AOE 4065 Space Design

Transcription

1 AOE 4065 Space Design Aerospace and Ocean Engineering Department Virginia Polytechnic Institute and State University Blacksburg, Virginia 16 NOVEMBER 2001 Communications, Computers, and Command and Data Handling Subsystem Submitted By: Cai Y. Chen Brian J. Kober Christine R. Rogers F. Elliott Shoup Ryan W. Wood

2 TABLE OF CONTENTS LIST OF TABLES... iii LIST OF FIGURES...iv LIST OF ABBREVIATIONS...v LIST OF SYMBOLS...vi CHAPTER 1 - INTRODUCTION INTRODUCTION HISTORY OF COMMUNICATIONS IN SPACE HOW THE COMMUNICATION SYSTEM AFFECTS SPACECRAFT HISTORY OF COMPUTERS IN SPACE HOW THE COMPUTER SYSTEM AFFECTS SPACECRAFT OVERVIEW OF REPORT...7 CHAPTER 2 INTERACTIONS AND MODELING SUBSYSTEM INTERACTIONS Astrodynamics Guidance and Navigation Propulsion Attitude Determination and Control Power Thermal Environment Structures Mechanisms Launch Vehicle Mission Operations Ground Systems Cost Political and Legal Program Management SYSTEM MODELING COMMAND AND DATA HANDLING MODELING CHAPTER SUMMARY...21 CHAPTER 3 SYSTEM EXAMPLES GALILEO Communication System Command and Data Handling System CASSINI Communication System Command and Data Handling System MARS PATHFINDER Communication System Command and Data Handling System HUBBLE SPACE TELESCOPE Communication System Command and Data Handling System MARS ODYSSEY Communication System Command and Data Handling System CHAPTER SUMMARY...30 CHAPTER 4 - CONCLUSIONS LOW EARTH ORBIT ANALYSIS COMMONLY USED COMPONENTS...33 APPENDIX 1: Atmospheric Loss Data...35 REFERENCES...36 ii

3 LIST OF TABLES Table 2. 1: System Interactions...8 Table 2. 2: RF Frequency Ranges...15 Table 2. 3: Antenna Properties...18 Table 2. 4: Size, Mass, and Power Ranges for a Typical C&DH System...21 Table 4. 1: Uplink Input parameters (SGLS to LEO Satellite) 11, Table 4. 2: Downlink Input parameters (LEO Satellite to SGLS) 11, Table 4. 3: Link Budget, Uplink...32 Table 4. 4: Link Budget, Downlink...32 iii

4 LIST OF FIGURES Figure 2. 1: Transmitter-Receiver Diagram...18 Figure 2. 2: Ground-to-Satellite Communication Diagram...18 Figure 3. 1: Galileo Antenna Placement Figure 3. 2: Cassini Antenna Placement Figure 3. 3: Mars Pathfinder High Gain Antenna iv

5 LIST OF ABBREVIATIONS ADCS AIM ANT BIS BIU BPF C&DH CDEA DOD DSN EFC FCC GEO GPS HGA HPCC HST I/O IRAC JPL LGA LNA LO MOPS NASA RAM REE REU RF RFS RRH SGLS SSAT SSR UHF WTS Attitude Determination and Control System Integrated Attitude and Information Management System Antenna Subsystem Bus Interface System Bus Interface Units Bandpass Filter Command and Data Handling Command Data System Electronic Assembly Department of Defense Deep Space Network Engineering Flight Computer Federal Communications Commission Geo-Synchronous Orbit Global Positioning System High Gain Antenna High Performance Computing and Communications Hubble Space Telescope Input/Output Interdepartmental Radio Advisory Committee Jet Propulsion Laboratory Low Gain Antenna Low Noise Amplifier Local Oscillator Millions of Operations Per Second National Aeronautics and Space Administration Random Access Memory Remote Exploration and Experimentation Remote Engineering Units Radio Frequency Radio Frequency Subsystem Relay Radio Hardware Standard Ground Link Station S-Band Single Access Transmitter Solid State Recorder Ultra High Frequency Waveguide Transfer Switches v

6 LIST OF SYMBOLS A e c d f G AR G AT G FS H(x) λ M P Rx P Tx R s T s Effective Area of Antenna Speed of Light Distance Between Receiving and Transmitting Antennas Operating Frequency Receiving Antenna Power Gain Transmitting Antenna Power Gain Free Space Power Gain Average Information of Outcome Wavelength Amount of Outcomes Signal Power into the Transmitting Antenna Signal Power into the Receiver Data Transferring Rate Duration of Outcomes vi

7 CHAPTER 1 - INTRODUCTION This chapter gives a brief history of communication and computer systems used in the space industry, as well as discusses how each system is important to spacecraft. 1.1 INTRODUCTION Communication systems are a critical part of any project in the space industry. The communication system sends data collected by the spacecraft to a ground station. A practical satellite would never have been put into orbit without this ability. Even today when a satellite loses its ability to communicate with the ground station it is considered a failure. The communication system also allows for remote control over a spacecraft. No task can be performed on a satellite without an ability to communicate with a ground station and with the other subsystems. The computer system is also an invaluable part of a spacecraft. There have been spacecraft without onboard computers, but without an onboard computer the abilities of the spacecraft are severely limited. The computer allows for quick efficient communication between subsystems and controls many tasks that once could only be commanded from a ground station. An onboard computer also processes some of the raw data itself, eliminating the need to transmit a large portion of data that would occupy valuable communications time. These two crucial subsystems must therefore be selected with great care and consideration. 1

