Nanosatellites enable space-based applications for the many

By Paul Leys   |   April 13, 2022   |   Provided By Avnet Silica

Nation states and major multinationals used to be the only organisations that could launch and operate satellites. Since 2000, the situation has changed dramatically, bringing the cost for much smaller satellites, known as nanosatellites, down to below €500,000 - far less than the €500m needed to develop and launch larger conventional satellites.

The biggest change came when engineers at California State Polytechnic University (Cal-Poly) developed a form factor for very small satellites, which they called a CubeSat. Weighing little more than a kilogram each, CubeSats made it possible to reduce the launch costs for student and research projects. By demanding that every small satellite employed the same basic cubic format, it became much easier to pack many of them together into a single launch unit, alongside other much larger commercial satellites. This allowed many projects to share the cost.

Nanosatellite compactness brings clear benefits

The containerised format made it easier for providers to add CubeSat capability to their offerings, with the result that there are now many launch opportunities each year. High launch frequency supports rapid design, test and fly cycles and provides much greater flexibility for project lifecycles because design teams do not need to commit to a fixed launch date.

Whereas it could take more than five years to identify a requirement, design, test and place a conventional large satellite in orbit, companies can move from concept to deployment with a CubeSat or similar nanosatellite in less than a year. Though it took around a decade for momentum in nanosatellites to build, launch rates took off rapidly after 2013. In a three-month period from November 2013, almost a hundred nanosatellite payloads were placed in orbit.

Nanosatellite applications

The potential applications that nanosatellites are able to address have expanded far beyond the original plans of the Cal-Poly team - which were largely focused on supporting space-science student projects for low-Earth orbit (LEO) deployments. They can be used in various environment monitoring, Earth observation, atmospheric analysis and remote sensing tasks. Two CubeSats left Earth orbit in 2018, designed to provide communications relays for NASA’s InSight Mars lander mission and to act as a test of nanosatellite operation in deep space. 

A nanosatellite design can perform a range of duties, letting multiple research teams share the cost of a single launch. The 3Cat-1 developed by a team at the University of Catalonia, and launched in 2018, was used to explore the possibility of using commercial-off-the-shelf (COTS) electronic components in orbit. It hosted a variety of engineering and scientific experiments that included a novel form of solar cell, a MEMS oxygen sensor, a wireless-power transfer technique and an energy-harvesting system that could exploit the thermal gradient between two surfaces of the satellite.

A constellation-based approach

Communications and surveillance are among the leading commercial nanosatellite applications. A key asset of the small satellite concept is that it becomes cost effective to deploy a large nanosatellite constellation that can ensure constant coverage anywhere in the world. Large constellations take advantage of LEOs that are most commonly supported by launch providers (with altitudes up to 2000km). Though each one passes quickly over the surface of the Earth, the many nanosatellites that are comprised within a constellation ensure that at least one is always in view from the ground at practically any location.

The LEO deployment of large nanosatellite constellations allows rapid responses to changing conditions on Earth. Some satellites use onboard imagers to capture forest fires and serious weather events. As the imagers on these tiny satellites often have comparatively low resolution, an important tactic is to use them to alert larger satellites in higher orbits. These can then capture high-resolution images and perform multispectral analysis to aid workers on the ground.

Different nanosatellite constellations are beginning to work together, forming an Internet of Things (IoT) in space, as well as supporting the IoT on the ground. For example, a communications network that can track the transponders on ships and aircraft can alert a second network of imaging satellites to focus on craft that have just changed their transponder status - which can be a sign of illegal activity, such as piracy.

The wide array of nanosatellite formats

It should be noted that nanosatellites are not restricted to CubeSats, though many teams choose that form factor to make it easier to obtain launch slots and take advantage of the ecosystem that has formed around the standard. Even within the CubeSat form factor there are options to expand size beyond the basic 10 x 10 x 10cm envelope. 2U, 3U and even larger sizes are possible, with some proposed shapes extending above the typical maximum nanosatellite weight of 10kg and into the microsatellite category. By 2017, more than half of the nanosatellites launched used the 3U format. At the other end of the scale, picosatellites based on the PocketQube form factor have been developed. These weigh less than 1kg.

