Challenges for Electronic Circuits in Space Applications: Part 1
by Chris Leonard, Analog Devices, Inc.
This article provides insight into the challenges engineers encounter in implementing circuits for space level applications. Concerns about reliability, radiation effects and the ability to tolerate harsh environmental conditions require special attention during the design process to ensure mission success. The global space market is approximately $330 billion. We’ll review the satellite industry segment of the space market and the applications driving growth. Satellite services are a key driver for the overall industry and present the standard elements of a typical satellite platform and various types of payload applications that they support.
The second part of this series provides a review of the natural space radiation environment. We’ll identify and describe two primary types of radiation that can affect and damage electronic devices. The article discusses current technology trends and their interaction with the natural space radiation environment. To conclude the article series, we’ll present Analog Devices’ efforts in support of space level applications.
To set the stage for this discussion, let me propose this scenario: Imagine yourself as an astronaut sitting in the crew module of the NASA Orion spacecraft. You are stepping through your final equipment checklist for a voyage to Mars. You are sitting on top of a rocket, anticipating the final countdown to ignition. The ignition of the largest rocket ever designed, the NASA Space Launch System. You are sitting 384 feet in the air on a massive 130 metric ton configuration, the most capable and the most powerful launch vehicle in history. When you hear those famous words, “gentlemen, we have ignition,” you will have 9.2 million pounds of thrust propelling you into outer space. The Orion spacecraft is being designed to take humans to Mars and into deep space, where the temperature can approach 4,000°F, the radiation is deadly, and you will be travelling at speeds up to 20,000 mph.
Now ask yourself: what quality grade of electronic components were selected for the control systems of your spacecraft? High reliability and devices with space heritage are key factors in the selection of components for space level applications. NASA generally specifies Level 1, Qualified Manufacturer List Class V (QMLV) devices, and they will always ask if there a higher quality level available. Knowing the extensive selection process NASA uses for identifying electronic components for space flight applications, one should be confident sitting on top of that rocket.
The Harsh Environmental Conditions of a Spacecraft and the Hazards Posed to the Electronics
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 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 the performance. For instance, an automobile plastic dash can emit vapor that deposits a film on the windshield. This is a practical example I can attest to from personal experience. Using ceramic rather than plastic components eliminates this problem in electronics. Outgassing of volatile silicones in low earth orbit can 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.
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 volts (V) have been known to occur on satellites in geosynchronous orbits. If protective design measures are not taken, electrostatic discharge, a build-up 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 Low Earth Orbit (LEO) is comprised of about 96% atomic oxygen. Oxygen exists in different forms. The oxygen that we breathe is O2. Ozone O3 occurs in Earth’s upper atmosphere, and O (one atom) is atomic oxygen. Atomic oxygen can react with organic materials on spacecraft exteriors, gradually damaging them. Materials 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, which 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. Another obstacle is the very high temperature fluctuations encountered by spacecraft. A satellite orbiting around earth can be divided into two phases: a sun-lit phase and an eclipse phase. In the sun-lit phase, the satellite is heated by the sun and as the satellite moves around the back side 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.
It is interesting to note that during a lunar day and night, the temperature on the surface of the Moon can vary from around +200 to -200 degrees C. It makes you wonder how it was even possible for a man to walk on the moon. 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. So how do you dissipate the heat generated by the electronics? The accuracy and life expectancy of electronic devices can be degraded by sustained high temperatures. There are three ways of transferring heat: 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.
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 which 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. 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. Will your spacecraft mission occur during a solar minimum, a solar maximum period, or both? The key point here is that 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.
Analog Devices has been supporting the Aerospace & Defense markets for over 40 years with high reliability devices. Areas of focus are electronic warfare, radar, communications, avionics, unmanned systems and missile and smart munitions applications. Today’s focus is on the space market. Analog Devices has the depth and breadth of technologies that spans the complete signal chain from sensors, amplifiers, RF and microwave devices, ADCs, DACs, and output devices that provide solutions to the challenging requirements of the aerospace and defense industry.
