The continued evolution of communication satellites

Abstract

Communications satellites have evolved at a steady rate since their birth nearly 40 years ago, with the power requirements of geostationary satellites doubling every 5 years or so. By the early-80's this led to the nearly complete replacement of spinning satellites by three-axis stabilized satellites for most applications, as solar array size became so great that designers could not tolerate the inefficiencies of an array that did not track the sun. In the late-90's the quest for power is again forcing architectural changes to the spacecraft platform, as the solar array has become the dominant cost element and the battery has become the dominant mass element of the typical satellite platform. The dominant design constraint is no longer mass, power or propulsion fuel but thermal: how to get rid of the excess power dissipated as heat is now the limiting factor on how much rf energy can be radiated towards the earth. This paper addresses: 1. The reasons behind this growth (cost per bit), 2. How satellite manufacturers are coping with it (the most powerful satellites currently under construction generate 15 kW of power at end-of-life, while Loral is designing a 25 kW spacecraft), 3. The implications on launch vehicles (every major commercial launch vehicle supplier has announced plans for significant growth) and 4. The new communications architectures and services that will be enabled by the increase in power capabilities 5. Although all elements of the spacecraft will be discussed, emphasis is placed on the Power, Thermal and Propulsion Subsystems, because that is where the largest gains are being realized. The introduction of Electric Propulsion for both orbit raising and stationkeeping affects the overall mission design, which will also be discussed. It will be shown that, in order to keep up with the demand for greater capacity, the satellite designer has up to this point in time been able to compound the improvements in many seemingly unrelated areas to give vastly greater communications capability per satellite than the first commercial three-axis stabilized satellites a little over 20 years ago.