The small satellite is one of the fast growing sectors in space industries. Small satellites usually refer to satellites below 500 kg, including minisatellite (100–500 kg), microsatellite (10–100 kg), nanosatellite (1–10 kg), picosatellite (0.1–1 kg), and femtosatellite (<0.1 kg).
Cube Satellites, aka CubeSats, are a class of nano satellites that have gained popularity recently, especially for those that consider CubeSats as an emerging alternative to conventional satellites for space programs. This is because they are cost-effective, and they can be built using commercial off the-shelf components. Moreover, CubeSats can communicate with each other in space and ground stations to carry out many functions such as remote sensing (e.g., land imaging, education), space research, wide-area measurements, and deep-space communications.
The antenna is one of the key components onboard small satellites as its design determines the performance of all the wireless systems including telemetry, tracking, and control, high-speed data downlink, navigation, intersatellite communications, intrasatellite communications, wireless power transfer, radars, and sensors, etc.
CubeSats are constrained by limited space available for each subsystem, available DC power, and the non-availability of sufficiently large RF aperture for communication and radar payloads. Their mass ranges from 1 to 6 kg and are low power of few watts. Antennas for cubesats is a challenge as the antenna needs to meet several restrictions related to the CubeSat’s size, i.e., ≤10 × 10 × 10 cm3 , weight, i.e., ≤1.3 kg, and power, i.e., ≤2 W while yielding high gain and wide bandwidth.
Moreover, CubeSats have also been considered for deep space missions. While LEO spacecraft may have maximum communication range of only 2,000 kilometers, Cubesats for deep space missions must support at least a 2 million km link back to earth. Since CubeSat RF output power resources are limited, e.g. 5 watt RF, a higher gain antenna is needed to compensate for the factor of 1000 increase in range.
Many antenna types with different operating frequency bands are proposed for CubeSat applications. Some of the antenna designs include patch antennas, slot antennas, dipole and monopole antennas, reflector antennas,
reflectarray antennas, helical antennas, metasurface antennas and 3 millimeter and sub-millimeter wave
antennas. In addition, antennas are also classified according to their operating frequency bands, e.g., VHF,
UHF, L, S, C, X, Ku, K/Ka, W and mm/sub-mm wave bands.
Planar antennas such as patch and slot antennas are easy to fabricate, have low profile, low cost and easy to integrate
with other Radio Frequency (RF) and microwave circuits. These features make them ideal for CubeSats addressing most
of the challenges and constraints of CubeSat. In addition, as compared to deployable antennas, i.e., helical and reflector
antennas, planar antennas occupy smaller real estate and do not require deployment. This is important as it provides more space on a CubeSat for solar cells and decreases the probability of deployment failure.
Two types of proposed planar antenna designs (e.g., patch and slot) have been proposed for CubeSats. Proposed slot antenna designs for CubeSats are very limited because they provide linear polarization and have low directivity which results in weak signal strength and low gain.
The main limitation of existing antenna designs such as dipole and patch antennas that are used for LEO CubeSats is
their low gains which makes them unsuitable for deep space communications. In addition, moving from LEO to deep
space communications requires Ka-band or X-band antenna designs that can provide high gains of 42 and 30 dBi respectively
Recently, designs of patch antenna arrays that consist of many sub-array elements and are fed by different feeding networks are proposed to enhance the antenna gain and to electronically steer the antenna’s radiation beam. This is important as it maintains the communication link during the CubeSat’s maneuver. The challenge is how to achieve a
superior gain by implementing small patch antenna arrays on limited space on CubeSat.
Reflector based antennas
Reflector antennas have a large profile and can provide gains higher than 30 dBi at operating frequency ranging from
0.3 to 300 GHz. Recently, reflector antennas have received considerable attention for higher orbits and deep space CubeSat applications at orbits above LEO and in deep space due to their superior gains which can provide long distance communications. The main challenge is the large size of the reflector antennas making them hard to be integrated
on the limited CubeSat volume. To address this challenge, all the reflector-based antenna designs require a deployment mechanism.
