HISTORICAL APPROACHES

Abstract

As will be appreciated from Chaps. 8 to 10, satellites in LEO have relatively limited coverage footprints on the surface of the globe by comparison with their cousins higher altitude orbits. Bearing in mind this footprint limitation, a system designer wishing to achieve a reasonable level of communications performance is automatically driven towards a constellation involving multiple satellites. Traditionally, both satellites and their launch vehicles have been expensive, and this raises obvious questions about the financial wisdom of constructing multi-satellite communications constellations in LEO—would smaller numbers of satellites in higher orbits not represent a more logical investment? And yet the most prolific satellite series in history, the Russian Strela-1 system, is a communications constellation which over its lifetime saw the launch of some 350 or so relatively short-lived satellites. And in the 1990s, Iridium, Globalstar and Orbcomm all invested large sums in the creation of LEO communications systems. The explanation behind the apparent contradiction relates to the user communities that these satellite systems were endeavoring to serve, and the locations of those users on the surface of the Earth. These user communities were either mobile, with small, low-power hand-held receivers, or, (in the case of the Strela-1 system), espionage agents who presumably had no desire to advertise their presence by erecting a satellite dish on the roof! In most cases, such terminals will not be “cooperative,” (in the sense that the user will not necessarily be able to ensure a clear line of sight to the satellite, or use a highly directional antenna to track the satellite as it moves across the sky). In order to establish a satisfactory link budget to such an uncooperative terminal, it is necessary to ensure that the Effective Isotropic Radiated Power (EIRP) from the satellites is sufficient to overcome these limitations. Specifically, the system designer must make certain that the free space path loss, (which is dictated by the range between the transmitter and receiver), does not render the system infeasible. The early Russian Strela-1 satellites were simple, mass produced devices. Approximately spherical, and lacking attitude control, they were equipped with relatively low gain, low frequency antennas, and were launched in batches of 8 into a 1,500 km altitude, high inclination orbit. Lacking a propulsion system, they were deployed at intervals of a few seconds from the Cosmos launch vehicle, thereby gaining slightly different initial orbital parameters which would cause them to drift around their orbit plane relative to one another over time. More than one plane of these satellites was supported, but the lack of a station keeping system meant that they were, for statistical reasons, unable to guarantee uninterrupted coverage. The system was, instead, used to support a store-and-forward communication system for Russian agents worldwide. The Strela-1 constellation was eventually superseded by a more sophisticated system called Strela-2, (later marketed commercially under the name Gonets in the West). This constellation was composed of larger gravity-gradient stabilised satellites which could perform real-time communication, if both user and receiver were within the coverage footprint of the satellite, but could also relay data in a store and forward fashion if this were not the case. Since they were gravity stabilized, the satellites could exploit higher-gain, directional antennas, operating at higher frequencies than the Strela-1 system, and hence offering higher data rates. Like its predecessor, the Strela-2 system operated in high inclination orbits, also approaching an altitude of 1,500 km. The choice of orbital altitude may have been dictated in part by the desire to keep the satellites below the worst effects of the Van Allen radiation belts, although, (since all Russian satellites during this era were pressurized designs), their electronics would have received a degree of shielding from the pressure vessel in which they were housed. However, the Van Allen radiation belts certainly represent a constraint on the orbital options open to the LEO communications system designer if a reasonable design lifetime is to be achieved. It is tempting to treat orbital altitude as a completely free parameter along with the other orbital parameters such as inclination and right ascension, but in practice, the radiation doses that a satellite receives from protons trapped in the Earth’s electromagnetic field at altitudes above 1,500 km will have implications for the relative amount of shielding required by the satellites, or the effective lifetime of the hardware, or both. Due to the availability of lower latitude launch sites, access to GEO was easier for Western nations than it was for Russia. As a result, there was a greater focus on high altitude communications, and significant investment in LEO communications constellations did not take place until the 1990s. The increasing popularity of mobile communications led a number of providers to envisage global, satellite-based systems that would service regions where cellular towers were unavailable. Several concepts were proposed to meet this communications requirement, and three reached the stage of actually launching satellites, Iridium, Globalstar and Orbcomm. These networks took different approaches to 23 Space Logistics and Manufacturing