Constellations of satellites are being proposed in large numbers; most of them are expected to be in orbit within the next decade. They will provide communication to unserved and underserved communities, enable global monitoring of Earth and enhance space observation. The idea of a constellation of satellites appeared in the market about twenty years ago with companies like ViaSat, Boeing, Samsung, Yaliny, Globalstar and Iridium. In all cases, the target application was in the communication field, aimed at providing global connectivity with different strategies: medium Earth orbit (MEO) or low Earth orbit (LEO) constellations, and large or small numbers of satellites. They however had limited success due to the small market and high initial and maintenance costs.
With a tremendous forecasted increase in the launch rate for small satellites (from pico-sized to mini-sized), constellations are getting attention again from sustainable businesses. Constellations have their greatest potential in the communication field. The upcoming era of the Internet-Of-Things requires the communication infrastructure to handle huge amounts of data and to guarantee service in any geographical position. Constellations, however, also have great potential in weather science, safety/security and disaster monitoring.
The number of satellites in orbit is going to increase more than linearly with about 8000 spacecraft in orbit in 2024 due to constellations only. More than hundred companies have been found proposing constellations with varying numbers of satellites. Satellite sizes range mainly from nano to micro-sizes, i.e., from 1 kg up to 100 kg. Most of them are expected to be active in orbit before 2025. The most common number of elements for the constellations is below 150 units.
Earth Observation Constellations
In a Low Earth Orbit (LEO), a satellite completes about 14 or 15 polar orbits every day, but not always over the same area (this only happens in equatorial orbits). The rotation movement of the Earth makes the satellite complete a ‘sweep’ over the entire surface of the Earth, so each step is different from the previous one. At the same time, there are cycles of orbits that are repeated at regular intervals.
The small satellites launched to date are almost always placed in LEO, most of which fall into Sun-Synchronous and Non-Polar Inclined orbits. A Sun Synchronous orbit is the most popular for small satellites as it places the satellite in
constant sunlight. This is convenient for satellites that image earth in visible wavelengths and allows for simpler power subsystems. Additionally, LEO provides the lowest $/kg delivered to orbit.
When designing the Earth observation missions, the first step is to know the requirements of the project, with some basic questions:
- Which geographical areas will the constellation serve?
- How often will the satellites receive and transmit information? for example, a satellite can be expected to pass through the same region every seven days. In that case, the satellite would repeat this cycle of orbits every seven days, allowing the same area to be observed on a regular basis.
If we consider uses such as monitoring crops, snow melt, or desertification, if the orbit cycle is seven days, every week you will obtain accurate and comparable images of each location through which the satellite passes. For other services, such as port traffic control (to mention another Earth observation mission), which requires more frequent images, this waiting period is too long, so it would not be a viable mission with only one satellite.
For communication to take place, two parts are necessary. On the one hand, we have the space segment, with the satellites in orbit, but it is important to also take into account the ground segment. At this point, it is necessary to determine how many stations will be needed to collect information and where they will be located. This decision will depend on the type of service to be provided
In general, in the design of a satellite constellation for SatCom services, it is important to assess a number of parameters and to evaluate their respective trade-offs. The principal performance parameter is the coverage, as the first requirement to guarantee the communication link is to reliably cover the regions of interest. Typically, the coverage of the satellite is assessed taking into account various practical restrictions, such as the minimum elevation angle for the user terminal and required service availability.
Another fundamental performance parameter to be considered is the link latency, which is directly related to the constellation altitude. While high altitude constellations, such as GEO ones, allow wide coverage, they suffer a much higher latency compared to the lower altitude ones. The fundamental trade-off is that the GEO satellites are farther and therefore are characterized by a longer path length to Earth stations, while the LEO systems promise short paths analogously to terrestrial systems. The path length introduces a propagation delay since radio signals travel at the speed of light. Depending on the nature of the service, the increased latency of LEO, MEO and GEO orbits may impose some degradation on the quality of the received signals or the delivered data rate. The extent to which this influences the acceptability of the service depends on several factors, such as the degree of interactivity, the delay of other components of the end-to-end system, and the protocols used to coordinate information transfer and error recovery.
