Electronics Guide

Grounding Architecture

Spacecraft grounding architectures fall into several categories, each with distinct EMC implications:

Single-point ground: All ground returns connect to a single point on the spacecraft structure, theoretically eliminating ground loops. This approach works well at low frequencies but becomes impractical at high frequencies where distributed capacitances create multiple current paths.

Multi-point ground: Subsystems ground to the spacecraft structure at multiple points, minimizing ground lead inductance. This approach is superior at high frequencies but requires careful attention to controlling low-frequency ground loops.

Hybrid ground: Most spacecraft use hybrid approaches, with single-point grounding for power returns and low-frequency analog signals, and multi-point grounding for RF and high-frequency digital systems. The transition between regimes typically occurs around a few megahertz.

Key grounding design principles:

• Power returns should be low-inductance conductors, not depending solely on spacecraft structure
• Signal ground and power ground should join only at defined points
• Sensitive analog circuits should have isolated ground systems tied to spacecraft ground at a single point
• RF system grounds should connect directly to local structure with minimum inductance

Bonding Requirements

Electrical bonding ensures that the spacecraft structure provides a low-impedance path for fault currents, EMI return currents, and electrostatic charge dissipation:

DC bonding: Maximum resistance values are specified between components and structure, typically 2.5 milliohms for EMC-critical bonds and 25 milliohms for structural bonds. These values ensure adequate current paths for fault protection and ESD dissipation.

RF bonding: At radio frequencies, surface transfer impedance rather than DC resistance determines bonding effectiveness. Joint design must maintain intimate metal-to-metal contact across the frequency range of interest, typically requiring bare metal surfaces, appropriate fastener patterns, and controlled finish specifications.

Bond verification: Bond resistance measurements are performed during integration and may be repeated periodically during environmental testing to verify that bonds remain intact after thermal cycling, vibration, and other stresses.

Structural Current Control

Despite best efforts to control return current paths, significant currents inevitably flow through spacecraft structure:

Structural impedance: The spacecraft structure is not a perfect conductor, and at high frequencies its impedance is dominated by inductance rather than resistance. Current distribution across structural elements creates voltage gradients that can interfere with sensitive circuits.

Seams and joints: Structural joints can have higher impedance than continuous panels, particularly at high frequencies. Current preferentially flows around joints, creating magnetic field concentrations that can couple to nearby cables and circuits.

Mitigation approaches: Critical circuits should be located away from high-current structural paths. Cable shields should be terminated to local structure rather than relying on conductivity through multiple joints. Sensitive sensors may require isolated ground planes with controlled connection points to the main structure.

Power Bus Characteristics

Spacecraft power buses have evolved through several generations:

Unregulated buses: Direct connection to battery terminals, with voltage varying from fully charged to discharge levels. Equipment must tolerate wide voltage ranges but power system EMI is minimized by the absence of high-frequency switching.

Regulated buses: Switching regulators maintain constant voltage, but their high-frequency switching creates conducted and radiated emissions. Regulation frequencies typically range from 50 kHz to several MHz.

Hybrid architectures: Many spacecraft use both regulated and unregulated buses, with critical loads on regulated buses and power-tolerant loads on unregulated buses to minimize power system complexity and emissions.

Power bus EMC requirements typically specify:

• Conducted emissions limits on power lines (ripple voltage and current)
• Load impedance requirements (to prevent interactions with power system dynamics)
• Transient tolerance (turn-on and turn-off events, fault conditions)
• Inrush current limits (to prevent bus voltage sags affecting other loads)

Power Converter Design

DC-DC converters and power regulators require careful EMC design:

Switching topology: Converter topology affects EMI characteristics. Hard-switching converters generate faster edges and broader spectra than soft-switching designs. Full-bridge and push-pull topologies can achieve some degree of EMI cancellation through symmetry.

Input filtering: Input filters attenuate conducted emissions from the converter to the power bus. Filter design must consider the source impedance (which varies with power system state) and maintain stability across operating conditions.

Output filtering: Output filters reduce voltage ripple delivered to loads. Higher-order filters provide better attenuation but may interact with load impedance to create instability.

Layout and shielding: High-current switching loops should be minimized and contained within shielded enclosures. Power magnetics should be oriented to minimize coupling to adjacent circuits.

Power Distribution Wiring

Power wiring distributes EMI as well as power throughout the spacecraft:

Twisted pairs: Power and return conductors should be twisted together to minimize the magnetic field area they create. Twist pitch should be appropriate for the frequency spectrum of concern.

Common-mode filtering: Common-mode chokes on power feeds attenuate common-mode currents that would otherwise flow on cable shields and spacecraft structure.

