How to Build a Satellite

How to Build a Satellite

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Satellites are essential tools in our  modern world. They facilitate seamless   global communication, gather valuable scientific  data, and even allow us to explore the cosmos and   other planets in the solar system. These orbiting  spacecraft are true marvels of engineering. Not   only must they survive extreme forces and  vibrations during launch, they also need   to operate reliably in the harsh environment  of space. In this video we'll dive into the   fascinating world of satellite technology, and the  clever engineering used to ensure the success of   each mission. Any satellite can be thought of  as two separate systems, the payload, and the  

bus. The payload is the equipment used to carry  out the mission the satellite has been launched   for. An earth observation mission could have a  payload consisting of cameras and radar equipment,   a communications mission would likely be equipped  with transponders and high gain antennas,   and a scientific research mission might carry  various probes and sensors. The satellite bus  

is essentially everything else. It includes  the structure and all of the systems needed to   operate the satellite and support the payload.  Satellites come in a wide range of shapes and   sizes. This one has a mass of just under 100  kilograms, which places it in the microsatellite   category. Large satellites can weigh well over a  thousand kilograms. And those in the picosatellite   category can weigh less than one kilogram. One  notable format in the nanosatellite category is  

the CubeSat. Developed around the concept of  a standardised cube-shaped unit, called 1U,   CubeSats are designed to be modular and low-cost.  They can scale up from 1U, with 3U, 6U and 12U   being common sizes. Although they were initially  developed for educational purposes, CubeSats have   evolved dramatically, and are now used for all  sorts of applications including state-of-the-art   scientific missions and technology demonstrations.  Regardless of the size of the satellite, the bus  

will usually have the same seven fundamental  subsystems - the mechanical structure, the   on-board computer, the electrical power system,  the attitude determination and control system,   the propulsion system, the communications system,  and the thermal control system. Let's start with   the mechanical structure. It needs to be strong  and stiff to survive launch, but also as light   as possible to save on launch costs. Engineers  work hard to reduce mass through smart design,   thorough analysis, and careful material  selection. As a result satellites use materials   that have good stiffness and strength properties  relative to their weight, like Aluminum alloys,   and carbon-fibre reinforced polymers. Honeycomb  composite panels, made of a lightweight core  

sandwiched between thin Aluminum alloy or carbon  fibre faces, provide surfaces for mounting   equipment. One important factor when selecting  materials for space applications is outgassing,   where the vacuum of space causes materials to  gradually release gases trapped within them,   which can then condense on sensitive instruments,  potentially endangering the mission. Metals like   aluminum tend not to outgas significantly,  but all materials used on the spacecraft,   from big structural parts all the way down  to the adhesives used to strap fasteners,   need to be checked for outgassing. A process  called bake-out can be used to reduce risk,   where the flight hardware is  heated under vacuum conditions   to accelerate outgassing before  it's integrated into the satellite.   The satellite structure includes a separation  system that attaches to the launch vehicle   adapter. The total volume of the satellite is  constrained by the space allocated to it in   the fairing of the launch vehicle. Deployable  mechanisms are often used for solar panels,  

instrument booms and antennas to get around this  limited envelope, with the trade -off that this   introduces new possible points of failure. At the  core of every satellite is the on-board computer,   the brain that controls and coordinates all of the  satellite's functions, including processing data,   monitoring the health of the satellite through  various sensors, and issuing commands to   instruments and systems. The hardware is typically  built around several printed circuit boards,   housed in an Aluminum alloy enclosure. One major  challenge with putting electronics in space is   their exposure to cosmic radiation, high-energy  particles that can penetrate the structure of the   satellite, disrupt circuits and in the worst case  lead to system failure. The on-board computer,   dense with electronic components, is particularly  vulnerable. Although radiation levels are  

relatively low in low earth orbit, satellites  venturing into higher orbits, particularly those   that pass through the Van Allen radiation belts,  are at significantly higher risk. To mitigate   this engineers often use radiation-hardened  components and shield sensitive parts—or even   the whole device—with layers of thick aluminum.  One of the roles of the on-board computer is to   manage the electrical power system that generates,  stores and distributes the power needed to operate   the satellite. A one square metre area directly  facing the sun just above Earth's atmosphere will  

receive 1.3kW of solar power, so it's no surprise  that by far the most common method of generating   power for satellites is the use of solar arrays.  They can be mounted to the body of the satellite,   but in many cases deployable panels are used  to maximise the generated power while allowing   the satellite to fit within the fairing of the  launcher. Some satellites even use articulated   solar arrays that can be pointed directly at the  sun. Most modern satellites use multi-junction  

