NAVAL SPACE

6-2 and refining both what has to be done and what mission concept will do it at the lowest cost. Eventually, another question “Does the system meet the user’s needs?” is answered. Since using space assets is so expensive, cost is a fundamental limitation to nearly all space missions. Consequently, good mission objectives must incorporate user needs and characteristics such as global coverage that will be exploited to achieve them. Otherwise, it will probably be more cost-effective to accomplish the mission on Earth. THE SPACE SEGMENT The space segment of the architecture refers to the spacecraft(s) which when placed in orbit will accomplish the mission. A spacecraft typically consists of two segments: a payload, which includes the hardware and software elements that perform the mission; and a spacecraft bus that supports the payload by providing orbital maintenance, power, command and control, temperature control, data handling, etc., to insure that the payload operates properly. Space vehicles are complex, expensive systems operating in a harsh t environment. To design a spacecraft requires a complete understanding of the mission, including the payload’s size and characteristics, plus significant system restraints such as orbit, lifetime, and operations. The design process involves identifying these functions and selecting the best approach for each function. All spacecraft, regardless of mission, have similar subsystems that allow the vehicle to function. For example, without a communications subsystem, commands could not be processed, or without an electrical subsystem such elements as thermal and attitude control could not be maintained. It is through the subsystems that spacecraft receive the resources to function properly. The Communications Subsystem The communications subsystem can link the spacecraft to the ground element or to other spacecraft in the same orbit. Data flowing from the spacecraft is called downlink, while data flowing from the ground element is called uplink, and data flowing between spacecraft is called cross link. These subsystems usually consist of a receiver, transmitter, and a variety of antennae. The data transmitted can be either satellite health and status information, commands, ranging, or mission data. Since distances between orbiting spacecraft and ground stations typically vary between a few hundred to several thousand miles, unique engineering problems are created that need to be resolved to ensure that mission operators can communicate with the spacecraft. For example, because of the altitudes involved, the propagation of radio signals between the Earth and the spacecraft are affected by the ionosphere and the atmosphere. This causes both signal distortion, or noise, and attenuation. As a consequence the transmitted signal can be extremely

6-3 weak by the time it reaches a ground station. Receivers need to be designed to compensate for this limitation. Also, receivers must be capable of calculating the doppler effect associated with a spacecraft. Except for those in geosynchronous orbit, spacecraft are usually moving at high rates of speed relative to the ground station. As a spacecraft appears above the Earth’s horizon and comes into view of the ground station, the frequency of the received signal increases and then decreases as the spacecraft passes overhead and fades over the horizon. Design of spaceborne communications equipment is also constrained by such factors as hardware and software capabilities, power requirements, size limitations, weight restrictions, reliability standards, and resistance to multiple types of radiation present in the space environment. Additionally, different types of spacecraft require different types of antennae to perform the communications function. Spacecraft that maintain a stable attitude in space can utilize high- gain antennae that transmit/receive thin, pencil shaped beam signals. Spacecraft that spin however, are normally limited to omnidirectional or spherical beam shaped antennae. Spacecraft antennae are often large, but relatively delicate structures. Since they are mounted on the outside of the spacecraft, they are exposed to greater extremes of heat and cold, all of which must be considered during their design phase. The Data Management Subsystem Data management is crucial to the operation of a spacecraft. Inadvertent errors in the performance of the data management subsystem can cause the abrupt termination of the mission or total loss of the spacecraft. To prevent catastrophe and sustain mission performance, data is processed in three different ways. • Command Processing. Command Processing permits the spacecraft to be configured in response to commands transmitted from the ground station. Examples of these commands include applying or removing power from a system, altering operating modes, or uploading complete computer programs into the onboard memory. • Telemetry Processing. Telemetry Processing enables data to be transmitted from the spacecraft to the ground station. Accurate and timely telemetry is required for system operators to determine the health and status of the spacecraft and take corrective action as necessary. • Data Processing. Data Processing and Storage allows the communication system to interface with all the processors in the spacecraft through various kinds of input/output channels, or over the spacecraft data bus. These processors are designed to operate

