ZigBee - A Smart, Viable, Wireless Architecture for Spacecraft Avionics
Thom Stone
AUG 24, 2012 07:44 AM
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Zigbee is gaining popularity as the foundation for smart infrastructures being widely deployed today, however at NASA Ames Research Center, it's also being investigated for use in wireless sensor networks aboard spacecraft.

Ames Research Center is part of a multi-center effort sponsored by the NASA Engineering and Safety Center (NESC) to investigate wireless sensor networks (WSN) for spacecraft.

 Smart home, hospital, and energy technologies are based on this low-cost, low-power, wireless mesh network standard. Both wireless and smart technologies have much to offer for the space business.

The weight of cables, ties, patches, connectors, tubes, and channels of traditional wired infrastructures represent at least 10 percent of the mass of most spacecraft. Wireless can significantly reduce this. Wireless sensor-nets (WSN) for spacecraft also:

·         Can be deployed where wires can't

·         Can avert disasters

·         Make adds, moves, and changes easier

·         Can be used for mobile applications such as bio-monitors for astronauts or live specimens

·         Can provide for failover of important components, providing redundancy not available in wired systems

·         Can connect new generations of smart sensors designed for intelligent applications, and

·         Can electrically isolate elements like hosted payloads from the rest of the spacecraft.

 

Buying some Texas Instrument (TI) ZigBee evaluation kits helped the team better understand the short-range wireless network standard. The design goals are low-power operation, use of the open Industrial Scientific and Medical (ISM) band (same as IEEE 802.11) and a layered protocol model. The IEEE 802.15.4 standard is used as the underlying physical layer, providing a self-organizing and self-healing dynamic mesh network.

Like Wi-Fi, ZigBee uses direct sequencing, dividing the AISM band into channels and using one. The network bandwidth is 104 kilobits per second—sufficient for sensor deployment.  In addition to the hardware attributes, a library of software, a rich architecture, evolving standards, and large developer community, coordinated by the ZigBee Alliance, allows for complex interactions between devices and external communications and control. 

 

ZigBee is something of a Facebook for gadgets because it:

·         allows communications between distant devices,

·         is simple to join a network,

·         uses few resources,

·         supports applications and multi-device cooperation to form "smart" networks, and

·         allows for sharing of global information and for gathering information and controlling devices remotely.

 

Our evaluation had two phases.  The first was to test Zigbee's basic performance and functionality. The second was to evaluate the ability to adopt ZigBee to a spacecraft's needs by looking at integrating sensors both commercially available and self fabricated into ZigBee networks, plug and play options, and available middleware to store and display data.

Our testing was enhanced by the availability of hardware and software monitoring tools. We began by testing the range, immunity from RF noise, ease of use, mesh, and self-healing features.

We measured range and could reach 41.1 meters between a sensor and coordinator with zero errors. We found that putting a sensor in a metal draw and behind metal doors had little impact.  We also could extend the range to 65.2 with use of a router node. There is no limit to the number of routers that can be used in a ZigBee network. Routers allow for complex topologies where more than one path can exist from an end node to the network coordinator.

We found that if a router node went down, the network would fail over to an alternative router in between 2 and 6 seconds. We found that while it is possible to jam ZigBee, it is remarkably immune from interference from other networks. ZigBee uses "listen before talking" like Wi-Fi so it interferes less with other systems. We tested against Wi-Fi, Bluetooth, microwave ovens, and 2.4G wireless phones, and found minimal disruption with normal traffic loads.

After we were satisfied that ZigBee met reliability and functionality requirements, we looked at ZigBee's ability to support spacecraft functions. We were able to integrate existing sensors and to fabricate our own sensors and integrate them into ZigBee. We also adopted a candidate plug-and-play architecture, IEEE 1451, for ZigBee with some success. We found that middleware and display software were available and somewhat easy to integrate.

In all we found the ZigBee platform adoptable to fit our needs. We plan to further investigate available resources from the ZigBee Alliance, especially the frameworks and profiles from smart technologies. Funding is still limited, but we are still advocating to include ZigBee-enabled transducers on spacecraft. This large step will be the beginning of an effort to develop an avionics and space framework within the ZigBee standards community to enable the smart spacecraft of the future.

 

Thom Stone is a senior computer scientist with Computer Sciences Corp. He is attached to the NASA Research and Engineering Network project at Ames Research Center. Mr. Stone has been at ARC employed by various contractors since 1989. He was an engineer with the NASA Science Internet project office where he led the project that bought reliable Internet connections to remote locations including US bases in Antarctica including McMurdo Station and Amundson Scott South Pole Station. He was principal engineer for communications for the NASA Search for Extraterrestrial Intelligence (SETI) project and was a senior engineer for the Space Station Biological Research Project. Before his involvement with NASA, Stone was employed in the computer and communications industry and taught telecommunications at the undergraduate level.

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