The relatively low orbit of most nanosatellites leads to short flight time compared to those operating at much higher altitudes. Atmospheric drag will cause the satellite to slow and lose altitude over time until it drops out of orbit and burns up. Typically, a nanosatellite designed for use in long-lived constellations will need to be replaced every two to four years. Though this increases the operating costs, the trade-off is that each new launch can use upgraded hardware. This, in turn, provides operators with a route to enhanced capability over time with far fewer worries about component obsolescence in order to maintain the fleet.

Nanosatellite propulsion

Phenomena such as atmospheric drag mean nanosatellites will need to be able to manoeuvre themselves into different orbits and attitudes over time, as well as performing insertion into the desired orbit immediately after deployment. Conversely, they must make themselves safely de-orbit into the atmosphere at the conclusion of their mission. The space available in the chassis puts stringent limits on how much can be dedicated to the weight and bulk of a nanosatellite’s propulsion system. However, suppliers and researchers have developed several options that range from compressed-gas propellants to more experimental pulsed-plasma generators. Electrical systems, such as ion thrusters, which convert a solid propellant to charged gas, are becoming more commonly used. However, the best nanosatellite propulsion technology will depend on the specific mission parameters.

Implementing effective communication

Modules operating in the sub-Gigahertz UHF bands designed for nanosatellites can generally deliver data up to 100kbit/s with an active power consumption of under 5W. Higher data rates, at the cost of a higher power envelope, are available with S-band and higher-frequency operation. The limit on channel capacity and power has put a greater emphasis on the use of advanced signal processing and machine learning to extract important features from the raw data they collect and only to transmit significant changes to the ground station as they pass overhead.

Design considerations

Though they have a relatively short lifetime compared to the geostationary satellites that support the global positioning system (GPS), the ability to work in the harsh environment of space brings challenges to nanosatellite design.

Radiation is one of the main issues facing any designer of an electronic system that is to be deployed in space - even at LEO altitudes where there is some protection from our planet’s strong magnetic field against high-energy charged particles emanating from the Sun. There remains a high threat from cosmic rays, many of which are absorbed by the atmosphere before they can strike ground-based systems. Ionising radiation can result in the release of a cloud of charge particles into the silicon substrate underneath transistors and memory cells. This charge tends to collect in registers and memory cells, which can lead to a change in value that causes an error in the form of a single-event upset. However, a discharge close to a transistor junction can lead to permanent latch-up created by the formation of a parasitic bipolar transistor.

The designers of larger, longer-life satellites typically turn to components designed for use in space that can handle high radiation, as well as the temperature extremes that result from the outer surface being exposed to direct sunlight when out of the Earth’s shade. These devices are rated according to Class V of the qualified manufacturers list (QML) developed by the US for its military programmes. However, if QML-rated parts were used throughout a design, many nanosatellite projects would become uneconomic.

Some component suppliers have reacted to this by developing a QML-like range of products that have more relaxed testing regimes, reducing their cost to the customer. It can also make sense to mix COTS components with those designed for rad-hard operation. The main system controller may operate rad-hard elements and even feature dual- or triple-modular redundancy. This will then supervise a set of less mission-critical components that have lower resilience. Those less critical subsystems may also use a more restricted level of redundancy, such as error detection and correction in the memory arrays, to let them operate in the presence of single-event upsets but not a full latch-up event.

A number of companies have recognised that many of the organisations looking to build nanosatellites do not have direct experience of space-based electronics. They can provide design support and guidance, showing where best to focus high-redundancy and resilient circuit-design techniques in order to deliver low-cost but effective hardware. This is further enabling the democratisation of space.

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Paul Leys

Paul Leys is the Market Segment Manager for the Aerospace and Commercial Avionics business at Avnet ...

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