6-4 autonomously to enable the spacecraft to perform its mission with minimum ground station control. The Power Subsystem. The power subsystem provides all the electrical energy used by the spacecraft and is normally provided by three sources: • Solar energy, • Stored energy carried by onboard batteries, or • A combination of the two. The power system must be able to provide reliable power for all spacecraft loads. These loads include sophisticated spacecraft instruments, sensors, transmitters, and any mechanical device that requires power to operate. Since spacecraft loads are powered by electricity, the interface between the power subsystem and the other spacecraft subsystems is an electrical distribution system. This distribution system consists of power buses, wiring, load protectors, and connections. Another source of electricity is from stored energy. This power source uses batteries and fuel cells, and is characterized by combining two chemical elements for a subsequent release of electrical power. All spacecraft employ batteries as power storage devices. Typically, the primary power producing system is used to charge the onboard batteries, and during times when power cannot be generated, draws this energy from the storage devices. If the main power source is based on solar cells, the batteries will charge when the spacecraft is in the sun, and then discharge when the spacecraft is in the dark. Batteries used for spacecraft applications are generally made of NiCd (nickel and cadmium) or NiH (nickel and hydrogen). Fuel cells are an electrochemical device in which the chemical energy of a conventional fuel and oxidizer are stored external to a battery and are fed to it as needed. Fuels cells, because they consume both stored fuel and oxidizers, would generally not be considered for spacecraft missions of long duration. A simplified diagram of a gaseous hydrogen and oxygen fuel cell is shown in Figure 6-1. The disadvantage of both systems is they add a lot of weight to a spacecraft as compared to the amount of power produced.

6-5 Figure 6-1. Simplified diagram of a gaseous hydrogen and oxygen fuel cell. Thermal Control Subsystem. The thermal control subsystem keeps the thermal condition of a spacecraft within a specified range. High temperatures may cause electronic components to fail, and low temperatures may cause moving mechanisms or fuel lines to freeze. The thermal control system must be able to react throughout the environmental and operational configurations expected. As discussed in chapter 3, thermal control in space is difficult because of the space environment, which exposes spacecraft to extreme temperature differences. Spacecraft surfaces exposed to direct sunlight experience extremely high temperatures, while surfaces in the dark experience low temperatures. Low Earth orbiting spacecraft continuously cycle in and out of the Sun’s radiations, absorbing and radiating energy in phases. Geostationary satellites may be exposed to or eclipsed from solar radiation continuously for weeks or months. Internally, a spacecraft may generate large quantities of heat during times of peak operation and little during dormant periods. Thermal control devices fall into two categories, passive and active. Passive. Passive devices simply shield, insulate, or change their thermal characteristics depending on the existing temperature of a satellite. The external coating of a spacecraft determines the craft’s absorptivity (how much external energy is absorbed), and emissivity (how much internal thermal energy is radiated into space) characteristics to control temperature. An analogy to submarines is the anechoic coating on the surface of the submarine that is used to reduce the submarine’s sound characteristics. Many spacecraft are wrapped in thermal blankets to retain internal heat, and some spacecraft are equipped with heat-activated louvers, which open or close to expose different external surfaces to radiate or retain internal heat.

6-6 Active. Active devices usually involve some sort of working fluid to carry heat from one location within the spacecraft to another. Refrigeration devices, electric heaters, heat pumps, and heat pipes are examples of active thermal control devices. These devices have the ability to more precisely control spacecraft temperatures, but their disadvantage is they increase weight and complexity. Attitude Control Subsystem Depending on the mission, a spacecraft may have varying requirements for pointing accuracies. This is a function of the attitude control subsystem. Basically, a spacecraft must be able to determine its own attitude with respect to some reference, and be able to modify this attitude as necessary to perform a desired mission. Attitude determination for a spacecraft involves three functions: • Acquire position data relative to the spacecraft, • Compute direction vectors to the reference system, and • Compare attitude control sensor data to the reference direction vectors. The process of determining the vehicle’s attitude requires large numbers of computations. The computations can be performed by the spacecraft’s ground segment, but normally this is accomplished on board the spacecraft itself. The terms roll, pitch, and yaw are often used in discussions of attitude control. Yaw is measured toward the center of the Earth, roll is measured in the direction of a spacecraft’s orbit, and pitch is measured perpendicular to the yaw-roll plane (See Figure 6-2.). The data necessary to control these conditions is accomplished through a number of different sensors. The data is then acted upon by various attitude control devices (mechanisms). Figure 6-2 Graphical representation of roll, pitch, and yaw.

6-7 Sensors To acquire position data relative to spacecraft, the spacecraft must be able to measure its position relative to some object. The four types of measurement devices used for this task are Earth Sensor, Sun Sensor, Star Sensor, and Magnetic Field Sensor. • Earth Sensor. An Earth sensor typically uses the Earth’s infrared radiation (this is the radiation associated with the earth’s temperature), to detect the contrast between the black cold void of space and the radiation from the Earth. The direction to the Earth’s center can be inferred by two (or more) detectors sensing the Earth’s edge at a substantial angular separation. Using this method, spacecraft at an altitude of 500 miles can measure pitch and roll angles to a 0.1-degree accuracy. • Sun Sensor. Sun sensors typically use an arrangement of solar cells to measure sun-line angles. Generally, three to six sun sensors are mounted on the surface of the spacecraft, and the sun-line angles from these sensors are used to calculate the spacecraft’s position relative to the sun. • Star Sensor. In a typical star sensor, a space sextant measures the angle between two stars and a near body to determine two lines of position. A series of these measurements is made to secure an accurate position. There are two general categories of star sensors, star scanners, and star trackers. Star scanners are used on spinning spacecraft while star trackers are used on non-rotating spacecraft. • Magnetic Field Sensor . Magnetic field sensors typically use a three vector magnetometer to measure the three components of the earth’s magnetic field. These magnetometer measurements are combined with a math model of the magnetic field to determine the attitude of the spacecraft. Attitude Control Mechanisms Now that you know some of the basic concepts and terminology of attitude control, let us discuss some of the more common methods used by operational spacecraft. The four commonly used mechanisms for establishing and maintaining attitude are: • Gravity-gradient stabilization • Spin stabilization • Thruster control • Reaction Wheel Control

