Types of Non-Terrestrial Network (NTN) Platforms: Satellites and HAPS
Introduction Satellites as NTN Platforms
Satellites as NTN Platforms Non-Terrestrial Networks (NTNs) use communication platforms that operate above the Earth's surface to provide wireless connectivity over areas where conventional terrestrial networks may be difficult or uneconomical to deploy.
From the 3GPP NTN perspective, two major types of NTN platforms are considered:
Satellites
Uncrewed/Unmanned Aerial Systems (UASs), including High-Altitude Platform Stations (HAPSs)
These platforms have significantly different operating characteristics, altitudes, coverage patterns, propagation delays, and mobility behavior.
Understanding the different NTN platforms is therefore fundamental to understanding the architecture and operation of satellite-based and airborne communication systems.
1. Satellites as NTN Platforms
Satellites are spaceborne vehicles that orbit the Earth along a specific orbit.
Based on their orbital characteristics, satellites can broadly be classified into:
Geosynchronous Orbit (GSO) satellites
Non-Geosynchronous Orbit (NGSO) satellites
A GEO satellite is an example of a GSO satellite.
Examples of NGSO satellites include:
LEO satellites
MEO satellites
HEO satellites
Each orbit provides different characteristics for NTN communication.
2. Geosynchronous Orbit (GSO) and GEO Satellites
A satellite whose orbital movement matches the rotation of the Earth can appear stationary to an observer on the Earth's surface.
This type of satellite is associated with the Geosynchronous Orbit (GSO).
A Geostationary Earth Orbit (GEO) satellite is a particularly important example of a GSO satellite.
GEO satellites are located at an altitude of approximately:
35,786 km above the Earth's surface.
Because the satellite's orbital movement matches the Earth's rotation, a GEO satellite appears fixed to a stationary observer on Earth.
This characteristic makes GEO satellites particularly useful for applications where relatively stable coverage is desirable.
3. GEO Satellites in NTN
GEO satellites have been used extensively for communication applications.
Organizations such as Intelsat own and manage GEO satellite systems.
The very high altitude of GEO satellites provides a very large coverage area.
However, the large distance between the satellite and Earth also introduces significant propagation delay.
This makes the characteristics of GEO-based NTN different from those of lower-altitude satellite systems.
From the 3GPP NTN perspective, GEO and LEO have been particularly important, representing two very different points in the satellite-orbit spectrum.
4. Non-Geosynchronous Orbit (NGSO) Satellites
Satellites that do not remain synchronized with Earth's rotation are generally categorized as Non-Geosynchronous Orbit (NGSO) satellites.
Important examples include:
LEO
MEO
HEO
Unlike GEO satellites, NGSO satellites generally move relative to an observer on the Earth's surface.
This movement has important implications for:
Coverage
Handover
Mobility management
Timing
Frequency compensation
Network planning
5. Low Earth Orbit (LEO) Satellites
Low Earth Orbit (LEO) satellites operate relatively close to Earth.
Their altitude is generally in the range of approximately:
300 km to 1,200–1,500 km
depending on the specific system.
The LEO orbit is also used by well-known space systems.
For example:
International Space Station (ISS)
Hubble Space Telescope
operate in LEO.
6. LEO Satellite Speed and Orbital Period
LEO satellites move around the Earth at approximately:
7.8 km/s
A LEO satellite can complete an orbit around Earth in approximately:
90 minutes
This high orbital speed means that a LEO satellite does not remain above the same geographical location.
As a result, the coverage associated with a LEO satellite can move across the Earth's surface.
This introduces additional requirements for NTN systems, including:
Handover
Mobility management
Beam management
Tracking
Timing adjustments
Frequency compensation
7. Growing Interest in LEO-Based NTN Systems
There has been significant interest in LEO-based satellite systems.
Examples of major LEO-based deployments include:
Kuiper by Amazon
Starlink by SpaceX
OneWeb
LEO systems are attractive for many communication applications because their lower altitude can provide lower propagation delay compared with GEO systems.
However, maintaining broad and continuous coverage generally requires multiple satellites operating as part of a constellation.
8. Medium Earth Orbit (MEO) Satellites
Medium Earth Orbit (MEO) satellites operate at altitudes approximately ranging from:
5,000–7,000 km to 20,000–25,000 km
The exact altitude range depends on the particular orbital configuration.
MEO satellites occupy the region between LEO and GEO.
One well-known application of MEO satellites is the Global Positioning System (GPS).
MEO systems can provide a balance between:
Coverage
Propagation delay
Number of satellites
Orbital characteristics
Their characteristics therefore differ from both LEO and GEO systems.
9. Highly Elliptical Orbit (HEO) Satellites
A Highly Elliptical Orbit (HEO) satellite follows an orbit that is highly elongated.
As a result, the altitude of the satellite varies significantly throughout its orbit.
