Sandra K. Johnson

Open Architecture Standard for NASA's Software-Defined Space Telecommunications Radio Systems

NASA is developing an architecture standard for software-defined radios used in space- and ground-based platforms to enable commonality among radio developments to enhance capability and services while reducing mission and programmatic risk. Transceivers (or transponders) with functionality primarily defined in software (e.g., firmware) have the ability to change their functional behavior through software alone. This radio architecture standard offers value by employing common waveform software interfaces, method of instantiation, operation, and testing among different compliant hardware and software products. These common interfaces within the architecture abstract application software from the underlying hardware to enable technology insertion independently at either the software or hardware layer. This paper presents the initial Space Telecommunications Radio System architecture for NASA missions to provide the desired software abstraction and flexibility while minimizing the resources necessary to support the architecture.

CoNNeCT's approach for the development of three Software Defined Radios for space application

National Aeronautics and Space Administration (NASA) is developing an on-orbit, adaptable, Software Defined Radios (SDR)/Space Telecommunications Radio System (STRS)-based testbed facility to conduct a suite of experiments to advance technologies, reduce risk, and enable future mission capabilities. The flight system, referred to as the “SCAN Testbed” will be launched on an HTV-3 no earlier than May of 2012 and will operate on an external pallet on the truss of the International Space Station (ISS) for up to five years. The Communications, Navigation, and Networking reConfigurable Testbed (CoNNeCT) Project, developing the SCAN Testbed, will provide NASA, industry, other Government agencies, and academic partners the opportunity to develop and field communications, navigation, and networking applications in the laboratory and space environment based on reconfigurable, software defined radio platforms and the Space Telecommunications Radio System (STRS) Architecture. Three flight qualified SDRs platforms were developed, each with verified waveforms that are compatible with NASAs Tracking and Data Relay Satellite System (TDRSS). The waveforms and the Operating Environment are compliant with NASAs software defined radio standard architecture, STRS. Each of the three flight model (FM) SDRs has a corresponding breadboard and engineering model (EM) with lower fidelity than the corresponding flight unit. Procuring, developing, and testing SDRs differs from the traditional hardware-based radio approach. Methods to develop hardware platforms need to be tailored to accommodate a “software” application that provides functions traditionally performed in hardware. To accommodate upgrades, the platform must be specified with assumptions for broader application but still be testable and not exceed Size, Weight, and Power (SWaP) expectations. Ideally, the applications (waveforms) operating on the platform should be specified separately to accommodate portability to other platforms and support multiple entities developing the platform from the application. To support future flight upgrades to the flight SDRs, development and verification platforms are necessary in addition to the flight system. This paper provides details on the approach used to procure and develop the SDR systems for CoNNeCT and provide suggestions for similar developments. Unique development approaches for each SDR were used which provides a rare opportunity to compare approaches and provide recommendations for future space missions considering the use of an SDR. Three case studies were examined. In two cases, the SDR vendor (General Dynamics and Harris) was the integrated platform and waveform provider. In these cases, the platform and waveform requirements were considered together by the vendor using high level analysis to support the division of the requirements. In the Harris SDR case, the platform and waveform specification was then integrated into a single document. This case study was for a first generation platform, which offers significant processing and reconfigurablility, but is not optimized for SWaP. This provides a test bed platform for many investigations of future capabilities, but requires additional SWaP than optimized flight radios. In the GD case, the specifications were provided separately. The GD SDR leverages existing platforms with minor changes to the Radio Frequency (RF) portions. The most significant change to the CoNNeCT GD SDR from previous platforms was the addition of a reconfigurable processor. The capability tests the next generation SDR, but offers limited capacity and reconfigurability. In the case of the JPL SDR, the platform was developed by JPL and Cincinnati Electronics. Goddard Space Flight Center (GSFC) provided a waveform that was developed on a ground-based development platform, and Glenn Research Center (GRC) ported the waveform to the flight platform and performed the integrated test and acceptance of the subsystem. This last case also leverages an existing platform development, and offers more capacity for reconfigurability than the second case.