6-8 Gravity Gradient Stabilization The gravity gradient technique uses the torque produced by slight differences in the direction and magnitude of the earth’s gravitational field to orient a spacecraft. The origin of the gravity-gradient torque is shown in Figure 6-3, which is a sketch of a satellite consisting of two equal point masses, m1 and m2, separated by a rigid massless rod of length l. As you can see, the gravity force on m1 is greater than the gravity force on m2 because m2 is further away from the Earth than m1, and the moment arm of the gravity force on m1 is greater than the moment arm of the force on m2. Therefore, for both these reasons, the net torque will tend to reduce the angle theta to zero, establishing the spacecraft in a stable position. Figure 6-3. Origin of the gravity-gradient torque.

6-9 Spin Stabilization Spin stabilization uses the same principle as a gyroscope for stabilization. A spin-stabilized spacecraft tends to hold its spin axis orientation fixed in space. This attitude can be maintained for hours or days without control activity, which has important advantages for simplicity and reliability. For passive spin stabilization, the spin axis must be the principal axis of maximum moment-of-inertia. The reason for this is that spinning about any other axis is unstable; any disturbance may cause the spacecraft to move into a lowest energy position, which is aligned with the axis of maximum moment-of-inertia. Spinning about an axis of minimum moment-of-inertia is possible with active control of nutation, where nutation is the bobbing motion of the spin axis between inner and outer cones (see Figure 6-4). A very popular variation on spin stabilization is called dual-spin, one large section of the spacecraft is spinning and another large section is non-spinning (often referred to as "de-spun"). This scheme has many of the advantages of spin stabilization, where the large angular momentum of the spinning section tends to hold the attitude in space without rapid attitude control activity. The de-spun platform provides an ideal location for Earth pointing communication antennas. Figure 6-4. Spin axis between inner and outer cones.

6-10 Thruster Control The three-axis attitude control by thrusters is often used for attitude control on manned space vehicles (NASA’s Space Shuttle is a good example). This method provides substantial control authority, and the ability to counteract large disturbance torques. In this method small nozzles, generally referred to as thrusters, are located at various positions on the spacecraft. The attitude of the spacecraft is controlled by firing these thrusters at the proper time, and for the proper duration. Reaction Wheel Control A "reaction wheel" is an internal rotating device that exerts a torque on a spacecraft by the reaction effect (for every action, there is an equal and opposite reaction), as the rotating element is accelerated. The direction of the torque vector is parallel with the axis of wheel rotation. To produce a torque vector in an arbitrary direction in the body requires a minimum of three reaction wheels. Generally four or more wheels are used to provide component redundancy for the attitude control system. Reaction wheels take the place of the thrusters discussed in the previous section. They have the advantage that no mass is consumed, and the spacecraft will not suffer eventual loss of attitude control due to fuel depletion. The magnitude of the torque effect can be easily modulated over the range from zero torque to maximum by electronic control of the reaction wheel motor current. This makes it possible to design linear control systems with finer attitude performance control than can generally be obtained with thruster control. One of the major problems associated with this method is eventually the speed of one or more wheels may reach the physical limits of the drive electronics, or the structural limits associated with the wheel. If a wheel reaches its limit and further requests for acceleration cannot be satisfied, the wheel is "saturated," and the satellite is in danger of losing attitude control. Therefore, some means of "unloading" the wheels, i.e., getting them to run closer to zero, is necessary. The Hubble Space Telescope does its "unloading" by selectively energizing onboard electromagnets that interact with the earth’s magnetic field to "unload" the reaction wheels. Sources of Attitude Disturbance Because space is a vacuum, it would seem that the attitude of a spacecraft (once in its proper orbit) can easily be maintained. But space is not a perfect vacuum, and there are environmental factors that affect the attitude of a spacecraft. These factors include aerodynamic torque, solar radiation, magnetic field, and gravity gradient induced torque. • Aerodynamic Torque. Aerodynamic torque is the result of the drag force from the Earth’s atmosphere. The lower the altitude of a spacecraft, the larger the effects of atmospheric drag.