The altitude can range approximately from:
400 km to 50,000 km
depending on the particular HEO system.
This large variation in altitude produces an important consequence for communication systems.
10. Propagation Delay in HEO Systems
Because the altitude of a HEO satellite changes significantly during its orbit, the propagation distance between the satellite and the user also changes.
Consequently, the propagation delay can vary significantly.
The propagation path loss also changes as the satellite moves between different altitudes.
This creates additional challenges compared with satellite systems where the orbital altitude is relatively constant.
These characteristics are among the reasons why HEO systems require different considerations from LEO and GEO systems.
11. HEO and the 3GPP NTN Scope
Although HEO satellites are an important type of satellite orbit, 3GPP has primarily focused on LEO and GEO satellites as two extremes while defining NTN specifications.
Therefore, HEO satellites have not been explicitly investigated to the same extent within the 3GPP NTN specifications discussed here.
The significant altitude variation of HEO satellites creates additional challenges related to:
Propagation delay
Propagation path loss
Timing
Link characteristics
Network management
Therefore, HEO represents a different design space compared with the primary LEO and GEO focus of the initial 3GPP NTN work.
12. Comparison of Satellite Orbits
Satellite Type | Approximate Altitude | Major Characteristic |
LEO | 300–1,200/1,500 km | Low altitude, high orbital speed |
MEO | 5,000/7,000–20,000/25,000 km | Intermediate orbit |
GEO | 35,786 km | Appears fixed from Earth |
HEO | ~400–50,000 km | Highly elliptical, variable altitude |
These altitude ranges are approximate and depend on the specific orbital configuration.
13. Multi-Orbit Satellite Constellations
Instead of using only one type of satellite orbit, operators can pursue multi-orbit satellite constellations.
The objective is to combine the different advantages of various satellite types.
A multi-orbit network may combine:
GEO + MEO + LEO + Terrestrial 5G
Each layer can provide different capabilities.
For example:
GEO can provide very large-area coverage.
LEO can provide lower propagation delay.
MEO can provide intermediate characteristics.
Terrestrial 5G can provide high-capacity local connectivity.
This multi-layer approach can create a more flexible and resilient communication network.
14. Inmarsat Orchestra
One example of a multi-orbit communication concept is Orchestra, promoted by Inmarsat.
The concept combines:
GEO satellites
LEO satellites
Terrestrial 5G networks
The objective is to create a communication network that can use different connectivity layers to provide an efficient and cost-effective solution.
The broader concept demonstrates how future networks do not necessarily need to rely on a single type of connectivity.
Instead, different network layers can complement each other.
15. Multi-Orbit Systems and Resilience
Multi-orbit satellite systems can also provide important advantages for resiliency, including in military applications.
If a communication system depends on only one connectivity layer, disruption of that layer can significantly affect service.
A multi-orbit architecture can provide alternative connectivity options.
For example:
LEO unavailable → GEO connectivity
or
Terrestrial connectivity unavailable → Satellite connectivity
This type of architectural diversity can improve overall network resilience.
16. European Space Agency and Multi-Orbit Systems
The European Space Agency (ESA) has also initiated studies related to multi-orbit satellite systems.
These activities reflect the growing interest in combining different orbital layers rather than treating each satellite orbit as an isolated communication system.
Multi-orbit approaches can potentially improve:
Coverage
Resilience
Service availability
Flexibility
Network efficiency
17. UAS Platforms in NTN
The second major category of NTN platforms considered by 3GPP is Uncrewed/Unmanned Aerial Systems (UASs).
From the 3GPP NTN perspective, a UAS platform can operate at an altitude of approximately:
8 km to 50 km
An important example of a UAS-based NTN platform is the:
High-Altitude Platform Station (HAPS)
18. HAPS as an NTN Platform
A HAPS is an airborne platform that operates at high altitude, generally within the stratospheric region.
For example, a HAPS may be located at an altitude of approximately:
20 km
Unlike a LEO satellite, a HAPS can be designed to maintain a relatively fixed position over a geographical area.
Therefore, from the perspective of an observer on Earth's surface, the HAPS can appear approximately stationary.
This characteristic can simplify certain aspects of coverage and network operation.
19. Examples of HAPS
HAPS platforms can include different types of airborne vehicles.
Examples include:
Balloons
Aircraft
Solar-powered aircraft
These platforms can remain at high altitude and provide communication coverage over a large geographical region.
20. Google Loon
An example of a high-altitude balloon concept was Google's Loon project.
The project experimented with balloons designed to operate at high altitudes and provide connectivity.
Although the Loon project itself is not a representation of the entire HAPS ecosystem, it demonstrates the potential of high-altitude airborne platforms for communication applications.
21. HAPS in Military Applications
HAPS technology has also been widely considered for military applications.
