The communication system is an essential part of a spacecraft, enabling spacecraft to transmit data and telemetry to Earth, receive commands from Earth, and relay information to one another. A device that both receives and transmits is called a transceiver. In contrast, a transponder essentially uses the same technology as a transceiver, but is also capable of providing ranging information, either between spacecraft or with respect to Earth. Spacecraft-to-spacecraft communications is sometimes referred to as an InterSatellite Link (ISL). Traditionally, communication between Earth and spacecraft is based in the radio spectrum (from about 30 MHz to 40 GHz). The different communication bands that are typically used for spacecraft include 1:
- Very High Frequency (VHF): 30 to 300 MHz
- Ultra High Frequency (UHF): 300 MHz to 3 GHz
- L band: 1 to 2 GHz
- S band: 2 to 4 GHz
- C band: 4 to 8 GHz
- X band: 8 to 12 GHz
- Ku band: 12 to 18 GHz
- K band: 18 to 27 GHz
- Ka band: 27 to 40 GHz
- Optical (Laser Communication): 100 to 800 THz
The radio spectrum used for spacecraft communications is also shown graphically in Figure 9.1.
While the use of radio frequency (RF) for communications is still the state of the art at the time of this publication, advances have been made in recent years towards using higher carrier frequencies (which generally results in higher data rates), up into the X- through Ka-bands. Higher data rates are more readily achievable with higher frequencies because data rate is proportional to bandwidth used for communication, and bandwidth is more readily available in the higher frequencies. There is currently significant crowding of the lower RF frequencies, especially in S-band from cell phones 2.
Received signal power will decrease as the transmission distance gets larger, thus larger spacecraft on deep space missions almost always use dish antennas because of their ability to focus radio transmissions into a precise directional beam. Thus spacecraft must be able to point accurately. The large physical size and high pointing requirements of a parabolic dish antenna make such an antenna difficult to integrate with a CubeSat. Developers have sought alternatives, especially as the attitude determination and control of CubeSats gets better (refer to GNC Chapter). For example, an inflatable dish antenna is one proposed solution 3.
Thus far, CubeSats have not operated beyond LEO, and this has allowed them to take advantage of (lower gain) whip or patch antennas in their communication systems. Due to their low directionality, these antennas can generally maintain a communication link even when the spacecraft is tumbling, which is advantageous for CubeSats lacking accurate pointing control. Whip or tape antennas, such as the one shown in Figure 9‑2, are easily deployable from a CubeSat and are generally used for VHF and UHF communications. Patch antennas, such as the one shown in Figure 9‑3, are small and robust and do not require deployment. They are generally used from UHF through S-band on CubeSats, and are being explored for use in X-band arrays on CubeSats 4, and beyond. A key advantage of higher frequency (especially for CubeSats) is that antenna aperture decreases but gain remains similar. This is advantageous for ground systems too. One major disadvantage is that higher frequencies get readily absorbed by the atmosphere. In the Ka-band, water droplets heavily attenuate the signal, resulting in “rain fade” so greater transmitting power is required to close the link. However, this does not present a problem for intersatellite links, which do not pass through the atmosphere.
Another trend that aids in the improvement of RF based communication systems is the development of software defined radio (SDR). By using Field Programmable Gate Arrays (FPGAs), SDRs have great flexibility that allows them to be used with multiple bands, filtering and modulation schemes, without much (if any) change to hardware 2. Furthermore, such characteristics can be changed in-flight by uploading new settings from the ground. SDRs are especially attractive for use on CubeSats as they can be made increasingly small and efficient as electronics become smaller and require less power (see Figure 9‑4). Since 2012, NASA has been operating the Space Communications and Navigation (SCaN) Testbed on the International Space Station, which was created for the purpose of SDR TRL advancement, among other things 5.
Laser based communication (“lasercom”) has already been demonstrated with larger spacecraft such as LADEE 6. The era for lasercom on cubesats is just beginning, with technology demonstration missions planned for 2015 and 2016.
In the following sections, TRL 6+ technology that is relevant to the cubesat form factor is listed in tables organized by operating frequency.
