Systems Flashcards
Identify all microwave remote sensing techniques having high power requirements that can be used by instruments on an Earth orbiting satellite
Altimeter, synthetic aperture radar, radar
An engineer tests a space debris detector for mounting on the ISS to determine the optimal design. Select all design parameters
- sensor housing material
- data rate generated by the sensor
- location of the sensor on the ISS
- area of the sensor exposed to space environment
Using a diagram, explain the process undertaken by the ESA to assess proposals for missions submitted to its Earth Explorer programme
Call for proposals -> phase o (Proposals 1 to n) -> mission assessment grounds (nomination of independent advisors to ESA) -> decision -> phase A (mission A, B, C etc) -> decision -> phase B/C/D (mission choice)
A mission objective of a remote sensing mission is to provide centimetre precision, sea surface height data. Identify an appropriate instrument and explain why it is required. Comment of the likely impacts for s/c subsystem
SAR/Interferometric Radar Altimeter (SIRAL). Climate change.
Increased power requirement, need for precise attitude control
Explain the concept of a typical s/c systems programme customer supply chain
Top level customer -> prime contractor -> tier 1 subcontractors -> tier 2 subcontractors -> low level suppliers
Customer specifies needs through business agreement which goes to the supplier which responds with ID. The customer reviews and accepts/changes - cycle until accepts and then supplier provides product
Explain how the ECSS framework applies to s/c development. what is its purpose and what provisions does it make to assist in the development process?
ECSS sets out formal processes and standards for European space context via standards, handbooks and technical memoranda.
Express what to do in terms of regulatory provisions but not how
Branches relating to space project management, assurance, engineering, sustainability
You are the operator of a large fleet of telecommunications spacecraft and wish to procure an additional spacecraft for your fleet. Specifically, what detailed activities would you expect throughout phases 0 to F in the project lifecycle and what are the primary objectives of each phase for each mission segment? Include the interactions between project partners.
Phase 0 - call for proposals - mission analysis
Phase A - mission statement, obtaining needs from customer to implementation manager, feasibility
Phase B - preliminary definition
Phase C - detailed design
Phase D - testing and production
Phase E - operation
Phase F - end of life/disposal
Using a diagram, explain where the LEO and GEO protected regions are located
Region A - LEO - spherical region extending from Earths surface to Z = 2000 km
Region B - GSO - segment of spherical shell between Z = Z_geo -200 km and Z = Z_geo + 200km and +/- 15 deg latitude
Comment of the effectiveness of a drag sail to reduce collision risk
Increases volume of space s/c occupies which is good for reducing collisions
An engineer tests an optical instrument for a Mars rover to determine if it will withstand the dust environment on the surface of the planet. For each characteristic below, say whether it can be considered to be a design parameter or a noise factor
- Instrument housing material
- Number of dust particles per cubic metre
- Ambient temperature
- Ambient pressure
- Lens thickness
- Time of exposure to the dust environment
- Size of dust particles
- instrument housing - DP
- no of dust - NP
- ambient temp and press - NP
- lens thickness - DP
- time of exposure - NP
- size of dust - NP
The Very Large Telescope located at high altitude in the Atacama Desert in Chile (latitude 24.6 degrees S) requires 24-hour satellite coverage for emergency communications. By discussing the advantages and disadvantages, compare and contrast two orbit options for this system.
GEO - good coverage at low latitudes, would be fixed in one position over Chile. Would require a fixed ground station and high power requirement, expensive to reach geo
HEO - good regional coverage, satellite needs to be tracked and would need more than one
Requirements engineering is one of the most important aspects of spacecraft systems engineering, allowing the spacecraft to fulfil the mission objectives. Describe five of the nine possible categories of requirements, define each category and give one example of a requirement from each category
- functional - what it must do - programming
- configurational - the parts its composed of - components
- interfaces - interfaces between parts and external world - GPS
- physical - characteristics - mass
- environmental - conditions it has to perform its function - acceleration, altitude
- quality factors - how well it performs its function - usability, maintainability
- operation - how must it operate - autonomy, control
- support - support it needs to perform function - maintenance, logistics
- verification - method to verify requirements - inspection, test
Explain how it is possible for a spacecraft to be compliant with all subsystem technical requirements when verified, yet fail to meet mission objectives. Use clear systems engineering keywords and phrases
A product is built as per specification but the specifications themselves fail to address the users needs
You represent the propulsion subsystems supplier for the BepiColombo mission to Mercury. During qualification verification testing of your ion thrusters (which are at a relatively low TRL) your system fails to meet the customer requirements, delivering only 3900s specific impulse, rather than the required 4000s. What action must you take at this point in the test program, and what options do you have? What contingency may the prime contractor have made for this scenario?