8 1.2 HISTORY OF COMMUNICATIONS IN SPACE Sir Arthur C. Clarke first introduced the idea of satellite communications in the October, 1945 issue of Wireless World. His article outlined plans to place three communication stations in a geo-synchronous orbit (GEO), an altitude of miles, to cover the majority of Earth s surface. He feared that his proposal would be too far fetched to be taken very seriously, especially in 1945 when the only rocket the world knew was the German V-2. Once rocket development turned toward space, people from around the world were given the means to communicate with each other. 1 Twelve years after Clarke s article, on October 4 th, 1957, the Soviet Union launched Sputnik and ushered in the space age, although it did nothing more than beep. 11 On January 31 st, 1958, the United States launched Explorer 1, which had a scientific payload that studied the upper atmosphere. To transmit the collected data, Explorer used a 60-mW/108.3-MHz transmitter and a 10-mW/ MHz transmitter connected to a total of 6 antennas. Two of the antennas were located in the body of the satellite and four extended radially about the circumference. 5 Clarke s prediction of a geo-synchronous communication satellite was realized in 1963 by the Hughes Space and Communications Company s Syncom satellites. Syncom 1 never reached GEO, but a refined Syncom 2 reached GEO on July 26 th, The satellite s successful operation was demonstrated by a radio-phone call from Lakehurst, New Jersey to the U.S Navy ship Kingsport anchored at Lagos, Nigeria. Syncom 2 s antennas had specifications of 6 db on transmit and 2 db on receive. It had two 1815 MHz transmitters and two 7363 MHz receivers. Syncom 3 was put in a true GEO of less than 1 2

9 inclination on August 19 th, 1964 and broadcast the Olympic Games in Japan to the United States for the first time. 18 In 1978, the U.S. Department of Defense (DOD) launched a NAVSTAR satellite to an altitude of 12,000 miles. Although it was intended for military use only, it was soon realized that the information NAVSTAR transmitted would be useful in civilian applications. NAVSTAR used multiple satellites to send MHz signals to receivers on Earth. Those receivers used the data to calculate the global position of the receivers. In 1994, the DOD completed its constellation of 24 NAVSTAR satellites, now known as the Global Positioning System (GPS) HOW THE COMMUNICATION SYSTEM AFFECTS SPACECRAFT The communication system is an important component in satellite design. Every satellite has at least two requirements for its communication system. The communication system needs the ability to receive transmissions from a ground station or other source, and also the ability to transmit data to a remote receiver. There are two kinds of network systems that enable a satellite communication system to perform its tasks. They are the Deep Space Network and the Tracking and Data Relay Space System. A satellite communication system must be designed based on the specification of either network in order to use a particular network. Having high data transferring rates, or having X frequency bands is critical for using these networks. The structure of a satellite can affect the communication system in many ways. For example, the way the antenna is mounted to the main bus of the satellite has an effect on how well the signals can be received. Usually, one of the priorities of designing the satellite is making sure that the satellite is in the best position to send or receive a signal. 3

10 Other systems of the satellite, such as the propulsion system or the power supply, are chosen so that they do not inhibit the operation of the communication system. Choosing the type and size of antenna for the satellite is important. A stronger antenna will enable the satellite to receive the signal from a greater distance and help overcome the restriction of limited frequency bandwidth in signal. 1.4 HISTORY OF COMPUTERS IN SPACE When the National Aeronautics and Space Administration (NASA) was founded in 1958 the standard computer was the UNIVAC. The UNIVAC was a collection of spinning tape drives, noisy printers, and featureless boxes that could fill a room. UNI- VAC was expensive to purchase and operate and constantly needed a small army of technicians to run. Within 15 years NASA had one of the world s largest collections of these computers. Within another 10 years, the giant ground-based mainframe was succeeded by groups of medium-sized computers for spaceflight operations, and multiple machines replaced the single on-board computer. To the amazement of those who knew the computer industry in 1958, NASA eventually flew computers into orbit, to the moon, and to Mars running autonomously for the lifetime of the spacecraft. These remarkable achievements in computer systems used by NASA mirrored those in the commercial arena. 20 Today, NASA uses computers on the ground and aboard both manned and unmanned spacecraft. In the first 25 years of its existence, NASA conducted five manned spaceflight programs: Mercury, Gemini, Apollo, Skylab, and the Space Shuttle. The latter four programs used spacecraft that contained on-board digital computers. However, 4

