Putting the Pieces Together
Software-defined radio technology is key piece of the network-based space communications puzzle
By Michael Moore, Ph.D., Jeremy Price and Ben Abbott, Ph.D.
In January 2004, President George W. Bush outlined a new initiative for manned space exploration. The vision calls for the development of a Crew Exploration Vehicle (CEV). Initially, the CEV will serve as a replacement for the aging space shuttle and will assume the tasks previously performed by the shuttle, such as ferrying astronauts to and from the International Space Station. However, as its name implies, its true purpose is exploration. The CEV will return humans to the surface of the moon and eventually have a role in a manned mission to Mars.
With such a broad job description, the CEV must be designed with substantial flexibility in mind, especially in its communications system. In its travels, the CEV may be required to interact with many different communications systems, each with different waveforms and protocols. Because minimizing size, weight and power are paramount in space systems design, installing separate radios to cover each communications need is less desirable than building a reconfigurable communications system that can be adapted and re-purposed over the lifetime of the vehicle. Besides the CEV itself, supporting systems such as the extra-vehicular activity (EVA) suits worn by astronauts also could benefit from designed-in flexibility in their communications subsystems.
The need for flexibility is not limited to NASA, nor is the need for adaptability exclusive to radios using wireless communications channels. The military needs to be increasingly agile and responsive in launching its tactical satellite missions. To meet evolving needs while making the most of limited budgets, space communication assets must be designed to be operationally reconfigurable to meet as-yet-undefined needs. The speed at which threats evolve may necessitate designing and building a satellite even as details of its mission are being designed.
With these and other needs in view, scientists and engineers at Southwest Research Institute (SwRI) are evaluating and developing key technologies that will enable reconfigurable transceivers and flexible network architectures to be used for applications in space environments.
Reconfigurable communications technology could reduce the total size, weight and power of the overall communications systems designed for the NASA Constellation program. Systems such as the CEV and the advanced EVA suit must have communications subsystems that can work with existing shuttle-era systems while also supporting future mission requirements. One solution is to create reconfigurable network equipment, such as radios, that can communicate with current systems and then be reconfigured to communicate with systems such as deep-space satellite relays and lunar and Martian surface habitats.
The Reconfigurable Network Communications Puzzle
The traditionally separate areas of radio communications and network communications are melding together. All assets are now seen as network-enabled, and thus network-accessible, devices. Creating a seamless network that crosses vehicle boundaries and can be reconfigured to meet changes in the mission or battlespace requires solving many technology problems. The technologies that must be addressed to form a seamless, integrated network are all pieces in a reconfigurable network communications puzzle; the entire puzzle must be put together well for it to operate as a whole.
Reconfigurable transceivers are one part of the puzzle. This can readily be seen both in NASA’s need for communications among Earth, orbiting, en-route, lunar surface and Mars surface assets, and also in the military’s vision for network-centric warfare. Another piece is wireless network waveforms that implement a particular wireless algorithm on a reconfigurable transceiver. A third is architecture standards, which provide common languages and infrastructures on which waveforms can be implemented on reconfigurable transceivers. The puzzle also includes the space vehicle network, which is a wired backplane network that carries data between the components on the vehicle while also interfacing with the wireless network. Configuring, controlling and monitoring the network components and overall performance require network management. Seamless data routing insures that the data are moved across the network components reliably and with the necessary quality of service. Security is a cross-cutting requirement that must be addressed at the network-system level to be effective.
Network design ties all of the other pieces together. Here, designers model the mission requirements, trade off potential designs in terms of costs and benefits and determine the best network design to support the mission. Solving the space network communications puzzle involves addressing all of the aspects of network design to create an integrated solution, including both wired and wireless network channels, data routing, network management and interfaces to network applications and global infrastructure. The cross-disciplinary capabilities at SwRI help the Institute’s team address these challenges.
A transceiver sends and receives digital data that has been encoded in analog radio frequency signals. The method of encoding the digital data into analog signals is referred to as the radio waveform. Traditional, fixed transceivers can only operate with a single waveform. Transceivers that can be reconfigured, or updated with software and firmware to implement other waveforms, are more flexible because they can communicate with more than one radio system.
Enabling the transition from fixed to flexible, network-enabled transceivers is a key technology of software-defined radio (SDR).
Software-defined Radio Technology
Traditional radios are built with mostly fixed analog electronics. With the transition to modern digital communications, digital electronics have become commonplace in radios. To support the high performance required to convert digital data to analog radio signals while keeping cost, size, weight and power requirements realistic, it had been necessary to implement the digital parts of these radios in fixed-capability components such as application-specific integrated circuits (ASIC). Modern digital signal processing technologies, with higher processing capability per power consumed, have made it more reasonable to construct the digital sections of radios with programmable devices. These advances made possible the contemporary field of SDR. The benefit of SDR over fixed-capability digital electronics is that the waveform implementationthe implementation of the algorithm that converts between digital data and analog radio signalscan be independent of the hardware implementation.
SwRI engineers have performed internally funded research, as well as client-funded research on SDR projects, for a number of years. The SwRI SDR team recently completed a project for NASA’s Johnson Space Center (JSC) to create a prototype SDR that interoperates with the existing space-to-space communications system (SSCS).
The project demonstrated the feasibility and effectiveness of SDR technologies for space applications by implementing an SDR that is interoperable with an existing space communications system, and that has a clear path to space-capable hardware. The SSCS was chosen for a prototype application because this waveform may be required on the new NASA crew exploration vehicle. The SSCS SDR prototype was implemented by a team of engineers from SwRI and JSC. The SwRI team implemented the user interface and the encoder/decoder and network controller components, and the JSC team implemented the mod/demod component with integration assistance from the SwRI team. The SwRI and JSC teams worked together to integrate the rate conversion components and to integrate the SSCS SDR. The SSCS SDR has interoperated with the existing SSCS space radios in a JSC laboratory and is an example of how SwRI and external teams can work together to build an integrated system.
Recently the SwRI SDR team began a new project to assist JSC in a trade study to evaluate SDR architectures that could be used to build a reconfigurable transceiver for the Lunar EVA suit. This project may have a bearing on the Constellation communications system design effort by NASA. SwRI researchers will evaluate the costs and benefits of various implementation approaches and assist in forming the design strategy for the EVA suit communications subsystem.
Future Directions for SDR Research
NASA has initiated the SDR architecture team to evaluate the suitability of SDR technology for their manned and unmanned space flight missions and to develop a standard architecture for NASA space radios of the future. This team is defining the space telecommunications radio system (STRS) architecture, which standards bodies are evaluating as a potential standard architecture for space radios. The team is promoting a plan to move space-borne SDR in a flight-experimental phase by deploying an SDR testbed as a scientific payload on the International Space Station and potentially flying other reconfigurable radios on future shuttle flights to execute reconfigurable communications demonstrations in space. SwRI is a likely partner in this work if the plan is executed by NASA, and the waveform developed on the SSCS SDR project is a potential waveform for the space SDR demonstration project.
Questions about this article? Contact Moore at 210-522-5944 or at firstname.lastname@example.org.
Published in the Summer 2007 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.