Future exploration missions in deep space will use scientific tools capable of generating massive amounts of data. To be able to process them, the onboard computers will require high computational powers. The most significant limit to the increase in performance of these devices is the extreme environment in which they have to work.

The space environment has particular conditions that can influence and, in some cases, degrade the mechanical characteristics of space-based materials and therefore negatively influence the overall behavior of the structures’ operation.

The flow of space radiation consists primarily of 85% protons and 15% heavy nuclei. The effects of radiation can lead to degradation, interruptions, and discontinuities in the performance of the device. The main requirement for space qualified components is the ability to ensure reliable long-term operation.

Space radiation
Space, including the uppermost area of the earth's atmosphere, is populated by many high-energy particles that can damage semiconductor devices. For example, there are electrons and protons in the Van Allen belt, galactic cosmic rays, x-rays and ultraviolet rays. In general, there are two significant categories of effects (cumulative and single-event) that produce changes in the operating parameters of microelectronic circuits (Figure 1).

Figure 1 Scheme of a massive ion attack on the CMOS memory cell cross section

The charged particles and gamma rays create ionization which can alter the parameters of the device. These changes are estimated in terms of the total ionizing dose (TID) parameter. The absorbed ionizing dose is commonly measured in rad, an absorbed energy of 100 ergs on a gram of material (the rad was replaced over the years by the gray: 1 rad = 0.01 Gy)1. Since the loss of energy per unit of mass varies from one material to another, the material on which the dose is deposited is always specified in the unit of measurement [for example rad (Si) or rad (GaAs)].

TID is a cumulative effect of long-term exposure to electrons and protons due to Solar activity. The gradual degradation of component parameters such as supply and dispersion currents, threshold voltages, and propagation time are characteristics of TID faults. The requirements for the duration of missions with spacecraft and satellites and the altitude of the orbits, determine the level of ionizing radiation to which the components must comply. Typical levels are between 10 and 100 krad (Si).

The cumulative effects are the damages that irreparably accumulate over the years, to the point of making the electronics unusable within the devices in space. These damages are predictable in the laboratory and with this information we are able to establish a useful average life for each aircraft.

The single-event effects (SEE), on the other hand, are unpredictable and can appear at any time depending upon the placement of the electronic equipment. The SEEs are grouped into two categories: transient effects (or soft errors) such as Single Event Transient (SET) and Single Event Upset (SEU); catastrophic effects such as Single Event Burnout (SEB), Single Event Gate Rupture (SEGR) and Single Event Latch-up (SEL).

The mechanism underlying every SEE consists of the accumulation of charge in a sensitive area of ​​a device following the passage of the particle. In a semiconductor device in the path, by Coulomb interaction, will free an electron-hole pair with a diameter varying from a few hundred nanometers to a few microns.

Depending on several factors, the particle can cause unobservable effects (SET), transient perturbations of the microprocessor circuit operations, changes in logic states (SEU, SEL), or permanent damage to the device or integrated circuit (SEGR, SEB).

The prevention solution is to avoid placing satellites in the areas of the Van Allen belt; they can also turn off during periods of increased solar wind flow. Shields are made and used against radiation (although they can sometimes be heavy); but above all, rad-hard components are implemented in the design and tested in the laboratory (Figure 2).

Figure 2 Here's an example of a rad-hard component. Source: Aeroflex

Satellite architecture
Modern telecommunications satellites have a structure designed to optimize the process of placing them in a proper orbit and to better enable their function. A satellite is composed of a central part where most of the electronic equipment, the propulsion system, and the relative tanks are located (Figure 3).

The solar sensors are used and are able to identify the position of the sun which is the main point of reference for satellite positioning. The propulsion system is used to keep the satellite in its position. In the service platform or module, there are the functions for controlling the pointing of the satellite direction, propulsion, thermal regulation, and power. Typical components of the control system include inertial measurement units (IMU) and the electronics needed to process signals and monitor the position of the satellites. The gyroscopes ensure pointing stability. For telecommunications systems, the payload would include, antennas, low noise amplifiers, and local oscillators. In particular, for GPS navigation satellites, this includes atomic clocks, signal generators, and amplifiers.

Figure 3 The structure of a satellite is shown in this diagram. Source: NASA

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Maurizio Di Paolo Emilio is a Ph.D. in Physics and a telecommunication engineer.