Satellite Orbits

Satellite Orbits: Choosing the Right Altitude

The orbit a satellite follows determines everything about its communication capabilities — coverage area, signal delay, visibility duration, number of satellites needed for continuous service, and launch cost. Understanding orbital mechanics is essential for designing satellite communication systems that meet specific service requirements.

Kepler's Laws: The Foundation

Johannes Kepler's three laws of planetary motion govern all satellite orbits:

First Law: Every satellite orbit is an ellipse with Earth at one focus. Most communication satellites use circular orbits (a special case of ellipse where both foci coincide).

Second Law: A line from Earth's center to the satellite sweeps equal areas in equal times. This means satellites in elliptical orbits move faster when closer to Earth (perigee) and slower when farther away (apogee).

Third Law: The square of the orbital period is proportional to the cube of the semi-major axis:

T² = (4π²/GM) × a³

Where T = orbital period, G = gravitational constant, M = Earth's mass, a = semi-major axis.

This gives: T = 2π × √(a³/μ) where μ = GM = 3.986 × 10¹⁴ m³/s²

Geostationary Orbit (GEO)

Altitude: 35,786 km above equator Period: Exactly 24 hours (synchronous with Earth's rotation) Velocity: 3.07 km/s

Key property: A GEO satellite appears stationary from Earth — it stays at the same point in the sky permanently. This means ground antennas can be pointed once and never moved.

Advantages:

• Fixed antenna pointing (simple ground equipment — your satellite TV dish)
• Three satellites cover nearly all of Earth (except polar regions)
• Continuous service (always visible from fixed location)
• No handoff needed as satellite does not move relative to user

Disadvantages:

• High propagation delay: 2 × 35,786 km / 3×10⁸ = 239 ms one-way (480+ ms round-trip with processing)
• High launch energy required (expensive launches)
• Limited orbital slots (only ~1800 positions available, spaced 0.1-2°)
• Cannot cover polar regions (above ~75° latitude)
• High path loss due to distance

Applications: DTH television, weather satellites, GEO communication (Intelsat, SES, Hughes)

Low Earth Orbit (LEO)

Altitude: 200-2000 km Period: 88-127 minutes Velocity: 7.8-6.9 km/s

Advantages:

• Very low delay: 1-13 ms one-way (excellent for voice and interactive data)
• Lower path loss (closer to Earth) → smaller user terminals possible
• Lower launch cost per satellite (less energy needed)
• Better coverage of polar regions

Disadvantages:

• Satellite moves rapidly across sky (visible only 5-20 minutes per pass)
• Requires constellation of many satellites for continuous coverage (e.g., Starlink: 4000+)
• Complex tracking antennas or phased arrays needed
• Frequent handoffs between satellites
• Short orbital lifetime (atmospheric drag at lowest altitudes)

Applications: Starlink (internet), Iridium (voice/data), Earth observation, GPS (technically MEO)

Medium Earth Orbit (MEO)

Altitude: 2000-35,786 km (typically 8000-20,000 km) Period: 2-24 hours

Compromise between GEO and LEO:

• Moderate delay (30-80 ms one-way)
• Moderate constellation size (8-24 satellites for global coverage)
• Moderate path loss
• Good for navigation systems where precise timing is essential

Applications: GPS (20,200 km, 24+ satellites), GLONASS, Galileo, O3b (8,062 km, internet)

Highly Elliptical Orbit (HEO)

Examples: Molniya orbit (apogee 40,000 km over Russia, perigee 500 km) Period: 12 hours (Molniya), 24 hours (Tundra)

Advantage: Provides long dwell time over high-latitude regions that GEO cannot cover. Used by: Russian communication satellites serving Siberia, Sirius XM radio

Orbit Selection Criteria

Modern LEO Constellations

The 2020s have seen a revolution in LEO satellite communication:

• Starlink (SpaceX): 4,000+ satellites at 550 km, 100-300 Mbps to users
• OneWeb: 648 satellites at 1,200 km, business broadband
• Amazon Kuiper: 3,236 planned satellites at 590-630 km
• Telesat Lightspeed: 188 satellites at 1,000 km, enterprise/government

These constellations aim to provide fiber-like performance (low latency, high bandwidth) with satellite's coverage advantage (everywhere including oceans, deserts, aircraft).

Key Takeaways

1. Orbital altitude determines the fundamental trade-offs: GEO offers simplicity with high delay; LEO offers low delay with constellation complexity.
2. GEO satellites at 35,786 km appear stationary and cover 1/3 of Earth, making them ideal for broadcasting and fixed services.
3. LEO satellites (200-2000 km) provide 1-13 ms delay and enable small terminals, but require constellations of hundreds to thousands for continuous global coverage.
4. Kepler's third law T²∝a³ determines orbital period — only one altitude (35,786 km) gives exactly 24-hour period for geostationary operation.
5. Modern LEO mega-constellations (Starlink, Kuiper) are transforming satellite from a niche to a mainstream broadband technology.
6. The choice of orbit is the first and most consequential design decision in any satellite communication system.