I2S - Lecture 6-8 (Satellite Design) Flashcards
How many satellites are in orbit today?
9.900 active satellites as of May 2024
What are the definitions of payload and subject?
- Payload = combination of hardware and software that interacts with the subject
- Subject = the portion of the outside world that the s/c is looking at / interacting with
Name some types of s/c missions
- Communications
- Remote Sensing
- Navigation
- Weapons
- In-situ science
What are transmission characteristics of the Earth’s atmosphere?
EM spectrum has many bands for which Earth’s atmosphere is opaque (needs to be avoided)
What is the ground sampling distance (GSD)?
Ability to resolve fine detail on the surface of Earth observing systems
State observation payload types
- Visible Systems - high spatial resolution, operates only in daylight
- Infrared sytems - subject to atmospheric transmission windows, operates both day and night
- Microwave Radiometers - low resolution, but collect information over large areas
- Radar systems - require own illumination, penetrate most atmospheric disturbances
Name systems that require electrical power
- Payload
- Computer
- Communications
- Guidance, Navigation, attitude control
- Sensors
What is PMAD?
Power Management and Distribution
Power usage is divided into …?
- Baseline power (required continuously)
- Peaking power (required for shorter period)
- Dormant power (req. to keep the system alive)
- Burst or Transient power (needed for momentary surges)
Compare different power source comparison
- Chemical (batteries, fuel cells): short time, low power
- Solar arrays: long time, lower power
- Nuclear reactors: long time, high power
Name characteristics of solar arrays
- High specific power (25 - 300 W/kg)
- Require storage (batteries) during eclipse
- Good option for near Earth operation
Compare solar cell technologies
- Silicon - cheap but not terribly efficient
- Gallium Arsenide - more efficient, lower temp coefficient
- Multibandgap - even more efficient with several semiconductor layers
What are the key system requirements for solar arrays?
- Avg electrical power for payload
- Peak elect. power for payload
- Mission life
- Orbital parameters
- Spacecraft configuration
What is the difference between cells and batteries (terminology)?
- A primary cell is used once and discarded
- Secondary cell and can be recharged and used repeatedly
- Batteries consists of many cells
What is the battery lifetime?
The number of times the battery is cycled through charge and discharge equals the mission duration divided by the day/night period
What is the DOD?
Depth of discharge, fraction of the battery’s total capacity that is used.
Since the numbers of battery cycles descreases as the depth of discharge increases
What is the purpose of thermal control?
To control the operating temperature of spacecraft systems
Thermal management includes…
- Heat rejection, minimize thermal input and maximize thermal radiation
- Heat conservation, maximize thermal input and minimize thermal radiation
- Transient amerlieration, components dont overheat during transient high thermal load, or dont freeze during low thermal load
What are thermal inputs for a s/c?
- Solar radiation
- Albedo radiation
- Planetary radiation (infrared)
- Internally generated heat
Heat transfer mechanisms in space?
- Internal: Convection (liquid/gas), Conduction (solid), Radiation
- External: Radiation (infrared)
Name thermal control components and purpose
- Materials and coatings (paints, mirrors, optical solar reflectors)
- Multilayer insulation
- Electric heaters
- Space radiators (waste heat into space)
- Cold plates (mount for electronic equipment) (fluids)
- Doublers (passive heat exchange surface) (aluminum)
- Heat pipes (fluid based transfer)
- Sensors, isolators, coolers,….
Minimizes radiative heat transfer from/to a s/c component
What is TT&C?
Tracking, Telemetry and Control
Why do satellites communicate with Earth?
- Transmit telemetry
- transmit payload data
- receive commands
What is the advantage of crosslink?
Satellites communitcate with each other, therefore they dont need to be in contact with gound station all the time
Factors to consider in design…
- Orbit
- RF Spectrum (frequencies are limited)
- Data rate
- duty factor
- link availability and access time
- threat
What are the frequency selection drivers?
- Spectrum availability and FCC allocation
- Relay/Ground station frequency
- Antenna size
- Atmospheric attenuation
- Noise temperature
- Modulation and coding
How reduce the data rate?
- Increase duty cycle
- collect only above-threshold data
- record amplitude changes only
- data compression
What is the Link Design Process?
- Define requirements for each link
- design each link
- size the payload
Name some external noise sources
- Galactic noise
- Clouds, rain in path
- Solar noise
- Earth (290k)
- Man-made noise
- Nearby objects
- satellite structure
What is modulation?
Modulation modifies an RF carrier signal so that it contains input signal information
What is the BER?
The bit error ratio (also BER) is the number of bit errors divided by the total number of transferred bits during a studied time interval.
Name two compression algorithms
- Lossless compression - ensures data is exactly the same (heavy data)
- Lossy compression - does not promise that received data is the same as data sent, better compression ratios
Why compress data?
Need to send more data than bandwidth accommodates, since it is limited by the link equation and international regulation
Name some antenna types
- Parabolic reflector
- Helix
- Horn
- Biconical Horn
How expensive is the ground segment?
Represents approx. 25% of total cost
Name communications bands with frequency
- S Band - 2-3 GHz
- C - 3-6 GHz
- X - 7-8 GHz
…
What are factors for the Ground Station?
- Location in relation to s/c orbit
- Geopolitical
- Distance to control centers
- Weather
- Seismic activities
- Obstacles
What is the ranging function of a ground station?
A ground station can measure range and position of a s/c to update the orbital propagation computations, vie RF or laser observations
What is the ADCS?
Attitude Determination and Control Subsystem
What is the attitude error?
Low frequency s/c misalignment
What are the dominant disturbances to s/c attitude?
- Solar Radiation pressure
- Atmospheric drag
- Magnetic field (interaction with earth magnetic field)
- Gravity-Gradient (difference in gravitational acceleration between different points of the s/c)
Name s/c control methods
- PASSIVE CONTROL
Gravity Gradient control, momentum bias wheel, passive magnetic control - SPIN CONTROL TECHNIQUES
Spin stabilization, dual-spin stabilization, 3-axis control techniques
What contains the 3-axis control technique?
Combination of momentum wheels, reaction wheels, control moment gyros, thrusters, solar or aerodynamic control surfaces, magnetic torquers
What is SWaP?
Size, Weight and Power
three key budgets to keep track of during s/c design