Furthermore, satellites at lower altitudes move faster, which leads to higher Doppler frequency offset/drift and can be crucial for the design of the user equipment, especially for wideband links. This trade-off in the altitude choice clearly needs to be addressed taking into account the type of service to be provided. Concerning the cost of constellations, the principal parameter is clearly the number of satellites, thus it is important to achieve the desired performance keeping this number as low as possible. Also, the number of orbital planes affects the overall cost, as changes require large amounts of propellant.
Ultimately, once the constellation altitude is selected based on the specific service to be provided, the constellation design aims at guaranteeing coverage in the regions of interest, using the lowest possible number of satellites and orbital planes. After that, the satellite payload and architecture are designed by taking into account the system requirements.
The basic structure of a satellite communication system consists of a space segment that includes the satellite constellation, a ground segment including GW stations and large ground facilities for control, network operations and backhauling, and a user segment with the user terminals deployed on fixed and mobile platforms (e.g. airplanes and ships). As the coverage area of MEO satellites is typically larger than the coverage area of LEO satellites, LEO constellations require a substantially larger number of supporting GWs compared to MEO constellations. In contrast, GEO satellites require only one GW for backhauling due to their fixed position.
This large number of satellites in orbit has also thrown many challenges.
For example, the current ground segment infrastructures will probably not be able to monitor and control such a large number of satellites. Therefore ground service providers need to invest for new infrastructure development. By one estimate, the 4,400-satellite version of Starlink will require 123 ground-station locations and about 3,500 gateway antennas to achieve maximum throughput. The gateway antennas must be larger and will require significantly more power than user terminals do.
Spectrum Issues and Solutions
A second concern involves communication, with the RF spectrum becoming possibly overcrowded and the required data-throughput increasingly larger.
Frequencies are coordinated through the International Telecommunications Union (ITU). The United Nations Agency for Information and Communication Technologies has a comprehensive database with all satellite networks, to avoid interference and allow them to carry out their missions in an optimal way.
During the planning phase of a satellite launch, the international coordination process takes place. As frequencies are a limited resource, information regarding the type or model of the object to be launched into space must be published through the International Telecommunications Union. In this way, other countries or companies can assess whether it may interfere with any of the objects currently in orbit.
Communication will always be critical point until operating a constellation fully autonomously becomes possible. Constellations are designed usually for real time—24/7-purposes, requiring data down/up-load at any time. These requirements can lead to challenges of RF spectrum congestion and Radio Frequency (RF) spectrum partitioning. An overcrowded RF spectrum may indeed cause physical interference of adjacent RF signals. At the same time, the traffic capacity of the communication infrastructure shall grow in parallel with the data volume travelling in the RF channels.
Solutions against the increasingly crowded spectrum are devoted mainly towards spectrum sharing and enhancement of regulation. Using this approach, the temporarily unused spectrum could be reallocated for a more efficient use. A completely different approach to avoid RF spectrum overcrowding consists of moving to the optical part of the spectrum. Optical communication promises higher data rates using smaller and lighter terminals, even though due its high sensitivity to atmospheric conditions it is more suited for free-space inter-satellite links rather than satellite-to-ground.
The operator of a large LEO constellation must monitor and manage the status and functions of thousands of satellites. One of the solutions proposed is autonomous and semiautonomous control and management of spacecraft, reducing staffing requirements. For example Optimization of automatic satellite tracking (such as telemetry download) through increasing automation onboard the spacecraft, especially for the collision avoidance assessment and maneuver planning, which is now largely manual.