The first reflector antenna was proposed for deep space missions operating at Ka-band. The authors used an unfurlable meshed reflector with 32 ribs. The proposed reflector antenna consists of a feed horn, four struts, a hyperbolical reflector, and 0.5m deployable mesh reflector. It occupies a size of 100mm × 100mm × 150mm when it is folded. As soon as the CubeSat reaches the specified orbit, the 0.5m mesh reflector deploys. The proposed antenna design achieved an efficient of 60% and a superior gain of 42.8 dBi at an operating frequency of 34 GHz (Ka-band). . Its main limitation, however, is its complex deployment mechanism which increases the probability of deployment failure and hence failure of the whole mission.
Inflatable antennas were developed and comprehensively tested at S-band and X-band for Deep space communication. Additional work was also reported by another team at W-band. Although the spherical surface aberration can be compensated by adjusting the feed location or using a corrective lens, it is unlikely that the surface accuracy can be maintained at frequencies above S-band.
Metasurface antennas could potentially also be a good solution for high gain antennas. They provide the ability to
deploy a large aperture antenna without deploying a feed at a focal distance from the antenna aperture. Feed mechanics and geometry is often the biggest challenge as antenna aperture increases and in particular for deployable antennas.
Recently, a variety of deployable antenna technologies have been developed to address this need. Deployable reflectors enable an antenna to be compact in stowed configuration and become fully deployed in orbit. With an increasing demand for large-aperture (hundreds of square meters or more) space-borne antennas, deployable membrane antennas have been attracting interest in space research areas. In comparison with traditional rigid antennas, membrane antennas can easily achieve larger scale with lighter weight, smaller stowage volume, and lower cost.
Membrane antennas were highly investigated by John Huang at the Jet propulsion Laboratory for small satellites as
they allow achieving large aperture with excellent stowage volume. Membrane antennas can be patch arrays or
reflectarrays and are a natural option for CubeSats.
At present, there are two main kinds of space-borne membrane antenna structures: parabolic and planar membrane antenna structures. These membrane antenna structures generally involve a membrane surface, support structures, and a tensioning system.
Parabolic membrane antennas
Parabolic membrane antennas are folded or wrapped when in stowed condition. When the spacecraft reaches its orbit, the antenna is deployed to the desired reflector surface according to flight procedures. At present, there are five methods for deploying and forming large parabolic membrane antennas, which are inflation, inflation-rigidization, elastic ribs driven, Shape Memory Polymer (SMP)-inflation, and electrostatic forming
Folded panel reflectarray (FPR)
For these applications, high-gain antennas on-board the CubeSats are needed and deployable reflectarrays are believed to offer excellent performance along with significant size and cost reductions.
In 1996, John Huang introduced the idea of using deployable reflectarray composed of flat panels that could also potentially be combined with solar cells in the back of the reflectarray. This concept takes advantage of flat reflecting
surface relying on a simple mechanical deployment with spring-loaded hinges. His concept was implemented for the first
time for the technology demonstration CubeSat ISARA (Integrated Solar Array & Reflectarray Antenna)
Folded panel reflectarray (FPR) technology provides another way to realize a deployable high gain antenna. A reflectarray antenna consists of a special reflecting surface along with an illuminating feed. The reflecting surface comprises an array of phase control elements, such as microstrip patches, printed on a circuit board using standard photo etching processes.
The phase control elements are adjusted to collimate the reflected feed illumination, much as a parabolic reflector would. However, unlike a parabolic reflector, the reflectarray panels are flat, which permits them to be folded and stacked for compact stowage.
FPR antennas offer several notable advantages compared to deployable parabolic reflectors, including stowage efficiency, beam pointing and beam shaping flexibility, rapid development, and lower cost. Further, the printed circuit board construction readily accommodates solar cells, enabling integration of the antenna with solar array panels, either on the back side as done for ISARA or on the reflectarray side by using optically transparent reflectarray elements. However, FPR antennas are narrow band devices (typically a few percent bandwidth) and the aperture size is limited by the practical number of folds.
Tensioned membrane inflatable reflectarray
A tensioned membrane inflatable reflectarray offers an alternative antenna architecture that permits the use of a flat, instead of a curved, antenna surface . This antenna concept uses two thin Kapton membranes which are pulled flat by a perimeter truss structure, similar to a drum head.