Identify four microwave remote sensing techniques that can be used by instruments on an Earth-orbiting satellite
Altimeter, scatterometer, SAR, radiometer
How are missions selected?
Call for proposals-> suggested proposals that get assessed in a user consultation and carried through to check feasibility before a final decision
How do missions come about?
A need for a mission ie. Responding to a specific area of public or environmental concern
What criteria do you think are important?
Feasibility of required technology, cost, objective,
What industrial studies are required?
- end to end implementation concepts for each mission
- preliminary feasibility of required technology
- preliminary feasibility of programme constraints
What programmes exist (especially ESA)?
ESA - Cosmic Vision & Earth Explorer (Core and Opportunity), Earth Watch
NASA - explorer program, discovery, mars scout, new frontiers
What is the difference in ESAs programmes?
Cosmic Vision - space research, addresses 4 questions:
- what are the conditions for planet formation and the emergence of life?
- how does the solar system work?
- what are the fundamental physical laws of the universe
- how does the solar system work
- how did the universe originate and what is it made of
Earth Explorer - earth observation
- core missions to respond directly to specific areas of public concern
- opportunity to address areas of immediate environmental concern
What missions have already been selected (especially Earth Explorer and Living Planet programmes)?
Core - GOCE, ADM Aeolus, EarthCARE, Biomass
Opportunity - SMOS, CryoSat, Swarm, FLEX
How do the Earth Explorer missions address the science and operational challenges identified by the programme?
Core
- GOCE - determines gravity anomalies and geoid
- ADM Aeolus - furthers knowledge of earth’s atmosphere and weather systems
- EarthCARE - measures 3D structure of clouds and aerosols and observes solar and terrestrial radiation
- Biomass - provides info on state of our forests and how they are changing
Opportunity
- SMOS - global observations of soil moisture over land and salinity over sea, understanding water cycle
- CryoSat - measures thickness of floating sea ice to detect annual variations and survey ice sheets for changes
- Swarm - study core dynamics, geo-dynamo processes and core/mantle interaction, magnetism of lithosphere, conductivity of mantle. Data is used to study suns influence on earth
- FLEX - map vegetation fluorescence to quantify photosynthesis activity, understand how photosynthesis affects carbon and water cycles, and understand plant health and stress
Learn more about earth system and processes specifically about climate change. Collecting data to monitor effects and predict future changes and protect the environment. Provide observations for operational use - weather forecasting
What kind of instruments/payloads do ESAs missions use?
GOCE - accelerometers, GPS, laser retro-reflector
ADM-Aeolus - LiDAR
EarthCARE - imager, LiDAR, radar
Biomass - synthetic aperture radar (SAR)
SMOS - microwave radiometer (using aperture synthesis)
CryoSat - SAR/Interferometric Radar Altimeter (SIRAL)
Swarm - magnetometers, accelerometers, GPS, laser retro-reflector
What drives payload selection?
Mission objective
What is remote sensing?
Measurement of object properties on Earths surface using data acquired from aircraft and satellites
Measurement at a distance, not in-situ
What are the key characteristics of remote sensing instrument?
Spatial resolution - spacing between ideal samples on Earth
Spectral resolution - ability to resolve spectral features
Radiometric resolution - number of bits per pixel
Temporal resolution - revisit/re-imaging time
What parts of the electromagnetic spectrum are used for remote sensing applications?
Visible - solar
Near InfraRed - solar
Shortwave IR - solar
Mid wave IR - solar and thermal
Thermal or long wave IR - thermal
Microwave, radar - thermal (passive), artificial (active)
What types of remote sensing instruments are there?
IR/visible (passive) - radiometers, spectrometers, LiDAR
Microwave (active) - radar (altimeter, scatterometer, SAR), radiometers
What scanning methods are there?
Line scanner - 1 sensor
Whisk broom - a couple, still moves
Pushbroom - a line of sensors
How is a pixel characterised in a remotely sensed image?
Ground-projected Sample Interval (GSI)
Ground-projected Instantaneous Field of View (GIFOV)
Number of bits used to code signal Q
What are the principles of Outer Space Treaty?
