-14-B2,3,8,x26093 IMPLICATIONS OF SKY RADIANCE ON DEEP-SPACE OPTICAL COMMUNICATION LINKS Kevin Shortt German Aerospace Center, Germany, kevin.shortt@dlr.de Dirk Giggenbach German Aerospace Center, Germany, dirk.giggenbach@dlr.de Thomas Dreischer RUAG Schweiz AG, Switzerland, thomas.dreischer@ruag.com Carlos Rivera ISDEFE, Spain, crivera@isdefe.es Robert Daddato, Andrea Di Mira, Igor Zayer European Space Agency, Germany robert.daddato@esa.int, andrea.dimira@eumetsat.int, igor.zayer@esa.int As the number of deep-space missions that are turning to optical communications to support science operations increases, system designers are taking a more in depth look at the link budgets that govern such links. Noise sources, such as the radiance arising from scattering in the Earth’s atmosphere and light reflected from planetary bodies in close visual proximity to spacecraft, become particularly critical given the photon-starved channels normally associated with deep-space links. In the case of the Earth’s atmosphere, sky radiance becomes a significant factor when considering daytime operations especially when operators need to support spacecraft contacts close to the Sun. This paper encapsulates the implications of sky radiance on deep-space optical communication scenarios and provides an overview of the current efforts underway in Europe to further quantify its impact on future mission operations. I. INTRODUCTION Photons received as a result of sky radiance have a major impact on optical communication links to deep- space probes where only a handful of signal photons are received. The problem is further compounded when one wishes to support communication links with a line of sight in close proximity to the Sun. It is important to have a good understanding of the impact of sky radiance in order to i) apply the appropriate margins in the communications link budget; ii) design the ground station terminal accordingly; and iii) derive a feasible concept of operations (ConOps) By examining the sky radiance at any given ground station site, we can optimize the amount of time that site can support a communications link and thus maximize the amount of data return from the spacecraft. In the European context, this research is particularly timely given that the European Space Agency, as well as other European players, have set their sights on a number of deep-space missions in the coming years. These missions would benefit greatly from the advantages that optical communications systems provide in the way of maximizing the data return. In addition, this home grown research will further complement the activities already undertaken by JPL [1] and provide opportunities for future mission cross- support. Much of the theory for today’s sky radiance models was founded in the 1960’s (e.g. [2]), predominantly arising from research in the military. In 1989, Eric P. Shettle wrote a seminal paper [3] weighing the pros and cons of the different aerosol models in existence at that time and highlighted their applicability to different layers of the atmosphere. Today there are a number of software packages available to model sky radiance, such as MODTRAN and libRadTran, but they all trace back to these same origins. [4][5] From a practical standpoint, efforts have been made over the years to either derive simplified databases based on the aforementioned