Operating Systems for Wireless Sensor Networks in Space

1
Operating Systems for Wireless Sensor Networks
in Space

• Abdul-Halim Jallad and
• Tanya Vladimirova

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Outline of Presentation

• Applications of wireless sensor networks in space
• Formation flying missions overview
• Requirements analysis of operating systems for
  formation flying missions
• Testbed development
• Conclusions

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Wireless Sensor Networks Convergence of
Technologies
Wireless communications optical and RF
communications enable networking between nodes
Embedded computing Small and low-cost processors
that are networked together facilitate
collaboration through information and resource
sharing
Sensors Miniaturization and micromachining makes
tiny and low-cost sensors available commercially
Wireless sensor networks
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Wireless Sensor Networks in Space
1) Manned Spacecraft missions e.g. crew health
monitoring
4) Inter-planetary Exploration
Figure from http//sensorwebs.jpl.nasa.gov/
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Multi-Satellite Missions Terminology

• A Virtual Satellite is a spatially distributed
  network of individual satellites collaborating as
  a single functional unit, and exhibiting a common
  system-wide capability to accomplish a shared
  objective.
•  

• A Distributed Space System (DSS) is a system that
  consists of two or more satellites that are
  distributed in space and form a cooperative
  infrastructure for science measurement data
  acquisition, processing analysis and
  distribution.

• A Constellation is a group of satellites that
  have coordinated coverage, operating together
  under shared control, synchronised so that they
  overlap well in coverage and reinforce rather
  than interfere with other satellites' coverage.

• A Cluster is a functional grouping of spacecraft,
  formations, or virtual satellites.

• A Sensor Web is a system of intra-communicating
  spatially distributed sensor crafts that may be
  deployed to monitor environments. Sensor webs may
  involve many non-space elements and are therefore
  not completely covered by DSS.

• A Formation is a multiple-spacecraft system with
  desired position and/or orientation relative to
  each other or to a common target. Formation
  flying is the term used for the tracking and
  maintenance of a desired relative separation,
  orientation or position between or among
  spacecraft.

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Formation-Flying MissionsTypes

• Signal Combination
• Distinct sensors on separate nodes collect data
  from different sources and merge this data
  on-board of the formation to extract global
  information of a particular phenomenon e.g. Earth
  observation-1 mission.

• Signal Coverage
• A Sensor Web with identical sensors on the nodes
  with the purpose of covering wide areas of
  surface (e.g. multi-point sensing).

• Signal Separation
• Measurements from the same source are collected
  by spatially distributed sensors on-board
  different nodes in the formation e.g. large
  synthetic apertures.

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Formation-Flying Missions The Information System
Sensors and Actuators These may be divided into
three classes spacecraft specific,
formation-flying specific and payload specific

• On-Board Computing
• Hardware is to be power and memory efficient
  while being fault-tolerant.
• Software includes
• mission software
• middleware
• an operating system to support distributed
  services.

• Inter Satellite Communications
• Intersatellite links are different from
  terrestrial WSN wireless links in two main
  aspects
• large distances involved and
• predictability

Formation- Flying Missions Information System
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Model Application
Mission Model
The Network
Aims of Research

• For the purpose of narrowing down the scope of
  this investigation we focus on a particular type
  of FF missions virtual satellites
• Application
• Sensor web Imaging
• Signal Separation Synthetic apertures
• The satellite nodes
• Mass lt 1 Kg
• Area lt 1 cm3
• Power lt 2 Watts
• Orbit Low Earth Orbit (LEO) 600Km

• Separation distances in the order of kilometers
• Use of directional antennas.

• To investigate the advantages and disadvantages
  of distributed computing on-board of
  formation-flying (FF) missions
• To study possible implementations of distributed
  computing on-board FF missions
• To propose an optimal operating system
  architecture for such missions

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Formation-Flying Mission Information System
Architecture
Application
App1
App2
App3
Power Management
Algorithms
Modules
Services
Virtual Machine
Middleware
Middleware management
System Threads Address space Files
Transport
Operating System
Network
Data Link
Physical
Hardware Drivers
Sensor Driver
Hardware
Hardware
Sensor
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OS Design for Formation-Flying Missions
Main Functions
Process Description and Control

• Process description and control
• Fault-tolerance e.g. process replication
• Memory considerations
• Concurrency
• FF missions are distributed systems and involve
  concurrency
• Memory management
• Use of bulk memory
• Program memory wash
• Input/output management
• File management
• Fault-tolerance
• Networking
• Space protocol for ISL and ground space links
• Security
• Scheduling
• Real-Time scheduling
• Low-power scheduling

Scheduling
Concurrency
Memory Management
Input/Output Management
File Management
Networking
Security
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OS Design Factors for Formation-Flying Missions
Factors

• OBDH
• The architecture of the on-board data handling
  system (e.g. distributed, centralized,
  multi-processor etc.) affect the operating system
  design
• ISL
• The OS needs to consider the bandwidth, power
  consumption and unreliability of the
  inter-satellite links while making distributed
  decisions
• Formation Flying (FF)
• The effect of the relative dynamics brought by FF
  on the OS design needs to be investigated
• On-board Software
• The nature of the applications running on-board
  and its distribution among the FF nodes may have
  a direct impact on the OS design
• Constraints
• The limited size and therefore available energy
  for computation and communication is an important
  factor that the OS design has to consider