Usually the membrane constitutes the substrate sustaining the antenna elements and their feeding network, typically a patch array with microstrip lines. The limitations of this approach are generally high losses, sophisticated feeding network designs and, thereby, low reliability and high cost.
In a reflectarray, each element accomplishes the task of compensating for the different phase delays of the field impinging on the different points of the planar reflecting surface, in order to obtain contributions to the re-irradiated field that are all in phase in a given direction. The membrane is constituted by a Kapton foil. This material is particulary suited for spaceborne applications and can be easily metallized.
ESA selected TICRA to develop a deployable reflectarray for CubeSat applications with In-Orbit Demonstration. The objective of this activity is to develop a deployable passive reflectarray for CubeSats that is targeted for In-Orbit Demonstration. This development will improve the European capabilities within reflectarrays and is an important step to have a European reflectarray in orbit. TICRA will be responsible for all radio frequency (RF) related tasks in this activity. This includes the design and analysis of the reflectarray and its associated feed as well as the RF testing of the entire antenna. Once the project is complete, TICRA will be one of the few companies in the world able to provide complete reflectarray solutions for CubeSat applications.
Reflectarray Antenna demonstrations for High Bandwidth Cubesats
In 2013, The Integrated Solar Array and Reflectarray Antenna (ISARA) mission successfully demonstrated a reflectarray antenna that increases downlink data rates for CubeSats from the existing baseline rate of 9.6 kilobits per second (kbps) to more than 100 megabits per second (Mbps).
The reflectarray antenna consists of three panels, electrically tied together through hinges, which have an array of printed circuit board resonant patches on them. The size of the patches are adjusted so that the phase of the reflected feed illumination collimates the radiation in much the same way a parabolic dish re ector would. Unlike a parabolic dish, however, the reflectarray panels are at, which enables them to be folded down against the CubeSat. On the opposite side of the printed reflectarray antenna, solar cells have been added. This makes the overall antenna/solar array panel assembly slightly thicker, but the panels are stowed in the “dead space” between the launch rails that would have otherwise been left empty. This combination of antenna and solar cells makes for a very efficient use of CubeSat volume, leaving plenty of room for payloads such as science instruments or imaging systems.
The ISARA technology has been validated in space during a five-month mission that measured key reflectarray antenna characteristics, including how much power can actually be focused on a small, low cost ground station. To start the in-orbit test, ISARA successfully deployed its reflectarray antenna, as can be seen in a remarkable sheye lens photo taken after deployment. The spacecraft then used its high precision attitude determination and control system to stabilize itself and the onboard Global Positioning System (GPS) receiver to obtain accurate orbital location information. An Ultra High Frequency (UHF) communications system provided satellite command and control.
During the in-orbit test, ISARA’s reflectarray antenna transmitted a Ka-band signal to a ground station located at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. The spacecraft’s onboard location and orientation data was used to point the antenna beam toward the ground station. The received signal confirmed the antenna gain, which is a key measure of how efficiently the antenna focuses power. The in-orbit gain data was found to match pre-flight ground measurements, proving that the antenna works as designed in space.
ISARA is the first reflectarray in space. It demonstrates a gain of 33.0dBic at 26GHz for Low Earth Orbit communication, which translates into an efficiency of 26%. It suffers from a low efficiency feed and large gaps and hinges, resulting in an increase of the side lobe level and reduced gain.
With the success of the ISARA mission, the reflectarray antenna technology was now available for use on other missions that need high bandwidth telecommunications. The ISARA technology enables CubeSats and other small satellites to serve as viable platforms for performing missions that were previously only possible on larger and more costly satellites. For a modest increase in mass, volume and cost, the high data rate this technology enables will pave the way for high value science missions and formation flying missions that use distributed CubeSats and small satellites.
The first major application of ISARA technology was MarCO, the first interplanetary CubeSat mission, in which two briefcase-sized CubeSats flew to Mars alongside the InSight spacecraft in November 2018 (jpl.nasa.gov/cubesat/missions/marco.php).
This work was extended to an X-band telecommunication system using a reflectarray deployed from the 6U CubeSat,
launched as a secondary payload with the NASA InSight Mars lander mission to provide auxiliary telecommunications during the EDL portion of that mission.