The author would like to highlight that the presented tables are not intended to be exhaustive but to provide an overview of current state of the art technologies and their development status for this small spacecraft subsystem.
State of the Art
VHF and UHF
VHF and UHF frequencies are mature bands used for cubesats communication, with several radio developers to choose from. TRL~7 and higher technologies are listed in Table 9.1. Note that Clyde Space’s VUTRX transceiver was developed by F’SATI (the French South African Institute of Technology) at CPUT (Cape Peninsula University of Technology) 7. More information on BitBeam radios can be found in 8, while more information on L3 Communications’ Cadet Radio can be found in 9.
|Table 9-1: Developers and Products for use in VHF/UHF|
|Lithium-1||Astronautical Development LLC||9|
|CSK Phasing Board||Astronautical Development LLC||9|
|VUTRX||Clyde Space Ltd.||9|
|UHF Transceiver Type II||Endurosat||9|
|ETT-01EBA102-00||Emhiser Research, Inc.||9|
|NanoCom AX100||GOMSpace ApS||8|
|NanoCom ANT430||GOMSpace ApS||9|
|NanoCom SDR||GOMSpace ApS||7|
|P/N 17100||Haigh-Farr, Inc.||9|
|Helios Deployable Antenna||Helical Communications Technologies||6|
|Deployable Antenna System for CubeSats||ISIS B.V.||9|
|Cadet||L3 Communications, Inc.||9|
Typically, a small patch antenna (see Figure 9‑6) or whip antenna is used to transmit VHF and UHF. Aside from the TRL 9 antennas listed in Table 9.1, other deployable, higher gain antennas (Figure 9‑5 are being developed, including a TRL 6 deployable quadrifilar helical UHF through S-band antenna by Helical Communication Technologies (HCT), and a deployable helical UHF antenna by Northrop Grumman Aerospace System 10.
Endurosat has developed a UHF antenna at 435 – 438 MHz that are compatible with Endurosat Z solar panels. This antenna has a total mass of 0.105 kg and were flown on Edurosat-1 (launched May 2018).
In L-band, CubeSats can take advantage of legacy space communications networks such as GlobalStar and Iridium by using network specific transponders to relay information to and from Earth. An additional advantage is that these networks remove dependence on dedicated ground station equipment, as discussed further in the ground support equipment (GSE) section.
Examples of network-specific transponders are shown in Table 9.2. NearSpace Launch’s EyeStar-D2 Satellite Duplex radio has flight heritage as of 2015, but no large file transfer was possible during the flight due to an unplanned 2~rpm spin rate 11. Since then, NearSpace Launch has successfully operated EyeStar-D2 on AFRL’s SHARC. Also, sci_Zone, Inc. is developing its next generation of simplex radio, STX3, as well as a duplex radio, and both will use the Globalstar constellation 1. The multi-band HCT quadrifilar helical antenna mentioned earlier can also operate in L-band.
|Table 9-2: Developers and Products for use in L-band|
|Helios Deployable Antenna||Helical Communications Technologies||6|
|9602 SBD||Iridium Communications, Inc.||9|
|EyeStar-S2||NearSpace Launch, Inc. (NSL)||9|
|EyeStar-D2||NearSpace Launch, Inc. (NSL)||9|
|Antenna SYN7391-B (Iridium)||NAL Research Corporation||Unkn.|
|STX2 Simplex||sci_Zone, Inc.||9|
Examples of TRL 7+ S-band communication technology are shown in Table 9.3. A CubeSat compatible S-band transmitter is shown in Figure 9‑7. Note that the Clyde Space products SANT and STX were developed by F’SATI at CPUT. Haigh-Farr’s S-band antennas are scheduled to fly on the CPOD 3U CubeSat mission, which is planned to launch in mid-2018.