Operating System
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On-Board Data Handling for Pico-Satellites
OBDH
system-on-a-chip may involve various
technologies including mixed-signals
(analog/digital) on a single substrate
Ultra-low Power
SOC
Reconfigurable hardware
Advanced Packaging
Multi-processor Systems
SiGe on SOI
ASICs
FPGAs
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Types of Operating Systems
Operating System Description Pros Cons Example/ Mission
Monolithic Almost any procedure can call any other procedure. Efficient Lack modularity OS Linux Mission None
Microkernel (client/server) A few essential functions are embedded in the kernel. Other services run as processes in user mode. Flexible Well suited for distributed systems Less efficient than monolithic OS QNX, VxWorks Missions TiungSAT-1, PROBA
Virtual Machines Exact copy of bare hardware. Portable Low-performance OS Embedded Java Virtual machine Mission None
Component-Based The Operating system consists of a set of independent components representing system resources Portable Efficient Well suited for distributed systems OS TinyOS Mission None
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The TinyOS Component-Based OS
TinyOS
TinyOS Component
TinyOS Application

• Operating system specifically designed for
  wireless sensor networks
• Applications consist of scheduler and a graph of
  components
• Higher-level components issue commands to and
  respond to events from Lower-level components
• Components contain Set of command handlers, Set
  of event handlers, A fixed size storage frame,
  Collection of simple threads which can be
  scheduled.

Components can be implemented in hardware or
software. Events propagate upward in the
hierarchy Commands propagate downward in the
hierarchy.
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Operating System Design for Swarms of
Pico-Satellites
Design Requirements
Component-Based Model
Execution-Model

• Fault tolerance
• Small foot-print
• Low-power consumption
• Support for reconfigurable computing.
• Distributed system support
• Scalability
• Support for inter-satellite link communications

Thread-based model
Event-based model
Component library

• Tasks perform computations
• Tasks are implemented as finite state machines
• States of tasks are transitioned through events

• The system uses a main thread, which hands off
  tasks to individual task-handling threads
• High context switch overhead

Conclusion The component-based structural model
provides flexibility, reusability and is suitable
for distributed systems design while the
event-based behavioural model provides speed, low
power and memory efficiency.
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Distributed Computing for Formation-Flying
Missions Testbed
Windows XP PC
Visualization
STK
Matlab
Satellite Tool Kit
STK Advanced AO
TCP/IP server
Simulink
STK/ Connect
Ethernet
GR-PCI-XC2V-FT
XSV800
XSV800
LEON-3 Multiprocessor OBC
LEON-3 Multiprocessor OBC
LEON-3 Multiprocessor OBC
RS232
Linux development platform
DDD
GCC Compiler
DSU Monitor
Programming Environment
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System Emulation
Distributed System Emulation Hardware
Node Emulation Hardware

• GR-PCI-XC2V-FT
• XC2V3000 Virtex-II FPGA
• Ethernet PHY interface
• LEON-FT core Support
• On-board memory
• SRAM
• SDRAM
• Flash PROM

• XSV800
• XCV800 Virtex FPGA
• Ethernet PHY interface
• On-board memory
• SRAM
• Flash Prom

• Mica2 motes
• 916MHz Multi-channel Radio Transceiver
• ATMEL128L 8-bit low-power processor
• Compatible with TinyOS (specifically designed for
  sensor networks).

Figure from the LEON-PCI-XC2V Development board
user manual
Figure from the www.xess.com website
Figures from mica2 datasheet
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Pico-Satellite Computing Platform

• The chosen processor is the LEON-3 soft IP core
• 32-bit SPARC V 8 architecture
• Could be used in a multi-processor system
• Soft core (suitable for developing system-on-chip
  prototypes)
• Power-down mode is supported
• Embedded Hardware Debug Support Unit (DSU).

LEON-3 in a multi-prosessor configurationFigure
from www.gaisler.com
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Conclusions

• Wireless sensor networks are a promising
  technology for space applications including
  orbital formation-flying (FF) missions and
  inter-planetary exploration.
• This research focuses on implementation of
  distributed computing on-board FF missions
  employing the wireless sensor networks concept.
• The various factors that affect the operating
  system (OS) design of FF missions may be divided
  into two categories
• Traditional OS requirements e.g. code efficiency
  and real-time performance.
• Specific requirements for FF missions e.g.
  fault-tolerant distributed computing, orbit
  dynamics etc.
• A novel OS for multi-satellite FF missions should
  have the following features
• An event-based execution model allowing to
  achieve low-power consumption and to fulfil the
  concurrency requirement with minimal amount of
  code.
• A component-based structural model allowing to
  achieve the modularity requirement and enabling
  the hardware/software boundary crossing, which
  provides support for reconfigurable and
  distributed computing.
• The TinyOS is selected as the baseline OS to be
  studied and adapted for use in distributed FF
  satellite missions.