solar cells, constructed from multiple layers of  different semiconductor materials. These cells   are able to capture a wider range of wavelengths  than less expensive single-junction cells, making   them more efficient. While orbiting the Earth,  satellites experience periods of eclipse when   in Earth's shadow, where the solar arrays are unable to  generate any power. As a result satellites need   to carry batteries that are charged by the solar  panels during periods of exposure to sunlight, and   discharged to provide power during the eclipse.  The electrical power system is controlled by  

the satellite's power control unit, which in turn  interfaces with the on-board computer. It monitors   the charge level of the batteries, controls  charging and discharging, and regulates voltage   to ensure a stable power supply for all satellite  systems. Next is the Attitude Determination and   Control system. It's used to determine and  adjust how the satellite is oriented relative  

to a reference frame. This orientation is called  the satellite attitude. Satellites are constantly   monitoring and adjusting their attitude. This  is crucial immediately after separation from the   launch vehicle, when they may experience tumbling,  and the ground station needs to regain control.  

But it's also important for normal operation. In  a single orbit a satellite might need to point the   payload at a specific location, point solar panels  to face the sun, and point an antenna towards a   ground station. The attitude determination  and control system is made up of sensors,   used to determine the current satellite  attitude, and actuators, used to make the   necessary attitude adjustments. The strategy used  for attitude determination is selected based on   the pointing requirements of the mission and the  size of the satellite, but normally makes use of   several different sensor types, because each one  has its own limitations. Satellites often carry an   Inertial Measurement Unit that contains gyroscopes  and accelerometers. The three gyroscopes in the   IMU provide a continuous measure of any changes  in satellite orientation. But this isn't enough  

to obtain good pointing knowledge because  gyroscopes only provide a relative measurement,   and they can drift over time, resulting in  an error in the predicted attitude. A common   approach for precise attitude determination is to  combine gyroscope measurements with data from star   trackers. Star trackers provide highly accurate  reference measurements of the satellite attitude   that can be used to periodically correct the  continuous attitude measurement provided by the   gyroscopes. Depending on the mission requirements,  star trackers might operate anywhere from every   10 seconds to every 20 minutes. A star tracker  is a camera that captures an image of the sky,   and uses an algorithm to identify bright  stars. Their position in the image is compared  

with a catalogue of known stars, allowing the  orientation of the satellite to be determined.   Star trackers can be very accurate, but may not  work well if they're pointed close to the sun,   or if the satellite is spinning too fast  relative to the camera shutter speed,   because the captured image will be blurry. And  options for accommodating them on the satellite   are limited because they need to have a clear  field of view, and can be affected by thermal   distortion. This all needs to be taken into  account by the engineers designing the system.   Ultimately attitude sensors are selected based  on the requirements of the mission. A satellite   with less stringent pointing requirements might  avoid star trackers and make use of sun sensors   or magnetometers instead. Sun sensors are simple  devices that use photodetector cells to estimate  

the attitude of the satellite relative to the sun.  But they don't work during the eclipse portion of   an orbit. And magnetometers, sometimes included in  the IMU, measure the strength and direction of the   magnetic field surrounding the satellite. These  measurements are compared with a model of the   expected magnetic field of the Earth, given the  satellite's position, to estimate its attitude.   That covers attitude determination, but what  about attitude control? Most modern satellites   use three-axis stabilisation, an approach to  attitude control where actuators are used to   precisely control satellite orientation around  three axes. One common actuator is the reaction   wheel, which consists of a flywheel attached to  an electric motor. Using the motor to change the  

rotational velocity of the reaction wheel causes  the satellite to rotate about its centre of mass   in the opposite direction. A minimum of three  reaction wheels mounted in orthogonal planes   give the satellite full three-axis control,  allowing minute adjustments to be made to   the satellite attitude. One problem with reaction  wheels is that, after a certain number of attitude   adjustments, the incremental increases in the  rotational velocity of the flywheel can bring it   close to its maximum allowable rotational speed.  This is called saturation, and it means that  

reaction wheels cannot be used as the only method  for attitude control. Magnetorquers are often used   alongside reaction wheels to control attitude, and  also provide a way of desaturating the reaction   wheels. A magnetorquer is essentially a  coil of a conductive wire wound around   a magnetic core. Passing current through the  coil creates a magnetic field, and when this   field interacts with Earth's magnetic field a  torque is generated. This torque can be used to   control the satellite attitude, or if actuated  at the same time as the reaction wheels it can   be used to slow them down. Magnetorquers are  very reliable because they have no moving parts.  

