The Multi-Orbit Challenge for Broadband Communications

by Bruce Elbert  

Austin, Tex., November 1, 2024--The primary benefit of multi-orbit interoperability is to be able to guarantee the connection to the network under any circumstances, including user location, motion and timing of demand for service. Also, the immediate choice of satellite or orbit may be based on the cost of the connection and data transfer. Herein, we identify alternatives to achieve multi-orbit networks and rate them as to their capability, feasibility and costs. While many new technologies, like AI, facilitate integration, the physical link and the ability to hold and transfer to alternatives are central to how well such schemes will work in practice. Most certainly, multi-orbit integration is possible, but at what cost and complexity? We take a communications perspective rather than a pure technology orientation.

Characteristics of LEO, MEO and GEO

Multi-orbit Multi-network integration can occur with respect to the Space Segment, User Terminal, Gateway, or even the Terrestrial network. Illustrated in Figure 1 are six elements to provide a satellite communications service. That left-most element, the Control Plane, can be provided by a manual or automated system with or without human operators. There would be various degrees of integration between these blocks to support different users and services. The key characteristics of these orbit regimes from basic properties are given in Table 1. Low Earth Orbit (LEO) High requires fewer satellites and provides somewhat better coverage of the earth than LEO Low. But, LEO Low has become popular because of reduced free-space loss and that inactive spacecraft will re-enter without removal through on-board propulsion or external means. Max Range is measured from either the transmitting or receiving earth station (whichever is farther) and the satellite. Time delay is for a single hop (round trip is double this value) for the longest path between the earth transmit site, the satellite, and the earth receive site. This delay is only for propagation and does not include processing (ground or space) or for multiple hops and possibly intersatellite links. Service simplicity is a qualitative indication of the way users are served by the space segment. Terminal properties considers the antenna size, transmitter power and orbital considerations like radio frequency interference. Small terminals are preferred to reduce costs and exposure to the elements. Lastly, terrane blockage is caused by local obstacles like buildings, trees and hills that diminish service availability, based on motion of satellites and user terminals.

Figure  1. High-level architecture of a broadband satellite system. Interconnections depend on the data and control plane structure.

One obvious approach for a given application is to select the best orbit constellation and provider. GEO HTS can compete well with MEO and LEO but the subtilities can impose challenges in terms of a massive investment and risk of failure in a single launch. The maritime market, particularly cruise ships, began with GEO. This was a reasonable fit since most cruises operate in the lower latitudes. MEO was successfully introduced to this market around 2020 and owing to reduced latency was rapidly adopted. But, the LEO constellation has become a good competitor, mainly in terms of price. The economic aspects of these orbits are presented next.

Economics of Orbit Regimes

We assess the economics of these orbits in Table 2 for hypothetical constellations at different altitudes. These systems present increasing capacity by a factor of 100 as the quantity of satellites increases by up to 1,000. Note the utility of LEO satellites wherein a small fraction are able to provide payable service due to wastage of capacity over oceans and uninhabited regions.
    

Table 1. Characteristics and orbital range based on basic properties that affect the service.

GEO and MEO are the low investment leaders while both LEO categories represent substantially larger commitments. On the other hand, the capacity thus provided is so much larger with LEO so that the bottom line becomes nearly a wash as far as cost per Gpbs. However, the cost per Gbps is highly sensitive to the assumed utility percentage, especially for LEO. The similarity of cost per Gbps of the space segment would mean that the overall cost differences relate primarily to the terminal itself. Going to multi-orbit introduces requirements for multiple antenna beams and pointing requirements, as well as differences in specific frequency, waveform and RF power.

Multi-Orbit Integration Requirements and Tools

Some of the key applications likely to demand Multi-orbit integration are presented in Table 3. The requirements have not been met by a single orbit regime, hence their need for more resources. This sets us up to consider tools that can be applied to close the gap.

Table 2. Estimated cost of the broadband space segment across the three orbit regimes.

The Multi-orbit integration tools available today rely on the respective integration and management features of the underlying constellations and systems. A good example is Aalyria SPACETIME, a cloud-based scheme that creates a digital twin of the respective systems and provides a means of supporting users who need access to be best available resources. Also, Hughes Echostar offers solutions across both GEO and LEO through their well-integrated ground segment architecture. A detailed review and analysis of these and other products and suppliers is beyond the scope of this article. Some of solutions can be tried on an experimental basis with the support of the associated supplier. This would be a way to gain deeper knowledge about the capabilities as it may be able to address a specific need. Some solutions require wide-spread adoption and a long-term commitment. 

Table 3. Requirements and expectations for current services delivered from any orbit.

Future Prospects for Multi-orbit Multi-network Integration

The architecture and system design need to be described in sufficient detail so that this kind of study can be performed in a meaningful way. We cover how to anticipate challenges, based on past experience, and we suggest some solutions currently available. In the end, the engineer has to examine and address the details in the interfaces and integration challenges. One not identified and addressed early enough can render the system or service unusable or unacceptable in some manner. On the other hand, a thorough systems engineering process with verification will always work.

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Bruce Elbert  is the Founder and President of Application Technology Strategy LLC (www.applicationstrategy.com). He is a satellite industry expert, communications engineer, project leader and consultant with over 50 years experience in communications and  space-based systems in the public and private sectors.  Areas of expertise include space segment design and operation in all orbit domains, systems architecture and engineering, ground segment systems engineering, development and operation, overall system performance improvement, and organizational development. He can be reached  at: bruce@applicationstrategy.com