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Abstract-Wireless sensor networks (WSN) based on the IEEE 802.15.4 Personal Area Network standard are finding increasing use in the home automation and emerging smart energy markets. The network and application layers, based on the ZigBee 2007 PRO Standard, provide a convenient framework for component-based software that supports customer solutions from multiple vendors. This technology is supported by System-on-a-Chip solutions, resulting in extremely small and low-power nodes. The Wireless Connections in Space Project addresses the aerospace flight domain for both flight-critical and non-critical avionics. WSNs provide the inherent fault tolerance required for aerospace applications utilizing such technology. The team from Ames Research Center has developed techniques for assessing the fault tolerance of ZigBee WSNs challenged by radio frequency (RF) interference or WSN node failure. 1The ZigBee Network layer forms a mesh network capable of routing data around failed nodes. A two-tier ZigBee network is tested in the lab and various failures induced in sensor and router nodes, simulating realistic fault conditions. A ZigBee network analyzer is used to view the packet traffic and measure the response to these induced faults at the Network layer. Certain faults are induced using Radio Frequency (RF) interference or disruption of the Physical layer, so RF signal levels are monitored during the experiments. The speed at which an orphaned sensor node is detected and an alternative route formed is an important characteristic for fault-tolerant sensor networks. Our working definitions of metrics describing WSN fault tolerance are presented along with a summary of on-going test results from our development lab.A brief overview of ZigBee technology is presented along with RF measurement techniques designed to gauge susceptibility to interference caused by other transmitters such as wireless networks. Since 802.11 and 802.15.4 technology share the 2.4 GHz ISM band, spectrum management is used to ensure every network has a reasonably clear channel for communications. Quantitative RF characterization of the WSN is performed under varying duty cycle conditions to understand the effect of wireless networks and other interference sources on its performance. Furthermore, multipath interference caused by delayed reflections of RF signals is a significant issue, given that the WSN must run in confined metallic spaces, which produce 1 U.S. Government work not protected by U.S. copyright. IEEEAC Paper #1480, Version I, Updated December 9, 2010 high levels of reflected multipath RF energy. The results of RF characterization and interference testing of our prototype WSN in the lab are presented and summarized. The architecture and technical feasibility of creating a single fault-tolerant WSN for aerospace applications is introduced, based on our experimental findings.

Data from mobile and stationary sensors 1,2 will be vital in planetary surface exploration. The distribution and collection of sensor data in an ad-hoc wireless network presents unique challenges. Some of the conditions encountered in the field include: irregular terrain, mobile nodes, routing loops from clients associating with the wrong access point or repeater, network routing reconfigurations caused by moving repeaters, signal fade, and hardware failures. These conditions present the following problems: data errors, out of sequence packets, duplicate packets, and drop out periods (when the node is not connected). To mitigate the effects of these impairments, robust and reliable software architecture tolerant of communications outages must be implemented. This paper describes such a robust and reliable software infrastructure that meets the challenges of a distributed ad hoc network in a difficult environment and presents the results of actual field experiments testing the principles and exploring the underlying technology.

The Mobile Exploration System Project (MEX) at NASA Ames Research Center has been conducting studies into hybrid communication networks for future planetary missions. These networks consist of space-based communication assets connected to ground-based Internets and planetary surface-based mobile wireless networks. These hybrid mobile networks have been deployed in rugged field locations in the American desert and the Canadian arctic for support of science and simulation achvihes on at least six occasions. This work has been conducted over the past five years resulting in evolving architectural complexity, improved component characteristics and better analysis and test methods. A rich set of data and techniques have resulted from the development and field testing of the communication network during field expeditions such as the Haughton Mars Project and NASA Mobile Agents Project.

In 2015 NASA plans to launch a payload to 280 Km altitude on a sounding rocket from the Wallops Flight Facility. This payload will contain several novel technologies that work together to demonstrate methodologies for space sample return missions and for nanosatellite communications in general. The payload will deploy and test an Exo-Brake, which slows the payload aerodynamically, providing eventual de-orbit and recovery of future ISS samples through a Small Payload Quick Return project. In addition, this flight addresses future Mars mission entry technology, space-to-space communications using the Iridium Short Messaging Service (SMS), GPS tracking, and wireless sensors using the ZigBee protocol. SOAREX-8 is being assembled and tested at Ames Research Center (ARC) and the NASA Engineering and Safety Center (NESC) is funding sensor and communications work. Opensource Arduino technology and software are used for system control. The ZigBee modules used are XBee units that connect analog sensors for temperature, air pressure and acceleration measurement wirelessly to the payload telemetry system. Our team is developing methods for power distribution and module mounting, along with software for sensor integration, data assembly and downlink. We have demonstrated relaying telemetry to the ground using the Iridium satellite constellation on a previous flight, but the upcoming flight will be the first time we integrate useful flight test data from a ZigBee wireless sensor network. Wireless sensor data will measure the aerodynamic efficacy of the Exo-Brake permitting further onorbit flight tests of improved designs. The Exo-Brake is 5 m 2 in area and will be stored in a container and deployed during ascent once the payload is jettisoned from the launch vehicle. We intend to further refine the hardware and continue testing on balloon launches, future sounding rocket flights and on nanosatellite missions. The use of standards-based and opensource hardware/software has allowed for this project to be completed with a very modest budget and a challenging schedule. There is a wealth of hardware and software available for both the Arduino platform and the XBee, all low-cost or open-source. Along with the Exo-Brake hardware and deployment discussion, this paper will describe in detail the system architecture emphasizing the successful use of opensource hardware and software to minimize effort and cost. Testing procedures, radio frequency interference (RFI) mitigation, success criteria and expected results will also be discussed. The use of Iridium short messaging capability for space-to-space links, standards-based wireless sensor networks, and other innovative communications technology are also presented.