AstroSat Workshop th 12 August 2016 CubeSat Overview
OBJECTIVE Identify science justified exo-atmospheric mission options for 3U up to 12U CubeSat class missions in Low Earth Orbit. 3 Development Epochs: 1-3yrs, 3-5yrs, 5-10yrs approx Target in Epoch 1-3yr is 3U to 6U sized craft. Select at least one achievable, scientifically meaningful mission for the 1-3yr Epoch and do it!
What is a Cubesat? When in doubt resort to Wikipedia.. A single 10cm cube is known as 1U. Configurations are possible in 1U, 2U, 3U, 6U, 12U and up to 27U formats. For astronomical purposes, we won t use less than 3U and will limit to 12U target is 6U.
What do they look like? Launch pod Shown are examples of a 6U format our most likely solution?
How do they work? Like all space craft, we need to think of a CubeSat as comprised of Service Subsystems and a Payload of some kind being the primary reason for going into space. The alignment of Service Subsystem performance with Payload demands is an iterative process limited by engineering, physics, time and money! The modular nature of CubeSats, therefore, means that each mission is a multi-dimensional custom compromise of mission requirements vs available subsystem components vs budget vs schedule vs launch opportunity etc.
How do they work? continued The main subsystems of Cubesats are the same as for big satellites, just smaller. The electrical/electronic subsystems can be discrete, modular or highly integrated depending upon the equipment supplier and mission needs. NOTE: The Mechanical structure constrains external dimensions to fit within standard dispensers or launch P-PODs. External dimensions RIGIDLY constrain the available payload volume and shape approximately to multiples of U s. Mechanical Structure Thermal Control Command & Data Handling Telecommunications Electrical Power Attitude Determination & Control Propulsion if required Payload/s
Mechanical Structure EXAMPLE: A well utilised 3U stack showing a typical configuration of layered subsystems and payload equipment supported within a standard structure suitable for P- POD deployment. A Finite Element Model (FEM) of the entire Spacecraft is used to design and predict mechanical environments in all test and mission phases.
Mechanical Structure cont d A 6U Format Examples of standard structural skeletons Diagram by A. Marinan and K. Cahoy
(its cold!) Thermal Control (its hot!) Principally passive thermal control is preferred. Limited active control possible. Heat transport is only via conduction and radiation. Thermal design starts at circuit board component level. Spacecraft in LEO are in a constantly transient thermal state due to regular sunlight/eclipse cycles and typically one pitch roll per orbit. Thermal design must balance heat flows to maintain a shirtsleeves internal temperature (ie ~-5 C to +40 C). Less variation is better. Payloads requiring very precise thermal control may find difficulty with Cubesats. Thermal control of external surfaces is achieved by the use of controlled thermooptic finishes and multilayer super insulation. Internal thermal control is mainly by way of high IR emissivity finishes and conductive pathways to space viewing radiators. A Thermal Mathematical Model (TMM) of the spacecraft is used to design and predict thermal environments in all test and mission phases.
Command and Data Handling Consider a Cubesat like an expensive semi autonomous remote control toy. The various subsystems respond to uniquely coded commands and may provide uniquely coded confirmation responses. Commands enter the satellite via the Telemetry, Tracking and Command (TT&C) transceiver. Modern Cubesats have sufficiently powerful on-board computers to perform command encoding/decoding as well as data handling, storage and buffering. On-board storage today is massively capable and is used to store data until it is downloaded either in a single pass or progressively. Spacecraft health and status is also monitored and reported by the command and data handling subsystem via the TT&C transceiver. The on-board computer can perform attitude control calculations or this may be performed by a second/auxiliary processor. Computing performance is a significant power sink so dimensioning C&DH performance appropriately is an important system design parameter.
Command and Data Handling contn d Example: This diagram shows an example of a distributed computing configuration using two processors instead of one. There are many different configurations emerging because so many developers are producing hardware today. Diagram by S. A. Asundi and N. G. Fitz-Coy
Telecommunications Telecommunications may be divided into Telemetry, Tracking and Command (TT&Cnarrow/medium bandwidth) and Data Downlink (ideally broadband). TT&C systems operate in VHF, UHF and S-Bands in ITU assigned frequency bands and deals with health status, command and control of the satellite. Consultative Committee for Space Data Systems (CCSDS) standard most widely used. Data downlinks most normally operate in S and X bands. Data downlink rates vary according to power, coding schemes and frequency bands. More power, better coding schemes and higher frequencies all improve data rates. Because ground stations are required and are in small numbers of discrete locations, down link data rates need to be designed for available ground station access windows. Higher data rates permit more data to be downlinked when access is available. Higher data rates require more electrical power. Conservative S-Band data rates are 1-2Mbps or better. Conservative X-Band data rates are 50Mbps or better. A Link budget is essential, designed by an experienced communications engineer.
