Conceptual Design of an Earth Observation Satellite

Transcription

1 Conceptual Design of an Earth Observation Satellite Ehraz Ali 1 Ehraz_ali@yahoo.com Abid Ali Khan 3 Babar Saeed 2 Farooq Bin Akram 4 Abstract The essence of this study is to design a micro scale Earth Observation Satellite. The design process started with the conceptual design in which the disciplinary knowledge of orbital mechanics, Astrodynamics and spacecraft design was applied. Followed by the mission objectives as basic constraint, the design process was initiated by a bottom up approach. The first phase involves the basic requirement of any satellite that is the orbit selection. The space mission subsystem design approach was the second phase in which the required sub-systems were selected for completing the mission requirements. The optical instrumentation, attitude control subsystem, communication, telemetry, tele-command, navigation, thermal control and on board data handling subsystems were given the appropriate importance. The satellite configuration was selected and subsystems were modeled in SOLID EDGE software. The complete geometric layout was modeled using the same software. Finally the launch vehicle selection was incorporated in order to accomplish the orbit transfer of satellite. The design feasibility study was applied to explore the design space and select the best inclination and altitude for the satellite. The proposed design was validated for successful orbital simulation using the Satellite Tool Kit (STK 9) software. The proof of design concept was accomplished keeping in view the special considerations. These involve Earth coverage parameter, orbit perturbations, orbit maintenance, launch environment, space debris and Satellite environment in space. Keywords Astrodynamics, Orbital Mechanics, Spacecraft, satellite, telemetry, subsystem, navigation, simulation, Earth coverage. I. INTRODUCTION Spacecraft design is a separate discipline of aeronautics different from the analytical disciplines such as aerodynamics, structures, propulsion, control and rocket engineering. A spacecraft designer needs to be well versed in all these and many other specialties. Those involved in design can never quite agree as to just where the design process begins. The designer thinks it starts with a new concept, the sizing specialist knows that nothing can begin until an initial estimate of the weight is made. The customer civilian or military feels that the design begins with the requirements to accomplish a new demand [1-3]. Actually they all are correct; design is an iterative process, as shown in the figure of design wheel. Figure 1.Design Wheel [1] At different phases of design, alterations are made in accordance with the requirements and constraints that involve manufacturability, cost, material, environment and mission objectives. A general consensus is made by keeping all the trade-offs and insuring the accomplishment of the objectives [2]. The study aims at designing a cost effective and optimal design solution to complete the mission objectives specifically involving the Earth observation. An Earth observation platform orbiting the Earth with high resolution imaging devices directly pointing downward can provide vital information to the user. The application of such a satellite can be surveillance, Climatic changes, environmental monitoring, mineral exploration, oceanography, disaster management and many other respective fields. II. SPACECRAFT DESIGN APPROACH There is variety of approaches adopted by space technology specialists while designing a new spacecraft [2]. A spacecraft design begins with a new concept that will determine the requirements of the customer may it be military or civilian in its nature. One of the possible approaches is depicted in the figure below,

2 Figure 2. Bottom up design approach The complete design process is team effort put forward by dedicated engineers and scientists to accomplish the design requirements within constraints and the best possible performance characteristics. Design is an art through which engineers and scientists reflects their innovative ideas to fulfill the needs of future [1]. It all begins with a simple paper sketch that leads to the modern inventions. III. Figure 3. Pencil Sketch of the proposed design SATELLITE MISSION REQUIREMENTS AND INITIAL SIZING Space mission analysis and design begins with one or more broad objectives and constraints and then proceeds to define a space system that will meet them at the lowest possible cost. Broad objectives and constraints are the key to this process. Space missions range widely from communications to planetary exploration, to proposals for space exploration and to burial in space [2]. The major mission objectives of the proposed Earth Observation satellite are, Image capturing platform providing images in visible light. Infrared imaging for environmental and climatic changes. Multi spectral images. Global coverage and coverage of Pakistan (30 00 N, E). Day to day update to Ground Mission Control Centre. Satellite Mission Requirements dictate the complete design process from orbit selection, subsystem level design and launch vehicle selection. To estimate the size and main characteristics of the mission payload several approaches are available from the historical trends and scaling techniques. The techniques adopted or initial mass estimation for the proposed design are, 1. Analogy with existing systems 2. Scaling from existing systems 3. Budgeting by components (component buildup method) All three approaches were used and initial estimates were calculated, the summary of results is tabulated below, Table 1.Summary of initial mass estimation Analogy with existing systems Scaling from existing system Component buildup method Mass dry =246.8kg Mass dry=162.4 kg Mass dry = 104kg Mass payload = kg Mass payload = kg Mass payload = 36.4 kg Budgeting by components method also called as components buildup method is the safest approach while estimating an initial guess for the space mission design. The space subsystems along with structure, sensors and all components are selected and parameters are estimated based on the mission objectives. The same approach is used in conceptual design of aircraft to estimate initial weight. The pie chart represents the approximate payload design and estimation based on component wise contribution, Figure 4. Component pie chart for Earth observation satellite IV. SATELLITE SUBSYSTEM DESIGN AND 3D MODELING After knowing the initial mass for a satellite, the geometrical configuration is the next step. The configuration starts from an initial conceptual sketch that dictates the overall look of the spacecraft. Different modeling tools and software are used in