|Table 9-3: Manufacturers and Products for use in S-band|
|Beryllium 2||Astronautical Development LLC||9|
|SANT||Clyde Space Ltd.||9|
|STX||Clyde Space Ltd.||9|
|S-band Patch Antenna||Endurosat||9|
|Helios Deployable Antenna||Helical Communications Technologies||6|
|HISPICO||IQ Wireless GmbH||9|
|SLINK-PHY||IQ Wireless GmbH||8|
|TXS||ISIS B. V.||8|
|S-Band Patch Antenna||Surrey Satellite Technology||9|
|CSR-SDR-S/S||Vulcan Wireless, Inc.||9|
Regarding lower TRL technology, L3 Communications’ Cadet Nanosat Radio (see Table 9.1) is also configurable to be used in S-band, although this has not been demonstrated at the time of publication. LJT & Associates have developed the LCT2-b, an S-band transponder to work with the Tracking and Data Relay Satellite System (TDRSS). The LCT2-b S-band BPSK TDRSS transmitter has already flown on the SOAREX-VI flight experiment 13. Similarly, Surrey Satellite Technology US LLC developed an S-band quadrifilar antenna, S-band downlink transmitter, and S-band receiver with flight heritage on spacecraft that are less than 180 kg in mass, though to the knowledge of the author they have not flown on a CubeSat mission. Haigh-Farr also offers high TRL technology for S-band communications.
Many antennas are available in S-band, including a stacked patch S-band antenna being developed by NewSpace Systems and the HCT quadrifilar helical antenna mentioned in the VHF and UHF section. AntDevCo, IQ Wireless, Surrey Satellite Technology and many others make S-band patch antennas that could be compatible with CubeSats. ISIS B.V. resells the S-band patch antenna, and transmitter and receiver for IQ Wireless’ HISPICO communication system. Syrlinks is a strong competitor in the European market and also offers patch antennas in the S- and X-bands, among many other high TRL products.
The unlicensed ISM (Industrial, Scientific, and Medical) bands have been utilized for CubeSat communications as well. Notably, a group at Singapore’s Nanyang Technological University used a 2.4-GHz ZigBee radio on its VELOX-I mission to demonstrate that COTS land-based wireless systems can be used for inter-CubeSat communication 14. . Similarly, there are investigations underway for using wireless COTS products, such as Bluetooth-compatible hardware, for intra-satellite communications 15.
Furthermore, companies that traditionally design communications for larger spacecraft are now modifying some of their products for use on smaller spacecraft. One example is the COM DEV S-band transceiver 16. COM DEV was acquired by Honeywell in 2016, but many legacy COM DEV products are still available in their Honeywell incarnation.
X-band transmitters (such as that in Figure 9‑8) have recently become a reality for CubeSats because of the advent of commercially available Monolithic Microwave Integrated Circuits (MMICs). There has been much effort recently from industry, universities and government centers alike to develop communications systems at this wavelength 17.
|Table 9-4: Manufacturers and Products for use in X-band|
|Evolved X-band wire antennas||Antenna Development Corporation, Inc. (AnyDevCo)||9|
|Quadrifilar Helix Antenna||Antenna Development Corporation, Inc. (AnyDevCo)||9|
|X-band Patch Antenna||Endurosat||9|
|XLINK||IQ Wireless GmbH||9|
Table 9.4 displays TRL 9 CubeSat compatible X-band communication hardware. Note that AntDevCo’s “evolved” wire antennas were designed using X5 Systems’ AntSyn (Antenna Synthesis) software. The corresponding flight heritage (ST5 mission) is not of the CubeSat form factor, but each of the five spacecraft still fit into the small satellite category with a mass of 25 kg. AntDevCo also develops X-band patch antennas. It should also be noted that Planet Labs uses a proprietary X-band radio 18.
Surrey Satellite Technology developed a high-gain X-band antenna and corresponding pointing mechanism (see Figure 9.8), and an X-band transmitter that have flight heritage on spacecraft less than 180 kg in mass, though to the knowledge of the author have not flown on a CubeSat mission.
JPL has also developed a CubeSat compatible transponder, IRIS V2, suitable for deep space communications in X-, Ka-, S-bands, and UHF 19, while CU Boulder and Goddard Space Flight Center jointly developed an X-band SDR that is now being sold by Blue Canyon Technologies 4. Figure 9‑9. The license to design IRIS radios has been given to Space Dynamics Laboratory and future iterations will be delivered by SDL. CU Boulder and Goddard Space Flight Center jointly developed an X-band SDR that is now being sold by Blue Canyon Technologies 2. Lower TRL technologies include an X-band transmitter from NewSpace Systems. A team from Utah State University is working on an X-band antenna array that is integrated with solar panels, a novel idea that could greatly save space 3.
