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The Satellite has a large number of electronic systems and subsystems which are critical to the functioning and its mission. The spacecraft is divided into two sections: the platform or bus and the payload. The platform consists of the five basic subsystems that support the payload: the structural subsystem, the telemetry subsystem, tracking and command subsystems, the electric power and distribution subsystem, the thermal control subsystem, and the attitude and velocity control subsystem. The structural subsystem is the mechanical structure and provides stiffness to withstand stress and vibration. It also provides shielding from radiation for electronic devices.
The telemetry, tracking, and command subsystems include receivers, transmitters, antennas, sensors for temperature, current, voltage, and tank pressure. It also provides the status of various spacecraft subsystems. The electric power and distribution subsystems convert solar into electrical power and charge the spacecraft batteries. Satellites are designed to remain in space for months or years and must have access to power. Solar power is the most likely source of energy to be used in space.
The thermal control subsystem helps to protect electronic equipment from extreme temperatures. And finally, the attitude and velocity control subsystem is the orbit control system that consists of sensors to measure vehicle orientation and actuators (reaction wheels, thrusters), and to apply the torques and forces needed to orient the vehicle in the correct orbital position. Typical components of the attitude and control system include Sun and Earth sensors, star sensors, momentum wheels, inertial measurement units (IMUs), and the electronics required to process the signals and control the position of the satellite.
The payload is the equipment in support of the primary mission. For GPS navigation satellites, this would include atomic clocks, navigation signal generators, and high-power RF amplifiers and antennas. For telecommunications systems, the payload would include antennas, transmitters and receivers, low noise amplifiers, mixers, and local oscillators, demodulators and modulators, and power amplifiers. Earth observation payloads would include microwave and infrared sounding instruments for weather forecasting, visible infrared imaging radiometers, ozone mapping instruments, visible and infrared cameras, and sensors.
These electronic systems and subsystems are required to be of high quality and able to function reliably in the harsh Space environment. Traditional geostationary satellites have enormous lifetime requirements – up to 20 years or more. You can’t go up there and replace parts after they wear out, so you need to guarantee reliability over a very long time. That dramatically raises the cost.
Electronics for military, avionics, and aerospace are crafted to withstand the extreme temperatures, vibration, shock, and radiation effects commonly encountered at lift-off and outside the earth’s atmosphere.
Harsh Space environment and electronics technologies
The first hurdle for space electronics to overcome is the vibration imposed by the launch vehicle. The demands placed on a rocket and its payload during launch are severe. Rocket launchers generate extreme noise and vibration. There are literally thousands of things that can go wrong and result in a ball of flame. When a satellite separates from the rocket in space, large shocks occur in the satellite’s body structure. Pyrotechnic shock is the dynamic structural shock that occurs when an explosion occurs on a structure. Pyroshock is the response of the structure to high frequency, high magnitude stress waves that propagate throughout the structure as a result of an explosive charge, like the ones used in a satellite ejection or the separation of two stages of a multistage rocket.
Pyroshock exposure can damage circuit boards, short electrical components, or cause all sorts of other issues. Understanding the launch environment provides a greater appreciation for the shock and vibration requirements, and inspections imposed on electronic components designed for use in space-level applications. Outgassing is another major concern. Plastics, glues, and adhesives can and do outgas. Vapor coming off of plastic devices can deposit material on optical devices, thereby degrading their performance. Outgassing of volatile silicones in low Earth orbit (LEO) cause a cloud of contaminants around the spacecraft. Contamination from outgassing, venting, leaks, and thruster firing can degrade and modify the external surfaces of the spacecraft. Using ceramic rather than plastic components eliminates this problem in electronics.
Another obstacle is the very high-temperature fluctuations encountered by a spacecraft from -55°C to +125°C. A satellite orbiting around Earth can be divided into two phases; a sunlit phase and an eclipse phase. In the sunlit phase, the satellite is heated by the Sun and as the satellite moves around the backside or shadow side of the Earth, the temperature can change by as much as 300°C. Because it is closer to the Sun, the temperature fluctuations on a satellite in GEO stationary orbit will be much greater than the temperature variations on a satellite in LEO. For instance, the surface temperature of the Moon ranges from approximately –200°C to +200°C.