11 fifteen years of unmanned earth orbit and deep space missions were carried out without general-purpose computers on board. The first computers flew on manned spacecraft in the early 1960 s. These systems operated in real-time mode, handling essentially asynchronous inputs and outputs and continuous processing, similar to a telephone operator who does not know on which line the next call will come. The requirement for real-time processes led to new requirements. Software needed to be reliable. If the software failed, the vehicle was no longer controllable. Memory needed to be stable so that if the power were lost then the program in storage would not disappear. Because modern random-access memories (RAM) are usually unpredictable, older technologies such as ferrite core continue to be used on spacecraft. 20 The future of on-board computer systems relies on the dedication of many research organizations. One such organization is the Jet-Propulsion Laboratory (JPL) in Pasadena, California. The scientists and engineers there are creating a powerful, reliable and efficient onboard computer to guide spacecraft beyond our solar system in the High- Performance Computing and Communications (HPCC) program. This low-energy, high performance machine will collect and analyze enormous amounts of data and then transmit the results to Earth-bound scientists. The machine will be capable of simultaneously controlling many in-flight operations currently directed by Mission Control. This prototype operates at 30 million operations per second (MOPS) per watt and utilizes 20 nodes capable of executing almost 32-bit MOPS each. Today s computers experience computational errors caused by cosmic radiation. As part of HPCC s Remote Exploration and Experimentation (REE) project, engineers 5

12 are developing software that corrects radiation-induced errors without any intervention from Mission Control. Currently, NASA s most powerful radiation-hardened computer matches the capability of an IBM 486 PC, which was introduced in These computers are programmed to carry out operations at prearranged times and if anything goes wrong, they fall into safe mode and wait for the ground crew to redirect them. Other problems with current computer technology on spacecraft include the data transfer rate. Currently, the rate at which spacecraft can transmit data from the outer planets to earth is barely more than a 56k modem. The data rate is somewhat faster for spacecraft transmitting from Earth orbit, but it is still not fast enough to keep up with onboard instruments. The HPCC program will attempt to develop a powerful onboard computer that can collect, analyze and reduce data. By shipping reduced, rather than raw, data scientists could retrieve much more information HOW THE COMPUTER SYSTEM AFFECTS SPACECRAFT The command and data handling (C&DH) system uses the onboard computer to perform two primary functions. The system receives commands and/or processes sensor information and distributes commands to the other spacecraft systems. The commands typically originate from a ground station or from other subsystems on the spacecraft. The onboard computer is capable of validating, decoding, processing, and distributing commands to the other subsystems. A single computer can provide control over all spacecraft systems efficiently by combining C&DH into one subsystem. Onboard computers are also required for data handling purposes. Onboard sensors take data relating to attitude, environment, location, as well as many other mission- 6

13 related subjects. Onboard computers also allow telemetering, which means that they are capable of transmitting sensor data and interpreting data for internal spacecraft command. To select a proper computer for a mission, the other systems on the spacecraft must be defined, and requirements established. The complexity of the system is directly based on the needs set forth by other spacecraft systems. The speeds of telemetry processing and command rates are determined by the required speed to effectively operate all other spacecraft systems OVERVIEW OF REPORT This report gives an overview of the steps taken in choosing an effective computer and communication system for a spacecraft. Chapter 2 models the methods used in the design. Chapter 3 describes the process of choosing and designing the communications and computers. Chapter 4 concludes the report by analyzing the information within the report. 7

14 CHAPTER 2 INTERACTIONS AND MODELING This chapter analyzes the interactions between the communication and C&DH systems and the other systems present on a typical spacecraft. The last section of this chapter models some of the attributes of a communication C&DH system, allowing the needs of the spacecraft to be calculated so that appropriate systems can be selected. 2.1 SUBSYSTEM INTERACTIONS Table 2.1 lists the interactions between the communications and C&DH subsystems and the rest of the subsystems present on a spacecraft. Interactions are rated so that a 2 means the systems have strong interaction, a 1 means moderate interaction, and a 0 means no interaction. Table 2. 1: System Interactions Communications C&DH Astrodynamics 2 2 Mission Analysis 2 2 Mission Geometry 2 2 G&N 2 2 Propulsion 1 1 ADCS 1 1 Communications 2 C&DH 2 Power 2 2 Thermal 1 1 Environment 2 2 Structures 2 2 Mechanisms 1 2 Launch Vehicle 0 0 Program Management 0 0 Cost 2 2 Mission Operations 2 2 Ground Systems 2 2 Political and Legal 2 0 8

15 2.1.1 Astrodynamics Communications has a strong interaction with astrodynamics. The process of choosing an orbit is significantly influenced by whether the satellite is in view of a ground station and for how long. The time the satellite is in view of one or more ground stations is to be maximized. In this same way, aspects of C&DH also have significant interaction with astrodynamics. Commands need to be sent from ground stations to be received onboard and distributed among relevant subsystems onboard. The mission analysis and mission geometry aspects also have strong interactions with communications and C&DH Guidance and Navigation Both communications and C&DH have strong interactions with guidance and navigation. Ground stations need to know where the satellite is in its orbit at all times so that if it strays from its orbit, corrections can be made. These corrections need to be communicated to the satellite from ground stations in addition to onboard sensors so that these messages can be relayed to other onboard systems to perform the appropriate tasks Propulsion Communications has some interaction with propulsion systems. Ground stations may have to communicate instructions to the onboard computers pertaining to the propulsion system. The C&DH system also has interaction with propulsion. Onboard computers distribute the aforementioned commands or commands from onboard sensors to activate or deactivate the propulsion system in order to keep the satellite on the correct path. 9