But they only work in relatively low orbits where  the Earth's magnetic field is sufficiently strong.   Another option is simply to control attitude by  using the propulsion system to fire thrusters.   The propulsion system plays a crucial role in  making adjustments to a satellite's orbit. This   could be to move the satellite to a new orbit,  or for station-keeping - corrective manoeuvres   performed to maintain an existing orbit. But  it can also be used for attitude control.  

Common methods for propulsion are cold gas  propulsion, chemical propulsion, and electric   propulsion. All work using the same principle -  thrust is generated by accelerating mass through   a nozzle. The simplest form of propulsion is  the cold gas thruster, which is the controlled   expansion of a stored pressurised gas through  thruster nozzles. The propellant is usually  

an inert gas like nitrogen or helium. Valves,  pressure transducers and pressure regulators   are used to monitor system pressure and control  the propellant to each of the thrusters. Higher   thrust can be generated with chemical propulsion,  which uses a controlled chemical reaction. In a  

monopropellant system the propellant is a liquid,  which decomposes into a hot gas when it comes   into contact with a catalyst. A pressurant, an  inert gas stored in a separate tank, supplies   the pressure that forces the propellant from the  tank to the thrusters. A bipropellant system uses   two propellants, a fuel and an oxidiser, which  are mixed and ignited in a combustion chamber,   producing exhaust gases that are expelled through  the nozzle to generate thrust. Electric propulsion  

uses electrical energy to generate thrust. These  systems work by ionizing a propellant—often a   gas like xenon—and then using electric and  magnetic fields to accelerate the ions to   high velocities before expelling them through a  nozzle. The propulsion system is selected based   on the thrust requirements and the mass of the  satellite. Bipropellant systems are complex but  

offer high thrust, useful for large satellites.  Monopropellant systems have lower thrust but   simpler system design. Electric propulsion has  lower thrust than the chemical propulsion options,   but is very efficient, requiring less propellant.  And cold gas thrusters offer low thrust but very   simple design and precise control, making them  a good choice for attitude control systems. As   the only way for the satellite to exchange  information with Earth, the communications   system is of vital importance. It has two  separate capabilities - Downlink, and Telemetry,   Tracking and Command. The downlink system is  used to beam the data generated by the payload  

down to earth. The satellite uses its attitude  control system to point a downlink antenna   towards a ground station, and start transmitting  data. The exact way this is done depends on the   satellite - some transmit data once per orbit  for each pass over the same ground station,   others transmit to multiple ground stations,  and those in geosynchronous orbits can use the   same ground station continuously. Satellites  transmit information to ground stations using  

electromagnetic waves. These are characterised  by their wavelength and frequency. Visible light,   X-rays, and high energy gamma rays are all just  electromagnetic waves with different frequencies.   Satellite communications use electromagnetic  waves in the radio frequency, or RF, part of   the spectrum, mostly between 1 and 40 GHz.  For convenience this part of the spectrum is   split into bands with designated names. As the  frequency increases the required power increases,   but the data rates are also higher. And higher  frequencies are more susceptible to degradation  

by atmospheric attenuation. The data transmitted  from a satellite is essentially a stream of bits,   which can have a value of either 1 or 0. To  transmit this information over vast distances   using electromagnetic waves, the data needs to  be encoded onto a carrier signal - a continuous   sinusoidal electromagnetic wave that has a  frequency in one of the frequency bands we   just discussed. This is done by changing  certain properties of the carrier wave,   in a process called modulation. Amplitude  Modulation encodes the data to be transmitted   by changing the amplitude of the carrier wave.  Frequency Modulation changes the frequency of  

the carrier wave instead. And Phase Shift Keying  adjusts the phase of the carrier wave. Most high   rate communication links use one of the several  forms of Phase Shift Keying modulation, which   provide high data throughput and low error rates.  The modulated signal is generated on board the   satellite by the transmitter hardware. It's then  routed through a series of switches and filters,   before arriving at the downlink antenna for  transmission to the ground station. Once received   by the ground station the signal is demodulated  and processed to obtain the valuable payload data.  