A satellite can be exposed to below-freezing temperatures and extremely hot temperatures within a short period of time, taxing all materials. Extreme temperature not only shortens the life of a component, but it also affects its everyday operational functionality. The accuracy and life expectancy of electronic devices can be degraded by sustained high temperatures. There are three ways of dissipating the heat generated by the electronics: convective, diffusive, and radiative. In the vacuum of space there is no thermal convection or conduction taking place. Radiative heat transfer is the primary method of transferring heat in a vacuum, so satellites are cooled by radiating heat out into space.
Casual electronic components cannot bear such temperatures or such a wide range of temperatures for long. Commercial off-the-shelf (COTS) components that are designed to withstand the required temperature range must be used. Here again, ceramic packages can withstand repeated temperature fluctuations, provide a greater level of hermeticity, and remain functional at higher power levels and temperatures. Ceramic packages provide higher reliability in harsh environments.
High levels of contamination on surfaces can contribute to electrostatic discharge. Satellites are vulnerable to charging and discharging. For that reason, space applications require components with no floating metal. Satellite charging is a variation in the electrostatic potential of a satellite, with respect to the surrounding low-density plasma around the satellite. The extent of the charging depends on the design of the satellite and the orbit. The two primary mechanisms responsible for charging are plasma bombardment and photoelectric effects. Discharges as high as 20,000 V have been known to occur on satellites in geosynchronous orbits. If protective design measures are not taken, electrostatic discharge, a buildup of energy from the space environment, can damage the devices.
A design solution used in geosynchronous Earth orbit (GEO) is to coat all the outside surfaces of the satellite with a conducting material. The atmosphere in LEO is comprised of about 96% atomic oxygen. Oxygen exists in different forms. The oxygen that we breathe is O2. O3 occurs in Earth’s upper atmosphere, and O (one atom) is atomic oxygen. Atomic oxygen can react with organic materials on spacecraft exteriors and gradually damage them. Material erosion by atomic oxygen was noted on NASA’s first space shuttle missions, where the presence of atomic oxygen caused problems. Space shuttle materials looked frosty because they were actually being eroded and textured by the presence of atomic oxygen. NASA addressed this problem by developing a thin film coating that is immune to the reaction with atomic oxygen. Plastics are considerably sensitive to atomic oxygen and ionizing radiation. Coatings resistant to atomic oxygen are a common protection method for plastics.
The vacuum of space is a favorable environment for tin whiskers, so prohibited materials are a concern. Pure tin, zinc, and cadmium plating are prohibited on IEEE parts and associated hardware in space. These materials are subject to the spontaneous growth of whiskers that can cause electrical shorts. Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin is used as a final finish. Devices with pure tin leads can suffer from the tin whiskers phenomenon that can cause electrical shorts. Using lead-based solder eliminates the risk of shorts occurring when devices are used in high stress applications.
Finally, the space radiation environment can have damaging effects on spacecraft electronics. Outside the protective cover of the Earth’s atmosphere, the solar system is filled with radiation. There are large variations in the levels of and types of radiation a spacecraft may encounter. Missions flying at low Earth orbits, highly elliptical orbits, geostationary orbits, and interplanetary missions have vastly different environments. In addition, those environments are changing. Radiation sources are affected by the activity of the Sun. The solar cycle is divided into two activity phases: the solar minimum and the solar maximum.
The natural space radiation environment can damage electronic devices and the effects range from a degradation in parametric performance to a complete functional failure. These effects can result in reduced mission lifetimes and major satellite system failures. The radiation environment close to Earth is divided into two categories: particles trapped in the Van Allen belts and transient radiation. The particles trapped in the Van Allen belts are composed of energetic protons, electrons, and heavy ions. The transient radiation consists of galactic cosmic ray particles and particles from solar events (coronal mass ejections and solar flares).