16 2.1.4 Attitude Determination and Control Communications and attitude determination and control system (ADCS) have some interaction. If the satellite is pointing in the wrong direction, it will be unable to transmit signals to ground stations. Therefore the correct attitude must be maintained. The C&DH system also has some interaction with the ADCS. The satellite needs to use the onboard computer to interpret sensor data and ground station commands to make orientation changes to the satellite Power The communication system and the C&DH systems interact strongly with the power system. The communication system consists of three components: the transponder, the filters and switches, and the antennas. Of those three, only the transponder requires power. The transponder consists of the transmitter, and the receiver. The power needed by the communication and C&DH systems varies with the system. Ku-Band transponders require only 24.3W, with 20W needed for transmitting. S-Band transponders require 57.5W, with 40.0W to the transmitter. An X-Band transponder needs 45.4W, with 35.0W for the transmitter. The transmitter requires more power than the receiver because of the need for increased signal strength resulting from the signal s modulation. 8 The power required by the C&DH system also varies. Different spacecraft have different operations they must perform, and the C&DH system must be designed to meet those requirements. The more complex the operations are, the more power the C&DH system requires. The power ranges anywhere from 7W to 25W, with typical systems using 13W to 18W. 10

17 2.1.6 Thermal The communication and C&DH systems must operate within certain temperature ranges. Without an atmosphere to help regulate the temperature difference between night and day, the thermal system needs to keep the other systems within their design temperature limits. In Earth orbit, the spacecraft is exposed to temperatures between 250 F and 250 F (-157 C and 121 C). 22 For components of the communication system mounted on the outside of the spacecraft, such as antennas, the operating temperature range is wide. A parabolic reflector can operate between 160 C and 95 C, a GPS antenna can operate between 95 C and 70 C, and a Telemetry, Tracking and Command antenna can operate between 65 C and 95 C. The antennas need some form of thermal protection since they cannot operate within the full temperature range of space. Inside the spacecraft, not only do the solar thermal differences affect the computer and command units, but the heat dissipated by the power units also heat up the components. One way to remedy this is to mount the command and communication units away from the power source, but by doing this, power may be lost in the extra wiring length Environment The communication and C&DH systems must also be protected from the environment of space, much like being protected from thermal effects. In fact, a major environmental effect is the thermal effect from the sun. The ionosphere is another factor that affects communications. The ions absorb and scatter the electromagnetic communication waves, disturbing or altogether interrupting communication between the ground and the spacecraft. 11

18 2.1.8 Structures The communications subsystem interacts strongly with the structures subsystem. The interaction comes from the need of the communications subsystem to be pointed in the direction of communication. The placement of the communication subsystem is as important as the quality of the communication subsystem. The limited choices for placement of the communication subsystem require an interaction between the communications and structures subsystem teams at each step of the design. The computer subsystem also interacts strongly with the structures subsystem. The computer needs a secure location and needs a location as free from radiation as possible. The computer also needs a central location to minimize the command transmission distance, which reduces the weight of the wire in the satellite. The limited choices for placement of the computer subsystem require an interaction between the communications and structures subsystem teams at each step of the design Mechanisms The communications subsystem interacts moderately with the mechanisms subsystem. The interaction comes from the fact that the communication antenna must be deployed and oriented. Aside from the initial deployment and the occasional adjustment the communications subsystem does not interact with the mechanisms subsystem. Since mechanisms are not as critical as the structures to the communications subsystem the mechanisms and communications subsystem teams need to interact at the end of the design. 12

19 The C&DH subsystem has a strong interaction with the mechanisms subsystem. The onboard computer must interpret automated commands or commands from the ground station and relay them to the mechanisms so they know when to function Launch Vehicle The communications subsystem does not interact with the launch vehicle selection. The communications subsystem does not contribute a substantial portion to the size or mass of the satellite. The computer subsystem also does not interact with the launch vehicle selection. The computer subsystem does not contribute a substantial portion to the size or mass of the satellite Mission Operations The communication and computer subsystems have strong interactions with mission operations. The first step in sizing and choosing a C&DH system is to identify the functional requirements of a space mission. Capability of a C&DH system is based on specific needs presented by a mission. Reliability, lifetime, resistance to radiation, command capability and processing rate are all aspects of a C&DH system that need to be tailored to fit the needs of spacecraft and mission Ground Systems The communications and computer subsystems have a strong interaction with ground systems. Communication equipment on a spacecraft must be designed to interact with ground stations for the purpose of command (of the spacecraft) and data exchange. This interaction has implications for the power of the communications system, and the 13

20 capability of the C&DH system. Location of ground stations must be determined based on orbit geometry and spacecraft communications equipment. Appropriate ground stations must also be equipped with suitable command and communications systems to successfully interact with the spacecraft Cost All communications, C&DH system decisions have an impact on the cost of the spacecraft. Size and capability of a communications and computing system has a direct interaction with the cost of the spacecraft systems, and the ground systems selected for mission operations. Complexity and capability adds cost in the design, the building and the support of any communications or computing system Political and Legal The computer and data handling subsystems have no interaction with political and legal aspects of the mission. The communications subsystem however, has a strong interaction with political and legal aspects of a space mission. Legal constraints exist for space-based communications systems. In the United States, the Federal Communications Commission (FCC) enforces these laws for commercial applications, while the Interdepartmental Radio Advisory Committee (IRAC) enforces the laws for military applications. Mission designers must operate through the FCC or IRAC to obtain permission to operate radios at specific frequencies and orbits and to use specific ground stations. The purpose of this regulation is specifically to avoid interference between spacecraft communications, ground communications and any other influence that may cause radio communications problems

