HAP-LEO Link Communication Systems Based on Optical Technology

Abstract

One among the most recent configurations in satellite architectures, is enhanced with respect to the traditional ones, by the presence of HAPs (High Altitude Platforms). Those balloons are located at stratospheric altitudes, and make possible to separate the link from LEO to ground into two segments: the former crosses the highest levels of atmosphere and can be developed by optical technology, the latter is more sensitive to scattering and absorption, since it crosses lower atmosphere, so it must be carried out in RF domain where more consolidated technology can be used. The insertion of HAPs in the global architecture enhances the system performance in terms of LEO-Ground link capacity, connectivity and flexibility; in addition the wide band-width offered by optics makes more and more effective those improvements. Even better results can be obtained if a proper coding system is designed. 1 Scenario Description For a HAP-LEO link an optical communication system based on 1550 nm technology is proposed. This wavelength ensures a wide capacity and the consolidated technology used in terrestrial links. In addition EDFA (Erbium Doped Amplifiers) optical amplifiers work in that band, offering some important improvements in terms of system performance. The optical communication subsystem should be designed starting from the knowledge of the operative scenario. In the case we consider, data stream is mainly transmitted from LEO to HAP: LEO can collect information during its orbit, for example from high resolution Earth Observation imageries. Instead of transferring data to a Ground Station with a RF link in the visibility window, it is possible to increase the downloading effectiveness by locating a HAP just above the destination Ground Station [1,3,4]. As a matter of fact, the HAP location at 20 km allows to avoid the most limitative atmosphere layers making possible to carry out an optical link from LEO to HAP (Fig. 1). With a 10 Gbit/s link, even in a very short time window for download it is possible to transfer up to some Terabit per day. HAP can transmit stored data to Ground Station until the next time LEO becomes visible to HAP and it starts its downloading again. Between two consecutive accesses there is a time delay, that sometimes is some hours long, depending on the HAP latitude. That means HAP has a quite long time to empty its memory, so the link to Ground does not require a wide band [5]. During this silent delay, HAP 728 Satellite Communications and Navigation Systems can transmit data even to another HAP exploiting optical technology, as well. That can improve the downloading speed and the network connectivity.