Designers ensure that the satellite has sufficient shielding between the spacecraft body and the electronics to reduce the energy that gets into the electronics, but it’s never enough. There is also a requirement for Radiation resistant electronic design that is robust to any kind of transients. Since the parts we select must be radiation hard, or radiation tolerant at the very least, it makes finding parts very difficult. Due to market and government reasons, radiation hard parts generally lag the state of the art by about 10 years or more.
To make it even more difficult, radiation is a very stochastic effect, explains Brady Salz of Astranis. If you could very precisely shoot a bunch of atoms at every single part of a chip, we could say ‘Ok this much energy is required to cause an upset of this type’. But it’s pretty hard to precisely shoot atoms in the real world. We go to radiation facilities and put the components in a radiation beam to test to get some kind of averaged susceptibility and probability curves. We combine that with other space modeling tools which allow us to get insight on our expected radiation performance during a mission.
The typical CubeSat is equipped with solar cells, however, generating power with only a few cells means that the power supply is limited. Under these circumstances, large or power-hungry circuits are not easy to justify. Power consumption and size requirements can be reduced by using Integrated Circuits (ICs).
It’s possible that different ICs will require different voltage levels in a satellite. Separate voltage busses might power ICs with different voltage demands. In some cases, it is not practical to use low voltage levels as the voltages could drop significantly enough at the end of a bus to adversely affect operation. In such cases, choosing components with similar voltage level requirements might be preferable. By paying attention to the operating voltage levels of various ICs and components, designs avoid the need to provide several different power supply levels and corresponding voltage regulators. Using components with similar voltage level requirements reduces cost and energy dissipation.
A typical satellite mission lasts for years, and repairs are uneconomical. For critical missions, redundant systems are often installed in case a part or system malfunctions or dies. Thus, if something fails to operate properly, the redundant system can be implemented either automatically or deployed via radio signal from earth. Interestingly, a side-effect of designing for operation in space is the flexibility of being able to continue a mission even after disconnecting primary systems.
Missile environments generally cover these parameters: High and low temperature, Vacuum, Shock, Acceleration and Extreme vibration. Designers would generally agree that no one design will be optimum under all conditions for this kind of environment. Therefore, a design compromise must be achieved to reduce the extremes sufficiently and meet performance requirements reliably.
Refining defense contracting practices, supply chains
With the changing world of defense and security, there are sea changes in the making at the Department of Defense (DoD) that will have lasting effect upon the defense electronics supply chain sure to impact the manner in which electronics manufacturing services (EMS) providers conduct their business in this sector. The nature of war itself is changing and the tools warfighters rely on are evolving to become soldier-centric; dependent on constellations of integrated information systems and mechanisms at their instant (and local) command and control. It will become as though an entire battlefield – every nuance of it – can be seen through a single eyepiece; allowing the individual soldier to become a force multiplier.
The myriad devices required to accomplish this will depend upon modernized acquisition and supply chain practices and a well-prepared group of opto-electronics manufacturers; because the future battlefield is a high bandwidth, on demand, and diverse mix of integrated RF / microwave and optical signal paths. U.S. secretary of defense Robert Gates said in April 2010, “The perennial procurement and contracting cycle… of adding layers and layers of cost and complexity onto fewer and fewer platforms, that take longer and longer to build, must come to an end.”Under Secretary of Defense for Acquisition and Logistics, Ashton B. Carter, September 2010, in the form of a ‘Memorandum for Acquisition Professionals‘. (PDF, defense.gov) The memorandum outlines 23 initiatives aimed at refining defense contracting practices and emphasizes performance-based contract awards for relevant warfighter applications.
There are vastly different environments in space. The requirements for a launch vehicle are much different from that of a geostationary satellite or a Mars rover. Each space program has to be evaluated in terms of reliability, radiation tolerance, environmental stresses, the launch date, and the expected life cycle of the mission.
Technologies for EMS providers targeting defense
Of importance to the EMS industry is the multitude of new U.S. military technology applications the asymmetric enemy portends; all of which contain significant opto-electronics assembly content. Below are some key military technologies outlined in the handbook, ‘U.S. Army Weapons Systems 2010-2011′ (Skyhorse Publishing) while noting some points relative to the electronics printed circuit board and EMS industries for current, and wannabe, defense contractors with intent.