Recent advances in analytics, combined with improved computing power and artificial-intelligence algorithms, can assist with these functions while reducing response times and operating costs. Given the low cost and high speed of computer processors today, it is now feasible to equip every satellite in a constellation with enough processing power to be able to direct its own activities. Control would thereby be distributed throughout the entire constellation, minimizing the reliance of the whole system on the limited and fallible resources of any single component, and minimizing human ground management. A well-designed and flexible social structure would enable the constellation to coordinate its activities as a whole, distributing tasks and allocating responsibilities to the appropriate member satellites, without losing the benefits of individual autonomy.
Automatic failure detection, so that the operator does not need to manually check the satellite’s status of health. In the latter case, expert systems (intended as an ensemble of algorithms, and machines aiding the operator’s decisions, usually associated with artificial intelligence, are applied for high level tasks—prediction, planning, diagnosis, repair, etc.) shall be deployed on ground. Another challenge is satellite delays which constraint real time strategies.
Some have proposed intersatellite-link, which is usually not affordable for low-cost strategies. Moreover, it implies intensive intersatellite communication contributing to RF spectrum crowding. Expert systems shall be designed to assist operators and keep the workload constant during constellation operations, e.g., mitigating the heavier workload during launch and early orbit phase. Splitting between payload operations and spacecraft operations, possibly with dedicating ground segments to each of the two.
A small satellite constellation is renewed on a regular basis. Far from being a problem, this is an opportunity and one of the main advantages in comparison to conventional satellites. This upgrading process overcomes problems of obsolescence and makes it possible to always have the very latest technology in space, with the possibility of modernising, improving or expanding the services offered by the constellation.
Replacing the satellites in a constellation is usually dealt with in two different ways:
- With spare parts already prepared on the ground, meaning that they are ready to be launched at any time, so that replacement is almost immediate.
- By launching more satellites than necessary, meaning that some provide the service while others are ‘dormant’ in the event that one of the others should fail. There are even propulsion systems that allow them to be activated in orbit and moved to their new position. Logically, it is not possible to make any kind of movement, but in the same orbital plane it is possible to make some satellites in the constellation overtake others or to slow them down until they reach the desired position.
Lastly, but probably most importantly, rise of the space traffic and debris which may prevent the safe and successful operation of spacecraft with higher probability of collisions. At the same time, constellation management shall be enhanced to make an efficient use the new infrastructure. This requires new operational architectures towards higher automation, either onboard or on ground, involving, for instance, artificial intelligence and virtual reality.
Debris are already a problem that is being faced actively with surveillance networks (e.g., the JSpOC, Joint Space Operation Center) and avoidance maneuvers from the spacecraft operators. Researchers advocate the need for updating state-of-the art space debris modelling as a result of the evolving debris environment. One of the Debris mitigation policies is making the spacecraft reenter at the end of its life, prventing one of the main cause of space debris. Alternative solutions to the debris problem include active removal and space-based surveillance networks. On-Orbit servicing is a further option to decrease the amount of failed or dead satellites that become a debris. Its implementation, however, is still costly and technically challenging.
Space Traffic Management
Among the issues to be faced in the “constellation race” the space traffic management is probably the most critical, yet not directly faced.
Regulatory aspects of space traffic management are instead poorly covered. Currently, once a free orbital slot has been identified, the common practice consists of seeking for a technically and economically viable solution to reach such a slot. Not much attention, instead, is payed to interferences affecting other operators, that might be caused while the spacecraft are reaching the target orbit or during de-orbiting at the end of life. Though up to now it has been safe to assume that space is so large that satellite operations do not interfere with each other, this may not be true in the near future. For instance, with thousands more spacecraft in orbit, an Earth observation satellite may find unexpectedly another one in its field of view, or a region of space may become so overcrowded as to impact the quality of space observations from ground.
Besides the regulatory part, there are also technical challenges to be overcome. Researchers have proposed machine learning through support vector machines both to monitor satellite health and address management issues, highlighting a great potential of neural networks for enhancing space traffic management.
