Electronics Guide

Geostationary Orbit (GEO)

GEO satellites orbit at approximately 35,786 km altitude, completing one orbit per sidereal day and appearing stationary relative to Earth. Characteristics include:

• Coverage: Single satellite covers approximately one-third of Earth's surface.
• Latency: Round-trip delay of approximately 250 ms limits interactive applications.
• Path loss: Large distance results in significant free-space loss.
• Orbital slots: Limited positions available, subject to international coordination.
• Station keeping: Requires fuel for maintaining position against gravitational perturbations.

GEO is dominant for broadcasting, fixed satellite services, and maritime/aviation communications.

Medium Earth Orbit (MEO)

MEO satellites orbit between 2,000 and 35,786 km altitude. Key applications include:

• Navigation: GPS (20,200 km), Galileo (23,222 km), GLONASS (19,130 km).
• Communications: O3b constellation (8,062 km) provides lower-latency connectivity.

MEO offers lower latency than GEO but requires multiple satellites for continuous coverage.

Low Earth Orbit (LEO)

LEO satellites orbit below 2,000 km altitude. Characteristics include:

• Low latency: Round-trip delays of 20-40 ms enable interactive applications.
• Lower path loss: Smaller terminals can achieve adequate link budgets.
• Constellation required: Many satellites needed for continuous coverage.
• Orbital period: 90-120 minutes, requiring frequent handovers.
• Doppler shift: Significant frequency shift as satellites move relative to ground.

Modern LEO constellations like Starlink and OneWeb aim to provide global broadband internet.

Highly Elliptical Orbit (HEO)

HEO satellites have elongated orbits with perigee close to Earth and apogee far away. Molniya orbits provide extended coverage of high-latitude regions poorly served by GEO satellites. The satellite appears to hover over the coverage area for most of its 12-hour orbit.

Orbital Mechanics Considerations

• Inclination: Angle between orbital plane and equator affects coverage.
• Eccentricity: Deviation from circular orbit.
• Eclipse periods: Time when Earth blocks sunlight, requiring battery operation.
• Orbital debris: Growing concern requiring debris mitigation measures.
• End-of-life disposal: Regulations require moving satellites to graveyard orbit or deorbiting.

Satellite Frequency Bands

• L-band (1-2 GHz): Mobile satellite services, GPS, satellite phones. Good propagation through atmosphere and foliage.
• S-band (2-4 GHz): Mobile services, some broadband. Similar propagation to L-band.
• C-band (4-8 GHz): Traditional satellite TV, telephony. Resistant to rain fade but requires larger antennas.
• X-band (8-12 GHz): Military and government communications.
• Ku-band (12-18 GHz): Direct broadcast satellite TV, VSAT. Higher throughput but susceptible to rain fade.
• Ka-band (26.5-40 GHz): High-throughput satellites, broadband. Highest capacity but most affected by rain.
• V-band (40-75 GHz): Emerging band for future high-capacity systems.

Uplink and Downlink

Satellites receive signals on uplink frequencies and transmit on different downlink frequencies to prevent interference. Typically, uplink uses higher frequencies where rain fade affects the more powerful ground transmitters, while downlink uses lower frequencies to benefit the weaker satellite transmitter.

Spectrum Coordination

The International Telecommunication Union (ITU) coordinates satellite spectrum globally. Satellite operators must file orbital positions and frequency plans, coordinating with potentially affected systems to prevent interference.

Uplink Budget

• Earth station EIRP: Transmit power plus antenna gain minus feed losses.
• Path loss: Free-space loss at the operating frequency and distance.
• Atmospheric losses: Gaseous absorption and rain attenuation.
• Satellite G/T: Receive antenna gain relative to system noise temperature.

Downlink Budget

• Satellite EIRP: Transponder power plus antenna gain.
• Path loss: Similar considerations as uplink.
• Atmospheric losses: Often less critical than uplink due to lower frequency.
• Earth station G/T: Determines receive capability.

Link Margin

The difference between available and required C/N (carrier-to-noise ratio) provides margin for:

• Rain fade: Attenuation during precipitation events.
• Equipment aging: Degradation over time.
• Pointing errors: Antenna misalignment.
• Interference: Adjacent satellites and terrestrial sources.

Typical rain fade margins range from 3-6 dB for C-band to 10+ dB for Ka-band in tropical regions.

Bent-Pipe Transponders

Traditional transponders receive signals, frequency-convert, amplify, and retransmit without demodulation. Simple and reliable but offer limited processing capability. Signal quality depends on the entire link (uplink plus downlink).

Regenerative Transponders

Regenerative transponders demodulate, process, and remodulate signals. Benefits include:

• Link separation: Uplink and downlink errors don't compound.
• Onboard switching: Route signals between beams and frequencies.
• Processing capability: Error correction, protocol conversion.

Modern high-throughput satellites increasingly use regenerative payloads.

Transponder Components

• Receive antenna: Collects uplink signals.
• Input filter: Selects desired band, rejects interference.
• Low-noise amplifier: Amplifies weak received signals.
• Frequency converter: Translates uplink to downlink frequency.
• High-power amplifier: Traveling wave tubes (TWTAs) or solid-state power amplifiers (SSPAs).
• Output filter: Suppresses spurious emissions.
• Transmit antenna: Directs signals toward coverage area.