21 Program Management The communications and computer subsystems have no interaction with program management. The communications and computer subsystems operate independently of the business and managerial aspects of the program. 2.2 SYSTEM MODELING Data handling is a major component of the communication and command systems. It consists of how the on-board computer processes the data collected by the spacecraft s sensors, and also how the data interacts between satellite and ground station. Other components in communication and command systems include data telemetry, providing the commands, and housekeeping. All communication component performances depend on how well the communication signal can be processed. In outer space, Radio Frequency (RF) signals are the most common signals used in communication. Existing spacecraft rely on the Deep Space Network (DSN) to communicate with ground stations or in between other satellites. The DSN excepts varieties of RF frequencies, such as L, S, X, and K bands. Table 2.2 contains the operating frequency range for each band. Table 2. 2: RF Frequency Ranges Frequency Band Frequency Range (MHz) L S X K The data sampling rate of satellites depends on the different types of frequency bands and the antennas used. The data-sampling rate can be modeled using the following equation: 15

22 R s =H(x)/T s; H(x)=Σ I=1 to M 1/M*Log 2 (M); (1) In equation 1, R s is the data transferring rate, H(x) is average information of outcome, and M is the amount of outcomes, and is represented in binary form. Lastly, T s is the duration of outcomes. Equation 1 states that if T s is relatively small, the data transfer rate is large. The properties of the frequency band and antenna, both on the spacecraft and ground stations, need to have the capability to reload the RF signals continuously to have a high transfer rate between ground stations and spacecraft. Reloading the RF signals continuously means the duration time will be smaller, thus, a high transferring data rate will be attained. Selecting different types of frequency bands and antennas in spacecraft design is critical, because the properties of the frequency bands and antennas affect the quality of signals received and/or transmitted. In satellite communication, the quality of signals is determined by the power gain of the signal. Having a high power gain in communication signals means clearer signals can be received by the satellite. In this case, selection of the antennas is critical in space communication; different types of antennas have different amounts of power gain. The overall signal power gain is P Rx /P Tx =G AT G FS G AR (2) where P Rx is the signal power into the transmitting antenna, G AT is the transmitting antenna power gain, G FS is the free space power gain, G AR is the receiving antenna power gain, and P RX is the signal power into the receiver. 16

23 Different antennas have different relationships between different properties. One such relationship is between the effective area of an antenna and the antenna power gain of the antenna, given by equation 3. G AT =4πA e /(λ 2 ) (db) (3) where λ=c/f is the wavelength, c is the speed of light, f is the operating frequency in Hz, and A e is the effective area of the antenna. The operating frequency has an effect on power gain, as equation 2 shows. Higher operating frequency means higher power gain. Table 2.2 shows that the K band RF signal has the highest operating frequency range, so it will probably be the best type of RF signal to use in terms of power gain. Free space power gain is determined from G FS =(λ/(4πd)) 2 (db) (4) where d is the distance between receiving and transmitting antennas. The receive power is described in following equation. P Rx =G AT A e P Tx /(4πd 2 ) (db) (5) where G AT is the transmitting antenna power gain, P RX is the signal power into the receiver, and d is the distance between receiving and transmitting antennas. The properties of the antennas are also important in having better signal power gain in satellite communication. The effective area of the antenna has an influence on the receiving power of the signal. Higher effective area results in higher receiving power. Table 2.3 lists the properties of different types of antennas, where A is the mouth area. 17

24 Table 2. 3: Antenna Properties Type of Antenna Power Gain, G A Effective area, Ae (m 2 ) Isotropic 1 4λ/2π Dipole or Loop λ/2π Half-Wave Dipole λ/2π Horn 10A/λ A Parabola 7A/λ A Turnstile λ/2π Figure 2.1 gives a block diagram of a communication system between the receiving and transmitting antennas. Baseband signals in Receiver Transmitting antenna gain G AT Baseband signals out Receiving antenna gain Free space gain Transmitter G AR G FS Figure 2. 1: Transmitter-Receiver Diagram Figure 2.2 gives a simplified block diagram of the transmissions between the communication satellite and the ground station. BPF LNA BPF LNA Bandpass filter Low noise amplifier LO Local oscillator Figure 2. 2: Ground-to-Satellite Communication Diagram 18

25 According to Figure 2.2, the receiving signals from the antenna goes through a series of signal modulations. First the signal goes through a Bandpass Filter, which will filter out the noise, then it passes through the Low Pass Filter, which amplifies the signal s magnitude. Then the signal goes through the local oscillator, which converts the signals into data, such as commands for satellite operations. 2.3 COMMAND AND DATA HANDLING MODELING The C&DH system is usually the last aspect of a spacecraft to be chosen. The processor is not actually designed and built based on the spacecraft specifications, rather it is selected based on the spacecraft s mission objectives it must carry out. Reliability is a major factor in choosing a C&DH system. A brand new computer that can perform every function on the spacecraft autonomously is most likely to be passed over in favor of a computer with a successful history in similar missions. That is not to say that the same computer is used on every flight. Each spacecraft has a different set of objectives, and as technology advances, so do the scope of the missions, and the computers must evolve accordingly. Minor adjustments are made to existing equipment to compensate for the complexity of the mission. The more complex the mission is, the more complex the computer must be. 23 Some spacecraft may not even need an actual computer. Simple tasks such as storing commands received from ground stations are carried out by sending a time tag with the commands. 23 When the system s time reaches the specified time in the time tag, that specific command is carried out. Using time tags as a method of storing commands is helpful when the spacecraft is out of view of the ground stations for a period of time. The time tag can be set to a time when the ground station is on the other side of the Earth 19

