At the turn of the century, an in-orbit satellite would have cost more than $17m. These same capabilities can now be put into the same orbit for $200,000.
Satellites rotate around a gravitational body – such as the Earth – at a distance proportional to their speed, but the orbit may not be circular, and the exact path of the satellite can provide advantages in terms of capability and availability.
Some orbits will provide better coverage for specific countries, times of day or antenna designs. They may be suited to relaying data between other space assets, avoiding known belts of electromagnetic radiation or reducing the revisit time to specific areas.
Developing such orbits requires a detailed understanding of gravitational fields in four-dimensional space – including time – and several orbits are protected by patent in their most useful applications.
Many new constellations of satellites are being launched into low Earth orbit (LEO), less than 1,000km above the Earth’s surface. This provides good observation and communication services. But these satellites are also skimming our planet’s atmosphere, and can only survive for about five years before de-orbiting and burning into dust.
These constellations therefore require a constant replacement cycle – in contrast to satellites in geostationary orbit, which might use the same technology for 20 years. New services must be judged on the upgrades they plan to deploy – and their financial ability to support the constant refreshes – rather than the capabilities they are already deploying.
Communications satellites in LEO offer low-latency communications, while commercial Earth observation satellites are photographing every spot daily to provide time-based analysis. Much of the technology used in these “new space” projects is borrowed from the smartphone industry – efficient batteries, compact antennas and impact-resistant electronics – but much has been developed to address the problems specific to working in space.
A circular orbit – around the equator of a planet – offers considerable utility, but as scientists gained experience working in space, it became clear that alternatives could open up new opportunities. Different altitudes – low Earth orbit (LEO), medium Earth orbit (MEO), geostationary Earth orbit (GEO) – provide one variable, but elliptical (stretched out) orbits can support new use cases. Combinations of orbit can be used to maximise service availability and minimise idle time – a critical factor in the profitability of a satellite constellation.
Several well-known orbits are already in common use. Sun synchronous is an orbit, generally used within LEO altitudes, which places the satellite over specific spots on the Earth at the same time of day, every few days. This is used heavily by Earth observation (and spy) satellites to provide consistent shadowing of an observed region.
An orbit in which the satellite passes over both poles is known as a polar orbit. This allows the planet to circle beneath it, enabling the satellite to pass over every point on the surface, and to pass over each pole every 100 minutes or so to collect data from static sensors or photograph specific locations.
Molniya is a highly elliptical orbit, which increases the time that the satellite spends over a particular latitude. This is generally used to increase availability for communications systems covering northern latitudes, such as Canada and Russia.
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A geostationary orbit matches the rotation speed of the Earth and traces the equator, providing satellites that seem to hover above a specific point on the surface. Such an orbit requires an altitude of 35,786km to match the speed of the Earth, resulting in high latency, but such an orbit enables dish antennas to be pointed at the satellite.
Different orbits and orbital altitudes offer different features. Hybrid services combine LEO, MEO and GEO orbits to optimise the coverage, bandwidth and latency of a connection. GEO, for example, offers wide-area coverage, while LEO can provide low-latency communications.
A communications protocol, such as TCP/IP, may be broken down into layers that can be handled by satellites in different orbits – the channel being reassembled at the receiving equipment.
Management of data traffic, which is sensitive to latency, can be handled by satellites in LEO, while bulk traffic flows are delivered from satellites in MEO or GEO orbits. This enables the use of legacy assets, as well as reducing the need for high-capacity satellites in dense LEO orbits.
Network protocols for satellite communications
The TCP/IP protocol used for internet communications requires that received packets are acknowledged within a specific timeframe and consistency. This makes TCP/IP very difficult to use with MEO constellations and almost impossible to use over GEO, without modification.
By providing the acknowledgements over LEO while the bulk data is transmitted over MEO, a combined constellation can deliver data using standard TCP/IP communications. This enables, for example, interaction with a video service to be performed over LEO, while the selected video is delivered from a satellite in GEO.
Similar methods can be applied to other network protocols, providing the benefits of both orbits without the drawbacks of either. Such a combined constellation offers:
- Simplified application development: The ability to use standard internet protocols without going through a required proxy or protocol conversion simplifies the development of applications, as well as removing potential areas of incompatibility that can slow deployment times.
- Reduced constellation density: MEO (and GEO) satellites have a much larger footprint, being able to provide services across wide areas without customers needing low-angled antennas, which are more prone to interruption from buildings and trees. This results in a smaller, and more economical, constellation being able to provide a comparable service.
- Greater availability of spectrum: Splitting the control plane from the data plane allows more efficient use of the radio spectrum, because these two components of the network stream can use different radio frequencies.
- Broader coverage: The larger footprint from higher altitude allows coverage to be extended into areas without receiving ground equipment, and without needing satellite-to-satellite communications systems.
- Exploitation of existing assets: Many companies have existing assets in GEO orbits. Complementing these with LEO services can enable these assets to compete with wholly LEO constellations.
The potential for satellite comms
Various companies are seeking ways to increase the value of existing (deployed) orbital assets. The value of satellite data is falling rapidly, as broadcast television is increasingly supplanted by on-demand services, and LEO constellations threaten the market for rural internet access.
These companies may be hopeful that hybrid systems will have a long-term future, but much will depend on the development of low-cost endpoint equipment capable of supporting hybrid networks. Given that they have a 15-year life, and few new communications systems are being deployed in GEO, the existing assets may be operational for another decade. This means the greatest opportunity is in the next five years.
As a final point, satellites in orbit often require large reflectors or antennas, and almost always need extensive solar arrays to power the mission. Such objects are difficult to launch, so various technologies and techniques are needed to reduce the size during launch.
Techniques being used include folding arms, springs, origami techniques and memory metals. The best approaches offer a significant reduction in size, while keeping weight low, but, most critically, can offer very high reliability.
Because reliability is so paramount, the technology behind such systems is rarely complicated, so interesting innovation is mostly in the ways in which the components are folded and unfolded, and the method used to push them into deployment.
This article is an excerpt from the Gartner report, “Emerging technologies: emergence cycle for satellite systems”. Bill Ray is a vice-president analyst at Gartner.