6-11 • Solar Radiation Induced Torque. Solar radiation induced torque is generated because light from the Sun produces a very slight pressure effect. At spacecraft altitudes above 600 miles, aerodynamic torque is small compared to solar radiation induced torque. • Magnetic Field Induced Torque. Magnetic field induced torque is caused by permanent magnetization of spacecraft hardware components interacting with the Earth’s magnetic field. Care must be taken in the design and construction of a spacecraft to minimize the magnetic effects of onboard components. • Gravity Gradient Induced Torque. For spacecraft that are not gravity-gradient stabilized, the gravity-gradient effect is a source of attitude disturbance. Figure 6-5 shows a graphic representation of the torque spectrum for a spacecraft with a projected area of 25 square meters and masses m1 = m2 = 500 kilograms separated by 25 meters. The tick marks on the graph represent the spacecraft’s orbital altitude in kilometers. Figure 6-5 Graphic representation of the torque spectrum. THE GROUND SEGMENT For each space system, there is a ground infrastructure manned by trained operators who provide management and control. During the design phase, studies are conducted to determine whether a function is best performed on board the spacecraft or by the ground segment. The capabilities and limitations in computational power, electrical power generation

6-12 and storage, and spacecraft weight requirements are considered. Functions inappropriate for execution by the spacecraft are incorporated into the ground segment. A typical ground segment is made up of facilities, processes, data networks, computer hardware and software. A trained staff commands the spacecraft, monitors telemetry, and routes sensor data for appropriate utilization. Generic functions and processes of a typical ground segment follow. Ground Segment Functions The management of the ground segment can range from managing a simple one facility operation, to managing a large network with international cooperation of control centers, sustaining engineering, and training facilities. In each case there exists three primary functions: program support, mission operations, and operations planning. • Program Support. Program support provides the ground segment with all of the necessary administrative, operations coordination, and general technical support to sustain a mission. • Mission Operations. Operations takes in data from numerous sources, charts the spacecraft course, plans the necessary commands, and executes them. Operations personnel working at a control center require a working knowledge of spacecraft systems, how to operate them efficiently, and how to bypass them when they malfunction. Flight controllers are usually assigned by specific disciplines, such as electrical power, mechanical systems, data management, attitude control, and for manned spacecraft, environmental control. A team of flight controllers is coordinated by a central control center director who has overall responsibility for the safety of the spacecraft (or crew) and the completion of its mission. • Operations Planning. Operations planning determines how the overall program plan is to be implemented. For example, the precise time and duration of firing attitude control thrusters in order for the spacecraft to aim a spacecraft sensor at a particular spot on the Earth or in the heavens must be carefully planned. Planning information must be accumulated on the sensor, the target’s location, spacecraft control systems, and the orbital trajectory before a determination is made how to construct the commands to execute the operation. Operations planning choreographs the flight controllers’ activities so that each system or sensor is operated to maximize available resources (power or crew time), and to work with the other operating systems to meet the various mission requirements.

6-13 Commanding Command and control is the primary function of the ground segment. This function requires personnel to be in contact with the spacecraft, and deal with the spacecraft’s operations, utilization, changes in configuration, and anomalies. Through a communications network, space flight controllers send commands to the spacecraft which are then routed to the designated system or sensor. Some commands are sent from the ground for immediate execution by an onboard system (real-time commands). Additionally, commands can be stored onboard in computer memory for execution at a later time. These commands, which are normally referred to as stored program commands (SPCs), allow for autonomous operation. If commands are constructed or sent improperly, they could render a spacecraft useless. An example of improper commanding would be to fire attitude control thrusters in the wrong direction, at the wrong time or for an incorrect duration of time. This might cause: • An undesirable rotation of the spacecraft • Prevention of any further commands from being received by the spacecraft because spacecraft receive antennas are pointed in the wrong direction • Positioning of solar arrays away from the Sun resulting in loss of spacecraft power To avoid mistakes, commands sent to a spacecraft are reviewed and validated prior to their transmission. Command verification can be accomplished using a spacecraft simulator, where commands are sent to a ground representation or model of the spacecraft and the results monitored. Another validation method used is to compare commands to standardized or previously verified commands. Tracking and Monitoring Tracking a spacecraft’s position, and monitoring its health and status are critical factors in successful spacecraft operations. Spacecraft tracking is required for navigation purposes, coordinating activities with ground stations, or coordinating any space-to-space activities. Tracking can be accomplished using ground radars when the spacecraft is within range, but this method cannot determine precise spacecraft attitude. Tracking can also be accomplished by downlink of the spacecraft’s calculated navigational state and attitude. Whatever the means, it is imperative that the control center have up-to-date location and attitude information on the spacecraft. Monitoring is accomplished by transmitting the onboard instrumentation data via the communications network to a control center. Once in a control center, the raw "downlink" data is reformatted into engineering units and routed to the appropriate flight controller displays.

