Spacecraft in low, mid, and high orbits provide a diverse array of services from TV and radio broadcast to terrestrial and maritime communications, remote sensing, and navigation and timing. Not surprisingly, the satellite industry has been a steady consumer of RF and microwave components for more than five decades, and the pace is likely to increase. The driver of this good fortune is the “smallsat” that makes it possible for even companies and governments without enormous financial resources to create space-based services for the first time.
With the cost of placing constellations of satellites now within reach, the market pits major satellite builders and global satellite communications providers against start-up companies that are establishing themselves as service providers. In short, ventures flush with private capital are working to build large constellations of small satellites in low Earth orbit to provide low-latency communications, remote sensing, technology demonstration, and science and exploration.
He Had a Vision…
In 1945, Arthur C. Clarke described in Wireless World magazine (still alive today as Electronics World) a way that communications could be achieved via satellites (Figure 1). Said Clarke, “A true broadcast service, giving constant field strength at all times over the whole globe would be invaluable, not to say indispensable, in a world society,” though noting that “many may consider the solution proposed in this discussion too farfetched to be taken very seriously.”
No doubt many did, but Clarke was just 15 years ahead of his time because, in 1960, Courier 1, the first communications satellite with solar panels, was launched, followed in 1962 by Telstar 1, which effectively marks the beginning of satellite communications. It was owned by AT&T and launched by NASA and could relay telephone and television signals across the Northern Hemisphere.
Today, there are about 1,500 communications satellites in orbit as well as hundreds more for remote sensing, navigation and timing, and a mysterious but large number for defense applications. In its latest report covering 2019, the Satellite Industry Association (SIA) noted that 386 satellites were launched that year alone, an increase of 17% over the year earlier. Global revenue increased to $366 billion, of which $271 billion was from the commercial sector, nearly 75 % of the world’s space industry. Remote sensing revenues grew by 11% to $2.3 billion, and ground equipment revenues grew by 4% to $130 billion, led by continued sales of global navigation satellite system terminals such as smartphones and tablets.
NASA Invites Industry In
The impetus for this new market actually emerged when NASA ended the Space Shuttle program in 2004, reorienting its focus from “all things space” to the most challenging efforts that, among other things, will send manned and unmanned spacecraft further into the universe to discover the secrets it holds. This left the door open to private industry, where Elon Musk, Jeff Bezos, Richard Branson, and others poured billions of dollars into all facets of the industry from launch vehicles to satellites. Ventures from billionaires like Musk, Bezos, and Branson may be the most visible, but hundreds of companies throughout the world are also feverishly working to develop new satellites and applications that they can perform.
NASA has also reached out to industry to “commercialize” the International Space Station, a major policy change for the agency which previously refused to allow its side of the ISS to be commercialized; but now the agency has opened up parts of the ISS for use by companies including filming commercials or movies against the backdrop of space. NASA is also calling on the private space industry to send in ideas for habitats and modules that can be attached to the space station semi-permanently.
The results of NASA’s releasing its grip on the space domain have already been spectacular, as extraordinary advances in digital (and yes, analog) technologies are making it possible to build small satellites that can be designed, developed, and fabricated for as little as $35,000 and launched either in bespoke rocket compartments or as “rideshares” within rockets carrying other payloads.
Small satellites are not new, as Russia launched the first one in 2003, and the earliest CubeSat built to a standard form factor was conceived in 1999 by Bob Twiggs, creator of Stanford University’s Space Systems Development Laboratory, and Jordi Puig-Suari, a professor at California Polytechnic State University, San Luis Obispo. As legend has it, Twiggs got the idea from the clear acrylic box that Beanie Babies came in, making it possible to standardize smallsats for the first time. This allowed Twiggs and Puig-Suari to develop a launch concept called the Poly-Picosatellite Orbital Deployer (P-Pod) at Cal-Poly that allowed these standardized payloads to be stacked into a launcher tube (Figure 2). Creating the standard CubeSat dimensions and launcher are arguably the keys that opened the door to today’s smallsat boom because, without them, satellites would have been launched as they always had, one or two at a time, each one essentially a customized design.
Compared with traditional satellites, smallsats typically have shorter development cycles, require smaller design teams, and cost a fraction of their larger counterparts. CubeSats, a class of smallsats, has the additional benefit of containerization and a standardized form factor, allowing mass production and easier launch vehicle integration, which can reduce cost costs even more. These lower cost satellites’ expendability and simultaneous deployment in large numbers enable spatially-distributed data collection, communications, and other applications to be created with less financial risk than larger satellites.
There is no universally accepted definition of a smallsat, and as the market develops, variants are regularly appearing but they can generally be defined by their mass, volume, cost, and capabilities. As a rule of thumb, a femtosat weighs less than 0.1 kg, a picosat 0.1 to 1 kg, a nanosat 1 to 10 kg, a microsat from 10 to 100 kg, and a minisat from 100 to 1000 kg.
The smallest of smallsats, appropriately called CubeSats, weigh from 1 to 10 kg and are based on a basic 10 x 10 x 10 cm 1U cube. The CubeSat form factor allows many to be densely packed inside a rocket so dozens can be launched at once. The concept was first launched at Stanford University in 1999, where the basic unit was defined as a 1U cube and today ranges up to 16U (Figure 3).
The coverage provided by a CubeSat mission depends on the number of satellites, the number of orbital planes, elevation angle, inclination, altitude, orbital plane spacing, and orbit eccentricity. All current CubeSats have been deployed in low earth orbit (LEO) to provide the low latency required for communications and other applications. LEO operation also reduces constraints on the radiation hardening of devices that becomes more challenging the further from Earth the satellite is placed in space.