- Force protection technologies include devices like the electromagnetic rail gun that could replace the Phalanx Systems on navy ships. The Rail Gun system involves 80+ megajoule capacitor banks and solid state switching up to 5.5 mega-amps in 10 milliseconds.
- Command, control, communications, computers, intelligence, surveillance, and reconnaissance (C4ISR) technologies include: Active Electronically Scanned Array (AESA) and Synthetic Aperture Radar (SAR) systems; Joint Tactical Radio System (JTRS) technologies, stealth satellites; hypersonic aircraft, RFID tagging, and certain UAVs.
- There are also calls for precision lethality technologies like Joint Direct Attack Munitions (JDAM) and other missiles, as well as retrofitting pre-existing ‘dumb’ munitions inventories with Precision Guided Kits (PGK). JDAM involves GPS and actuator motor technologies.
- Miniaturization applications include shrinking the size of conventional missiles and avionics; replacing rigid circuits with rigid-flex circuit boards in aircraft power distribution, and micro sensors.
- Unmanned vehicle technologies range from man-portable programs such as Dragon Eye, to catapult-launched vehicles such as Scan Eagle, to the mid-altitude loitering Predator, to the high altitude loitering Global Hawk.
- A number of military blimps such as the ISIS (Integrated Sensor IS Structure) program (Lockheed) will also emerge. Blimp technology includes managing high frequency materials in phased array radar configurations. Northrop Grumman’s unmanned vehicle, Fire Scout, involves actuator motors, rigid flex circuit boards, antenna suites, data links, and shielded avionics.
Manufacturing
Defense and aerospace industries throughout the world depend on precision electronics manufacturing and electronic assemblies’ solutions for their high-end applications. As Lead time is becoming shorter, and cost pressures are building up; the designer must try to find design approaches that are common to varied applications. All too often, however, a given package’s requirements are unique, and only portions of previous designs or new developments can be used.
We use Altium for schematic and layout, and a whole lot of simulation and modeling tools outside of that. I like Altium because, generally, it has good linking between the schematic and the layout. The backward and forward synchronization is really critical when you’re doing complex designs that need to interact with other programs and teams.
There’s also so many other tools you need to design a cutting-edge system nowadays. We have a lot of other tools in the chain. You need circuit simulation/SPICE tools before you even think about a PCB: LTspice and ADS are our favorites. Then you need to think about what those electrons are going to do in real life – ideal wires don’t exist. Tools like HFSS and SiWave are critical for verifying any of our high frequency components. It’s great that you can export and simulate your PCB models and gain some confidence in your design.
Distinct packaging techniques have been developed for such commercial equipment as radio and television, for aircraft communications and control systems for military ground equipment, and for missile and space vehicle guidance and instrumentation. It cannot be overemphasized that a packaging technique for the individual stage or circuit will have serious consequences for the product design. Every ounce removed, every fastener or accessory item eliminated, every increase in vibration response frequency will decrease the penalty in weight, volume, and cost necessary to meet the package’s environmental specifications. Within the space allotted to electronic equipment on a missile, aircraft, or space vehicle, the weight and volume of the various packages determine the quantity of data and the instructions that may be transmitted and received.
Where severe environments are to be encountered, it becomes mandatory that the packaging techniques satisfy all conditions. Use of less-than-optimum methods of joint connection, support, thermal paths, RF shielding, and layout incurs an overall package penalty. For any one application, components and connections must have a particular configuration. Because a “standard” packaging technique must be extremely flexible, a relatively simple connection method should be chosen to eliminate unnecessary variables and tolerances. In short, all material used and all operations performed must be completely functional and must become a part of the product. Ease of manufacture is a measure of the efficiency achieved in the various design areas. A well planned and integrated design will result in a minimum of manufacturing difficulty. Electronic components of high quality will also keep replacement and rework to a minimum.