NASA's space communications and navigation test bed aboard the international space station

The NASA SCaN Test Bed flight experiment payload aboard ISS will enable experimenters the unique opportunity to investigate SDRs, navigation, and networking in the space environment. Comprised of three SDRs from industry partners, CoNNeCT allows experimenters to develop and verify new waveforms compliant with the STRS SDR architecture standard, using verified ground systems and then have those waveforms uploaded to the flight SDRs to assess in situ performance and to better understand operational concepts for SDRs in space. In addition to the SDRs, the reprogrammable avionics software allows application software for on-board networking and routing experiments. The flight system communicates with TDRSS at both S-band and Ka-band and can receive within the GPS L-band for navigation waveform development and experiments.

Unique challenges testing SDRs for space

This paper describes the approach used by the Space Communication and Navigation (SCaN) Testbed team to qualify three Software Defined Radios (SDR) for operation in space and the characterization of the platform to enable upgrades on-orbit. The three SDRs represent a significant portion of the new technologies being studied on board the SCAN Testbed, which is operating on an external truss on the International Space Station (ISS). The SCaN Testbed provides experimenters an opportunity to develop and demonstrate experimental waveforms and applications for communication, networking, and navigation concepts and advance the understanding of developing and operating SDRs in space.

Lessons Learned in the First Year Operating Software Defined Radios in Space

Operating three unique software defined radios (SDRs) in a space environment aboard the Space Communications and Navigation (SCaN) Testbed for over one year has provided an opportunity to gather knowledge useful for future missions considering using software defined radios. This paper provides recommendations for the development and use of SDRs, and it considers the details of each SDRs approach to software upgrades and operation. After one year, the SCaN Testbed SDRs have operated for over 1000 hours. During this time, the waveforms launched with the SDR were tested on-orbit to assure that they operated in space at the same performance level as on the ground prior to launch to obtain an initial on-orbit performance baseline. A new waveform for each SDR has been developed, implemented, uploaded to the flight system, and tested in the flight environment. Recommendations for SDR-based missions have been gathered from early development through operations. These recommendations will aid future missions to reduce the cost, schedule, and risk of operating SDRs in a space environment. This paper considers the lessons learned as they apply to SDR pre-launch checkout, purchasing space-rated hardware, flexibility in command and telemetry methods, on-orbit diagnostics, use of engineering models to aid future development, and third-party software. Each SDR implements the SCaN Testbed flight computer command and telemetry interface uniquely, allowing comparisons to be drawn. The paper discusses the lessons learned from these three unique implementations, with suggestions on the preferred approach. Also, results are presented showing that it is important to have full system performance knowledge prior to launch to establish better performance baselines in space, requiring additional test applications to be developed pre-launch. Finally, the paper presents the issues encountered with the operation and implementation of new waveforms on each SDR and proposes recommendations to avoid these issues.

Dynamic communication system characterization of phased array antennas

Phased array antenna (PAA) systems offer many advantages when used on a low Earth orbiting (LEO) satellite system. Technology advancements to reduce the power, weight, and cost of these systems make phased arrays a competitive alternative compared to the gimbled reflector system commonly used in science missions. However, the operational characteristics of LEO communications links that employ a PAA are not fully understood. For example, theoretical and computer simulation analysis of a phased array antenna operating in a communication link predict 3-4 dB of degradation of BER performance for high rate systems. The degrading effect of large scan angles on high data rate communications is being investigated in a laboratory environment. In these experiments a LEO environment is simulated by dynamically scanning the array while simultaneously rotating the range pedestal in the complimentary direction. Such investigation reduces the operational risk of deploying high frequency PAA technology. This characterization bounds the degradation associated with operating the phased array systems in this environment, and provides for the opportunity to system engineer the communication link.