The number of companies and government agencies availing themselves of smallsats is growing, but the most visible is the Starlink program from SpaceX that is, at least initially, dedicated to providing commercial broadband service everywhere on Earth. SpaceX launched 60 satellites into low Earth orbit from a single Falcon 9 rocket, and as they deorbit in 5 years or less, they mitigate what would otherwise add to the increasing amount of space junk, and as the launch costs of these satellites are low and large-scale smallsat manufacturing is emerging, replacements for deorbited satellites can constantly refresh the constellation.
The Crucial Role of Miniaturization
Fixed satellites and High-Throughput sSatellites (HTS) are much larger than smallsats, so they have much more room for hardware. Smallsats have no such luxury, so they require as much miniaturization as possible. The Software-Defined Radio (SDR) is of significant benefit for smallsats as it allows physical layer functions previously performed by hardware to be defined in software.
Not only does this reduce the size, cost, and complexity of the radio, it enables on-orbit reconfigurability of protocols, access methods, waveforms, and security without changes in hardware. One of the primary drivers of SDR is direct RF sampling, in which the analog input signal is digitized as near as possible to where it enters the receiver, and this has almost completely redefined how transceivers are designed and constructed.
Much of what the SDRs can accomplish is related to their use of FPGAs that allow a wide variety of characteristics to be changed in flight by uploading new settings from the ground. In a CubeSat, where miniaturization and low DC power consumption are essential, the SDR has become the de-facto choice of companies designing and building smallsat communications and other payloads.
That said, all smallsats and CubeSats create an entirely new market for manufacturers of RF and microwave components that did not exist even a few years ago. Although it is too early to accurately determine the full benefits to the industry from the “smallsat revolution,” there is no doubt that its effect will be positive for many years to come.
For example, even though an SDR reduces the number of required analog components, it does not eliminate them because low-noise amplifiers, filters, discrete semiconductors and SoCs, timing devices, and various passive components are still needed, as are the various types of antennas that range from simple monopoles to microstrip patches, and increasingly optically transparent electrically conductive types that mount either above or below the deployed solar panels. As the latter approach takes no space in the satellite and is deployed along with the solar panels, it virtually eliminates the space utilization problem.
The Antenna Factor
All but a few smallsats are in low-Earth orbit and operate at VHF, UHF, and lower microwave frequencies where directional antennas that deliver gain are not needed. Consequently, a smallsat typically uses monopole antennas (Figure 4) and crossed-dipole antennas that have cross-polarization and provide the desired performance even when the spacecraft is tumbling— a benefit for CubeSats that lack accurate pointing control.
However, these antennas must be deployed outward from the satellite, and this requires packaging during launch and a way to deploy them when in orbit. This packaging and deployment hardware adds volume, weight, and complexity to a payload while also posing a potential risk if something goes wrong with the deployment mechanism.
By integrating the antenna into the structure of a CubeSat, all of these issues can be eliminated. One approach replaces sidewalls and railing rods of the CubeSat with RF radiators that double as supporting structures. The radiators are hollow rods with inner dimensions that function as a waveguide to carry RF energy at the desired frequency. Radiating slots are cut on two of the four sides of hollow tubes that are open to the outside environment. Antennas for multiple operating frequencies can be placed at each of the CubeSat’s four corners, and they also structurally support the CubeSat structure. Higher frequency antennas with increased gain and directivity may be embedded into the rails.
Patch antennas are also used in smallsats. They do not need to be deployed as they are integrated within all four walls of the satellite. This design adds the capabilities to control multiple antennas in the satellite faces depending on a spatial signal signature to maximize directivity for any link direction. However, they have invariably consumed space that is needed for the solar panels.
The latest technique to mitigate this problem is to combine these flat antenna structures with the solar arrays either above or below the solar panels. Those above are optically transparent and those mounted under solar panels need not be. Circular polarization is achieved by feeding two 90° rotated patches with a 90° phase offset signal. One of the challenges with the transparent approach is choosing the appropriate materials that allow the most solar energy to be captured without adversely affecting RF performance.
Ideally, they would be completely transparent, and a great deal of research is being conducted to accomplish this, or at least nearly so, through the use of conductive transparent films bonded to the solar panels themselves, and other techniques. A considerable number of variants of this approach have been developed in academia, by manufacturers, and at agencies such as DARPA, and most are directional, providing forward gain.
The Active Electronically Steered Phased Array (AESA) antenna architecture that is already replacing most rotating radar antennas in defense systems is another possibility. However, while AESA antennas have revolutionized applications such as radars and electronic warfare systems, their unique abilities may not be needed for the relatively simple missions performed by smallsats. They are also not inexpensive, require considerable onboard processing capability, and consume DC power, all critical factors for CubeSats. That said, as CubeSats begin to be used at higher frequencies, the AESA architecture and others (Figure 5) will be required to produce the high levels of gain necessary to blast signals through the atmosphere.
CubeSats may be small but their potential market is not. According to analyst reports, there are more than 100 companies developing CubeSat payloads throughout the world as well as government agencies and universities. Companies seeking to deploy CubeSat constellations do not even require expertise in payload buses or their contents because they can rely on manufacturers dedicated to this market to customize them based on standard designs.
So, while 5G and IoT make headlines, the smallsat market is quietly becoming the next big thing for manufacturers of many RF and microwave components, and as this market is just emerging, reaching out to the growing ecosystem of satellite builders can prove lucrative.