The other key element of the communications system  is the Telemetry, Tracking and Command subsystem,   which consists of a unit called the transceiver,  and various filters, switches and antennas. It   uses some really clever engineering to fulfil its  three different functions. The Command function   is what allows the operations team to control  the satellite. Commands are transmitted from  

the ground station, demodulated by the TT&C  hardware on the satellite, and then routed to   the on-board computer for implementation. The  Telemetry function transmits housekeeping data   from various sensors on the satellite down to the  ground station, like the temperature of critical   components, the power levels in the batteries, or  propellant levels in the tanks. Finally there's   the Tracking function, that provides information  about the position and speed of the satellite. The   ground station sends a signal to the satellite,  which the transceiver receives and sends back.   The turnaround time provides an estimate of the  distance between the satellite and the ground   station. And the Doppler effect frequency  shift provides an estimate of the velocity  

of the satellite. These two valuable pieces of  information help the operations team monitor the   position and trajectory of the satellite.  Although not an option for all orbits,   many satellites also carry GPS receivers to  enhance tracking capabilities, allowing for more   effective monitoring of position and trajectory.  Antennas, used to transmit and receive RF signals,   are a fundamental part of any communications  system. They come in many different shapes  

and sizes. To compare their performance it's  useful to think about a theoretical antenna,   called an isotropic antenna, that radiates  power uniformly in all directions.   Two points located the same distance from a  transmitting isotropic antenna will receive the   same signal strength, regardless of where they're  located around it. In reality, antennas neither   radiate nor receive power uniformly across all  directions, and in most cases it's not actually   desirable to do so. Instead, real antennas are  engineered to focus or receive energy in specific   directions. This directionality can be visualised  as a radiation pattern around the antenna,   and is quantified by a parameter called gain.  Gain is a measure of how much an antenna focuses  

energy in a particular direction, relative to an  isotropic antenna. An isotropic antenna has a gain   of 1 in all directions, but a real antenna will  have a gain greater than 1 in some directions,   and less than 1 in others. The total power  emitted or received is the same, it's just   focused in different directions. The antennas on  a satellite are selected to optimise performance  

for a specific task. Downlink systems use high  gain antennas which focus the transmitted signal   into a narrow beam, enabling high data transfer  rates. Antenna pointing mechanisms or the attitude   control system can be used to direct this beam  towards the ground station. TT&C systems on the   other hand prioritise reliable communications  instead of high data rates, because they only   transmit small packets of data. They use low gain  antennas with wider coverage to make sure the   satellite can communicate with the ground station  under all conditions, even if it's tumbling or its   attitude control system isn't working properly.  As a satellite zips around the Earth it's exposed   to impressive extremes in temperature. Surfaces  that face direct sunlight can get extremely hot,  

especially with the lack of a protective  atmosphere, with temperatures suddenly   plummeting as the satellite enters Earth's shadow.  Add the fact that many of the electronics on board   will be dissipating power, locally increasing  temperatures, and it's clear that controlling   the satellite temperature is a huge challenge.  Good thermal control is mission-critical - some   parts of the payload may need to be kept within  narrow temperature ranges to function as intended,   and batteries or electrical components that  see temperatures outside of their allowable   limits are at risk of failure. Thermal control  systems make use of many different technologies,  

and some clever engineering, to maintain stable  temperatures. Because there's no heat transfer   by convection in the vacuum of space, the only  way for the satellite to exchange thermal energy   with the environment is by radiation. Radiators,  large surfaces with high emissivity coatings,   are used to radiate heat from the satellite.  Heat pipes are used to transport thermal energy   from hot to cold areas. Thermostat-controlled  electric heaters switch on at low temperatures   to make sure certain components don't get too  cold. Multi-layer insulation blankets help   control temperature by reflecting solar radiation  when the satellite is in sunlight, and reducing   radiative heat losses when the satellite is in  shadow. Then there's special paints and coatings,  

and phase change materials that absorb or release  thermal energy by undergoing a phase change.   Even the attitude control system can contribute  to thermal control by changing the direction   radiators and other surfaces are facing. All  of these components need to work in harmony,   perfectly balancing absorbing, retaining and  dissipating heat, to make sure the temperature   of the satellite stays within acceptable ranges.  Space is hard - the harsh, isolated environment,   cost of launch, and technical barriers present  some formidable challenges, that can only be   overcome with clever engineering and extensive  testing, backed by decades of research. Whether  

you're designing hardware destined for orbit, or  working on an experimental project in your garage,   having access to the right design tools is an  important part of getting the job done. And that's   why I'd like to introduce you to this video's  sponsor, Onshape, a really powerful cloud-based   CAD platform that makes it quick and easy to  bring your designs to life. With its user-friendly   interface and intuitive set of tools, Onshape  makes modelling parts, building assemblies,   and producing drawings easy. And it all runs  in your web browser. Working with OnShape will  

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2024-05-23 18:01

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