26 for an Earth orbiting spacecraft, or when the spacecraft s view of Earth is blocked by the moon or another celestial body. Knowing how much data will need to be sent to and from the spacecraft is important in choosing a C&DH system. If the spacecraft is designed to be completely autonomous, the stored commands can be programmed before launch, eliminating the need for the spacecraft to process commands onboard. Most spacecraft are not designed to be autonomous and need specific commands for unforeseen events, such as a sudden change in the mission. The data transfer rate, constrained by the choice of communication system, dictates how much data can be sent in a given time frame. A second constraint is the speed at which the spacecraft can convert the digital commands to analog operations. 23 The computer industry refers this conversion speed as the computer s bus speed. Even though a 600MHz processor in a personal computer can process 600 million operations a second, the computer will ultimately run as fast as its bus speed. Spacecraft with a data transfer rate greater than the bus speed requires data buffers. Data buffers either temporarily store outgoing spacecraft data to send it all at once at the transfer rate, or store the incoming data to be send to the data handling system at a speed the system can handle. Data buffers are commonly used on deep space probes. The further out the probe travels, the more the transfer rate decreases, and the need for adapting the transfer rate of the ground station to the data rate of the satellite arises. 19 Being able to handle computations is not the only aspect of choosing a C&DH system. The space the system will take up in the spacecraft, the mass the system will add to the spacecraft, and the power it needs from the spacecraft are other factors that must be 20

27 considered. 23 Table 2.4 lists size, weight, and power requirements for a typical C&DH system. Table 2. 4: Size, Mass, and Power Ranges for a Typical C&DH System Command Data Handling Combined Size (cm 3 ) Mass (kg) Power (W) CHAPTER SUMMARY In this chapter, the interactions between the communication and C&DH systems and the rest of the spacecrafts systems were analyzed to get a model of how the systems work together. A closer look at the communications and C&DH systems was taken, and a combination of mathematical representations and prior experiences described the process on how these systems are chosen for a particular spacecraft. In chapter 3, five examples of communications and C&DH systems on existing spacecraft are presented. 21

28 CHAPTER 3 SYSTEM EXAMPLES Chapter 3 takes the information presented in chapter 2 and explains it further using five existing spacecraft as examples of different communication and C&DH systems. The spacecraft examples are the Galileo Jupiter Probe, the Cassini Saturn Probe, the Hubble Space Telescope, the Mars Pathfinder, and the Mars Odyssey. 3.1 GALILEO The Galileo space probe was launched on October 18, 1989 on a six-year voyage to Jupiter. IT was designed to continue the mission the Voyager probes started over ten years prior, to study Jupiter and its four major moons in more detail than any previous spacecraft. Galileo arrived at the Jovian system in December of Communication System Galileo s communications system is comprised of a 4.8m high gain antenna (HGA) and two 120 low gain antennas (LGA). Figure 3.1 shows the placement of the antennas on Galileo. The HGA was designed to provide approximately 34dB of gain at X-band (10GHz) for a 134kbps data transfer rate, but in 1992 the HGA malfunctioned and failed to deploy. Galileo s mission had to be modified to use its LGA, which could only provide 8dB of gain at S-band (2.8 GHz) with a data transfer rate of 8-16bps. In 1996, modifications were uploaded to Galileo to increase the data rate to bps by using data compression. 10 One major component of the Galileo mission was the atmospheric probe that was sent into Jupiter s atmosphere. 12 Ames Research Center developed a relay radio hardware system (RRH), consisting of a 1.1m antenna and two receivers to transmit the at- 22

29 mospheric probe s data to Earth. The 1.1m antenna was pointed using two ultra-stable oscillators so that the LGA link to Earth would not be broken. Figure 3. 1: Galileo Antenna Placement Command and Data Handling System Galileo s CD&H system is based around six COSMAC bit processors supplied by RCA, which Galileo uses as a single multiprocessor computer. The processors communicate with each other using two communication busses controlled by one of the processors. 3.2 CASSINI Cassini was launched on October 15, 1997 on a seven year journey to Saturn, and its moon Titan. Currently between Jupiter and Saturn, Cassini will deploy a probe that will study the surface and atmosphere of Titan while the spacecraft orbits Saturn itself, studying it with an intensity similar to Galileo and Jupiter. 23

30 3.2.1 Communication System Cassini s communication system is comprised of the Radio Frequency Subsystem (RFS) and the Antenna Subsytem (ANT). 3 The spacecraft s transmitter produces an 8.4 GHz X-band signal modulated with data from the C&DH system. Ground stations transmit a 7.2 GHz X-band signal, which is in turn received by Cassini. 4 Cassini is equipped with three antennas, shown in figure 3.2, one four-meter high gain antenna (HGA) and two low gain antennas (LGA-1 or -2). 3 The HGA provides fast data transfer, but must be directly focused on the Earth to do so. The LGA s have a slower data transfer than the HGA, but need not be focused on the Earth. The antennas were chosen based upon the distance from Earth to Cassini, frequency bands used, data rates used, and the power of the transmitter. 3 To switch between using the three antennas, Cassini uses waveguide transfer switches (WTS) to select the route the transmitting data is sent from the spacecraft. 3 Figure 3. 2: Cassini Antenna Placement 4 Diplexers are used to allow simultaneous transmission and reception on separate channels on the same antenna. 3 Cassini has two sets of diplexers, one for each trans- 24

