In this third lecture, the Professor will review the four regions of the “spacescape” and how the radiation levels are an important consideration in device selection.
Defining the Spacescape
The “spacescape” can be defined as comprising four distinct regions. In a whitepaper published in 2020, several key parameters are discussed that are considered in making the optimum crystal oscillator selection for each region – phase noise & jitter; size, weight & power (SWaP); stability; and radiation tolerance. When it comes to the latter, this chart defines to industry standard for total ionizing dose (TID) radiation in each region.
|Environment or Orbit||Applications||Industry Standard TID|
|Deep Space||Inter-planetary missions||300kRad+|
|GEO||Com, Mil, Scientific||100 kRad|
|MEO||Com, Nav||100 kRad|
|LEO||Low Cost Megaconstellations||30-50 kRad|
Historically, most of the unmanned space programs were in GEO (Geosynchronous) orbits or beyond. GEO orbits are about 36,000 kilometers above ground level, high enough to allow a satellite to be positioned to be almost stationary over an exact earth surface location. Each of these kind of satellites costs millions of dollars and no one is willing to take any chances. Companies building these satellites use only the very best, most reliable components that are rad hard in every way. We commonly call these kind of applications “Full Space” and 20 years ago this was the most common kind of space application, and today the Full Space category still makes up a lion’s share of space programs. GEO satellites are at a high enough altitude to cross over the infamous Van Allen radiation belts, and so are exposed to somewhat higher levels of several types of radiation. We will discuss the many types of radiation in our next lecture.
MEO (Medium Earth Orbits) are between 2000 and 35,000 kilometers above the earth’s surface. The radiation requirements for electronics in use in MEO orbits is not that much different than for GEO. There are 2 MEO orbits of particular significance, called semi-synchronous orbits at about 20,000 kilometers above the earth’s surface. The importance of these orbits is that they allow a satellite to pass over the same two spots on the earth’s surface every day. Prominent examples of the use of semi-synchronous MEO orbits include the GPS (Global Positioning System), and other navigation satellites such as GLONASS, Galileo, and Beidou.
LEO (Low Earth Orbits) are only about 100 kilometers to 2000 kilometers above the earth’s surface. While almost all manned space flights use LEO orbits, the use of LEO orbit for communication and other commercial and military applications has only increased exponentially in recent years. These satellites are much smaller and lower cost than Full Space satellites. Commonly used in what are called Megaconstellations, which can be a fleet of dozens, hundreds or even thousands of satellites each. The first such Megaconstellation was the One Web project, with 648 satellites. Advantages of Megaconstellations include lower costs, faster development and deployment, ability to survive the loss of a few satellites without degradation of purpose and performance, and very low latency communication with earth, which is useful for real time video and human communication. Mission life of a LEO satellite can be only a few years, as opposed to 20 years or more for Full Space satellites. For use in LEO satellites, electronic components are expected to be lower cost, smaller in size, lower in power consumption and yet still be able to survive in their intended application. This niche is a booming growth business often referred to as New Space.
In the next and final lecture, I will discuss TID and other radiation effects in more detail.