Mobile networks cover about the part of the Earth where operators can build masts. Satellites cover the rest: oceans, mountains, aircraft, disaster areas. Since 3GPP Release 17 satellites are becoming an official part of 5G instead of a separate world.
1. Orbits — the key idea
| Orbit | Altitude | Round-trip delay (typical) | Satellites for global coverage | Examples |
|---|---|---|---|---|
| GEO — Geostationary | 35,786 km over the equator, appears fixed in the sky | ~500–600 ms | 3 | Inmarsat, Viasat, Eutelsat, Hispasat, Thuraya |
| MEO — Medium Earth Orbit | ~2,000–35,786 km (O3b/SES mPOWER ≈ 8,000 km, GPS ≈ 20,200 km) | ~100–150 ms | dozens | SES O3b mPOWER, GNSS constellations (GPS, Galileo, GLONASS, BeiDou) |
| LEO — Low Earth Orbit | ~160–2,000 km (Starlink ≈ 550 km, OneWeb ≈ 1,200 km) | ~20–60 ms | hundreds to thousands | Starlink, OneWeb (Eutelsat OneWeb), Amazon Kuiper (Leo), Iridium (780 km), Globalstar |
| HEO — Highly Elliptical | elongated orbit, e.g. Molniya | varies | few | Polar coverage (Sirius, Molniya) |
Why delay matters: radio waves travel ~300,000 km/s. 35,786 km up and back down twice (request + answer) is already ~480 ms before any processing — fine for TV, bad for calls and gaming. LEO fixes the delay but needs many satellites that move across the sky and constant handovers between them.
Orbital period: LEO ~90–120 min per orbit; a satellite is visible from one spot for only ~5–15 minutes. GEO matches Earth's rotation (24 h).
2. Frequency bands
| Band | Frequency | Typical use | Remark |
|---|---|---|---|
| L | 1–2 GHz | Mobile satellite, GNSS, Iridium, Inmarsat | Low rain fade, small antennas, little bandwidth |
| S | 2–4 GHz | Mobile satellite, 5G NTN (n256 / n255 / n254 bands in 3GPP) | |
| C | 4–8 GHz | TV distribution, backhaul | Large dishes, rain-tolerant |
| X | 8–12 GHz | Military, radar | |
| Ku | 12–18 GHz | VSAT, DTH TV, in-flight Wi-Fi, Starlink user links (10.7–12.7 GHz down, 14–14.5 GHz up) | |
| Ka | 26.5–40 GHz (satcom 17.7–20.2 down / 27.5–30 GHz up) | High-throughput satellites (HTS), Starlink gateways | Highest capacity, strongest rain fade |
| V / Q | 40–75 GHz | Future feeder links |
Rain fade: raindrops absorb high frequencies; the higher the band, the worse. Systems use adaptive coding and modulation (ACM) and uplink power control.
3. System architecture
User terminal ⇄ Satellite ⇄ Gateway / Earth station ⇄ Core network ⇄ Internet
(service link) (feeder link)
⇅ inter-satellite links (ISL, laser)
| Term | Meaning |
|---|---|
| Service link | Terminal ⇄ satellite |
| Feeder link | Satellite ⇄ gateway on the ground |
| ISL | Inter-Satellite Link (laser or RF) — traffic hops between satellites without touching the ground |
| Transparent (bent-pipe) payload | Satellite only amplifies and shifts frequency; processing is on the ground |
| Regenerative payload | Satellite decodes and processes signal on board (can contain a gNB or part of it) |
| Spot beam | Narrow beam covering a small area; many beams reuse frequencies |
| HTS / VHTS | High / Very High Throughput Satellite (hundreds of Gbit/s to Tbit/s per satellite) |
| VSAT | Very Small Aperture Terminal (small dish for businesses) |
| DTH | Direct-To-Home satellite TV |
| Gateway / Teleport | Earth station connecting satellites to the internet |
| Phased array antenna | Flat electronically steered antenna (Starlink dish) |
| Doppler shift | LEO satellites move at ~7.5 km/s; frequency shifts must be compensated |
| TLE | Two-Line Element: orbit data used to predict a satellite's position |
4. 3GPP NTN — satellite inside 5G and LTE
| Release | Feature |
|---|---|
| Rel-15/16 | Studies of NR in non-terrestrial networks (TR 38.811, TR 38.821) |
| Rel-17 (2022) | NR-NTN (5G NR over satellite, GEO/LEO/MEO/HAPS) and IoT-NTN (NB-IoT and LTE-M over satellite) |
| Rel-18 | NTN enhancements: coverage, higher bands (Ka), mobility, regenerative |
| Rel-19 | Regenerative payloads, store-and-forward, RedCap over NTN |
Key technical changes versus a terrestrial network: - Timing advance and HARQ must handle hundreds of ms of delay. The UE uses GNSS to know its position and pre-compensate. - Doppler pre-compensation, big cell sizes, earth-fixed vs earth-moving cells. - Handover triggered by time/location rather than only signal strength. - Satellite appears as a radio access network (gNB) — either transparent (gNB on ground) or regenerative (gNB on satellite).
Direct-to-Device (D2D / "direct-to-cell")
Ordinary phones connect to a satellite directly, no dish. | Approach | Idea | |---|---| | Standards-based (3GPP NTN) | Phones with NTN support in n255/n256 (L/S-bands) | | Existing phones, MNO spectrum | Satellite carries a cell-tower-in-space using the operator's own LTE frequencies (e.g. Starlink Direct to Cell with T-Mobile, AST SpaceMobile) — mostly text/low data at first | | Proprietary satellite messaging | Phone-specific emergency SMS over satellite (Apple/Google/ Samsung with Globalstar/Iridium/Skylo) | | Skylo | NB-NTN network for IoT and messaging using GEO satellites over standard 3GPP |
5. Other satellite systems
| System | Role |
|---|---|
| GNSS — GPS (US), Galileo (EU), GLONASS (Russia), BeiDou (China), QZSS, NavIC | Positioning and time (cell sites need GPS/PTP time); signals are weak, jammable and spoofable |
| Iridium / Globalstar / Inmarsat / Thuraya | Satellite phones, tracking, maritime and aviation safety |
| VSAT / DTH | Business internet, TV |
| IRIS² (EU) | Planned European secure multi-orbit constellation |
| HAPS | High-Altitude Platform Stations (balloons, solar aircraft ≈ 20 km) as "low-orbit towers" |
| COSPAS-SARSAT | Emergency beacons (ELT, EPIRB, PLB) |
| Weather and Earth observation | Not telecom, but share ground stations |
6. Strengths, limits, security
- Strengths: coverage everywhere, resilience (disasters), maritime, aviation, rural broadband, IoT tracking.
- Limits: capacity per beam, delay (GEO), terminals cost, weather (Ka), line of sight, regulation of spectrum (ITU Radio Regulations, EPFD limits protect GEO from LEO interference).
- Space debris and Kessler syndrome, astronomy impact, licensing by ITU and national regulators.
- Security: jamming and spoofing of GNSS, eavesdropping on unencrypted downlinks, ground-segment hacking (e.g. the KA-SAT attack in Feb 2022 which disabled tens of thousands of modems), supply chain.
Remember this: orbit decides delay (GEO ≈ 0.5 s, LEO ≈ 30 ms), band decides capacity and rain fade (L/S small, Ka big), and from 3GPP Rel-17 a satellite is just another radio access to the 5G core.