31 ponder. The second transponder acts as a backup, and is not powered when the first is on Command and Data Handling System The C&DH system is composed of the computer and input/output (I/O) units, as well as a solid state recorder system for data storage. 2 The Command Data System Electronic Assembly (CDEA) is centralized around an engineering flight computer (EFC), designed by IBM. IBM s EFC has a successful legacy, as well as many features required by Cassini to complete its mission, such as memory write and instruction execute protection, security to allow certain commands to enable only the functions with clearance, and a fail-safe that will reset the processor. 2 All of Cassini s I/O devices are designed at the Jet Propulsion Laboratory (JPL). The I/O units include a bus interface system (BIS), which takes the commands from the CDEA and sends them to their appropriate spacecraft systems. A part of the BIS are the bus interface units (BIU), which translate the commands from the BIS to commands the individual spacecraft system can understand. 2 Each system, such as propulsion and ADCS, has its own individual BIU. Data going from the separate systems to the CDEA are gathered by remote engineering units (REU). The REU s work the same way as the BIU s, only instead of translating commands, they translate spacecraft data (ie. temperature, position, spacecraft health, etc.) and translate them so that the BIS can send them to the computer, and possibly to the communications system for relay back to Earth. 2 Spacecraft stored their data using flight tape recorders until Cassini, which is the first to use a solid state data recorder (SSR). 2 Cassini s 30V SSR was manufactured by TRW, Inc. The SSR can store 2Gb or data, retaining 90% of its capacity after 15 years. 2 25

32 Four serial ports interface with the SSR; data recording, data playback, commanding, and status. 2 The SSR also stores the preloaded flight commands that Cassini uses en-route to Saturn MARS PATHFINDER The Mars Pathfinder was launched on December 4, 1996 and landed on Mars on July 4, It s mission was to study the Martian atmosphere and surface, as well as serve as a test for advancements made in planetary landers that would be utilized on future Mars missions Communication System The Mars Pathfinder relied solely on the Deep Space Network (DSN) to communicate with ground stations on Earth. 9 The Pathfinder used a high gain antenna operating on the X-band frequency. This antenna, shown in figure 3.3, had the capability to send and receive data between 6kpbs and 70Mbps. 13 Figure 3. 3: Mars Pathfinder High Gain Antenna 15 26

33 3.3.2 Command and Data Handling System Command and Data Handling of Pathfinder is controlled by the Integrated Attitude and Information Management system (AIM). This system uses a R600 computer with a VME bus. The computer had 120 Mb of memory, and it performed 22 million command operations per second HUBBLE SPACE TELESCOPE The Hubble Space Telescope was deployed by the Space Shuttle Discovery on April 25, Hubble was designed to be updated with new technology when available. Three service missions took place in December of 1993, February of 1997, and December of Two more service missions are scheduled for February of 2002, and July of The telescope allows scientists and astronomers to see parts of the universe that no other telescope can see Communication System The communication system on the Hubble Space Telescope (HST) is composed of two S-Band Single Access Transmitters (SSAT) and two high gain antennas. The SSAT s send the data up to the Tracking and Data Relay Satellite System, (TDRSS). From there the data is sent to a relay station in White Sands, New Mexico, before being finally sent to the HST science institute in Baltimore, Maryland. The SSAT is capable of transmitting data at rates between kb/sec Command and Data Handling System The original computer system on the (HST) was specially designed in the 1970 s for space based operations, known as DF-224. The DF-224 stands for Digital Fixed Point 27

34 2 s complement 24-bit word Rockwell computer module and is comparable in speed to a modern pocket calculator. 7 Since the DF-224 is a specially designed computer, special skills are required to program the computer. Special skills require additional funding, and because of this and system degradation, the computer was upgraded and eventually replaced. The DF-224 was upgraded in 1993 during the first servicing mission by adding a coprocessor based on the microchip. The computer was replaced on STS-103 in December 1999 using an Intel based microchip as its processor. The new computer is 20 times faster and has 6 times more memory than the old one. The data storage devices on the HST were also designed in the 1970 s and used 3 reel-to-reel tape recorders that could store 1.2 gigabytes of data each. A Solid State Recorder (SSR) replaced one of the tape recorders when the coprocessor was added to the DF-224. The SSR is the same size as the tape recorders but store data digitally and are capable of storing 12 gigabytes of data each. A second tape drive was removed and replaced with an SSR during the same servicing mission that replaced the DF MARS ODYSSEY The Mars Odyssey spacecraft was launched on April 7, 2001 carrying scientific payloads designed to make observations of, and to help better understand the climate and geographic history of Mars. It arrived at Mars orbit on April 24, Communication System Odyssey's telecommunications subsystem is composed a radio system operating in the X-band microwave frequency range and another radio system that operates in the ultra high frequency (UHF) range. The X-band system is used for communications be- 28