Wire segregation: Power wiring should be segregated from signal wiring by maximum practicable distance. When crossing is unavoidable, perpendicular crossing minimizes coupling.

Harness design: Careful harness routing avoids placing power wires adjacent to sensitive signal wires. Power harness branches should be filtered at their source to prevent EMI propagation throughout the spacecraft.

Solar Array Electrical Characteristics

Solar arrays are electrically complex structures:

Voltage levels: Modern high-power arrays may operate at 100 V or higher, creating potential for electrostatic discharge and electromagnetic field generation. The trend toward higher bus voltages increases EMC concerns.

String configuration: Arrays consist of series-connected strings of solar cells, with strings connected in parallel. String failures can create voltage imbalances that affect grounding and charging behavior.

Bypass diodes: Diodes protect shaded or failed cells from reverse breakdown but create potential points of failure and EMI generation if they fail or operate under unexpected conditions.

Switching transients: Connection and disconnection of array sections for power regulation creates transients that can couple to other circuits.

Electromagnetic Coupling Modes

Solar arrays can couple electromagnetic energy through several mechanisms:

Capacitive coupling: The large area of solar arrays creates significant capacitance to the plasma environment and to the spacecraft structure. High-frequency voltage variations couple through these capacitances.

Inductive coupling: Current loops formed by solar cell strings and return conductors generate magnetic fields that can couple to nearby circuits and cabling.

Common-mode currents: Voltage differences between the array and spacecraft structure can drive common-mode currents on power cables, coupling array noise into spacecraft systems.

Electrostatic discharge: Arrays can charge relative to plasma and spacecraft structure, with subsequent discharges generating broadband EMI.

Solar Array EMC Design

Effective solar array EMC design incorporates several strategies:

Grounding scheme: The array electrical ground point relative to spacecraft structure significantly affects charging and EMC behavior. Options include grounding the positive bus, grounding the negative bus, or center-grounding with respect to the structure.

Harness design: Power harness routing should minimize loop areas and maintain tight coupling between positive and return conductors. Shielding may be required for sensitive spacecraft.

Filtering: Common-mode filtering at the array-spacecraft interface attenuates noise transfer. Differential-mode filtering reduces ripple from switching regulators.

Plasma interaction control: Coverglasses, edge insulation, and other features control plasma interactions that could otherwise lead to charging and arcing.

Electromagnetic field management: Array layout can be optimized to minimize net magnetic moments and reduce field coupling to adjacent antennas and sensors.

Interface Definition

Clear EMC interface requirements prevent integration problems:

Conducted requirements: Maximum conducted emissions and minimum conducted susceptibility levels on power and signal interfaces ensure that payloads can share power buses and communicate through the spacecraft data system without interference.

Radiated requirements: Radiated emission limits and susceptibility thresholds define the electromagnetic environment each payload must tolerate and must not exceed.

Grounding interfaces: The number, location, and impedance of ground connections between payloads and spacecraft structure must be specified and controlled.

Shielding interfaces: Cable shield terminations, enclosure bonding requirements, and aperture control at payload-spacecraft interfaces need clear definition.

Electromagnetic Isolation

When payloads have conflicting EMC requirements, isolation techniques separate them:

Spatial separation: Physically separating sensitive instruments from noisy equipment reduces coupling. Antenna placement typically requires maximum separation from other apertures and electronics.

Temporal separation: Scheduling incompatible operations at different times eliminates interference during critical measurements. This approach trades timeline flexibility for EMC compatibility.

Frequency coordination: Ensuring that payload operating frequencies do not overlap with other spacecraft systems or create interference through harmonics and intermodulation.

Shielding: Additional shielding enclosures around sensitive payloads or noisy equipment when other isolation methods are insufficient.

Integration Testing

Systematic EMC testing during integration verifies compatibility:

Unit-level testing: Each payload undergoes standalone EMC testing before integration to verify compliance with interface requirements.

Segment-level testing: Groups of payloads are tested together to identify interactions not apparent in unit testing.

System-level testing: The complete integrated spacecraft is tested in a representative configuration to verify that all systems operate compatibly.

Self-compatibility testing: Specific tests exercise critical operating modes (such as science measurements during communication passes) to verify absence of interference.

ISL System Characteristics

Inter-satellite links have demanding EMC requirements:

Power levels: ISL transmitters may need significant power to span the distances between spacecraft, creating potential for self-interference and interference to other missions.

Receiver sensitivity: ISL receivers must detect weak signals while rejecting interference from onboard sources and other spacecraft in the constellation.

