Introduction

The structure is the skeleton of a satellite. The primary design criterion for a satellite structure is that it should be rigid enough to survive the launch ascent phase while being as light and compact as possible. Low weight usually means lower launch cost, and small volume is necessary if the satellite is to fit inside the confines of the launcher fairing. Perhaps the day will come when satellites are assembled not on the ground but in orbiting space stations, in which case this fundamental design criterion will become far less onerous.

A secondary criterion is that flimsy structures such as solar panels and antennae should suffer a minimum of deformation under the influence of dynamic forces and thermal stresses in geostationary orbit. Panels have a tendency to twist during attitude manoeuvres and could actually counteract the intended movements. Parabolic antenna reflectors may change geometry due to thermal stresses as the solar incidence angle varies; the result could be a loss in antenna gain due to defocussing.

The structure must also be sufficiently stiff to prevent permanent misalignment of highly directive equipment such as antennae, thrusters and attitude sensors. In this chapter we will study the architecture of a typical geostationary satellite structure, make an inventory of materials used, follow the design logic, and discuss a mathematical modelling method.

Structure Architecture

A fairly typical geostationary satellite structure is shown in Fig. 5.1. It is made up of a primary structure, a secondary structure and various appendages.

Introduction

A geostationary applications satellite must be orientated in space such that its antennae or radiometers view the earth continuously. The solar arrays should also face the general direction of the sun at all times. Given that the satellite-earth and the satellite-sun vectors move 360° relative to each other every day, the satellite has to be something of a contortionist to satisfy both pointing conditions.

The obvious design solution to the variable two-way pointing problem is to mount the antennae or telescopes on one part of the spacecraft and the solar arrays on another, allowing the two parts to rotate in opposite directions around a common shaft. Two-way pointing can be maintained as long as the orientation or attitude of the shaft remains approximately perpendicular to the earth and sun vectors. Hence the dual spin and the three-axis stabilized design concepts (Figs 10.1–10.4).

The attitude of a spacecraft is the orientation of its body axes in inertial space. The angles which define an attitude may be direction cosines or azimuth and elevation (Fig. 10.5); the latter convention was used in Chapter 2 to derive an expression for sun angle.

Although space is virtually void of matter, it is nevertheless full of forces acting on the spacecraft (see Chapter 4), and some of these cause the attitude to drift. If the two-way pointing requirement is to be met at all times, it is necessary to ensure attitude stabilization continuously, and perform measurement and control at regular intervals.

Twenty-two thousand miles above the equator, a very special family of man-made satelllites circles the earth. Basking in the sunshine, their wings dark blue and their bodies golden, they look like parrots perched side by side on an endless telephone wire. Most strain their ears to pick up messages from one part of the world and relay them to another. Some spend all their time observing the evolution of weather patterns in the atmosphere below. A few size up the earth to the nearest inch, while others perform scientific experiments. All of the satellites are hypochondriacal chatterboxes who mix tales about what they have just seen, heard or felt with frequent reports about their precarious health.

This is the family of geostationary satellites, so named because to an observer on the earth they appear to be fixed at one point in the sky. In fact they are not fixed at all but travel around the earth at the same rate as the earth turns about its axis. Unlike spacecraft in any other orbit, a geostationary satellite remains constantly within view of almost half the earth at all times, which is why it is so eminently suited for telecommunications and earth observation.

The spacecraft literature abounds with titles on payloads, such as telecommunication transponders, radiometers and scientific instruments. The rest of the spacecraft, called the platform, is usually only presented in outline, and the presentation of launch vehicles, orbits and programmatic issues is often schematic.

Introduction

From launch onwards, the quality of life of a satellite is abysmal. During the ascent phase it is subjected to violent acceleration, vibration, shock and decompression which stretch its endurance to the limit - and that is only the beginning of a satellite's troubles.

On earth, vacuum is employed to extend the storage life of foodstuffs. Out in space, vacuum has the opposite effect on satellites, for it shortens their lifespan. In the absence of an atmosphere, they are bombarded with charged particles and exposed to ultraviolet radiation. Different parts of a satellite reach temperature extremes at the same time and, because there is no temperature exchange through convection, such extremes cause structural stress leading to possible malfunction. The particle bombardment gives rise to electrostatic discharge which produces short or open circuits and burns out electronic components. Lubricants evaporate in vacuum and cause moving parts to seize up. Paints and sealants “outgas” (perspire) and settle on sensitive optical surfaces. Micrometeorites travel unimpeded through space and strike satellites with tremendous impact.

Fortunately, a satellite's environment is largely predictable. Much of the time and money spent on building a spacecraft goes on verifying its resilience against a known environment through elaborate quality control and testing. In this chapter we shall explore the environment surrounding a geostationary satellite during all its phases of flight.