6-14 Downlink data is stored and archived for later study of system operating trends, failure cause and effect investigations, or for scientific analysis. THE COMMUNICATIONS SEGMENT The first regular military use of satellite communications occurred in the early 1960’s when the Navy used the moon as a medium for passing messages between ships at sea and shore stations. This method of communications proved reliable when other methods failed. Today, spaceborne data relay and communication systems are essential for command and control in this era of global military operations. As discussed in chapter 3, information is carried between the ground and space segments of a space system via electromagnetic waves. The information to be passed is impressed on these waves through modulation using various schemes. The type of electromagnetic wave, modulation technique, and the transmission power level are all important factors in establishing a successful communications link and is discussed below. Radio Wave Propagation The electromagnetic spectrum is divided into different frequency bands, with radio and microwave bands being most commonly used for communication. Figure 6-6 shows how these bands are subdivided. Propagation properties and data carrying characteristics vary with frequency band.

6-15 Figure 6-6. Communications bands. A generic electromagnetic wave is described in Figure 6-7. Frequency, amplitude, and phase are all important factors in communications. There are four types of electromagnetic waves, known as carrier waves, used for communications. (See Figure 6-8.) • Direct wave • Ground wave • Sky wave • Space wave Figure 6-7. A generic electromagnetic wave.

6-16 Figure 6-8. Types of electromagnetic waves. Direct Wave Direct waves travel from the transmitter to the receiver along a direct, unimpeded path. This is known as line of sight communications, and almost any frequency can be transmitted in this manner. Direct wave communications have limited range. Ground Wave At frequencies less than a few megahertz, electromagnetic energy can interact with the material in the Earth. Such waves tend to follow the contour of the Earth’s surface. Ground waves can communicate over longer distances than direct waves, but the amount of information transmitted is limited. Sky Waves Frequencies higher than AM frequencies cannot propagate as ground waves, but may interact with the ionosphere. This type of interaction causes the electromagnetic energy to be refracted back toward the receiving station on the ground. Communications via sky waves can achieve very long ranges, essentially providing worldwide coverage.

6-17 Space Waves Above the critical frequency (generally above 30 Mhz), electromagnetic waves are not affected enough by the ionosphere to create sky waves. Communications using these frequencies must be LOS (line of sight) between a transmitter/receiver located on the Earth and a transmitter/receiver located in space. POWER BUDGET The amount of power an antenna delivers to a receiver, and the ability of the receiver to pick up the signal determines if successful communications will occur. The power (or link) budget analysis of the system evaluates the probability of successful communications. The power budget analysis takes into account transmitter power, transmitting antenna gain, receiving antenna gain, free space loss, and losses caused by spreading and incidental loss. Miscellaneous loss depends upon the environment through which the signal will pass. For example, communications using microwave frequencies are greatly hampered by rain showers or thunderstorms. Noise The amount of power received may still not determine if the receiver will understand the transmitted information. The reason for this is the presence of noise which can interfere with the transmitted signal. Noise is caused by the transmitting and receiving equipment, and natural sources such as the Sun, Earth, and atmosphere, as well as other celestial bodies. Signal To Noise Ratio Successful communication systems, commonly referred to as links, are designed to operate under all expected conditions of losses and noise. The measure of this is given by a receiver’s signal to noise ratio: S/N ratio = Power/Noise A receiver will have a minimum signal to noise ratio specified for acceptable operation. If the received power decreases, or noise increases such that the actual signal to noise ratio is less than the minimum specified, the modulated signal may not be intelligible or detected at all. Modulation Modulation is the means by which information is impressed on electromagnetic radiation. There are two major components to a communication signal. They are the baseband signal and the carrier wave.

6-18 Baseband Signal The baseband signal is the information to be transmitted. There are two forms of baseband signals: analog and digital. • Analog. Analog signals are the raw, continuous wave-like frequencies of sound or light. Television stations transmit analog signals that are used by a TV set to illuminate the tube in a certain way to produce a picture. • Digital. Digital signals are in the form of a string of 1’s and 0’s, which represent the original information and allow easier or faster data transmission. Carrier Waves Carrier waves are electromagnetic signals used to carry the baseband information between transmitter and receiver and are chosen for their propagation characteristics with due regard to the amount of baseband information to be transmitted. They are used because the baseband signal may not have sufficient propagation characteristics for successful communication. A carrier wave can normally only be impressed with baseband frequencies up to about 10% of the carrier wave frequency. General Modulation Techniques Baseband information is impressed onto a carrier wave for propagation between stations. In all cases some characteristic (amplitude, frequency or phase) of the carrier wave is modified to represent the baseband information whether it is in analog or digital form. • Amplitude Modulation (AM). In amplitude modulation, the baseband signal is used to change the amplitude of the carrier wave in such a way as to represent the baseband information. Frequency and phase are held constant. • Frequency Modulation (FM). In frequency modulation, the amplitude and phase of the carrier wave are held constant, while the frequency is modified by the baseband signal. The frequency variation from the known carrier wave is directly proportional to the frequencies of the baseband signal. • Phase Modulation (PM). In phase modulation, information is impressed upon the carrier signal by using the baseband signal to modify the phase of the carrier wave. Phase modulation is not used much in conventional (analog) communications, but is widely used in digital communication.

