Data Communication for Polar Research Expeditions

I. Motivation

Link Management Software - Overcoming the Call Drop Phenomenon Nodes connected to an Iridium network experience frequent call drops due to the motion of the satellites, network overload and bad weather. The link management software as shown in figure 3, has three essential modules:1. Control Software module - Recovery from call drops

Establishes and monitors the connection between the client (field) and server (local). In response to a call drop or power failure, the failed link is redialed and attached to the bundle, restoring the full capacity of the system.

III. Design and Implementation

1. Remote field subsystem: Consists of multiple Iridium modems interfaced to a rugged single-board computer and an antenna array to access the satellite network. It is configured as a PPP client and is usually located on the field (Antarctica).

Inverse Multiplexing•A solution to the bandwidth limitation of a single Iridium channel is to use multiple

links. However, use of multiple channels as independent links does not increase the available bandwidth per application. If each application/user sends data on an independent link, the bandwidth per application/user remains to be 2.4 Kbps. In order to overcome this limitation, inverse multiplexing technology is used.

•Inverse multiplexing (as opposed to traditional multiplexing) combines multiple low speed links into a single logical link that is the aggregate of individual link bandwidths, and thereby increases the available bandwidth per application. Using this technique, data packets from a single application (user) are split into multiple segments that are sent simultaneously over multiple links. The segments received at the destination are combined to reconstruct the original packet.

•Inverse multiplexing can be done at bit level (bonding) or at packet level. Multi-link point-to-point protocol (MLPPP) is a software implementation of packet-based inverse multiplexing

IV. System Performance in the FieldPerformance data of the 8-channel system was collected at field camps in Greenland and Antarctica during 3 field seasons.

Throughput• The variation of the throughput with the number of modems was found to be linear as shown in figure 9.• Throughput of 8-channels was found to be 18.6 Kbps• Throughput efficiency was found to be greater than 95%• Files up to 60 MB were transferred from Greenland to KUDelay• The end-to-end delay was found to be 1.4 sec• The satellite system has large delay and delay variation• The variation of delay during day times is shown in figure 9.

V. Research Impact and ApplicationsTechnological Impact

• Effectively utilizes older Iridium system to support modern applications by overcoming fundamental limitations• Demonstrated practical solution to the very challenging problem of polar data communication• Successfully implemented inverse multiplexing technique over satellite links for the first time• Developed integrated rugged system that can sustain harsh polar climate

Science Impact• Provided lifeline communication support to polar expeditions• Allows collaboration of field participants with other scientists, thereby increasing the effectiveness of field research• Transparent network architecture allows field participants to utilize advanced computing resources at host institutions• Provides a possibility for data telemetry from unmanned remote stations• Internet access in the field camps provides critical weather and health reports.

Outreach• Public outreach from polar regions was made possible• Provides near real time pictures and videos of the field

activities to the science community• Daily journal logs of science experiments were uploaded• K-12 students were involved in research through video and audio conferences.

Acknowledgements: This work was supported by NSF (grant #OPP-0122520), NASA (grants #NAG5-12659 and NAG5-12980), KTEC, and the University of Kansas.

II. Research Goals

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2. Logical link : Consists of a pool of multi-hop satellite channels between the remote and local subsystems. 3. Local subsystem: The local sub-system consists of a bank of Iridium modems interfaced to a Linux system for

control and management. It is configured as a PPP server and is usually located at the host institution (e.g. KU).

2. Graphical User Interface (GUI) module To facilitate field use, GUI provides an interface to configure and manage the system. It also offers a visual representation of the connection status of each modem in the system

3. XML database module - System performance statisticsRecords all the events of a connection (such as connection attempts, failures and drops) in an XML format to facilitate network performance analysis.

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Figure 1: Multiplexing vs. Inverse Multiplexing

Inverse Multiplexing

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Reliability• In order to determine the reliability of the system under the presence of call drops 48 hr experiments were conducted, in which all connection activity was recorded• It was found that the uptime (time connected) of the system with full capacity was 85%, providing reliable communication.

Mobile Performance• Mobile experiments were conducted to determine the

performance of the system on a vehicle moving across the ice in Greenland as shown in figure 8. • For speeds up to 15 mph an average throughput of 18.0 Kbps was obtained with 8 modems

8-Channel system Implementation

•Based on the above design, an 8-channel ~18 Kb/s system was developed. This integrated field unit as shown in figure 5, is a rugged, fully autonomous, plug-and-play system. An on-board computer controls the operation of the unit and a small LCD screen displays the status of the connection

•To support the PRISM field experiments, an 8-channel Iridium-to-Iridium system was implemented and rigorously tested at SUMMIT field camp in Greenland and WAIS camp, Antarctica during 2004 and 2005 respectively. See Figures 6 – 8.

•Field Research in Polar Regions

Recent research has revealed that the study of polar ice sheets significantly contributes to our understanding of the environmental challenges such as

global warming and rise of sea levels. Moreover, glaciological studies have pointed out that the melting polar ice sheets have a direct impact on the environmental changes with serious consequences. To

further our understanding on the interaction of Polar Regions with global climate change, an increasing number of science expeditions and field camps to the Arctic and Antarctic are supported by the scientific community.

•Polar Radar for Ice Sheet measurement (PRISM) project is one such research effort at the University of Kansas that performs radar measurements of the Polar ice sheets.

•Design and implement a reliable, autonomous, light weight and field deployable Iridium based data communication system with sufficient bandwidth to support the data requirements of polar field research.

•Develop an end-to-end communication network to support field activities through the University of Kansas

•Provide internet access in the Arctic and Antarctic to facilitate critical communication between the field personnel and the University researchers

•Support near real time data transfer to facilitate outreach to the K-12 student community

•The research involved addressing the following issues

– Overcome the bandwidth limitations of an Iridium link

– Address the call drop issues associated with low earth orbiting satellite systems

– Develop an integrated plug and play system that can be deployed in polar field environment

Figure 2. Multi-channel Iridium system design

Figure 3. Link Management Software

Figure 5. Integrated field Unit

Figure 6. Field implementation (Antarctica)

Figure 7. Antenna array setup (Greenland)

Figure 8. Field test on mobile platform (Greenland)

Figure 9. Throughput and delay variation

•Data Communication Challenge

Due to the remote nature of polar field camps, the data communication problem presents a unique challenge. Because of the lack of terrestrial facilities, the communication from Polar Regions depends heavily on satellite systems. However, the design of the most commercial satellite systems does not allow for the coverage to extend to Polar Regions. Hence, the current data communication is supported by special science and research satellites from NASA and other research agencies. Also these satellite systems are not suitable for small polar field experiments.

The only commercial satellite system that has true pole- to-pole coverage is Iridium. However, the bandwidth of an Iridium channel is 2.4 kbps, which is not sufficient to support the most basic requirements of polar scientists.

Electrical Engineering and Computer Science

End-to-end system designUsing the Inverse multiplexing technology, we developed and end-to-end system design based on client-server model. A multichannel Iridium system as shown in figure 2, can be logically divided into three subsystems:

Iridium system mounted in an autonomous vehicle

Experiments monitored from another vehicle through wireless link