- the exploration and use of space shall be carried out for the benefit and in the interest of all countries and shall be the province of all mankind
- space shall be free for exploration and uses by all states
- space is not subject to national appropriation by claim of sovereignty
- states shall not place nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies or station them in space
- the moon and other celestial bodies are for peaceful purposes
- astronauts regarded as envoys of mankind
- states are responsible for national space activities whether governmental or not
- states are liable for damage cause by their space objects
- states shall avoid harmful contamination of space and celestial bodies
How does the Outer Space Treaty influence on current/future space activities?
Prevents a human colony on mars - contamination
Prevents space mining - can’t make a profit
How was the Outer Space Treaty applied to previous events/accidents? Provide examples
International liability - Skylab debris landed in Australia - $400 littering fine
Operation Morning Light - clean up of radioactive debris in Canada from Kosmos 954. Cost Soviet Union C$3 million
What is space debris and why do we need to worry about it?
All man made objects including fragments, in earth orbit or re-entering the atmosphere, that are non functional
Eg. adaptor rings, paint flakes, upper stages
Even the smallest pieces can cause hyper velocity impacts - risk to spacecraft
What is space sustainability and is our use of space sustainable?
Using outer space in a way future generations can also use. No but it’s making progress to being more sustainable
How do the impacts of standards and regulations for debris impact on spacecraft design?
Limit debris released in normal operations - limits stage release, and other parts ie tethers, lens caps, fairings and adaptors
Designed to prevent accidental explosions and ruptures at end of life
Additional lifetime stage planned for disposal
What are the on-board sources of stored energy? And what actions do we need to minimise the potential or on-board orbit break-ups due to them?
Batteries - interrupt power supply and limit battery recharging
Electro-explosive devices, pyrotechnic devices, actuators - deactivate/remove electrical power
Reaction wheels and gyros - remove electrical energy inputs
Propellant tank (propellant and pressurant), propulsion lines - depressurise tank, empty tank and lines
Heat piper - demonstrate low probability of rupture
What kind of post-mission disposals are required?
S/c should be manoeuvred far enough away from GEO to not cause interference with other orbiting craft in GEO. The s/c should remain in orbit above GEO protected region
Minimum increase in rp of 235 km + (1000CrA/m) and eccentricity <0.003
Disposal from LEO - re entry but more expensive than just increasing orbit
What is the definition of protected regions (LEO/GEO)?
LEO - spherical region extending from earths surface to Z = 2000km
Geosynchronous - segment of spherical shell between Z=Zgeo -200 km and Z=Zgeo+200km and +/-15 deg latitude where Zgeo is 35786km
Why does the post mission disposal of GEO not target a re-entry? Could you prove it?
DV for disposal = 11m/s but 1.5km/s for reentry
Equivalent to 1/4 yearly station keeping budget therefore more expensive
What is robust design? Why do we need it?
A design that performance is insensitive to variations
Equivalent to quality. Reduce variability, increase quality, reduce cost
What would happen if a design of a s/c subsystem was only based on maximising performance?
High cost and can have a high variation
In the context of s/c systems design, why is it important to minimise variation in performance?
Any deviation in target value will result in a loss of quality
What is the difference between a design parameter and a noise factor?
Design parameter - variables under the control of the designer
Noise factor - variables that cannot or are too expensive to be controlled
Why is a Design of Experiments needed?
Testing all possible combinations of DP and NF to select best design parameters is expensive and time consuming. DoE is a structured method of determining relationship between process inputs and outputs to choose what info to gather to determine relationship with minimal effort
Why do we need HEOs and examples?
Molniya orbit - perigee fixed in Southern Hemisphere, apogee in northern. Critical inclination combined with high eccentricity means good coverage for high latitude ground stations. Used for high latitude telecoms in Russia
Don’t have an eclipse in commas operations, allows for targeting specific latitudes if stable orbit is designed
Advantages and disadvantages of HEO comm orbits
Advantages
- satellite at high elevation at high latitude ground sites
- no eclipse during commas operations
- flexibility of design allows targeting specific latitudes if stable orbit designed
- high eccentricities allows for a longer time spent in apogee region hence offer enhanced coverage for regions at apogee point
Disadvantages
- ground station must track s/c
- satellite switching protocol required
- more than 1 s/c needed for 24h regional coverage
- variation in satellite range and range rate - impacts commas payload design - variation in time of signal propagation, frequency variation due to Doppler, variation in received signal power, change in ground coverage pattern during each orbit
What is a constellation and why do we use it?