The MarCO mission was made possible by the folded panel reflectarray technology developed on ISARA. ISARA technology provided the only antenna capable of meeting two difficult MarCO requirements: stowage of the 60 cm x 30 cm antenna into a thin 1.25 cm slice to allow space for the other MarCO subsystems, and the ability for an antenna to scan 23° away from the spacecraft to provide for a direct communications line to Earth.
The transmit-only reflectarray demonstrates a gain of 29.2dBic (i.e. 42% efficiency). Higher efficiency was achieved by (1) removing the gaps between the panels, (2) using low profile hinges. (3) improving significantly the feed efficiency. MarCO near-real time bent pipe communication (i.e. 8kbps) at Mars distance (~156 million km) would have not been possible without this X-band deployable reflectarray as the Iris radio solid state power amplifier (SSPA) is limited to 5W. The antenna gain of MarCO reflectarray performed within 0.4dB of its design value during Insight EDL and provided flawless near-real-time coverage.
To achieve a smaller beamwidth for remote sensing science applications, a deployable reflectarray antenna compatible with 6U-class CubeSat was developed; it is currently the largest Ka-band cubesat-compatible antenna. While this antenna was designed primarily for Earth Science remote sensing, it can easily be redesigned for Deep Space communication. The Ka band high gain reflectarray antenna employs Cassegrainian optics to accommodate a deployment mechanism that stows the reflectarray panels and feed assembly into a highly constrained volume. Despite stringent Ka-band small
wavelength mechanical constraints, the linearly-polarized antenna demonstrates excellent performance at 35.75GHz with
a gain of 47.4 dBi.
DARPA Prototype Reflectarray Antenna Offers High Performance in Small Package
DARPA’s Radio Frequency Risk Reduction Deployment Demonstration (R3D2) was launched by Rocketlab in March 2019 to space-qualify a new type of membrane reflectarray antenna. The antenna, made of a tissue-thin Kapton membrane, packs tightly for stowage during launch and then will deploy to its full size of 2.25 meters in diameter once it reaches low Earth orbit.
Small satellite launcher Rocket Lab successfully pulled off its first flight of the year out of New Zealand in March 2019, sending an experimental communications satellite into orbit for DARPA. Dubbed the R3D2 mission, the flight sent up a small, 330-pound satellite into orbit, designed to test out a new kind of radio antenna. Made out of a type of material known as Kapton, the antenna is as thin as tissue paper, but able to grow in size while in space. The antenna launched on the rocket folded up inside a canister, and now that it’s in orbit, the antenna will unfurl and expand out into its full shape that’s more than seven feet wide. The design should provide more area to reflect radio waves.
R3D2 will monitor antenna deployment dynamics, survivability and radio frequency (RF) characteristics of a membrane antenna in low-Earth orbit. The antenna could enable multiple missions that currently require large satellites, to include high data rate communications to disadvantaged users on the ground. A successful demonstration also will help prove out a smaller, faster-to-launch and lower cost capability, allowing the Department of Defense, as well as other users, to make the most of the new commercial market for small, inexpensive launch vehicles. Satellite design, development, and launch took approximately 18 months.
“The Department of Defense has prioritized rapid acquisition of small satellite and launch capabilities. By relying on commercial acquisition practices, DARPA streamlined the R3D2 mission from conception through launch services acquisition,” said Fred Kennedy, director of DARPA’s Tactical Technology Office. “This mission could help validate emerging concepts for a resilient sensor and data transport layer in low Earth orbit – a capability that does not exist today, but one which could revolutionize global communications by laying the groundwork for a space-based internet.”
The launch took place on a Rocket Lab USA Electron rocket from the company’s launch complex on the Mahia Peninsula of New Zealand. Northrop Grumman is the prime contractor and integrated the 150 kg satellite; MMA Design designed and built the antenna. Trident Systems designed and built R3D2’s software-defined radio, while Blue Canyon Technologies provided the spacecraft bus.
“The mission could help validate emerging concepts for a resilient sensor and data transport layer in low Earth orbit, a capability that does not exist today, but one which could revolutionize global communications by laying the groundwork for a space-based internet,” Beck said.