Powered Flight Loads

During the ascent phase, a satellite is subject to compression forces due to quasi-static acceleration.

Introduction

Spacecraft controllers on the ground rely on telemetry to monitor the configuration and the health of a satellite. Telecommands provide the means to reconfigure, reorient and reposition the satellite by remote control. Tracking is a method of obtaining an update of the satellite's orbital elements.

The permanent loss of TT&C capability is the beginning of the end for a satellite. It will perhaps provide adequate service for hours, days or even weeks, but eventually it will drift away from its orbital location and nominal attitude. In the end it may degrade gracefully or suffer instant catastrophic failure as batteries discharge, propellants freeze, or electrical faults proliferate in the absence of corrective action from ground controllers.

Temporary loss of telemetry, telecommand and tracking is quite commonplace. Fortunately, the loss (called an “outage”) is seldom the result of a satellite anomaly, but is usually triggered by a failure of the control centre computer, the ground station equipment or the data link connecting the two. Even so, spacecraft controllers are invariably seized by a feeling of impending doom when outages occur, and they act swiftly to restore contact before the satellite has come to harm.

Subsystem Architecture

Figure 11.1 shows the layout of a typical TT&C subsystem. Telecommands from the ground are picked up by a receiver. After demodulation they are passed on to a telecommand decoder. The decoder verifies the validity of commands, interprets their contents, and issues execution signals to the appropriate subsystem.

Introduction

In Chapter 5–11 we covered the classical spacecraft platform subsystems. The present chapter introduces the payload section of this book. The payloads of interest in geostationary satellite applications perform the functions of communication (this Chapter) and meteorological imagery (Chapter 13).

All geostationary applications satellites communicate information, be it language, pictures or abstract numbers. When we talk about communications satellites, however, we limit the definition to spacecraft which relay voice, data, telex, facsimile, or television pictures from one part of the earth to another, as opposed to telemetry and imagery which originate in the satellites themselves.

Satellite communications is a new, vast and fascinating engineering science. Along with orbital mechanics and attitude and orbit control, it is also one of the most “difficult” spacecraft subjects to understand, mainly because the student is faced with a wealth of mathematical abstractions from the outset. In the present chapter we will provide an outline of basic communications theory and how it influences the architecture of a communications payload. The interested reader is advised to consult the books listed in the Bibliography for a more indepth coverage of the subject. The list is far from exhaustive, and suggestions for further reading are made in the Reference sections of those books.

Transmission Capacity Versus Power and Bandwidth

We will begin this chapter by examining how various transmission techniques allow a designer to optimize the communications capacity of a satellite by trading off two precious commodities, namely spacecraft power and frequency bandwidth.

Introduction

Atmospheric pressure, temperature, humidity, wind speed, and sea surface temperature are fundamental parameters for determining the weather. If it were possible to measure these five parameters simultaneously across the globe at regular time, distance and height intervals, the problems of medium- and long-range weather forecasting would be greatly reduced.

Before the advent of satellites, global weather measurements on such a scale would have been totally impracticable. Systematic meteorological observations used to be extremely sparse. Although most industrialized countries maintained a network of ground-level observation stations, vertical sounding was limited to launching occasional balloons from land and sea, and receiving sporadic reports from commercial aircraft. These horizontal and vertical measurements covered only a small portion of the earth, forcing meteorologists to bridge the gap with educated guesses.

In the late 1940s, sounding rockets equipped with cameras took pictures of the earth from altitudes of 100 km and more. These early photographs revealed a whole new family of physical relationships in the atmosphere. In the years that followed the launch of Sputnik in 1957, much effort was devoted to developing television cameras and radiometers suitable for satellite meteorology. Throughout the 1960s, the United States and the Soviet Union deployed a host of increasingly powerful weather satellites which transmitted visible and infrared images to ground stations on the earth. The first steps towards making global weather observations had been taken.

Low-orbiting Satellites

The early meteorological satellites were launched into low earth orbit by necessity, since rockets in those days had limited lifting capacity.

The limited extent of the stream of air in a wind tunnel, whether of open or of closed working section, imposes certain restrictions on the flow past an aerofoil or other body under test, and the determination of the magnitude of this interference is of considerable importance, since it is found that certain corrections must be applied to the aerodynamic characteristics of an aerofoil tested in a wind tunnel before they are applicable to free air conditions. This interference correction is independent of and additional to any correction which may be necessary to allow for the change of scale from a model aerofoil to an actual aeroplane wing.