A collection of satellites of usually similar design, performing similar functions simultaneously in similar orbits
Global coverage and temporal coverage
High spatial resolution, low latency, user experience
But high cost and complexity in no of sats/ground stations/launchers
What types of constellations are available and how can we achieve them?
Walker-Delta
Walker Star
Total of t satellites, with s satellites evenly distributed on p orbit planes. All orbits are circular, at same height, inclination and the ascending nodes of p orbit planes are evenly distributed around equator at intervals of 360/p for WD and 180/p for WS
What are the key design drivers affecting the selection of altitude for constellations?
Payload, latency, coverage, cost
Definition of a s/c system
A system is a set of interrelated subsystems and components which interact with one another towards a common purpose
Definition of s/c system engineering and why do we need system engineering approach on s/c design
Systems engineering is an interdisciplinary approach and is the means to enable the production of robust systems, on-time and on-budget
Plans and integrates technical solutions within schedule and budget
Role of ECSS on system engineering
European Cooperation for Space Standardisation- sets out formal processes and standards by which systems engineering is achieved in a European space context
Defines procedures and standards - expresses what to do but not how
Aims to lower life costs whilst improving quality, functional integrity and compatibility of all project elements by applying common standards for hardware, software, operations, info and activities in projects
Challenges of large-scale s/c systems engineering
Large complex teams
Cost plus funding
Performance requirements frozen from onset
Only space qualified tech can be tolerated
Complex system - checklist approach to analysis and testing
Importance of systems engineering
Plans and integrates technical solutions within a schedule and budget
The influence of systems engineering at the beginning of a s/c project (phase 0 to phase C) on subsequent development costs from phase C onwards
Phase 0 - mission analysis - support customer in identifying their needs, propose possible systems concepts
Phase A - feasibility - finalise expression of needs, propose solutions to meet perceived needs
Phase B - preliminary definition - establish prelim definition of selected option, demonstrate solution meets technical requirements to schedule, cost and organisation requirements
Phase C - detailed definition - establish system detailed definition, demonstrate capability to meet tech requirements
Phase D - qualification and production- finalise development by qualification and acceptance. Finalise prep or operations and utilisation
Phase E - operations/use - support launch campaign, support entities in charge of operation and exploitation, support anomaly investigation
Phase F - disposal - support entity in charge of disposal
Concept of s/c systems programme customer supply chain
CUSTOMER is a consumer in a BUSINESS AGREEMENT which is provided by a SUPPLIER which provides the PRODUCT which is received by a CUSTOMER
CUSTOMER specifies needs through PRD which is read and accepted by a SUPPLIER who responds with an ID which is reviewed and accepted by a CUSTOMER
Why does the periodic change in eccentricity of the disposal orbit take place over a long period?
Due to the combined effect of earth gravity, luni-solar and solar radiation pressure perturbations
Why does the space debris mitigation guideline for GEO disposal orbits incorporate a max eccentricity value?
A low eccentricity reduces change in de/dt due to luni-solar perturbations
Increases change in de/dt due to SRP
Reduces volume of space the spacecraft will occupy
**But ensures s/c will always remain above GEO protected region if altitude guideline is followed
What strategy could be used to minimise the initial change in eccentricity of GEO disposal orbits due to solar radiation pressure?
Choosing values of w and omega such that:
w + omega = lambda_sun
(In de/dt equation, sin term must equal 0)
Define what is meant by a requirement and explain how a requirements specification enables the verification philosophy
Requirements can be quantitative or qualitative contractual obligation to which hardware or software must comply to achieve full mission success criteria
Specifications are derived from mission objectives and passed down assembly chain in requirement subsets. They must dictate the requirement, traceability, at what stage verified, at what architectural level and the method
Identify the key space debris mitigation guidelines for reducing the amount of debris in LEO as outlined by UN
- limit debris released during normal operations
- minimise potential for post-mission break-ups resulting from stored energy
- prevention of on-orbit collisions
- minimise potential for break ups during operational phases
- avoidance of intentional destruction and other harmful activities
- post mission disposal - geosynchronous region
- post mission disposal - objects passing through LEO region
A second mission objective is to provide cm precise, sea level height data. A radar altimeter can be used. Explain why a radar altimeter is better than a LiDAR system. Comment on likely impacts of power and attitude control subsystems
LiDAR is affected by poor weather conditions due to its short wavelength and losing energy over distance whereas radar is not affected due to its larger wavelength and lower attenuation. Radar is also cheaper as tech has been around longer
More complicated power and attitude subsystems needed to keep attitude precise