6-19 • Pulse Modulation (P-M). In this modulation technique, the modulated carrier signal is sent in a series of discreet pulses. The height, width, or position of the pulses is altered in a definite pattern corresponding to the information being transmitted. Pulse Amplitude Modulation (PAM) is very similar to AM. Pulse Width Modulation (PWM) mirrors FM in that the width or duration of each pulse varies in direct proportion to the value of the modulating wave. Pulse Position Modulation (PPM) resembles PM in that the amplitude and width of the pulse are kept constant, but the position of each pulse in relation to the position of a recurrent reference pulse, is varied. Pulse Code Modulation (PCM) refers to a system in which values of a " quantized" modulating wave are indicated by a series of binary coded pulses. This will be further explained in the section on digital communications. Demodulation Demodulation is the recovering of the baseband signal from the received modulated signal. The basic process simply uses the opposite of that used to modulate the carrier signal in the first place. The demodulation process is made complicated because the received signal is usually weakened due to various losses, and because the information may have been corrupted during transmission. DIGITAL COMMUNICATIONS Digital communications offers some benefits over analog in the areas of transmission, reception, and handling techniques. Digital signals represent the original information using a binary coding system. In this system, strings of 1’s and 0’s are used to represent letters and numbers. Each individual 1 or 0 is known as a bit and 8 bits are known as a byte. A word is the standard unit in a binary coding system. Information is obtained by properly decoding the words of the binary coding system. Analog to Digital (A/D) Conversion How can an analog signal, like music or a voice, be accurately converted into a digital signal? This question will be answered in the following section. Figure 6-9 shows a sinusoidal analog signal being digitized. Let’s examine the main aspects, quantization, sampling, and transmission rate of this digitization process.

6-20 Figure 6-9 Sinusoidal analog signal being digitized. • QUANTIZATION. Quantization is the method of assigning a range of digital values to represent the original analog signal. The quantization process has two limitingfactors which are (1) the range of the analog signal value and (2) the number of bits available in the digital system. The signal shown in Figure 6-9 has been split into eight quantization levels, known as Q-levels. There are limits to the number of bits a system can manipulate, so the signal should be represented with as few bits as possible. The method of assigning a particular bit word to each quantization level is called coding. Figure 6-9 has been split into eight Q-levels, and eight different three-bit words are assigned in specific order to each level. The word 000 has been assigned to the bottom level and a binary 1 added for each subsequent level. • SAMPLING. The frequency at which a signal is assigned a quantization level (sampled) is called the sample period (Ts). The signal is sampled often enough to allow the receiver to reconstruct the original signal from the digital information. • TRANSMISSION RATE. The transmission rate represents the number of bits per unit time a digital system transmits, and is a measure of the capability of the system to pass information. The major limiting factor to transmission rate is the carrier wave’s ability to transport digital information. Most spacecraft use digital communication methods, and in many cases, the transmission rate is a major limiting factor of the space system’s capabilities.

6-21 Digital Baseband Signals The output of an A/D converter is a string of 1’s and 0’s representing the bits assigned to each successive sample of the original analog signal, and is usually represented by an output voltage where high voltage is a 1 and low voltage is a 0. The time the specific voltage is held to represent each bit is called the bit period (Tb). The pattern of 1’s and 0’s is the information to be transmitted, and will be used as the baseband signal to modulate the carrier wave. Digital Modulation Methods Digital modulation is a type of pulse modulation where the pulses are of equal amplitude and duration, and the information is transmitted by encoding the spacing between pulses as they are sent in sequence. There are three basic digital modulation techniques: 1. Amplitude Shift Keying (ASK) 2. Frequency Shift Keying (FSK) 3. Phase Shift Keying (PSK) • AMPLITUDE SHIFT KEYING (ASK). In this modulation technique, the carrier wave is either transmitted, or not transmitted each bit period. The relationship between the carrier wave, baseband signal, and resulting ASK signal is shown in Figure 6-10. Figure 6-10. Relationship between the carrier wave, baseband signal, and resulting Amplitude Shift Keying (ASK) signal.