These quality components together with a repeatable, uncomplicated manufacturing process will ensure inherent component quality. Resistance-welding is one such process; it is repeatable, simple, neat, and allows for the placement of circuit connections close to component bodies. Because of this close joining, high-density modules are possible. In frequency-sensitive circuits, shorter lead lengths are desirable. If a satisfactory job is done in each design area so that the product will meet all customer specifications, then the cost incurred is minimum. Factoring in cheaper or supposedly equivalent methods and materials is dangerous to the success of the product. In fact, such practices may well boomerang and raise the manufacturing costs from expensive revision and rework.
The module configuration has been called modular weldment, but it is more than a configuration; it is a system of integrated design and manufacture. Although the modules are designed to withstand the extremes of missile environment, the environmental integrity is achieved without the penalty of special devices. This means that modular weldment is directly applicable to aircraft, ground, and test equipment-with no cost penalty and sometimes even with a cost reduction.
Rigid-flex printed circuit boards
U.S. military appetite for printed circuit boards is $1.1 billion with roughly $300 million (and growing) for military rigid-flexible printed circuit boards. To the benefit of some EMS providers (and disadvantage of many others) engaging new military and defense opportunities for EMS providers will be as much about knowledge, competency and trust as it will be about footprint or size. Defense electronics is a mentality, not a market.
Of particular interest is the rapid adoption of rigid-flex printed circuit boards in military applications, which reduce connector content, reduce weight, and can be formed to more efficiently utilize space constraints. Today, most EMS providers are managing their businesses to assemble rigid printed circuit board assemblies. Meanwhile, the market for the U.S. military’s appetite for printed circuit boards is $1.1 billion with roughly $300 million of it (and growing) for military rigid-flexible PCB work. Its important for EMS providers to keep in mind the majority of this work also some value-add assembly. High frequency signal management will dominate new designs.
Power distribution and thermal management will be areas of continued development and, keep in mind, there are only so many more transistors that can be crammed into a piece of silicon. Reduced graphene oxides and other materials currently being explored will also allow a continuous increase in transistor densities that will lead to commensurate changes in substrate and final systems assembly considerations. Reliability will become the primary concern, compelling EMS providers truly committed to defense electronics to get involved at the outset of component choice and front-end systems design.
In House Manufacturing
Today, there are many standalone EMS facilities with local presence but with less market clout than a regional EMS providers with secondary facilities; lesser still than large, multi-national EMS footprints. But, to the benefit of some EMS providers (and disadvantage of many others) engaging new military and defense opportunities for EMS providers will be as much about knowledge, competency and trust as it will be about footprint or size. The implication here is it doesn’t matter what size a company may be to effectively engage the military. They’re looking for help and they’re looking for competent, trusted suppliers who can synchronize their capabilities with military needs. Most EMS providers think the ‘cake’ for the military is the delivered electronics assembly and all of the problem-solving expertise; compliance initiatives, certifications, and managing federal acquisition regulation (FAR) flow down paperwork are the collective ‘icing’ on the cake.This may mean changing paradigms from being mere electronics assemblers to becoming robust partners that do their homework and foster a trusted DoD-EMS supplier relationship.
When carefully designed defense electronics modules are slotted into contract manufacturing facilities for production they take their place in the queue along with orders from the telecommunications, medical equipment, and consumer electronics industries. Like any business, the contract manufacturer responds fastest to their big volume orders and can’t afford to invest in equipment used only by a small set of customers. The implications for defense are obvious; a small, unexpected order will wait for a schedule opening, while unique processes and tests require that boards are moved to labs or small, specialized shops for final preparation.
The best way to avoid these issues is with an in-house manufacturing capability. Having full control of the manufacturing process delivers a combination of responsiveness, flexibility, and quality that far exceeds anything achievable by contract manufacturing. It also ensures that those special capabilities, processes, and testing typical to rugged deployed commercial-off-the-shelf (COTS) electronics are handled correctly, such as:
− both leaded and lead-free soldering
− RoHS
− International Traffic in Arms Regulations (ITAR) and non-ITAR
− Class 3 of IPC and J-STD
− Environmental stress screening for every rugged board
− Three options for conformal coating: acrylic, urethane, and parylene
References and Resources also include:
https://www.microcontrollertips.com/challenges-in-designing-electronics-for-satellites/