3 designing the basic satellite configuration by the design engineer. The initial configuration is not a final and rigid decision of the designer; it can be altered for necessary amendments after successive operation of iterations. Satellite configuration can be cylindrical, spherical (ideal), cubical or any other geometrical cross section. However, based on figure of merit analysis cubical configuration is selected for the proposed design as it is efficient in internal packaging and more stable as compare to other configurations. After the initial estimations and selecting the configuration for the spacecraft, the next phase is to decide about the sub systems that will constitute the spacecraft. An unmanned spacecraft consists of at least three elements, a payload, a spacecraft bus and a booster adapter. Payload is mission dependent and dedicated equipment that is the prime objective of the spacecraft. The spacecraft bus carries the payload and provides housekeeping function. The booster adapter provides the load carrying interface between the spacecraft bus and the boost vehicle. These major three elements will constitute the overall spacecraft geometry. Several open internet sources were consulted and SURREY SATELLITE technology ltd was taken for selecting the necessary equipment for payload design. These selected subsystems were then modeled in SOLIDEDGE Software and integrated with the basic satellite configuration. The three main systems selected for Earth observation objectives are, 1. CHRIS(Compact High Resolution Imaging Spectrometer) Imager 2. WIDE SWATH Multi spectral imager 3. Thermal infrared (TIR) Camera For attitude control, tracking and communication subsystems, the equipment selected was, 1. Magnetometer 2. Reaction Wheels 3. S band Antenna 4. Star Tracker 5. Transmitters Figure 5. 3D model of proposed design (internal assembly and fully deployed) V. SATELLITE ORBIT DESIGN AND ORBIT SIMULATION Keeping in view the requirement for provision of high resolution imagery of Earth with global coverage for photo reconnaissance, Near-polar LEO is selected for the proposed design due to its effective use by the Earth observers, military or civilian in operations. Important orbital parameters calculated using the analytical equations are tabulated below in Table 2. The orbit selection criteria are tabulated in Table3. Satellite geometrical modeling was done using the SOLID EDGE software. The proposed design internal assembly was designed using the assembly module whereas the overall design was completed using the part module.

4 Parameter Orbital Height Figure 6.Near Polar Low Earth Orbit Table 2.Orbital parameters Value 750 km Orbital Inclination Orbital Period Orbital Speed min km/sec After estimating the necessary orbital elements, the satellite orbit simulation was accomplished using Satellite Tool Kit Software STK9. The results of orbit simulations are shown in the figure below, Figure 8.Ground tracks of AEROSAT1 The launch process can severely constrain the satellite design as it is the primary mode that exposes the satellite structure to intense internal and external environment. A launch system consists of basic launch vehicle incorporated with one or more stages depending upon the nature of mission and orbital altitude. The selected launch vehicle for the AEROSAT 1 is Long March 3B (LM 3B) made by China Great wall corporation. Pakistan is still not having any appropriate satellite launching site, so the commercially available launch sites in neighbor China Xichang, Taiyuan and Jiuquan can be used. The LM 3B is a powerful three stage launch vehicle using liquid propellants. The fuel mainly consists of liquid oxygen and liquid hydrogen. VI. RESULT AND DISCUSSION The satellite design process was completed after incorporating the basics of Astrodynamics, orbital mechanics and spacecraft design. The complete bottom approach of the design process was applied to accomplish the mission requirements. Design and modeling was done using the SOLID EDGE software and orbital simulation was done using Satellite Tool Kit software. Subsystem level design was incorporated to support the payload and provide required mission requirements. The study provides an opportunity to the learner to start with an initial conceptual study of satellite design aiming at specific mission requirements. Figure 7.Proposed design AEROSAT1 orbiting around Earth Satellite ground track is the trace of all points formed by the intersection of satellite s position vector with the Earth s surface. For the observation of ground tracks a flat map of Earth surface is considered as shown in figure (taken from Satellite Tool Kit software).

5 REFERENCES [1] Danial P Raymer, Aircraft design; A Conceptual Approach, 1992, AIAA Education Series. [2] Wiley J Larson, James R Wertz., Space Mission Analysis and Design. Kinwer Academy Publishers United States Air force Academy. [3] Peter Fortescue, John Stark, Graham Swinered., Spacecraft Systems Engineering. British library catalogue in publication data John Wiley and sons limited. [4] Roger R Bate, Donald D Mueller, Jerry E White., Fundamentals of Astrodynamics. Department of Astronautics and computer science, United States Air Force Academy Dover Publications. [5] Graham Swinerd How Spacecraft Fly, spaceflight without formulae., Coper Nicus books, Praxis publications limited. [6] China Academy of Launch Vehicle Technology, LM 3B User's Manual [7] R. Farely, Satellite Design Course, Spacecraft Configuration and satellite structure, Preliminary design course, University of Maryland, NASA Publications [8] Earth Observation Portal Directory from [9] Surrey Satellite Technology, [10] AGI Graphics Incorporation, Table3.Orbit selection Parameter LEO(low inclination) LEO (Nearpolar) HEO GEO Constellation Earth observation Good Excellent Poor Good(specific) Good Ease of orbit acquisition Good Excellent Difficult Difficult Multiple Radiation Exposure Average Average Extensive Extensive Average Cost Effective Yes Yes No No No