Pointing requirements: Narrow-beam antennas reduce interference but require precise pointing. Antenna side lobes can still create interference in unintended directions.

Multiple simultaneous links: Constellation spacecraft may maintain links with multiple neighbors, requiring careful frequency and polarization management.

Interference Management

ISL EMC design addresses several interference sources:

Co-site interference: The spacecraft's own transmitters and oscillators can interfere with ISL receivers. Filtering, shielding, and frequency planning minimize this interference.

Inter-spacecraft interference: Links between different pairs of spacecraft can interfere if frequencies overlap or if sidelobes from one link illuminate receivers of another. Constellation-wide frequency planning addresses this.

Payload interference: Scientific instruments or other payloads may generate emissions at ISL frequencies or be susceptible to ISL transmissions.

External interference: Ground stations, other satellite systems, and terrestrial sources can interfere with ISL reception.

ISL Antenna Integration

ISL antenna placement on spacecraft requires careful EMC consideration:

• Antenna separation: ISL antennas should be separated from other antennas to minimize coupling through near-field interactions
• Obstruction avoidance: Spacecraft structures should not obstruct ISL antenna beams or create reflections that alter patterns
• Thermal effects: ISL antennas may experience different thermal environments depending on spacecraft orientation, affecting performance and EMC characteristics
• Pattern coordination: Antenna patterns should be coordinated with spacecraft attitude profiles to ensure link availability and minimize interference

Ground Support Equipment

Ground support equipment (GSE) used during spacecraft assembly, integration, and testing must be EMC-compatible:

Power sources: GSE power supplies should meet or exceed flight power system EMC requirements. Poorly designed GSE power can mask flight equipment problems or create spurious test failures.

Test cables: Cables connecting GSE to flight hardware must be shielded and filtered to prevent introduction of external noise. Cable lengths and routing during test should approximate flight configurations where practical.

Control and monitoring: GSE computers and monitoring equipment should not generate interference that affects flight hardware or masks flight equipment emissions.

Facility grounding: The relationship between GSE grounding, facility grounding, and flight hardware grounding requires careful management to prevent ground loops while maintaining safety.

Communication Links

Telemetry, tracking, and command (TT&C) links connect the spacecraft to ground stations:

Uplink considerations: Command receivers must reject interference from other ground transmitters, spacecraft emissions, and multipath. Receiver sensitivity and selectivity requirements flow from link budget and interference analyses.

Downlink considerations: Telemetry transmitters must not interfere with spacecraft receivers or sensitive instruments. Power levels must be sufficient for the link budget while minimizing EMC impact on other systems.

Ground station EMC: Ground stations have their own EMC environments, with high-power transmitters, sensitive receivers, and local interference sources. The spacecraft-ground interface specification must account for ground station characteristics.

Frequency coordination: TT&C frequencies must be coordinated with other space and ground users to avoid interference. International regulations govern space communication frequencies and power levels.

Launch Site Operations

Launch site EMC considerations include:

RF environment: Launch sites have complex RF environments with radar, communications, and tracking systems. Spacecraft receivers must tolerate these during ground operations and ascent.

ESD protection: Propellant loading and other operations create ESD hazards. Spacecraft and GSE must be bonded together and to facility ground during these operations.

Umbilical interfaces: The umbilical connection between spacecraft and ground systems during countdown must maintain EMC integrity until separation. Connector design and cable routing affect EMC performance.

Fairing effects: The launch vehicle fairing typically provides electromagnetic shielding during ascent but may also create resonances or modify spacecraft antenna patterns.

Electrical Interface

The electrical interface between launch vehicle and spacecraft includes:

Power connection: If the launch vehicle provides power to the spacecraft during ascent, this connection must meet EMC requirements for both systems. Transient events during staging and separation are particular concerns.

Signal connections: Telemetry, command, and status signals between spacecraft and launch vehicle must be protected from interference. These signals often pass through noisy environments during boost phase.

Grounding: The spacecraft-to-launch-vehicle ground connection affects EMC performance and must be specified. This connection is typically maintained through the separation interface until deployment.

Separation events: Pyrotechnic separation systems generate substantial EMI from both the detonation event and associated electrical transients.

Electromagnetic Environment

The launch vehicle creates an intense electromagnetic environment:

Pyrotechnic emissions: Stage separation, fairing jettison, and spacecraft separation all involve pyrotechnic events that generate broadband EMI with fast rise times.

Ignition transients: Engine ignition creates electromagnetic transients from both the ignition system and the resulting combustion.

Charging: Triboelectric charging during fairing separation and atmospheric transit can create substantial charge accumulation.

