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Results of an Iridium-Based Data Communications System Providing Internet Access to Polar Expeditions

Mohammad, A., Frost, V. and Braaten, D.

Abstract:

The Polar Radar for Ice Sheet Measurements (PRISM) Project at the University of Kansas has developed and field-tested a versatile communications system for use by researchers in high-latitude Polar Regions. The PRISM project is developing advanced intelligent remote sensing technology that involves radar systems, an autonomous rover, and communications systems to measure detailed ice sheet characteristics, and to determine bed conditions (frozen or wet) below active ice sheets in both Greenland and Antarctica. We also have a very strong public outreach and educational program aimed at K-12 educators and students that requires data, voice and video feeds from Polar field locations in near real time. Hence, PRISM requires a robust communications system for use in the field between a base camp and the mobile remote sensing system, and from the field back to the University of Kansas and onto the Internet. The communication system that has been developed is truly mobile and is relatively inexpensive. We initially considered various satellite services during the design phase of this project. The two feasible options for high-latitude locations were Iridium, with its low-bit-rate service (2.4 Kb/s), and Inmarsat/Intelsat with broadband service. We selected the Iridium option for testing and development because it provided coverage in both Antarctica and Greenland. To achieve higher capacity communications, the multilink point-to-point protocol (MLPPP) implemented in Linux was tuned to operate efficiently over the Iridium satellite system. This mechanism combines multiple channels to obtain a seamless data connection with a capacity equal to the sum of the individual link rates. We used four Iridium modems to obtain an aggregate capacity of about 9.6 Kb/s. Standard Internet protocols (TCP/IP) were then used to provide end-to-end connectivity. The communications system field experiments were conducted at the NorthGRIP ice core drilling camp in Greenland (75° 06' N, 42° 20' W) from June 23-July 17, 2003. We measured the reliability, throughput and general performance of this system over periods of up to 24 hours. This included delay, loss, throughput, call drops/up-time, relative signal strengths, connection time and number of connection retries. The system was also successfully tested while moving across the ice sheet at speeds of up to 30 km/h. We were successful in uploading and downloading large files (.2MB to 7.2MB) to the Internet. In combination with a modified Wi-Fi deployment, wireless Internet access was also provided to the entire NorthGRIP camp for a few days. The field experiments have shown that standard Internet protocols combined with MLPPP and Iridium modems can provide Internet access for polar expeditions at useful data rates. The 4-modem configuration we tested was observed to be >90 % efficient, e.g., a 2.5 MB video file was transferred in 35 min. at 9.524 Kb/s. The system had an average up-time of over 90 % and thus is stable and suitable for autonomous operation. Mobile performance results were very similar to that of stationary systems. While the end-to-end network architecture developed to provide Internet access worked well, the system round trip time is significant (~1.8 seconds), which can impair real time interactions.

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