Designing For The Mil-aero Market—Satellites
Similar to the products developed for naval applications, satellites are subject to specific environmental conditions that must be addressed during the product development cycle. From Radhardened ICs to heat pipes and an extreme emphasis on reliability, PCBs designed for use in satellites are subjected to extraordinary operational issues. This article will describe the challenges associated with designing PCBs used in satellites and the uniqueness of that design environment.
Not Very Many, But Very Expensive
Akin to naval applications, the PCBs developed for satellites are characterized as being very expensive with low volumes. In addition, the number of fabricators and assemblers who can build these products are highly specialized and limited.
Note: when I am talking about military-aerospace satellites, they are the ones that are used for surveillance and the ones used to support a huge communications network. For instance, GPS was originally created for the military and they still maintain this system today.
The high cost of satellite PCBs are a given due to the technology incorporated in them as well as the aforementioned low volumes. Further, these PCBs tend to be project specific so it’s unlikely that a PCB built for one satellite/program could be readily transferred to another.
As noted previously, with mil-aero programs, the amount of paperwork associated with a PCB build can be double the cost of the board itself. In addition, board manufacturers and assemblers have to be certified for mil-aero projects and that is a time, labor and cost-intense proposition.
Look, Up in The Sky: It’s a Bird, It’s a Plane, It’s a Satellite
In terms of signal integrity issues for satellites they are the same as every other application environment because the same level of technology (IC components), with some specific implementations, is used. In addition, satellites contain all the same performance processors, microwave products and RF radios as those used for other product implementations.
The environment-specific challenges encountered in developing satellite PCBs include:
- The PCBs have to able to survive the launch shock.
- The ICs need to be Rad-hardened.
- The end products need to be of minimum weight.
- There are severe power consumption restrictions.
- There are heightened cooling requirements.
- Reliability is a huge concern.
I will address these challenges in order.
The launch shock criteria are pretty much a given when you factor in the multi-megatons of thrust necessary to lift a missile off a launch pad. Similar to being able to withstand operation in a highly corrosive environment for the navy, in satellites, PCB packages can look is if they have been overdesigned and overbuilt. The reasons are fairly clear. If a PCB suffers any damage during the launch process, there is no way for it to be repaired once it is in space.
Radiation hardening is the process of ensuring that electronic components and circuits are resistant to the damage or malfunctions caused by high levels of ionizing radiation, such as the particle radiation and high-energy electronic radiation that is found in outer space. PCBs must be designed so that they readily accommodate Rad-hardened ICs.
ICs are Rad Hardened when a thin layer of silicon is grown on a sapphire wafer. (This process is known as SOS, or silicon on sapphire). The silicon is usually deposited by the decomposition of silane gas on a heated sapphire substrate. Sapphire is an excellent electrical insulator that prevents stray radiation currents from spreading to nearby circuit elements. All of the ICs used in satellites are Rad-hardened.
It should also be noted that all military satellites are designed in such a way that they can survive an EMP (electromagnetic pulse). An EMP is a huge energy shock similar to that which is generated by a lightening burst. This is what causes static sound in a radio. When an EMP hits a satellite it can destroy the electronics within the satellite by inducing very high voltages in the wires. The fix for this are the fiber optics that are now incorporated into satellites and most new airplanes.
All of the technology incorporated into a satellite needs to be of minimum weight—right down to the PCBs and components incorporated on them. Those engineers involved in developing satellite technology search every aspect of a satellite to ensure that it satisfies the specified weight requirements. They also search for ways to reduce weight wherever and whenever possible.
The power consumption goal for all satellites is to consume as little power as possible for the greatest time span possible. All of the satellites that are in earth orbit are solar powered. This means that they have to have some batteries that supply power when the satellite is behind the earth. It also means that there has to be a fairly elegant power management system.
Those satellites that leave earth orbit as well as those that have to work on the dark side of the moon are nuclear powered. This is accomplished by having thousands of thermal couples that are all hooked together and surround a hot nuclear core.
Cooling the ICs on satellites is normally done with heat pipes. Heat pipe technology actually originated with satellites. A heat pipe is created by having a metal plate that has tubing inside it and is put on the top of an IC. The tubing leads out to an open area where there is another big plate. Inside the tube, there is a mesh and a liquid. This liquid is chosen such that the heat of the IC makes it into a vapor. The vapor goes down the middle of the tube to the other end where it condenses back into a liquid and then goes back in the mesh to the other end where the cycle repeats itself. The use of heat pipes on ICs has expanded way beyond satellite technology. For example, if it weren’t for heat pipes, some of the ICs in Internet products would not function properly due to very high power levels.
In those instances where heat pipes cannot be used, such as when there is a very large IC, extra layers of copper can be added in the PCB to create a heat sink. The additional layers of metal within the PCB work to conduct the heat away from the IC.
When it comes to reliability issues, the MTBFs have to be very high. This includes all of the satellite’s components such as the vehicle itself as well as the technology incorporated into it all the way down to the PCBs. The typical lifetime expectancy for a communications satellite is 10 years. The challenge is that the solar panels degrade while the rest of the satellite components remain intact and functional. In the case of the geosynchronous satellites used for TV, there are tiny little rocket motors on them that can be fired when they start to drift out of position. By firing the rocket motors, the satellites can be put back into their exact positions. In these instances, the life of the satellite runs out when those motors run out of fuel. Since the costs of repairing these satellites is not reasonable, it’s easier just to send up new satellites.
As humans, we have an innate fascination with space. Since the beginning of the Space Race in the late ‘50s, we have turned our eyes upward almost in disbelief to watch those pieces of hardware that man has put into space as they fly over us on earth. Lee Ritchey, Founder and President of Speeding Edge, built the first radios that were left on the moon as part of the Apollo program. I had the great fortune to serve as a CDM (Configuration Data Management) engineer for the Space Shuttle Launch complex that was nearly completed on Vandenberg Air Force Base in central coast California in the mid 80s. Several years later, I was the public relations consultant for Wind River Systems when the company’s VxWorks software technology was guiding the first Pathfinder exploratory spacecraft to Mars in 1997. NASA was focused on using COTS (commercial off-the-shelf) technology for the project. The computer onboard Pathfinder consisted of a Rad-hardened IBM RISC 6000 CPU with Wind River’s VxWorks as the operating system. The wheeled robotic Mars rover, Sojourner, was the first rover to operate outside the Earth-Moon system. For both of us, our experiences working on space program efforts are among the high points of our careers.
Design and manufacture of PCBs used in satellites have to take into account a number of environmental and performance parameters that are specific to space operations. A thorough understanding of these parameters can help to ensure that not only will the PCB work right the first time but every time throughout the entire lifespan of the satellite.