The satellite industry revenue was $208 billion in 2015. There are four segments in the satellite industry: satellite manufacturing, the satellite launch industry, ground based equipment and satellite services.
Satellite services is by far the largest segment and it continues to be a key driver for the overall satellite industry. So, what has a satellite done for you lately? I believe most people would be surprised at just how much modern life depends on satellite services. If the 1,381 satellites currently in operation happened to shut down, modern life would be significantly disrupted. Global finance, telecommunications, transportation, weather, national defense, aviation, and many other sectors rely heavily on satellite services. There are three primary segments in the satellite services market: satellite navigation, satellite communications and Earth observation. Navigation satellites are used for the global distribution of navigation signals and data in order to provide positioning, location and timing services. Examples of available services are traffic management, surveying and mapping, fleet and asset management and autonomous driving technology — driverless cars and trucks are expected to be the next big thing. Telecommunication satellites or SATCOM examples are television, telephone, broadband Internet, and satellite radio. These systems can provide uninterrupted communications services in the event of disasters that damage ground-based telecommunication networks. Both business and commercial aircraft in-flight internet and mobile entertainment are growing segments of the market. Earth observation satellites are used for the transmission of environmental data. Space-based observations of the Earth promote sustainable agriculture and aids in the response to climate change, land and wildlife management, and energy resources management. Earth observation satellites aid in the safeguard of water resources and improve weather forecasts, so there are a very wide and growing range of satellite services.
So what types of electronic systems are used on satellites? The basic elements of a spacecraft are 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, the mechanical structure, provides stiffness to withstand stress and vibration. It also provides shielding from radiation for the electronic devices. The telemetry, tracking, and command subsystem includes receivers, transmitters, antennas and sensors for temperature, current, voltage and tank pressure. It also provides the status of various spacecraft subsystems. The electric power and distribution subsystem converts solar into electrical power and charges the spacecraft batteries. 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) 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 satellite’s position.
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 a telecommunications system, the payload would include antennas, transmitters and receivers consisting of low noise amplifiers, mixers and local oscillators, demodulators and modulators, and power amplifiers. Earth observation payloads would include instruments like microwave and infrared sounding instruments for weather forecasting, visible infrared imaging radiometers, ozone mapping instruments, visible and inferred cameras, and sensors.
The integration of Analog Devices and Hittite Microwave a few years ago now allows us to cover the DC to 110 GHz spectrum. ADI solutions range from navigation, radar and communication systems below 6 GHz, satellite communications, electronic warfare and radar systems in the microwave spectrum and radar systems and satellite imaging in the millimeter wave spectrum. Analog Devices offers more than 1000 components covering all RF and microwave signal chains and applications. The combination of Hittite’s full spectrum of RF function blocks, attenuators, LNAs, PAs and RF switches, in conjunction with Analog Devices’ portfolio of high performance linear products, high speed ADCs, DACs, active mixers and PLLs can provide end-to-end system solutions.
Conclusion
In this first part of a two part series, we discussed the harsh and ever changing environment of deep space. There are vastly different environments in outer space; the requirements for a low earth orbit satellite mission can be vastly different from that of Mars Rover or a deep space probe. 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. The second part of this article series will explore the natural space radiation environment and the effects different types of radiation can have on electronic devices.
Author bio:
Chris Leonard is a member the Analog Devices Space Products marketing team, focusing on electronic components for space level applications. Chris joined Analog Devices in 1982 and has supported the design, development, and manufacturing of electronic devices focused on military and space level applications. With over 30 years in the semiconductor industry, he has held a variety of engineering roles in multichip module design, assembly, test and qualification operations, in addition to several years experience in bipolar wafer process technology. He has spent the last 20 plus years in a variety of marketing roles, primarily focused on military and aerospace markets.
Look for Part Two of this article in next month’s issue of MPD
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