Another interesting attempt is by Australian Government which is financing a conjunction assessment service featuring a ground-based laser “deviator.” The aim is to maneuver small uncooperative objects remotely from ground using a laser beam, which is theoretically feasible Aerospace but with great technical challenges due to the laser power needed.
Novel concepts of conjunction assessment services are also on their way. One of the solutions proposed was a prototype of a ground-based service that can interface with all subscribed satellite operators (scalable solution) was presented. Besides integrating different object databases and giving alerts similarly to JSpOC, it can compute the most suitable avoidance maneuvers. Thanks to the global situational awareness of the service, such a maneuver can ensure minimum fuel consumption while avoiding “cascade maneuvers.” Moreover, after suggesting the maneuver directly to the involved spacecraft operators, it can update its database and inform the other operators when the maneuver is accomplished.
“The biggest challenge will be affordability,” CCS Insight analyst Kester Mann said. “Space is a huge and risky investment. “And it may take many years before devices fall sufficiently in price to become appealing to the mass market. “This will be particularly relevant in emerging markets.” And that means costs will have to be recouped from consumers. SpaceX charged $99 (£75) per month for its initial trial offering in North America, plus a one-off $499 fee for the hardware. And that came with a warning that, at least at first, the service might not always work.
Satellites have traditionally been more akin to handcrafted items than to mass-produced goods. That kind of customization, combined with long life-span requirements, explains why a typical large communications satellite costs from $50,000 to $60,000 per kilogram. If large LEO constellations are to be financially viable, their manufacturing costs must fall by more than an order of magnitude from those of traditional satellites. That would probably be at least 75 percent lower than the costs any company has currently claimed it can achieve . To cut costs in this way, manufacturers must leverage every possible tool, from economies of scale to automation to reduced component costs across the value chain.
Spacecraft deployment must be accounted for since the beginning because it has a significant impact on the lifecycle cost. In fact, it affects both the number of launches and the complexity of the satellite to be launched. In principle one launch for every orbital plane is needed, also the complexity of the onboard propulsion system (if any) changes based on the post-launch operations to be performed. Researchers have proposed staged deployment, i.e. deploying the spacecraft gradually as they are needed by the market, which is shown to reduce the life cycle cost of a constellation significantly, of about 20% when applied to the Globalstar case study.
Current gateways for GEO satellite communications are quite expensive—typically from $1 million to $2 million each. They are not directly comparable to LEO gateways, which have lower power requirements, but the numbers do suggest that gateway costs must be much lower than those of current approaches to make ground-segment costs manageable. Modular antenna designs could help, since they would enable equally critical cost reductions in user-equipment antennas, but owners of large LEO constellations will also look for other efficiencies.
Constellation technology trends
The large LEO concepts are mainly planning to use Ka band. Some propose V band as well. These frequencies enable higher data rates, smaller antennas, narrower beams, and greater security. Higher frequencies are more vulnerable to weather and rain fade, which is the absorption of a radio-frequency signal by atmospheric rain, snow, or ice; frequencies higher than 11 gigahertz are more vulnerable than lower frequencies. Fortunately, expedients such as improved ground-station design, adaptive coding, and signal modulation can reduce this exposure. Improved spectral efficiency and spectrum-reuse rates can also increase the amount of data a system delivers.
The trend is to use multibeam Satellite resulting in greater power that can be delivered through each beam and higher throughput. Many constellations also employ Intersatellite links (ISLs) that reduce latency, improve connectivity and confer particular benefits to large constellations, including improved throughput. Improved data-compression methods reduce bandwidth requirements without reducing the quality of communications.
The trend in Ground equipment is to move away from parabolic-dish antennas to electronically scanned apertures (ESAs), also called electronically steerable antennas, can shift beams (and track and access large numbers of satellites) without physical movement. ESAs can also be designed for modular assembly, which could allow manufacturers to produce large numbers of basic parts for use in both constellation ground stations and consumer equipment, thereby improving economies of scale. Other important advances in ground equipment include new predictive analytics and network-optimization techniques that use available ground-entry points more effectively.
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