6-22 • FREQUENCY SHIFT KEYING (FSK). In this modulation technique, two carrier frequencies are used to represent a transmitted 1 and the other a transmitted 0. The relationship between the baseband signal and resulting FSK signal is shown in Figure 6-11. Figure 6-11. Relationship between the baseband signal and resulting Frequency Shift Keyed (FSK) signal. • PHASE SHIFT KEYING (PSK). In this modulation technique, a change in a bit state is indicated by a change in the phase of the carrier wave. The relationship between the baseband signal and the resulting PSK signal is shown in Figure 6-12. Figure 6-12. Relationship between the baseband signal and the resulting Phase Shift Keyed (PSK) signal. An important point in PSK systems is to ensure that the receiver knows the starting bit, otherwise the reconstructed digital string will be opposite to the transmitted string. This is usually done by first sending a code that will indicate what the first bit in any message should be.

6-23 Digital Demodulation For digital demodulation, the demodulator must only detect the presence of a transmitted 1 or 0, which is much simpler than the analog demodulation task of recovering the actual analog baseband signal. This allows digital communications systems to obtain a successful link with lower minimum signal to noise ratios. COMMUNICATIONS SYSTEMS Communications systems consist of a network of transmitters and receivers. Block diagrams of the major components of a transmitter and a receiver are shown in Figure 6-13. Figure 6-13. Block diagrams of the major components of a transmitter and a receiver. Transmitter In the transmitter, the source represents the originator of the information signal (Ft). It may be a microphone of a radio or telephone; a video receiver of a television camera; or an antenna of a remote sensor. The function of the source is to turn information into an electromagnetic signal suitable for transmission. If the signal is to be digitized, an A/D converter would be part of the preparatory electronics that makes the signal ready for modulation onto the carrier wave. This takes place in the modulator. The amplifier boosts the signal to the desired transmit power (Pt) and delivers it to the antenna, which produces the electromagnetic signal (Xt) for propagation. The channel represents the medium through which the signal travels between stations. In space, the channel is a vacuum.

6-24 Receiver On the receiving end, the antenna picks up the signal and its associated noise from the channel. Since the power level received is usually quite low compared to the transmitted signal, it is delivered directly to an amplifier to boost the signal strength. The front end electronics filters out some of the noise. The signal then goes to the demodulator where the baseband signal is recovered from the carrier wave. Finally, the back end electronics reproduces the transmitted information into its original form. It is here where the digital to analog (D/A) converter reconstructs the original message. THE LAUNCH SEGMENT Given mission requirement, constraints, and a myriad of other factors, a decision must be made on which launch system configurations can deliver the spacecraft to its mission orbit by attaining a certain position in space with a certain velocity. The launch process can severely constrain spacecraft design. Primary restrictions are the launch vehicle’s lift capability and the environment to which it subjects the satellite on ascent. A launch system consists of a basic launch vehicle incorporating one or more stages and an infrastructure for ground support. It provides the necessary force to displace the spacecraft. Ultimately, it places the payload into the desired orbit with a functional spacecraft attitude. The term payload includes all hardware above the launch-vehicle-to-spacecraft interface, excluding the payload’s protective fairing, which is usually part of the launch system. Thus, the launch- vehicle payload consists of the entire spacecraft and the booster adapter. The booster is the rocket we see sitting on the pad during countdown. It provides the necessary change in velocity to get the spacecraft into space. The booster blasts almost straight up to gain altitude rapidly and get out of the dense atmosphere which slows it down through drag. Then it begins to pitch over to start gaining horizontal velocity. Because of the limits of rocket technology, we can’t construct a single rocket which can deliver a spacecraft efficiently into orbit. Instead, a large booster is actually a series of smaller rockets which light and then burn out in succession, each one handing off to the other like runners in a relay race. These smaller rockets are called stages. In most cases, a booster will require at least three stages before reaching orbit. Normally, the booster can’t deliver the spacecraft to its final orbit by itself. Instead, when the booster finishes its job and burns out, the spacecraft remains in a parking orbit. A parking orbit is a temporary orbit where the spacecraft will stay until transferring to its final mission orbit. Once a spacecraft is in its parking orbit, a final “kick” must send it onto a transfer orbit and, eventually, on up to its final mission orbit where it starts to perform its mission.

