This article is written to describe electrical grounding design architecture options that have been implemented in spacecraft. Spacecraft grounding architecture is a system-level decision which must be established at the earliest point in spacecraft design. In this post we assume that there is no one single “correct” design for the spacecraft grounding architecture. After a long search, we discover that there have been many successful satellite and spacecraft programs from both NASA and ESA using a variety of grounding architectures with different levels of complexity.
However, some design principles learned over the years, apply to all types of spacecraft development. This article summarizes those principles in order to help guide spacecraft grounding architecture design.
Why is it important to take a good electrical grounding design into account?
The primary objective of proper grounding architecture is to aid in the minimization of electromagnetic interference (EMI) and unwanted interactions between various spacecraft electronic components and/or subsystems. Ideally, this architecture should help achieve successful and good electromagnetic compatibility (EMC).
Grounding procedures used in the design and assembly of electrical and electronic systems will protect personnel, circuits from hazardous currents and damaging fault conditions. Benefits are the prevention of potential damage to delicate space flight systems, subsystems and components, in addition to the protection of development, operations, and maintenance personnel.
Types of Grounding Systems
Single Point Star Ground
The single-point ground may be interpreted literally to mean that all circuit commons are grounded by means of wiring that lead to a single point on the chassis. Note the isolation of grounds (circuit commons) between assemblies in the image below. As a result, there is one, and only one, dc ground reference path for each assembly.
This is sometimes referred to as a “star” ground because all ground wires branch out from the central point of the star. Inductances of long wires and higher frequencies can negate the adequacy of the ground to the degree that the assembly may no longer have a zero potential reference with respect to the chassis.
This grounding system shows a multiple point (multi-point or multi-path) ground arrangement. Note that each circuit common is grounded directly to the chassis and also grounded indirectly to the chassis via the connections to the other assemblies. This is typical for radio frequency (RF) subsystems but should not be used for video or other signals containing low frequencies (less than roughly 1 MHz).
Multiple, single reference Ground System
The next picture shows a better chassis reference ground system. Each assembly strictly has only one path to the chassis (the zero voltage reference), and there are no deliberate structure currents. Compared to the star SPG configuration, each ground reference wire is short, providing minimum ac impedance between each circuit common and chassis. The important points are that each electronic item has one and only one path to chassis, and there is no deliberate chassis current. Also, all subsystems have a common dc voltage reference potential (the interconnected structure). This grounding architecture is typical for a modern spacecraft (S/C) that pays special attention to g.
Floating (Isolated) Grounds
The following picture shows a floating (isolated) ground system (generally not desirable). While systems are usually ground referenced in some manner, there is no theoretical reason why an assembly’s circuit common needs to be chassis referenced. However, practical considerations dictate that at least a static bleed resistor (resistor attached to chassis that is large enough such that is has no practical effect on the circuit, but it permits any stray charge to “bleed” to ground, thus providing a “soft” ground reference) be present, even if isolation from chassis ground is designed into the subsystem/system (circuit commons isolated from chassis are vulnerable to noise pickup through parasitic paths).
Daisy-Chained Ground System (Not recommended)
This is a poor practice in general, and it is shown only to emphasize that it should not be done. Shared return wires cause common mode voltage differences (circuits “talk” to each other through common mode impedance coupling). It may be tolerable if it is done within the confines of a specific system component such as an attitude control subsystem, and the subsystem provides the box-to-box wiring. If it is permitted for separately built assemblies that are later integrated together, unpredictable behavior may occur.