System Requirements for Optical HAP-Satellite Links N. Perlot * , E. Duca ** , J. Horwath * , D. Giggenbach * and E. Leitgeb *** * Institute of Communications and Navigation, DLR, Weßling, Germany ** Department of Electronic Engineering, University of Rome "Tor Vergata", Italy *** Institute of Broadband Communications, Graz University of Technology, Graz, Austria nicolas.perlot@dlr.de , elisa.duca@uniroma2.it , joachim.horwath@dlr.de , dirk.giggenbach@dlr.de , erich.leitgeb@tugraz.at Abstract—Between a high-altitude platform (HAP) in the stratosphere and a satellite, high-data-rate optical links are conceivable but the challenges are still not clearly identified. We thus review the main requirements to properly establish such links. The design of a HAP-LEO link is in several aspects different from that of a HAP-GEO link. Among others, we consider the possible limitations caused by atmospheric propagation, the Sun and the environment of the HAP terminal. Signal transmission appears markedly more difficult when the optical beam must propagate near the HAP horizon or below. I. II. A. INTRODUCTION The high-altitude platform (HAP) is likely to be a key element in the development of free-space optics (FSO) technologies. FSO generally suffers from important drawbacks in the atmospheric environment. A HAP is located above the clouds where the air density is low; it is quasi-geostationary and may come down to the ground for maintenance. The clear sight of a HAP toward space is opportune for optical links with satellites. Various communication scenarios can be considered. Large amount of data are generated on many LEO satellites and must be brought down to the Earth surface. The use of HAP as Earth-observation data relay has been investigated recently by several authors [1]-[4]. Data can also be generated by sensors onboard a HAP or may result from the reception by the HAP of several RF signals from the ground. In this context, an interesting scenario is the optical data transfer from one HAP to another over a relay GEO-satellite. The geo-stationarity of all terminals provides advantages: quasi-unlimited communication time, simplified tracking, constant received power and frequency. The possibility of implementing such a long link (80 000 km) without any electrical conversion on the GEO satellite is investigated in [5]. Although two HAPs can be separated by hundreds of kilometres and still have a permanent line of sight, inter-HAP links are not directly addressed in this paper. The feasibility of optical HAP links has been claimed many times. However the actual complexity of the implementation of reliable systems is not well known. This paper aims at identifying critical points specific to optical HAP-satellite links. We first review the basic characteristics of optical space communications. In Sect. III, the geometry of HAP links is detailed, as well as the importance of low-elevation links. Sect. IV deals with atmospheric-propagation effects. Sect. V examines the conditions under which the Sun can hinder a communication. The next sections are dedicated to requirements on spatial acquisition and on HAP terminals. The paper concludes with Sect. IX. BASIC OPTICAL SPACE SYSTEM Link Parameters For long-distance links, the optical beam should approach the ideal TEM 00 Gaussian beam. The geometrical parameters of a link are represented in Fig. 1. "Tx" and "Rx" denote transmitter and receiver. θ div is the 1/e 2 half beam divergence, L the propagation distance, and D the receiver diameter. With P Tx and P Rx respectively the transmitted and received optical powers, the basic link equation is Rx Tx Tx R P P x = Ω  . (1) Tx  and Rx Ω are respectively the directivities of the transmitter and receiver with respect to isotropic antennas. For a transmitted Gaussian beam we have 2 8 Tx div θ =  . (2) Note that the gain of a Gaussian beam is a factor 2 higher than that of a flat-top beam with half divergence θ div . Conversely, Rx Ω is given by the effective angular extent of the receiver: 2 4 Rx D L ⎛ ⎞ Ω = ⎜ ⎟ ⎝ ⎠ . (3) Fig. 1 Illustration of the link parameters.