Telecommunications cont d ITU Regulated Telecommunications Frequency Bands BAND FREQUENCY EXAMPLE SPACECRAFT Optus C1 Ka K 50 GHz 27 GHz 15.4 GHz MILSATCOM MILSTAR ACTS DBS Ka-Band Ku UP LINK DOWN LINK 14.8 GHz 12.7 GHz 10.7 GHz DOMESTIC COMMSATS OPTUS B, Intelsat etc. Ku-Band X 8.4 GHz 7.25 GHz MILITARY e.g DSCS II X-Band C 7.075 GHz 3.75 GHz ANIK, GALAXY, SATCOM, INTELSAT, TELSTAR S 2700 MHz 1930 MHz NASA TTC DEEP SPACE L UP LINK DOWN LINK 1710 MHz 1452 MHz GPS MOBILE BIG LEOs e.g. INMARSAT UHF VHF 960 MHz 118 MHz NOAA, LEO METSATS, TTC, ORBCOM, STARYS vola-016.cdr UHF
Electrical Power Subsystem As its name indicates, the Electrical Power Subsystem (EPS) generates, conditions, stores and distributes spacecraft electrical power. Cubesats generate electricity via solar cells. Storage today is almost exclusively via Li-Ion batteries. Design and dimensioning of the EPS is based upon mission needs and principally payload demands and those of the ACS. Solar collecting area can be augmented by deployable arrays. The ultimate Cubesat EPS capacity is limited by operational physics Solar illumination is cyclical in LEO so power consumption must be modulated or EPS dimensioning must be able to replenish consumed power in a single sunlight pass. EPS can be a major component of system mass. Limited Cubesat EPS capacity means all generation, interconnects, conditioning and distribution must be as efficient as possible. EPS performance degrades from Beginning Of Life (BOL) to End Of Life (EOL)
Electrical Power Subsystem cont d Typical Solar Array Panel Picture by Clydespace Typical EPS Block Diagram Diagram by C.S. Clark Cyclic Power Generation Diagram by C.S. Clark
Attitude Determination and Control - ADCS Any orbiting satellite should be considered as a body in free space unconstrained by motion based friction but constrained in Earth orbit. CubeSats used for observation missions employ a 3-axis stabilized Attitude Determination and Control Subsystem (ADCS). The ADCS uses knowledge sensors to input information into a platform specific control algorithm computed by the on-board processor in order exert force via the control authority devices. The precision required for pointing knowledge, absolute pointing accuracy, stability drift rates and jitter will determine the complexity of the ADCS. As a rule of thumb, sensors need an order of magnitude better knowledge than the pointing accuracy requirement. Control authority is achieved with orthogonal magnetic torque rods as a minimum but reaction wheels will be used for higher precision and control authority when slewing to a new observation. An observation action needs sensor knowledge of location, direction of motion and pointing direction in order for the ADCS to exert forces to point the imager. Equal energy needed to start and stop a slew motion. The stabilisation required and then tracking.
ADCS cont d Torque Rod An Advanced ADCS Block Diagram by Farhat, Ivase, Lu and Snapp Vacco Micro Propulsion module
Propulsion (if required) Propulsion is normally only implemented when the orbit altitude is low enough to require orbit maintenance to compensate for atmospheric drag. Where possible, chose an orbit above ~650km so the need for propulsion is reduced or removed. Propulsion technology for Cubesats is dominated by Cold Gas systems (ie propane, butane). The addition of propulsion adds mass, takes up volume and adds control system complexity. Most LEO altitudes offer sufficient control authority via magnetic torque bars but MEO and GEO altitudes may require propulsion to achieve body control. Cubesat propulsion systems would not be used for any significant orbital maneuvering. New Cubesat propulsion technologies are evolving.
Payload/s These are for the WORKSHOP to decide. Payloads will be dimensionally, volumetrically and parametrically constrained to fit within the capability of a Cubesat class platform. This WORKSHOP should focus upon astronomically relevant payload concepts. Example: 3U configuration for Exoplanet transit detection. A 6U could offer more volume, power, etc. Image credit to M.W.Smith MIT)
What Performance do they have? Performance varies dramatically depending upon the configuration and the system extensions such as additional solar array panels, reaction wheels etc. Key parameters achievable for a typical 3U configuration would be Payload Power: ~5W orbital average power Pointing: < ± 0.5 Stability: < 3 /minute Data Downlink: S-Band providing ~ 2Mbps, up to 50Mbps in X-Band. Payload volume: Varies but typically 90 x 90 x 200 in a 3U to 90 x 90 x 290 + in a 6U. More in larger formats INDICATIVE ONLY -- BOTH BETTER AND WORSE PERFORMANCE OCCURS.