Satellite Internet and LEO Constellations

Satellite internet provides network connectivity via artificial satellites in orbit around Earth. It is used where terrestrial infrastructure (fiber, cable, DSL) is unavailable, too expensive, or impractical—such as rural areas, oceans, aircraft, and disaster zones. Traditional satellite internet uses geostationary (GEO) satellites at ~35,786 km altitude, which remain fixed relative to Earth. However, GEO introduces high latency (about 240 ms round-trip just for the space segment) and limited throughput. Low Earth Orbit (LEO) constellations, at altitudes of 500–1,200 km, solve the latency problem by being much closer, achieving round-trip times (RTT) as low as 20–40 ms. LEO constellations consist of hundreds to thousands of small satellites working together to provide continuous coverage.

LEO satellites orbit at high speed (about 7.8 km/s) and complete an orbit in ~90 minutes. Each satellite has a footprint on the ground that moves rapidly. To maintain connectivity, a constellation uses inter-satellite links (ISLs) and ground stations (gateways).

Step-by-step data flow: 1. A user sends a request from their UT. 2. The UT locks onto a satellite using beamforming and establishes a link. 3. The satellite receives the signal, applies frequency conversion, and decides routing (via ISL or direct downlink). 4. If the destination is far, the satellite sends data via laser ISL to neighboring satellites, hopping across the constellation. 5. The data reaches a satellite that has a line-of-sight to a ground station. 6. The ground station forwards the traffic to the internet backbone. 7. The response follows the reverse path.

Handoff Mechanism: As a satellite moves out of range, the UT must hand off to the next satellite. This is similar to cellular handovers but much faster. The NOC uses ephemeris data (accurate orbital positions) to predict when to initiate handoff. The UT and satellite exchange signaling (e.g., using modified TCP or proprietary protocols) to transfer the session without dropping packets. Handoff latency is typically under 50 ms.

While satellite internet is managed by the provider, network engineers may configure customer premises equipment (CPE) and monitor performance. Typical commands on a router connected to a satellite terminal:

! Set MTU for satellite link (often lower due to overhead)
interface GigabitEthernet0/0
 ip mtu 1400

! Adjust TCP MSS to avoid fragmentation
ip tcp adjust-mss 1360

! Verify latency and jitter
ping 8.8.8.8 source GigabitEthernet0/0
show interface GigabitEthernet0/0
show ip route

For satellite modems, web-based GUIs show signal strength (SNR), frequency, and satellite ID.

LEO constellations are a game-changer for low-latency global internet. They rely on a mesh of fast-moving satellites, electronic beamforming, and inter-satellite links to provide coverage. Key exam points: altitude, latency values, frequency bands, handoff mechanics, and TCP impact.

A mining company in northern Canada needs internet for its remote camp. Terrestrial options are limited to slow satellite (GEO) with 600 ms latency, which breaks real-time applications. They deploy Starlink LEO terminals. Each terminal is a phased-array antenna mounted on a trailer. The company configures QoS on their router to prioritize VoIP and video conferencing traffic. They set MTU to 1400 and TCP MSS to 1360 to avoid fragmentation. They also implement a VPN back to headquarters using IPSec with dead peer detection interval set to 30 seconds to handle satellite handoffs. Performance: latency 25 ms, throughput 150 Mbps down, 20 Mbps up. The challenge is weather: heavy snowfall can attenuate Ka-band signals, causing fade. The system automatically adjusts modulation (ACM) to maintain link, but throughput drops to 50 Mbps during storms. They also experience brief drops during satellite handoffs (every 30 seconds), but the VPN reconnects quickly.

A major airline equips its fleet with satellite internet using an LEO constellation (e.g., OneWeb). Each aircraft has a radome housing a phased-array antenna. The airline partners with a service provider that manages the satellite link. The aircraft's onboard router uses a BGP peering with the satellite network to announce its reachable IP subnets. The satellite network uses traffic shaping to ensure fair bandwidth distribution among passengers. The airline configures a captive portal for authentication. The challenge is handoff between satellites: as the aircraft flies at 900 km/h, it switches satellites every 2-3 minutes. The handoff must be seamless to avoid dropping passenger sessions. The provider uses predictive handoff based on ephemeris data and aircraft GPS. The onboard router uses TCP acceleration (PEP) to improve performance over the satellite link, which has higher latency than terrestrial networks.

After a hurricane destroys terrestrial infrastructure in a coastal city, a government agency deploys a mobile satellite terminal (e.g., Starlink in a backpack) to restore emergency communications. The terminal is set up within minutes, automatically acquiring satellites. The agency's network team configures a small LAN with VoIP phones and a cellular extender. They use a VPN to connect to the state's emergency network. The satellite link provides 100 Mbps down, 10 Mbps up. The main issue is contention: many emergency teams sharing the same satellite cell. They prioritize critical traffic using QoS (EF for VoIP, AF41 for data). The link is also susceptible to rain fade; during heavy rain, they switch to a lower modulation (QPSK) to maintain connectivity at reduced speed. The system logs performance metrics (latency, packet loss, signal strength) for post-event analysis.