6-25 The extra kick of energy needed to transfer from the parking orbit to the final orbit comes from the upper stage. In some cases, the upper stage is actually integral to the spacecraft itself, sharing the plumbing and propellant which the spacecraft will later use to orient itself and maintain its orbit. In other cases, the upper stage is an autonomous spacecraft with the one-shot mission of delivering the spacecraft to its final orbit. In the latter case, the upper stage breaks off once its job is done. Regardless of how it is configured, the upper stage consists mainly of a rocket engine (or engines) and the propellant needed to change the spacecraft’s energy enough to enter the desired final orbit. Once the spacecraft reaches its final orbit, it may still need thrusters to keep it in place or maneuver around. Thrusters are relatively small rockets used to adjust the spacecraft’s orientation (which changes over time from external forces) and to help maintain the orbit size and shape. THE USER SEGMENT The user segment consists of the individuals and organizations that use data generated by a space segment in support of their missions. The user segment usually consists of multiple users spread over large geographic areas, but could be a single individual with a dedicated receiver performing a special operations mission. USER ACCESS METHODS The ground, communications, and space segments are easy to define and describe. They are concrete physical entities with well defined functional elements. The user segment is more abstract, and each users’ set of requirements are unique. To simplify our discussion of the user segment, we will use the type of access or interface that users have with a particular space system. These interfaces are as follows: • Direct Access. The user receives space system information directly from the space segment • Indirect Access. The user receives the space system data from an intermediate source Direct Access A GPS equipped aircraft that receives navigational information directly from the GPS constellation is a direct user. As long as the aircraft is within view of sufficient GPS spacecraft and the onboard receiver is working properly, the pilot will have extremely accurate navigational data at his/her disposal. The pilot needs to take no action (except for turning on the GPS receiver) to access the information, so the fact that GPS is a space-based navigation system is transparent to the pilot.

6-26 Indirect Access A carrier battle group staff that receives satellite imagery data from some shore-based distribution point has indirect access. For example, LANDSAT spacecraft provide multi-spectral imagery of the entire Earth’s surface. A battle group staff planning an amphibious operation would find this information very useful to determine soil composition and vegetation density of the planned beachhead. Because LANDSAT data requires considerable processing before it can be used by planners, this processing takes place at only certain locations. Therefore, LANDSAT data has to pass through many hands before it is available to tactical planners (see chapter 7). ACCESS METHODS EVALUATION What are the important characteristics that must be evaluated before deciding whether to use direct or indirect access? Some of these important characteristics are timeliness, quantity of data, quality of data, ease of use, cost, and security. Direct access is usually more timely, has more data, the capability for better quality data, is harder to use, costs more, and has higher security risks than indirect access. Mission requirements are generated by the user, whereas operational and systems requirements are generated by the supplier. From the user’s viewpoint, the space system should provide a full data stream in real time directly to him at any location in the world. From the supplier’s viewpoint, the space system should be built and operated at the lowest practical cost, with sensitive data protected from falling into the hands of an adversary. The space system that is fielded is usually a compromise between the users and the suppliers viewpoints after careful examination of the system, mission, and operational requirements. System compromise is i llustrated as follows. As mentioned earlier, a GPS equipped aircraft is an example of a direct user. The safe navigation of an aircraft in restricted airspace is a real-time problem, where the pilot must have a continuous stream of accurate navigation data. This user must have direct access to the space segment’s data in order to fulfill his/her mission. The carrier battle group’s staff on the other hand has indirect access to satellite multi-spectral imagery data. The staff needs timely access to the data, but it doesn’t have to be in real-time. Moreover, security restrictions must be applied to the dissemination of satellite imagery data. The security of this data is best achieved by using indirect access. For a given space system, the user segment can be a mixture of direct and indirect users. An example of this could be a space-based environmental monitoring system where one group of users doing weather prediction requires direct access, while another group of users doing climatic research only needs indirect access. Again the main point to remember is that the choice of user access method is a logical compromise consistent with satisfying the user’s mission requirements.

6-27 SUMMARY Development of space system architecture is a complex, expensive, and time-consuming process. Although the most appropriate solution must meet mission requirements, political, legal, economic, and technical issues all need to be addressed in the overall planning of any space system. This process must lead to a detailed, well-defined mission concept that thoroughly justifies a space-based asset.

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7-1 CHAPTER 7 NAVAL TACTICAL USE OF SPACE INTRODUCTION This chapter provides an overview of the space systems and their related surface segments of most interest to the Navy and Marine Corps. These sections are organized into the functional areas of space-based force enhancements: • Communications (command and control) • Space-based warning • Space based positioning and navigation • Space-based Intelligence, Surveillance, and Reconnaissance • Meteorological Monitoring • Space-Based Environmental Monitoring Each of these sections presents the space fundamentals involved and the major systems and constellations currently being used to provide these capabilities to the naval warfighter. MILITARY SATELLITE COMMUNICATIONS SYSTEM (MILSATCOM) The Military Satellite Communications System (MILSATCOM) includes Ultra-High Frequency (UHF), Super-High Frequency (SHF), and Extremely-High Frequency (EHF) capabilities. Its role is to provide reliable, secure communications in support of Department of Defense (DoD) Command, Control, Communications, Computers, and Intelligence (C 4I) in all levels of conflict, in accordance with the current national military strategy. Figure 7-1 displays the varied SATCOM frequencies found in the electromagnetic (EM) spectrum. MILSATCOM offers the naval commander some unique advantages. Those advantages include: • Worldwide coverage • Service to isolated areas • Rapid expansion to new locations • Reliable communications which exceed the range of line-of-sight (LOS) systems