Spot Beams

High-gain spot beams concentrate power over limited geographic areas, increasing EIRP and capacity. Frequency reuse across non-adjacent beams multiplies available bandwidth. High-throughput satellites may have dozens or hundreds of spot beams.

Shaped Beams

Reflector shaping or array weighting creates coverage areas matching geographic regions, concentrating power over land while minimizing ocean coverage. Regional beams serve specific markets efficiently.

Phased Array Antennas

Electronic beam steering enables flexible coverage, rapid beam repositioning, and adaptive interference nulling. Phased arrays are increasingly used in both satellites and ground terminals.

Antenna Systems

Earth station antennas range from small consumer dishes (0.5-1 m) to large teleport antennas (10+ m):

• Prime focus: Feed at focal point, simple but feed blocks aperture.
• Offset feed: Feed below aperture center eliminates blockage.
• Cassegrain: Subreflector directs energy to feed behind main reflector.
• Tracking systems: Motor drives maintain pointing as satellites move (LEO) or compensate for wind and thermal effects.

Low-Noise Block Downconverters (LNBs)

LNBs mount at the antenna focus and perform:

• Low-noise amplification of received signals
• Frequency conversion to L-band for cable transmission to indoor equipment
• Polarization selection (linear or circular)

Block Upconverters (BUCs)

BUCs convert L-band signals to transmit frequencies and amplify to required power levels. Solid-state BUCs are common for lower powers; traveling wave tube amplifiers (TWTAs) provide higher power.

Modems

Satellite modems implement modulation/demodulation and access protocols. Modern modems support:

• Adaptive coding and modulation (ACM)
• DVB-S2/S2X standards
• Spread spectrum modes
• IP acceleration and optimization

FDMA (Frequency Division Multiple Access)

Users occupy different frequency slots within transponder bandwidth. Simple but fixed allocation may waste capacity when demand varies.

TDMA (Time Division Multiple Access)

Users share frequency spectrum but transmit in different time slots. Efficient for variable traffic but requires precise timing synchronization.

CDMA (Code Division Multiple Access)

Users share spectrum and time using unique spreading codes. Provides interference resistance and security but requires power control.

MF-TDMA (Multi-Frequency TDMA)

Combines FDMA and TDMA for efficient dynamic allocation. Dominant in modern VSAT networks, enabling bandwidth-on-demand.

Radiation Effects

Space electronics face radiation from:

• Solar particle events: Bursts of energetic particles during solar activity.
• Van Allen belts: Trapped radiation in Earth's magnetic field.
• Galactic cosmic rays: High-energy particles from outside the solar system.

Radiation effects include total ionizing dose (TID) degradation, single-event effects (SEEs), and displacement damage. Mitigation includes radiation-hardened components, shielding, and error correction.

Thermal Environment

Satellites experience extreme temperature variations:

• Direct solar illumination raises temperatures
• Earth's albedo and thermal radiation contribute
• Eclipse periods cause rapid cooling

Thermal control systems use radiators, heat pipes, heaters, and multi-layer insulation (MLI) to maintain component temperatures within operating ranges.

Vacuum Effects

Operation in vacuum affects:

• Outgassing: Materials release volatiles that can contaminate optics and solar cells.
• Multipaction: RF breakdown in vacuum at relatively low power levels.
• Thermal management: No convection, only radiation cooling.

Reliability Requirements

Satellites must operate without repair for 15+ years in GEO. Design approaches include:

• Redundant systems (primary and backup components)
• Derating components below maximum ratings
• Extensive ground testing including thermal vacuum and vibration
• Proven heritage designs where possible

Direct-to-Home Television

DTH broadcasting delivers hundreds of channels to small consumer dishes. GEO satellites with high-power transponders and spot beams enable reception with 45-90 cm antennas.

Satellite Internet

Broadband internet via satellite serves rural and remote users. High-throughput satellites (HTS) with spot beams and frequency reuse deliver Gbps capacity. LEO constellations promise lower latency for interactive services.

Mobile Satellite Services

MSS provides connectivity to mobile users including maritime, aviation, and land mobile. Services range from narrowband messaging to broadband connectivity on ships and aircraft.

Navigation (GNSS)

Global Navigation Satellite Systems including GPS, Galileo, GLONASS, and BeiDou provide positioning, navigation, and timing services critical for transportation, surveying, and synchronization.

Deep Space Communication

Communication with interplanetary missions presents extreme challenges including huge distances (light-time delays of minutes to hours), limited spacecraft power, and need for large ground antennas. The Deep Space Network provides global coverage for NASA missions.

LEO Mega-Constellations

SpaceX Starlink, OneWeb, Amazon Kuiper, and others are deploying thousands of LEO satellites to provide global broadband with low latency.

Software-Defined Payloads

Reconfigurable digital payloads enable satellites to adapt coverage, bandwidth allocation, and services during mission life.

Optical Inter-Satellite Links

Laser links between satellites provide high-capacity backbone connectivity for constellations, avoiding terrestrial routing delays.

Non-Geostationary Orbit (NGSO) Growth

Regulatory and technical developments are enabling proliferation of NGSO systems, creating spectrum coordination challenges but expanding service options.