19-11-2012, 06:14 PM
Satellite Communication Applied in a Distributed Application
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Abstract
The paper describes the development of a prototype application for
access to vital weather information from the Northern Atlantic sea
region. The application gives meteorologists access to weather observations
measured on sea vessels. Today the available information is
very limited. On a daily basis, only 4-5 weather observations in the
whole arctic sea-region are conducted. It is therefore suggested to use
the available fishing-float and the coast-guard as a basis for a full
scale application. Communication is based on the Inmarsat-C global
satellite system. This kind of communication offers limited data
bandwidth, currently 600 bit/sec. In addition delays are introduced by
the coast/earth-station due to message-queues and protocol-transformations.
Based on the services offered by the Inmarsat-C system, a
simple prototype communication mechanism for data communication
between terrestrial computers and computers on sea-vessels has been
developed. It is, partly due to the nature of the satellite system, based
on unreliable multicast of messages. A message may consist of a
request which requires a reply (multicast RPC). Test results showed
that doing a request and getting a reply takes from 1.5 - 4 minutes.
The results depend on the type of message (supported by the Inmarsat-
C system) which is used. The sea-vessels send weather observations
in set intervals.
Introduction
This report is part of the StormCast research project [Hartvigsen88][Johansen91]
[Johansen94]. The project has been under continuous development since 1988 and is a
project with both meteorological and environmental aspects. The StormCast project’s goal is
to develope an architecture for construction of distributed monitoring applications. The
domain for these application is mainly weather- and environment. In this report the meteorological
aspect is addressed.
Weather observations from the Northern Atlantic sea region are currently reported from seavessels
in reduced amount and frequencies. The reporting is based on manual observations
and transmission via radio calling or telex. The transmission of observations via telex is
timeconsuming. Typically, it takes hours from the observation was done until the meteorologist
has the data on his desk. Parameters of interest is typically wind-speed, wind-direction,
air-temperature, relative humidity etc. One of the main reason for the need for more observations
from these areas is polar lows. This is small and intense low-pressures which arise at
the polar ice-front and from there moves rapidly towards the south. Such polar low-pressures
may have caused many shipwrecks.
The StormCast Project
The motivation for the StormCast project [Hartvigsen88]was the need for prototypes
on distributed applications to examine distributed operating systems as the Amoeba
distributed system. Consequently, development of a software architecture
has been a central part of the project. The architecture is a fundament on which meteorological
and environmental monitoring distributed application is to be built. The architecture
StormCast Page 3
Satellite Communication Applied in a Distributed Application
covers all areas of such applications, from monitoring and collection to data storage and
visualization. The architecture also addresses common computer science problems in the
field of distributed systems, such as increased availability and reliability.
Satellite Communication and Inmarsat-C
Satellites in geostationary orbits, i.e. 36000 km above the equatorial earth plane, have the
ability to cover up to 25% of the earths surface. A satellite system is of nature a broadcast
medium. Later we will see how this is utilized by constructing a simple multicast RPC communication
protocol. The Inmarsat-C description is from [Enersen89], [Hanebrekke90],
[Berg90] and [Inmarsat93].