35 tween Earth and the orbiter, while the UHF system is used for communications between Odyssey and any landers present on the Martian surface. (A secondary mission is to provide uplink communications for Mars landers.) Command and Data Handling System Odyssey's computing functions are performed by the command and data handling subsystem. The foundation of the command and data handling system a RAD6000 computer, a radiation-hardened version of the PowerPC chip used on most models of Macintosh computers. The computer has 128 megabytes of random access memory (RAM) and three megabytes of non-volatile memory. Non-volatile memory allows the system to maintain data even without power. This computer runs Odyssey's flight software and controls the spacecraft through interface electronics. 15 Interface electronics make use of computer cards to communicate with external peripherals. For redundancy purposes, there are two identical strings of these computer and interface electronics, so that if one fails the spacecraft can switch to the other. One such interface electronics card is used for communication between Odyssey's attitude sensors and its science instruments. A master input/output card collects signals from around the spacecraft and also sends commands to the electrical power subsystem. The interface to Odyssey's telecommunications subsystems is done through another card called the uplink/downlink card. 15 There are two other boards in the command and data handling subsystem, both internally redundant. The module interface card controls necessary switches to backup hardware and provides the spacecraft time. A converter card takes power from the electrical power subsystem and converts it into the proper voltages for the rest of the com- 29

36 mand and data handling subsystem components. The last interface card is a single, nonredundant, one-gigabyte mass memory card that is used to store imaging data CHAPTER SUMMARY In this chapter, combinations of communication and command and data handling systems are presented for five different existing spacecraft. Each of these spacecraft has a different mission and needs different communication and C&DH systems to meet its requirements. Deep space probes, such as Galileo and Cassini, require more powerful transmitters, to compensate for their distance from Earth, than Earth orbiting spacecraft. The onboard computers are chosen with the knowledge of what functions the spacecraft needs to perform. Chapter 4 discusses the actual process of choosing the systems to meet certain requirements. CHAPTER 4 - CONCLUSIONS This chapter examines a simple communications system using a satellite in low earth orbit and Standard Ground Link Stations. In addition, this chapter discusses the most commonly used components of communications systems. 4.1 LOW EARTH ORBIT ANALYSIS Using the equations in the previous chapters, potential communication requirements were generated for a satellite in a 400 km low earth orbit linked with a Standard Ground Link Station (SGLS). The example includes a link budget for both uplink and downlink. The data used to calculate atmospheric and environmental effects is shown in appendix 1. 30

37 Table 4. 1: Uplink Input parameters (SGLS to LEO Satellite) 11,1 SATELLITE ALTITUDE 400 KM TRANSMIT FREQ 1645 MHZ TRANSMITTER POWER 91 DB MODULATION LOSS 21 DB TRANS LINE LOSS 0 DB TRANS ANT GAIN 0 DBI TRANS ANT DIA 6 FT RAIN ATTENUATION 0 DB RECV ANT GAIN 0 DB RECEIVE ANT DIA 10 FT POLARIZATION LOSS 3 DB RECEIVE LINE LOSS 2 DB RECEIVE NOISE FIG 5 DB DATA RATE MB/S IMPLEMENTATION LOSS 2.4 DB REQUIRED Eb/No 10^-6 BER 24.5 DB Table 4. 2: Downlink Input parameters (LEO Satellite to SGLS) 11,1 SATELLITE ALTITUDE 400 KM TRANSMIT FREQ 1545 MHZ TRANSMITTER POWER 37 DB MODULATION LOSS 2 DB TRANS LINE LOSS 3 DB TRANS ANT GAIN 0 DBI TRANS ANT DIA 10 FT RAIN ATTENUATION 0 DB RECV ANT GAIN 0 DB RECEIVE ANT DIA 6 FT POLARIZATION LOSS 0.5 DB RECEIVE LINE LOSS 0.25 DB RECEIVE NOISE FIG 3 DB DATA RATE 1 MB/S IMPLEMENTATION LOSS 3 DB REQUIRED Eb/No 10^-6 BER 6 DB 31

38 Table 4. 3: Link Budget, Uplink ELEVATION ANGLE deg OFF-NADIR ANGLE deg SLANT RANGE km TRANSMIT POWER dbm MODULATION LOSS db LINE LOSS db TRANS ANT GAIN dbi PATH LOSS db ATMOSPHERIC LOSS db RAIN ATTENUATION db RECEIVE ANT GAIN db POLARIZATION LOSS db RECEIVE LINE LOSS db DATA RATE LOSS db*hz IMPLEMENTATION LOSS db ENERGY/BIT dbm/hz NOISE DENSITY dbm/hz SPREADING GAIN db REQUIRED Eb/No db MARGIN Table 4. 4: Link Budget, Downlink ELEVATION ANGLE deg OFF-NADIR ANGLE deg SLANT RANGE km TRANSMIT POWER dbm MODULATION LOSS db LINE LOSS db TRANS ANT GAIN dbi PATH LOSS db ATMOSPHERIC LOSS db RAIN ATTENUATION db RECEIVE ANT GAIN db POLARIZATION LOSS db RECEIVE LINE LOSS db DATA RATE LOSS db*hz IMPLEMENTATION LOSS db ENERGY/BIT dbm/hz NOISE DENSITY dbm/hz SPREADING GAIN DB REQUIRED Eb/No db MARGIN db