Module 2: How Satellite Services Work

Access models, packet paths, ground infrastructure, and real-world service categories.

Access Models: Relay, D2D NTN, Backhaul

Not all satellite services work the same way at the access layer.

Satellite Relay / VSAT Model

In the traditional relay model, the user connects through a dedicated terminal — typically a VSAT dish — which communicates with the satellite, which in turn relays traffic to a ground gateway connected to the internet or private core. This is the architecture behind services like HughesNet, Viasat residential broadband, and Inmarsat maritime VSAT. The user terminal handles all the RF complexity: tracking the satellite, managing adaptive coding, and buffering traffic. This model has been proven over decades and serves millions of subscribers globally. Enterprise VSAT networks connect bank branches, gas stations, and retail sites in areas without reliable fiber. Maritime VSAT (typically Ku or Ka-band) provides broadband to cargo ships, cruise lines, and offshore platforms at costs ranging from $2,000-5,000 per month depending on throughput commitments.

Direct-to-Device NTN

Direct-to-Device (D2D) NTN is the most disruptive access model. The smartphone or IoT device communicates directly with the satellite using standard 3GPP NTN protocols — no ground relay or specialized terminal is needed. The satellite either acts as a transparent repeater (forwarding signals to a ground-based gNB) or carries a regenerative payload with full base station processing onboard. Starlink Direct-to-Cell partners with T-Mobile to offer texting and eventually data to existing LTE phones. AST SpaceMobile deploys massive phased-array satellites to deliver voice-grade links to unmodified handsets. This model addresses the billions of people with phones but no cell coverage.

The three access models are not competing alternatives — they serve different use cases. VSAT excels for high-throughput fixed sites, D2D NTN unlocks coverage for billions of existing phones, and satellite backhaul extends terrestrial networks to underserved communities at lower cost than new fiber builds.

Satellite Backhaul

In the backhaul model, a terrestrial cell tower (eNB/gNB) uses a satellite link instead of fiber or microwave to connect back to the mobile core network. End users connect to a standard cell tower using their regular phones — the satellite hop is entirely transparent to them. This approach is particularly valuable for extending mobile coverage to rural and remote communities where fiber backhaul is prohibitively expensive. Community cellular networks in sub-Saharan Africa, Pacific islands, and remote South American regions frequently rely on satellite backhaul. The trade-off is added latency (depending on orbit type) and shared backhaul bandwidth, but for communities that would otherwise have zero connectivity, it is transformative.

Choosing the Right Model

The right access model depends on the use case. Fixed enterprise sites with high throughput needs favor VSAT. Emergency coverage for phones in disaster zones or remote hiking trails favors D2D NTN. Extending an MNO's coverage footprint to rural villages favors satellite backhaul. Many operators deploy multiple models simultaneously — Inmarsat offers both VSAT and D2D, while Starlink offers consumer VSAT terminals alongside its D2D partnership with T-Mobile.

Why it matters: This is the key to choosing the right architecture in Solution Explorer.

Life of a Packet

Traffic path depends on the chosen model, endpoint type, and gateway/core placement.

Forward Link (Gateway to User)

The forward link carries data from the internet or core network, through a ground gateway, up to the satellite, and down to the user terminal. In most VSAT systems, the forward link uses DVB-S2X (Digital Video Broadcasting - Satellite Second Generation Extension), a highly efficient broadcast standard that supports adaptive coding and modulation (ACM). The gateway transmits a continuous carrier that the satellite amplifies and retransmits. Multiple users in the same beam share the forward carrier using time-division multiplexing (TDM), with each user's terminal extracting only the packets addressed to it. Forward link throughput is typically higher than the return link because gateways have large antennas (7-13 m) and high transmit power.