The theory of the interference has been developed by Prandtl in his second aerofoil paper by considering the conditions which must be satisfied at the boundary of the stream. The continental wind tunnels usually have an open working section and the condition of constant pressure must be satisfied at the boundary of the stream. British wind tunnels, on the other hand, have a closed working section of square or rectangular cross section, and the boundary condition takes the form that the component of the velocity normal to the tunnel walls must be zero. This boundary condition can be satisfied analytically by the introduction of a suitable series of images of the model, and the interference experienced by the model is the induced velocity corresponding to the vortex systems of these images.

Circulation and vorticity.

The definition of the circulation round a closed curve in two dimensions (see 4·1) as the integral of the tangential component of the velocity round the circumference of the curve can be extended at once to the more general case of motion in three dimensions by removing the restriction that the curve must lie in a single plane. Also by dividing any surface bounded by this curve into a network by a series of intersecting lines it can be shown that the circulation round the curve is equal to the sum of the circulations round the elementary areas formed by the network.

The vorticity of a fluid element in two-dimensional motion was defined (see 4·3) as twice the angular velocity of the element. This definition is retained in the more general case of three-dimensional motion but the axis of rotation of the fluid element may now point in any direction. By following the direction of the axis of rotation of successive fluid elements it is possible to construct a curved line whose direction coincides at every point of its length with the axis of rotation of the corresponding fluid element. Such a line is called a vortex line.

The vortex lines which pass through the points of the circumference of a small closed curve C will form the surface of a vortex tube, of which the curve C is a cross section.

The aim of aerofoil theory is to explain and to predict the force experienced by an aerofoil, and a satisfactory theory has been developed in recent years for the lift force in the ordinary working range below the critical angle and for that part of the drag force which is independent of the viscosity of the air. Considerable insight has also been obtained into the nature of the viscous drag and into the behaviour of an aerofoil at and above the critical angle, but the theory remains at present in an incomplete state. The problem of the airscrew is essentially a part of aerofoil theory, since the blades of an airscrew are aerofoils which describe helical paths, and a satisfactory theory of the propulsive airscrew has been developed by extending the fundamental principles of aerofoil theory.

The object of this book is to give an account of aerofoil and airscrew theory in a form suitable for students who do not possess a previous knowledge of hydrodynamics. The first five chapters give a brief introduction to those aspects of hydrodynamics which are required for the development of aerofoil theory. The following chapters deal successively with the lift of an aerofoil in two dimensional motion, with the effect of viscosity and its bearing on aerofoil theory, and with the theory of aerofoils of finite span. The last three chapters are devoted to the development of airscrew theory.

An airscrew normally consists of a number of equally spaced identical radial arms, and the section of a blade at any radial distance r has the form of an aerofoil section whose chord is set at an angle θ to the plane of rotation. The blade angle θ and the camber of the aerofoil section decrease outwards along the blade. If the airscrew moved through the air as through a solid medium, the advance per revolution would be 2πr tan θ and this quantity would define the pitch of the screw. Actually this quantity will not have the same value for all radial elements of the blade and so it is customary to define as the geometrical pitch of the airscrew the value of 2πr tan θ at a radial distance of 70 per cent. of the tip radius. An airscrew rotates in a yielding fluid and in consequence the advance per revolution is not the same as the geometrical pitch and may in fact assume any value. The value of the advance per revolution for which the thrust of the airscrew vanishes is called the experimental mean pitch, and in many respects the characteristics of an airscrew are defined by the ratio of the experimental mean pitch to the diameter.

An ordinary propulsive airscrew experiences a torque or couple resisting its rotation and gives a thrust along its axis.

The theory of the lift force given by an aerofoil in two-dimensional motion has been developed by considering the flow of a perfect fluid governed by Joukowski's hypothesis that the flow leaves the trailing edge of the aerofoil smoothly. It is necessary now to examine the fundamental basis of this theory and the extent to which the assumed motion represents the actual conditions which occur with a viscous fluid.

All real fluids possess the property of viscosity and the conception of a perfect fluid should be such that it represents the limiting condition of a fluid whose viscosity has become indefinitely small. Now it is well known that the limit of a function f(x) as x tends to zero is not necessarily equal to the value of the function when x is equal to zero, and hence, to obtain the true conception of a perfect fluid, it is not sufficient to assume simply that the coefficient of viscosity is zero. The viscosity must be retained in the equations of motion and the flow for a perfect fluid must be obtained by making the viscosity indefinitely small.

Slip on the boundary.

The first point to be considered is the motion of the fluid at the surface of a body. In a viscous fluid the relative velocity at the surface of a body is zero and the body is surrounded by a narrow boundary layer in which the velocity rises rapidly from zero to a finite value.