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Multi-Link Iridium Satellite Data Communication System for Polar Research. Abdul Jabbar Mohammad (September 15, 2005). Motivation - PRISM. Polar Radar for Ice Sheet Measurements (PRISM) Field experiments in Greenland and Antarctica Data telemetry from the field to the University

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multi link iridium satellite data communication system for polar research

Multi-Link Iridium Satellite Data Communication System for Polar Research

Abdul Jabbar Mohammad

(September 15, 2005)

motivation prism
Motivation - PRISM

Polar Radar for Ice Sheet Measurements (PRISM)

  • Field experiments in Greenland and Antarctica
  • Data telemetry from the field to the University
    • Science data
  • Access to University and web resources from field
    • Internet, email
    • ssh, file transfer
  • Public outreach
    • Involvement of student community (K-12) in scientific research
    • Virtual experience of the polar expedition for the science community
  • Lack of conventional communication facilities
    • Satellite communication is the only viable solution
satellite data communication
Satellite Data Communication
  • Commercial Satellite systems
    • No coverage or intermittent coverage
    • Prohibitively expensive ($3 - $30 per MB)
  • Special Purpose Satellite systems
    • ATS3, LES9, GOES, TDRS 1,and MARISAT2
    • Broadband Access
    • Geo-synchronous : Limited visibility (10-13 hrs/day)
    • Low elevation angles : extremely large ground terminals

Inmarsat

Globalstar

iridium satellite system
Iridium Satellite System
  • Low earth orbiting satellite system
    • True pole-to-pole coverage
    • 66 Satellites in orbit
    • Altitude of 780 Km
    • Minimum elevation angle – 8.2 0
    • Average satellite view time – 10 minutes
    • Access Scheme – FDMA and TDMA
    • Maximum number of users – 80 users per a diameter of 318 Km
    • Low cost availability for research purposes ( NSF sponsored)
  • Data communication features
    • Type of data services – Iridium-to-PSTN, Iridium-to-Iridium
    • Throughput – 2.4 Kbps, primarily intended as a voice only system
    • Cannot support most of the data communication requirements of polar research
    • Not practical to be used as a main stream/ life-line communication system
iridium based data communication

Mux

App 1

High Bandwidth Link

App 2

App 3

Multiplexing

Low Bandwidth Links

Inv-Mux

App 1

Iridium Based Data Communication

Idea – Combine multiple Iridium channels in to a single logical link

Inverse Multiplexing

  • Distributes data from a single application over multiple links.
  • Increases the available bandwidth per application
  • Packet based inverse multiplexing solutions exist - Multi-link point-to-point protocol (MLPPP)

Inverse multiplexing

multi channel iridium system protocol stack
Multi-channel Iridium System – Protocol Stack

Remote System

Local System

point-to-point satellite links

4 channel iridium system design

Remote System

PPP client

Iridium Gateway

USB-SERIAL

I. Modem 1

Antenna Grid

I. Modem 2

I. Modem 3

I. Modem 4

Remote Subsystem

Local System

PPP Server

Multi-port PCI card

PSTN

Modem Pool

Local Subsystem

4-Channel Iridium System - Design
  • 4 Iridium – 4 PSTN data configuration
  • Discrete components
  • Patch antennas
  • Control software on a rugged Laptop
4 channel iridium system testing at ngrip greenland 2003
4-Channel Iridium System Testing at NGRIP, Greenland-2003

4-Channel Iridium System at NGRIP

Antenna setup at NGRIP

4 channel iridium system 2003 conclusions
4-Channel Iridium System -2003 Conclusions
  • Reliable console based management software
  • End-to-end network worked
  • 4-channel throughput = 9.26 Kbps; System efficiency > 90%.
  • Average time interval between call drops = 100 minutes
  • Average up-time ~ 90%
  • Average round trip time with Iridium-PSTN configuration ~ 2 seconds
  • Problems with USB-Serial converter in Linux
  • PPP level compression was resulting in corrupted modem termination – loss of packets
  • Primary modem failure resulted in the termination of all the other modems
  • Not fully autonomous
4 channel iridium system 2003 conclusions10
4-Channel Iridium System -2003 Conclusions
  • Identified areas for additional research
    • Increase the bandwidth - Scale the system from 4-channels to 8-Channels
    • Reduce delay – use Iridium-to-Iridium configuration in data-after-voice (DAV) modem
    • Improve management and Control - more structured system
    • Solve the primary modem drop problem
    • Develop a plug and play integrated system
    • Improve the user friendliness of the system
    • Reduce the antenna footprint
8 channel iridium system design elements
8-channel Iridium System – Design Elements
  • 8 channel Iridium-to-Iridium configuration
  • On-board computer to run the control software
  • GUI based management/control software
    • Allows on-field reconfiguration in different data modes:
      • a) Iridium-Iridium DAV mode,
      • b) Iridium-Iridium data mode,
      • c) Iridium-PSTN mode
    • System parameter tuning
    • Status monitoring
  • Inverted cone antennas - easily mounted on field and do not need a external ground plane.
  • Integrated field unit – plug and play
8 channel iridium system integrated unit
8-channel Iridium System – Integrated Unit

Bottom View

Top View

19”

24”

Front View

  • Dimension : 9x19x24 inch
  • Weight : 50 lbs
  • Operating temp : -30 to 60 c
  • Power input : 120 V AC
  • Replication Costs : ~$18,000

9”

8 channel iridium system client software
8-channel Iridium System – Client Software

Client Software consists of three modules:

Graphical User Interface

  • Easy Configuration and Operation
  • Does not require experienced users

Control Software

  • It is the core of the software
  • Automatic Modem Control

XML Database

  • Registers all call drops and retrials
  • Makes it possible for future analysis of network performance data
8 channel iridium system network architecture

Polar Camp, Greenland/Antarctica

100 Mbps Ethernet

(Default gateway)

(Default gateway)

user 4

PPP Server

PPP Client

ITTC Default Router

P-T-P Satellite link

eth0

ppp0

ppp0

eth0

100 Mbps Ethernet

User 2

User 1

user 3

user 1

User 3

Camp WI-FI

user 2

World Wide Web

ITTC Network, University of Kansas

8-channel Iridium System –Network Architecture
field experiments system implementation
Field Experiments – System Implementation

8-Channel system in a weather-port at SUMMIT camp in Greenland, July 2004

field experiments antenna setup
Field Experiments – Antenna Setup

4 ft

10 ft

8 Antenna setup at SUMMIT camp in Greenland, July 2004

results throughput
Results – Throughput
  • Average throughput efficiency was observed to be 95%
  • The above results are from the test cases where no call drops were experienced
  • In event of call drops the effective throughput of the system will be less than the above values
results throughput20
Results – Throughput

FTP throughput observed during data transfer between the field camp and KU

  • Average throughput during the FTP upload of large files was observed to be 15.38 Kbps
  • Due to call drops, the efficiency was reduced to ~80%
  • Detailed TCP analysis based on IPERF and FTP data is in progress
results round trip time
Results – Round Trip Time
  • Average RTT = 1.4 sec
  • Minimum observed RTT = 608 msec
  • Mean deviation = 800 msec
results reliability 14th july 12 hr test

Uptime %

89

95

96

97

97

97

97

98

Results – Reliability: 14th July 12-hr test
  • Call drop pattern during 8 Iridium – 8 Iridium DAV mode test for 12 hrs
  • Percentage uptime with full capacity (8 channels) is 89% and with at least one modem is 98%
  • Total number of primary call drops during 12 hrs = 4
  • Average time interval between call drops is ~ 180 mins
results reliability 22nd july 32 hr test

85

92

93

93

94

94

94

96

Results – Reliability: 22nd July 32-hr test

Uptime %

  • Call drop pattern during 8 Iridium – 8 Iridium DAV mode test for 32 hrs
  • Percentage uptime with full capacity (8 channels) is 85% and with at least one modem is 96%
  • Total number of primary call drops during 32 hrs = 24
  • Average time interval between call drops is ~ 72 mins
results mobile tests
Results – Mobile tests

Iridium antennas

Iridium system mounted in an autonomous vehicle (MARVIN)

Experiments monitored from another vehicle through 802.11b link

results mobile tests25

Uptime %

65

79

82

84

84

85

87

92

Results – Mobile tests
  • Call drop pattern during 8 Iridium – 8 Iridium DAV mode test for 2 hrs
  • Percentage uptime with full capacity (8 channels) is 65% and with at least one modem is 92%
  • Average time interval between call drops is ~ 45 mins
  • Average throughput = 18.6 Kbps, Average RTT = 1.8 sec
applications
Applications
  • Summer 2004 field experiments
  • Communications data upload – up to 40 MB files
  • Radar data uploads – up to 55 MB files
  • Text chat with PRISM group at KU
  • Video conference - real time audio/video
    • Individual audio or video conference works with moderate quality with the commonly available codecs
  • Outreach Use
    • Daily Journal logs uploaded
    • Daily Pictures uploaded
    • Video clips uploaded
    • Held video conference with science teachers/ virtual camp tour
  • Wireless Internet access
conclusions
Conclusions
  • Integrated 8-channel system
    • Works out of the box
    • Reliable and fully autonomous operation
  • The newly developed GUI based control software
    • Reduced the field setup time, increased the ease of operation
    • Suitable for operation by non-technical users
  • System performance based on field experiments
    • Average throughput with 8 channels is 18.6 Kbps, efficiency > 90%
    • Average round trip time using DAV modes is 1.4 sec, significantly less than 2 sec of Iridium-PSTN configuration
    • Average uptime with full capacity using DAV mode was 85 %; better than both non-DAV mode and PSTN mode
    • Percentage system uptime (at least one mode) was ~95% for all the modes
    • Average time interval between call drops is 60 mins and varies a lot.
  • In conclusion, the throughput and delay performance of the system using Iridium-Iridium DAV mode is better than other data modes.
lessons learned
Lessons Learned
  • The call drop pattern
    • Increased number of call drops in Iridium-Iridium mode
    • The average time interval between call drops reduced from 100 minutes in case of 4 Iridium-4 PSTN system to 60 minutes in case of 8 Iridium – 8 Iridium DAV system.
    • Varies with time and weather
    • Increased call drops in presence of strong radio interference
  • Modem firmware failures
    • random modem lock ups due to bug in firmware.
    • Newer firmware upgrades reduced the number of such lock ups
  • Primary modem failure
    • No longer drops all the modems
    • Just the one modem is lost
    • System operates with remaining 7 modems
continuing work
Continuing Work
  • Signal Strength issues
    • Reduce the number of call drops
    • Reduce signal attenuation at the server
  • Server Software
    • GUI based server management software
    • Increase reliability during field operations
    • Ease of operation and use by non-technical personnel
  • Delay Tolerant Networks
    • Communication networks tolerant to inherent delays
    • Set of protocol and architectures well suited to intermittent links
    • Supports communication in heterogeneous sensor webs such as polar sensor web
    • Adapt the evolving DTN technologies to address polar communication issues?