Return Link (User to Gateway)

The return link carries data from the user terminal, up to the satellite, and down to the gateway. Because user terminals have small antennas (0.75-2.4 m for VSAT) and limited transmit power, the return link uses MF-TDMA (Multi-Frequency Time Division Multiple Access), which allows many terminals to share the return bandwidth by transmitting in assigned time slots across multiple frequencies. Return link capacity per user is typically lower than forward link, reflecting the asymmetric nature of most internet traffic (more downloads than uploads).

In a GEO bent-pipe architecture, a single packet round-trip traverses roughly 72,000 km total (4 x 35,786 km legs), introducing approximately 550-600 ms of propagation delay alone — before any processing, queuing, or terrestrial transport delays are added.

TCP Over Satellite Challenges

TCP's congestion control algorithms were designed for terrestrial networks with low latency. Over satellite links, the high bandwidth-delay product (BDP) means TCP takes a long time to ramp up throughput and reacts poorly to satellite-induced packet loss (which it misinterprets as congestion). Performance Enhancing Proxies (PEPs) split the TCP connection at the gateway and terminal, using optimized protocols over the satellite segment. PEPs can improve effective throughput by 5-10x on GEO links. Modern approaches also leverage QUIC and multipath TCP to reduce the impact of satellite latency on application performance.

Encapsulation and Delay Budgets

In 3GPP NTN architectures, user traffic is encapsulated in GTP-U tunnels (GPRS Tunneling Protocol) just like terrestrial mobile networks. The satellite link sits transparently within the N3 interface between the gNB and the User Plane Function (UPF). Typical one-way delay budgets vary dramatically by orbit: GEO adds 270-280 ms of propagation delay, MEO (O3b altitude) adds 60-75 ms, and LEO (Starlink altitude) adds 12-30 ms. Network architects must account for these delays in timer settings, handover parameters, and HARQ (Hybrid Automatic Repeat Request) feedback timing.

The 3GPP NTN specifications introduce timing advance adjustments and HARQ disabling options specifically to accommodate the much longer round-trip times over satellite compared to terrestrial cells, where delays are measured in microseconds rather than milliseconds.

Why it matters: It connects business needs to concrete packet movement and interface decisions.

Ground Segment and Gateways

Ground stations anchor space links into cloud, internet, and enterprise cores.

Gateway Earth Stations

Gateway earth stations are the physical anchor points where satellite traffic transitions to terrestrial fiber networks. For GEO satellites, gateways typically use large parabolic dish antennas ranging from 7 to 13 meters in diameter, mounted on fixed pedestals since the satellite appears stationary. For LEO constellations, gateways require either mechanically steered antennas or electronically steered phased-array antennas that can rapidly track satellites as they cross the sky in 5-10 minutes. Starlink operates over 100 gateway locations (called ground stations) worldwide, each with multiple phased-array panels tracking different satellites simultaneously. A single gateway site may serve dozens of satellite beams and handle 10-40 Gbps of aggregate throughput.

Site Diversity for Rain Fade Mitigation

At Ka-band and above, heavy rain can attenuate the gateway-to-satellite feeder link by 15-20 dB or more, effectively killing the connection. The primary mitigation strategy is site diversity: deploying two gateway sites separated by 20-50 km so that a rain cell over one site is unlikely to affect the other simultaneously. Traffic automatically reroutes to the clear-sky site within seconds. This spatial separation exploits the fact that intense rain cells are typically 5-15 km in diameter. Some operators like SES and Viasat have invested in smart gateway networks with N+1 redundancy, where a pool of backup gateways can absorb traffic from any failed primary site.

The ground segment typically accounts for 30-40% of total satellite network capital expenditure. Hughes' Germantown, Maryland teleport and SES' Betzdorf, Luxembourg facility each house dozens of large antennas, redundant fiber paths, and full Network Operations Centers monitoring thousands of satellite parameters in real time.

