Satellite Communications

Satellite communications connects the world from geostationary orbit to modern LEO constellations.

Geostationary Orbit Fundamentals

Geostationary orbit (GEO) at 35,786 km altitude is the backbone of satellite communications. A satellite in GEO orbits at 3.07 km/s with a period of23 hours, 56 minutes, and 4 seconds(one sidereal day), appearing stationary relative to Earth's surface. This allows ground antennas to remain pointed at a fixed point in the sky, eliminating the need for tracking systems. A single GEO satellite covers approximately one-third of Earth's surface, so three satellites provide near-global coverage (except polar regions).

Round-trip signal delay to GEO is approximately 600 milliseconds (240 ms up, 240 ms down, plus processing), which is noticeable in voice conversations but acceptable for broadcast and data services.

Frequency Bands

Satellite communications uses a hierarchy of frequency bands, each with different propagation characteristics, antenna sizes, and typical applications:

• L-band (1-2 GHz): Mobile satellite services (Inmarsat, Iridium), GPS navigation. Low rain attenuation, works in all weather. Requires larger antennas for given gain. Used for maritime, aviation, and land-mobile services.
• S-band (2-4 GHz): Weather radar, TT&C (telemetry, tracking, command), mobile services. Moderate rain resistance. Used for ISS communications and Earth observation satellite downlinks.
• C-band (4-8 GHz): Traditional satellite TV and data. Well-established technology with high reliability. Up: 5.925-6.425 GHz, Down: 3.7-4.2 GHz. Low rain fade susceptibility, ideal for tropical regions.
• X-band (8-12 GHz): Military and government communications, Earth observation. Higher gain per antenna size than C-band. Up: 7.9-8.4 GHz, Down: 7.25-7.75 GHz.
• Ku-band (12-18 GHz): Direct-to-home TV (DTH), VSAT networks, maritime. Up: 14.0-14.5 GHz, Down: 11.7-12.2 GHz. Smaller antennas than C-band, but susceptible to rain fade above 15 GHz.
• Ka-band (26-40 GHz): High-throughput satellites (HTS), Starlink, Viasat. Up: 27.5-31 GHz, Down: 18.3-20.2 GHz. Highest capacity but most rain-fade susceptible. Frequency reuse via spot beams enables 100+ Gbps per satellite.

Link Budget Analysis

A satellite link budget accounts for all gains and losses between transmitter and receiver. The fundamental equation is:

C/N₀ = EIRP + G/T - L - k

• EIRP (Effective Isotropic Radiated Power): 42-63 dBW depending on satellite and band. Product of transmit power and antenna gain. A typical GEO Ku-band transponder produces 42-52 dBW EIRP.
• G/T (Figure of Merit): -2 to +22 dB/K. Ratio of receive antenna gain to system noise temperature. Higher G/T means better receive sensitivity. A 5m ground dish at Ku-band achieves ~18 dB/K.
• Free Space Path Loss (FSPL): 200-207 dB for Ku-band GEO. Calculated as FSPL = 20·log₁₀(4πd/λ), where d is distance and λ is wavelength. At 12 GHz and 36,000 km: ~205.5 dB.
• Thermal noise floor: k = -228.6 dBW/Hz (Boltzmann's constant)

Transponder Specifications

A typical GEO communications satellite carries 24-80 transponders. Each transponder:

• Bandwidth: 36 MHz (standard), 27 MHz, 54 MHz, or 72 MHz (wideband)
• Transmitter: Traveling Wave Tube Amplifier (TWTA) or Solid-State Power Amplifier (SSPA)
• Output Power: 40-250 W (TWTA), 2-20 W (SSPA)
• Gain: 55-65 dB depending on band and power level
• Noise Figure: 10-15 dB
• Intermodulation products: C/IM > 20 dBc (carrier-to-intermodulation ratio)
• AM/AM and AM/PM conversion: Nonlinear effects at saturation reduce output power and cause phase shift

VSAT Networks

Very Small Aperture Terminal (VSAT) networks provide satellite internet to businesses, ships, and remote locations. A typical VSAT has a 0.75-2.4m dish and provides 1-10 Mbps. These networks use hub-and-spoke topology, with all traffic routing through a central hub.

VSAT uplink power is typically 1-10 W, and the hub terminal uses a 3-7m dish. Return link (hub to VSAT) operates at lower power. Modern VSAT systems use DVB-S2 modulation with adaptive coding and modulation (ACM) to optimize link efficiency.

High-Throughput Satellites (HTS)

Modern HTS satellites use spot beams and frequency reuse to achieve capacities of 100+ Gbps - 10-20× traditional satellites. ViaSat-3 (Ka-band) promises 1 Tbps capacity. This enables consumer broadband, aviation, and maritime services competitive with terrestrial options.

HTS uses spatial frequency reuse: the same frequency channels are reused across multiple spot beams, each covering a small area (~100-500 km diameter). A single HTS satellite may have 50-100 spot beams, achieving a total capacity 10-50× that of a traditional widebeam satellite. The tradeoff is that ground terminals must be more sophisticated to select and track individual beams.

Satellite Specifications

• Launch mass: 2,000-6,500 kg (typical GEO commsat)
• Design life: 15-20 years
• Power: 5-25 kW (solar arrays + batteries)
• Station-keeping Δv: ~50 m/s/year (N-S) + 2-5 m/s/year (E-W)
• Orbit maintenance: Bipropellant (N₂H₄/N₂O₄) or electric propulsion (ion or Hall-effect thrusters)

Australian Satellite Programs

Australia has a notable history in satellite development despite not maintaining a national launch capability. The WRESAT-1 satellite, launched on 29 November 1967 from the Woomera Rocket Range, made Australia the third country (after Russia and the United States) to build and launch its own satellite. Built jointly by the Weapons Research Establishment and the University of Adelaide, WRESAT weighed approximately 45 kg and carried instruments to measure solar X-rays and ionospheric electron density.

In 1970, University of Melbourne students launched Australis-OSCAR 5, the first non-US amateur radio satellite. The spacecraft operated on 144/435 MHz and provided a full-duplex transponder for amateur radio operators worldwide. AO-5 was operational for over three years.

In 2002, the CSIRO launched FedSat, a 58 kg microsatellite carrying aKa-band transponder (26 GHz uplink / 18 GHz downlink) for high-capacity communications experiments. FedSat demonstrated the viability of low-cost microsatellites for Ka-band research, an approach later adopted by commercial high-throughput satellite operators.