High-altitude platforms can provide persistent observation, sensing, and communication capabilities over large areas.
An example mentioned in this context is the Northrop Grumman RQ-4 Global Hawk, which is operated by the United States Air Force.
The broader use of high-altitude unmanned aircraft demonstrates the potential of airborne platforms for applications requiring persistent operation over large areas.
22. Important Distinction: Drones and UAVs
An important distinction must be made between general UAV/drone technology and the specific NTN scope discussed by 3GPP.
While drones and Uncrewed/Unmanned Aerial Vehicles (UAVs) can be considered part of the broader non-terrestrial communication concept, drones operating below approximately 8 km are not within the scope of the 3GPP NTN activities described here.
The 3GPP NTN platform discussion instead focuses on higher-altitude UAS platforms, including HAPS.
Therefore:
General NTN concept:Can be broader and may include different airborne systems.
3GPP NTN scope:Focuses on defined satellite and higher-altitude airborne platforms.
23. HAPS Alliance
The HAPS Alliance is an organization focused on accelerating the commercial adoption of HAPS technologies and building a robust HAPS ecosystem.
Its membership includes companies from different industries, including:
Telecommunications
Aerospace
Other technology sectors
The organization promotes the use of HAPS in the stratosphere and works toward greater cooperation across the industry.
24. Objectives of the HAPS Alliance
The HAPS Alliance focuses on several objectives, including:
Advocating the advantages of HAPS to authorities
Building a cooperative HAPS ecosystem
Developing common product specifications
Promoting standardization
Supporting network interoperability
These activities are important because successful HAPS deployment requires cooperation among multiple parts of the telecommunications and aerospace ecosystem.
25. Satellite vs HAPS
Satellite and HAPS platforms have fundamentally different characteristics.
Feature | Satellite | HAPS |
Operating Environment | Spaceborne | Airborne |
Typical Altitude | Hundreds to tens of thousands of km | ~8–50 km |
Examples | LEO, MEO, GEO, HEO | Balloons, aircraft, solar-powered aircraft |
Mobility | Depends on orbit | Can potentially maintain a fixed position |
Coverage | Can be extremely large | Large but generally smaller than satellite coverage |
Propagation Distance | Very large | Much shorter |
Potential Delay | Depends strongly on orbit | Generally lower than satellite systems |
The exact characteristics depend on the specific platform and deployment design.
26. Why Multiple NTN Platforms Matter
There is no single NTN platform that is optimal for every communication requirement.
Different platforms provide different combinations of:
Coverage
Latency
Capacity
Mobility
Propagation characteristics
Deployment flexibility
Resilience
For example:
GEO → Very wide coverage
LEO → Lower propagation delay
MEO → Intermediate characteristics
HEO → Highly variable altitude and coverage characteristics
HAPS → High-altitude airborne coverage
This diversity provides operators with greater flexibility when designing NTN systems.
27. The Future of Multi-Layer Connectivity
Future communication networks can potentially combine multiple NTN platforms with terrestrial networks.
A conceptual architecture could include:
Terrestrial 5G/6G
↓
HAPS
↓
LEO
↓
MEO
↓
GEO
Each layer can provide different capabilities.
This multi-layer architecture can support the broader objective of providing connectivity across a wide range of geographical and application requirements.
Conclusion
The 3GPP NTN ecosystem considers two major categories of platforms:
Satellites and Uncrewed/Unmanned Aerial Systems (UASs), including HAPSs.
Satellite platforms can be categorized into different orbital types, including GSO and NGSO.
GEO satellites operate at approximately 35,786 km and appear fixed to an observer on Earth's surface.
LEO satellites operate at approximately 300–1,200/1,500 km, move rapidly around Earth at about 7.8 km/s, and can complete an orbit in roughly 90 minutes.
MEO satellites operate at approximately 5,000/7,000–20,000/25,000 km, while HEO satellites follow highly elongated orbits with significant altitude variation.
3GPP has primarily focused on LEO and GEO satellites as two important extremes when developing NTN specifications, while HEO has not been explicitly investigated to the same extent.
The second major platform category is UAS, particularly HAPS, which can operate between approximately 8 and 50 km. HAPS platforms can include balloons and aircraft, including solar-powered aircraft, and can potentially maintain a relatively fixed position over a geographical area.
Another important development is the use of multi-orbit satellite constellations, where GEO, MEO, LEO, and terrestrial networks can complement each other. Such architectures can provide benefits in terms of coverage, efficiency, flexibility, and resilience.
Ultimately, understanding NTN requires understanding not only the communication protocols but also the physical characteristics of the platforms carrying the NTN payload.
LEO, MEO, GEO, HEO, and HAPS each represent different approaches to delivering non-terrestrial connectivity, and their combination can become increasingly important as NTN evolves through 5G, 5G-Advanced, and future 6G networks.
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