Teleport Facilities and NOCs

Major teleport facilities combine multiple gateway antennas, fiber interconnection to internet exchange points, and Network Operations Centers (NOCs) under one roof. The NOC monitors satellite health (power, thermal, attitude), link quality (signal-to-noise ratios, bit error rates), traffic loads across all beams, and customer service level agreements. Teleports like Encompass Digital Media in Atlanta or Telespazio in Fucino, Italy operate around the clock with redundant power, cooling, and staffing. They also house the ground-based beam forming (GBBF) equipment for satellites that synthesize their beam patterns on the ground rather than onboard.

Fiber Backhaul Requirements

Even the most capable satellite is bottlenecked if its gateway lacks sufficient terrestrial connectivity. A modern HTS satellite delivering 500 Gbps needs gateway sites collectively connected by at least that much fiber capacity to the nearest internet peering points. Each individual gateway site typically requires 10-40 Gbps of fiber backhaul, with redundant paths from different providers. In regions where fiber is scarce, this creates a chicken-and-egg problem: the satellite is meant to bridge connectivity gaps, but its own ground infrastructure needs robust terrestrial links.

Many user-perceived satellite performance problems are actually ground segment bottlenecks — insufficient fiber backhaul at the gateway, congested internet peering, or overloaded NOC processing. Diagnosing satellite service issues always requires examining the full end-to-end path including the ground segment.

Why it matters: Many perceived "satellite issues" are actually ground or transport bottlenecks.

Use Cases, Players, and Constellations

Real deployments are shaped by service goals, ecosystem roles, and constellation choices.

Maritime VSAT

Maritime satellite connectivity is one of the oldest and most demanding use cases. Ships at sea rely on Ku-band and Ka-band VSAT for crew welfare (internet, video calling), operational communications (weather routing, fleet management), and regulatory compliance (GMDSS safety systems). Inmarsat Fleet Xpress combines Ka-band high-throughput with L-band backup for guaranteed availability. Monthly costs range from $2,000-5,000 for medium-throughput plans to $15,000+ for cruise ships and large vessels. Starlink Maritime has disrupted this market with lower-cost, higher-throughput offerings, though coverage gaps remain in polar regions and parts of the southern Pacific. ### Aviation Connectivity

In-flight Wi-Fi has gone from novelty to passenger expectation. Gogo (now part of Intelsat) provides Ku-band connectivity to North American airlines, while Viasat and Panasonic Avionics serve global airlines on Ka-band. Starlink Aviation uses LEO for lower-latency service on JSX, Hawaiian Airlines, and others. Aviation is uniquely challenging: aircraft move at 900 km/h, antennas must maintain lock through turbulence and turns, and the link must hand over between satellite beams and even constellations during a single flight. > The global satellite services market exceeds $30 billion annually. Maritime and aviation together account for roughly $4 billion, consumer broadband for $6-8 billion, and government/defense for $8-10 billion, with enterprise and IoT sharing the remainder. ### Consumer Broadband and Enterprise SD-WAN

Starlink serves over 4 million residential subscribers at approximately $120/month, offering 50-200 Mbps with 25-60 ms latency. HughesNet and Viasat continue serving millions of rural American homes via GEO with higher latency but proven reliability. This hybrid approach gives enterprises reliable connectivity at remote sites — mines, oil platforms, rural retail — without paying for dedicated fiber. ### IoT/M2M and Government/Defense

Satellite IoT is booming. Iridium Short Burst Data (SBD) tracks shipping containers, monitors pipelines, and reports weather station data from Antarctica. Orbcomm's LEO constellation serves logistics and cold-chain monitoring. NB-IoT NTN (3GPP Release 18) promises to bring standardized, low-cost IoT connectivity via satellite to billions of sensors. On the government side, military satellite communications (MILSATCOM) use dedicated systems like the US Wideband Global SATCOM (WGS) constellation and the Space Development Agency's (SDA) proliferated LEO transport layer. Broadcast/DTH (Direct-to-Home) television from DirecTV, Sky, and Dish Network remains a significant revenue stream, serving over 100 million households globally despite streaming competition.

Why it matters: It prevents one-size-fits-all assumptions when selecting a deployment path.