Laser communication for CubeSats has yet to become a TRL 9 technology, but it is quickly maturing and has been successfully demonstrated. Already, Aerospace Corporation, in cooperation with NASA Ames, has launched all three CubeSats in its AeroCube Optical Communication and Sensor Demonstration, see Figure 9‑10. In March 2018 a systems checkout was completed and the mission has entered the operational phase. Once the demonstration is deemed successful, the AeroCube’s optical communication technology will be placed at TRL7.
Fibertek launched a 6U lasercom system in 2018 as part of the NASA Ames Small Business Innovation Research program (SBIR), and are continuing to make substantial progress in the area of lasercom and lidar technologies. Sinclair Interplanetary is developing the DCL-17 (TRL 5), a self-contained optical communications terminal that incorporates a built-in star tracker and 1 Gbps laser downlink. Future lasercom endeavors include the NASA-sponsored Miniature Optical Communication Transmitter on October 8, 2015 (see Figure 9.9 ). Also, Fibertek is working on a (currently TRL 6) 6U lasercom system. For both of these ventures, lasers are hosted onboard the cubesat(s). A lower TRL lasercom concept involves an asymmetric optical link, whereby the laser hardware is on Earth and a modulating retroreflector is on the spacecraft (refer to the Asymmetric Lasercom section) 4.
Many other international entities are advancing in the area of CubeSat laser communications as well. The German Aerospace Center (DLR) is currently flying two lasercom terminals as part of its OSIRIS program. The Small Optical Transponder (SOTA) developed by the National Institute of Information and Communications Technology in Japan (NICT) has successfully demonstrated a laser space-ground link from a 50-kg microsatellite 5. The CubeL, a laser communication terminal for CubeSats designed by Tesat-Spacecom, is on track for implementation after passing the Critical Design Review in April 2018.
For all of these ventures, lasers are hosted onboard the spacecraft, but a lower TRL lasercom concept involves an asymmetric optical link, whereby the laser hardware is on Earth and a modulating retroreflector is on the spacecraft (refer to the Asymmetric Lasercom section).
Ku- to Ka-band
Ku-, K-, and Ka-band communication systems are the state of the art for large spacecraft, especially in spacecraft-to-spacecraft communications, but they are still young technologies in the CubeSat world. Developers working on CubeSat compatible Ka-band communication systems include Astro Digital, Micro Aerospace Solutions, NewSpace Systems and Tethers Unlimited.
Astro Digital, formerly known as Aquila Space, has already launched Landmapper-HD 1, a 20-kg 16U microsatellite that’s the first in a constellation of 20 imaging satellites. It has a 300 Mbps Ka-band downlink transmitter shown in Figure 9 11. The Landmapper-BC is the predecessor to the Landmapper-HD constellation and unfortunately lost four satellites to launch damage. Landmapper-BC 3 v2 was launched in January 2018, weighs 1-kg, and boasts a 320 Mbps Ka-band data rate. The next generation of Ka-band transmitter from Astro Digital will increase the data rate to 800 Mbps. A Ka-band transmitter is shown in Figure 9.11. Micro Aerospace Solutions has a TRL~5 Ku/Ka-band transceiver with deployable 60 cm CubeSat dish antenna 23. Tethers Unlimited has a TRL~5 K-band SDR called SWIFT-KTX.
|Table 9-5: Manufacturers and Products for use in Ka- to Ku-band|
|Ku-band Transceiver||NewSpace Systems||6|
|1 Gbit Transponder||NewSpace Systems||6|
|SDR Transceiver||NewSpace Systems||6|
At the higher frequencies, rain fade becomes a significant problem for communications between a spacecraft and Earth 24. Nonetheless, the benefits have justified further research by both industry and government alike. At the Jet Propulsion Laboratory (JPL), the ISARA mission (Integrated Solar Array and Reflectarray Antenna) demonstrates a high bandwidth Ka-band CubeSat communications capability that allows over 100 Mbps downlink rate 6. Essentially, the back of the 3U CubeSat is fitted with a high gain reflectarray antenna that is integrated into an existing solar array. The ISARA technology is currently in orbit and has recently completed a systems checkout. It will be elevated to TRL 7 following successful demonstration.