Plasma effects: Rocket exhaust plasma can affect RF propagation and create unexpected charging or coupling paths.

Interface Verification

Launch vehicle interface EMC is verified through:

Analysis: Analytical modeling predicts interface EMC based on system characteristics and heritage data.

Component testing: Critical components (separation systems, connectors, etc.) are tested individually to characterize their EMC behavior.

Integrated testing: Spacecraft-launch vehicle integrated testing exercises the electrical interfaces under representative conditions.

Similarity: For heritage interfaces, EMC performance may be verified by similarity to previous successful missions.

Surface Charging Control

Surface charging is controlled through material selection and design:

Conductive surfaces: Exposed spacecraft surfaces should be sufficiently conductive to dissipate charge accumulation. Surface resistivity below 10^9 ohm/square is typically required, with more stringent requirements for high-charging environments.

Material grounding: All surface materials must have reliable electrical connection to spacecraft structure. Conductive adhesives, grounding straps, or mechanical fastening provide these connections.

Avoid floating conductors: Isolated conductors can charge to different potentials than surrounding surfaces, creating discharge risks. All conductors should be grounded or designed with sufficient capacitance to limit voltage buildup.

Control dielectric areas: Large exposed dielectric surfaces (thermal blankets, solar cell cover glasses) should be designed with grounded conductors spaced closely enough to limit voltage development across the dielectric.

Internal Charging Control

Internal (deep dielectric) charging occurs when energetic electrons penetrate shielding and deposit charge in insulating materials:

Shielding design: Appropriate shielding thickness reduces the electron flux reaching internal materials. The optimal shielding thickness depends on the electron spectrum at the orbit of interest.

Conductive materials: Using conductive rather than insulating materials where functionally acceptable eliminates charge accumulation sites.

Charge dissipation: Materials with controlled conductivity allow deposited charge to dissipate before dangerous potentials develop. Dissipation time constants should be shorter than accumulation rates.

Geometry control: Sharp edges, triple points, and other geometric features that concentrate electric fields should be avoided or protected.

Discharge Protection

Despite best design efforts, some discharges may occur, and protection minimizes their impact:

Current limiting: Limiting discharge currents reduces the energy delivered and the resulting EMI. Series resistance in potential discharge paths limits current magnitude.

Filter protection: Filters on cable interfaces attenuate ESD-induced transients before they reach sensitive electronics.

Transient suppression: TVS diodes, MOVs, and other transient suppression devices clamp voltages at circuit interfaces.

Shielding: Continuous metallic shielding around sensitive circuits prevents direct field coupling from discharge events.

Software protection: Robust software design with error detection, correction, and recovery minimizes the impact of ESD-induced upsets.

Anomaly Investigation Process

Effective anomaly investigation follows a structured process:

Data collection: Gather all available information about the anomaly: telemetry, test logs, operating conditions, timeline of events, and any correlated environmental data.

Characterization: Define the anomaly precisely: what failed, when, under what conditions, and how reproducibly.

Hypothesis generation: Develop potential explanations for the anomaly, considering EMC mechanisms (conducted and radiated emissions, susceptibility, ESD) as well as other failure modes.

Testing: Design and conduct tests to discriminate among hypotheses, systematically eliminating possibilities until the root cause is identified.

Corrective action: Develop and implement corrections that address the root cause, not just symptoms.

Verification: Verify that corrective actions are effective and do not introduce new problems.

Common EMC Anomaly Sources

Experience has identified common sources of spacecraft EMC anomalies:

• Inadequate filtering: Power line filters that are undersized, improperly installed, or degraded allow conducted emissions to propagate
• Shield termination: Cable shields not properly terminated or damaged during integration allow interference coupling
• Grounding errors: Ground loops created by improper bonding or routing cause low-frequency interference
• Resonances: Unexpected resonances in cables, enclosures, or circuit boards amplify interference at specific frequencies
• Component failures: Degraded components (particularly filter capacitors) may not show as hard failures but degrade EMC performance
• Software interactions: Software operating modes may create timing relationships or signal patterns that cause interference only under specific conditions

On-Orbit Anomaly Investigation

On-orbit anomalies present unique investigation challenges:

Limited observability: Only telemetered data is available; direct measurement is impossible.

Environmental correlation: Correlating anomalies with space weather events (solar activity, radiation belt conditions, charging environments) may reveal EMC-related causes.

Trending: Long-term trending of parameters may reveal gradual degradation indicative of EMC-related damage.

Ground testing: When possible, identical or similar ground units can be tested to reproduce and diagnose anomalies.

Operational workarounds: If root cause cannot be determined or corrected, operational procedures may avoid triggering conditions.