On the Horizon
Asymmetric Laser Communications
Spacecraft parameters like power, mass, and volume are constrained by cost and current capability. Ground operations, on the other hand, are not subject to the same limitations. Asymmetric laser communications leverages this imbalance. Asymmetric laser communication utilizes a remotely generated laser (i.e. does not require an on-board signal carrier) and modulating retroreflector (MRR) to reflect and modulate a laser beam (encoding it with spacecraft data) back to Earth (see Figure 9‑12). The laser is located on Earth, where power and volume constraints are not as tight, while the communications payload on the spacecraft is limited to only a few Watts for operation. SPAWAR is developing this technology using a MEMS-based MRR 26, while NASA Ames Research Center is developing a similar capability using a modulating quantum well (MQW) device as the MRR 27.
When deployable solar panels are not an option, a CubeSat’s surface is prime real estate for solar cells. One way to maximize exposed surface area on a CubeSat is to create communications antennas that are optically transparent. Groups at the University of Houston 28 and Utah State University 29 have developed prototypes of these small, optically transparent antennas. Owing to progress from MMA Design, deployable antennas may become common in the CubeSat world. They are developing a revolutionary deployable antenna providing extremely high areal compaction and combining the positive attributes of currently available CubeSat antennas. They predict that the deployable antenna will enable performance for smallsats consistent with today’s large spacecraft 7. A similar design is seen at NASA’s Marshall Space Flight Center. The Lightweight Integrated Solar Array and Transceiver (LISA-T) is a deployable array on which thin-film photovoltaic and antenna elements are embedded 8.
Intercubesat Communications and Operations
There are multiple advantages to communicating between spacecraft. As CubeSat missions become more automated, constellations could exchange information to maintain precise positions without input from the ground. Data can be relayed between spacecraft to increase the coverage from limited ground stations. Finally, inter-CubeSat transponders may very well become a vital element of eventual deep space missions, since CubeSats are typically limited in broadcasting power due to their small size and may be better suited to relay information to Earth via a larger, more powerful mothership.
Though transponders are well established in the spacecraft world, networked swarms of CubeSats that pass information amongst each other and then eventually to ground have yet to be demonstrated. Developing networked swarms is less of a hardware engineering problem than a systems and software engineering problem, as demonstrated by NASA Ames Research Center’s Edison Demonstration of Smallsat Networks (EDSN) mission 32, see Figure 9.12. Unfortunately, the eight small satellites that comprise the EDSN mission were lost due to launch failure. Ames’ follow up, the two 1.5 U Network & Operations Demonstration Satellites (Nodes), deployed from the International Space Station in June 2016. The Nodes mission will be an opportunity to complete some of the tasks set forth in the EDSN mission. Similarly, the CubeSat Proximity Operations Demonstration (CPOD) mission “will demonstrate rendezvous, proximity operations and docking using two 3U CubeSats”33 and is led by Tyvak NanoSatellite Systems, Inc.
Engineers from NASA Marshall Space Flight Center are also developing inter-CubeSat communication using a peer-to-peer topology. The mesh network architecture is designed to allow for the exchange of telemetry and other data between spacecraft with no central router 9. Clyde Space is in the early stages of its ambitious project called the Outernet, a low-cost, mass-producible constellation of 1U CubeSats that will provide a near continuous broadcast of humanitarian data to those in need 10.
There is already strong flight heritage for many UHF/VHF and S-band communication systems for CubeSats. Less common but with growing flight heritage are X-band systems. The use of even higher RF frequencies and laser communication already has some flight heritage on CubeSats, but with limited (or yet to be demonstrated) performance. Ka-band systems for CubeSats are currently in development, but TRL status is still relatively low. On the other hand, laser communication is a spaceflight ready technology that will most likely see increased performance in the near future for onboard laser systems. Alternatively, a few groups are working on asymmetric laser communication, but it is still a relatively low TRL technology.