Written by Assoc.Prof. Roman Jašek, Ph.D. - European Association for Security Faculty of Applied Informatics, Tomas Bata Univerzity in Zlín, e-mail: jasek@fai.utb.cz; SLt. mgr. inż. Dominik Iwen - Polish Naval Academy in Gdynia and Faculty of Economics, University of Gdańsk, e-mail: dominikiwen@wp.pl; Dr. Janusz Tomaszewski, Adiunkt - Wyższa Szkoła Administracji i Biznesu im. Eugeniusz Kwiatkowskiego w Gdyni, e-mail: j.tomaszewski@wsaib.pl

INTRODUCTION
Barely 40 years have passed since the creation of TRANSIT system – the first satellite navigational system. In this time, thanks to the sudden development of science and technology, satellite systems have dominated navigation. They have also found application in many other fields connected with data transmission. Apart from navigation, they have covered broadly understood Maritime Economy. Maritime economy is a set of activities of functional entities whose objective is skilful acquisition, processing and division of sea resources. This also entails principles and mechanisms as well as institutional foundations of the operation of those entities. The principal subject of the management of the wealth source concerns sea resources whose effective acquisition at present is not possible, with regard to the scale of the demand, without the application of the most modern technological inventions.

Current communication cannot practically function without satellite systems. So far, two main, global satellite systems have been used, namely GPS and GLONASS, systems that provide navigational data to the users all over the world. Although sufficient for many consumers, their navigational possibilities have certain limitations: lack of international, civil supervision (both systems are military systems), GPS as well as GLONASS cannot meet all the requirements of civil aviation (e.g. accurate approach to landing), lack of possibility of relaying information on improper operation of the system that should not be in use at such moment. Hence, there is the demand of various groups of users for additional system improving the possibilities of GSP/GLONASS. In the context of the problem contained in the article, a system that responds to the needs of maritime economy whose key elements are maritime navigation and sea transport. Hence, there is a need to answer the following question: what applications can EGNOS system be used for?

SAFETY AND SATELLITE NAVIGATION SYSTEMS
In response to the needs of aviation, sea navigation and transport and of other users the Global Navigation Satellite System GNSS-1 has been created that assumes the increase of the potential of GPS/EGNOS in order to meet the requirements of civil aviation. It has been established on the basis of the European EGNOS system, American WAAS (Wide Area Augmentation System) and Japanese MSAS (Multi-national Satellite Augmentation System).These systems are only auxiliary elements and they do not constitute independent systems of satellite positioning. Contrary to other continents, Europe, the place of the operation of EGNOS, is characterised with a high and even density of traffic in its whole territory. This creates the need for a system to operate that would be equally accessible and accurate in all locations within Europe.

Three civil institutions creating ETG (European Tripartie Group) have been dealing with the execution of GNSS – 1 project in the territory of Europe since 1997: ESA (European Space Agency), EC (European Commission) and EUROCONTROL (European Organisation for the Safety of Air Navigation).

The system called EGNOS (European Global Navigation Overlay System) together with its counterparts WAAS in the US and MSAS in Japan is to constitute GNSS-1. It had been assumed that final tests would be completed and EGNOS would be fully operational in 2004. Meanwhile, the test version of ESTB (EGNOS test BED), whose capabilities are expanded together with the expansion of the proper system, have been made available.

European Space Agency has been dealing with the development of EGNOS. The system shall consist of three main segments: ground segment, space segment and user segment. The appropriate architecture as well as optimal location of particular elements have significant influence on the parameters of the operation that is the condition for using EGNOS for various applications. Ground segment is most developed and complex element of the system. Its components create a network of stations evenly distributed in the whole territory of Europe. The ground segment consists of the following elements: (fig. 1.):

• (RIMS) Range and Integrity Monitoring Stations,
• (MCC) Mission Control Centre,
• (NLES) Navigation Land Earth Station,
• (PACF) Performance Assessment and System Checkout Facility.

All the elements of ground segment are interconnected by EWAN Information Network (EGNOS Wide Area Network).

Fig. 1. The distribution of the elements of the ground segment

Source: M. Spagnulo, R. Giubilei, L. Bardelli, G. Leggeri, S. Badessi, An Innovative Satellite Payload for Advanced Navigation Systems. Proceedings of the 6th International Conference on Integrated Navigation Systems, S. Petersburg 1999.
Space segment is responsible for the transmission of data received from NLES stations to the users located within the range of operation of EGNOS. It is created by navigational transponders located at the three geostationary satellites:

• Inmarsat III, the Eastern Region of the Atlantic Ocean (AOR-E) – 15.5º W.
• Inmarsat III, the Indian Ocean Region (IOR) – 64º E.
• ESA ARTEMIS – 21.5º E.

Fig. 2. The distribution of satellites and the range of the operation of the transponders

Source: M. Spagnulo, R. Giubilei, L. Bardelli, G. Leggeri, S. Badessi, An Innovative Satellite Payload for Advanced Navigation Systems. Proceedings of the 6th International Conference on Integrated Navigation Systems, S. Petersburg 1999.

The European Space Agency is currently leasing the navigational transponders at two satellites Inmarsat III. Each transponder has a nominal life period of 13 years and should be functional until do 2008/2009.

User segment – EGNOS sends its information via geostationary satellites and the format of broadcast signals is regulated by SARPS (Standards and Recommended Practises) created by the International Civil Aviation Organisation (ICAO). Thanks to this, the EGNOS signal is sent at the same frequencies as GPS and is similar in its structure. Following minor modifications, a GPS receiver may be able to receive a message of EGNOS. The basic tasks of EGNOS signal receiver include:

• Processing radio signal – decoding and demodulation of data (navigational data “+” corrections of GPS/GLONASS),
• Correction of pseudo-distance measurements received from GPS/EGNOS system satellites through the application of corrections received in the EGNOS message(s),
• Processing of algorithms concerning the data about the reliability of the signal.

The reception of information sent by geostationary satellites has its limitations because the transmitted signal is subject to interference by obstacles in the configuration of the land surface (narrow canyons) as well as high constructions that make it difficult or even impossible for the information to be received. Therefore, there is a need to use another form of communication for this type of obstacles not to limit the use of EGNOS. One solution is the possibility to connect with EGNOS system. The European Space Agency created a method of satellite signal reception via the Internet – SISNet (Signal In Space through the Internet).

SISNet system is primarily designed to ensure, through wireless connections of GSM/GPRS, the accessibility of EGNOS messages from the Internet networks. The system is to meet the following conditions:

• It must not interfere with the structure and the operation of EGNOS system,
• The connections with SISNet server must be infallible and stable,
• There must be the possibility of the adaptation of GPS receivers to use SISNet services,
• The GPS receiver operating in SISNet mode should present higher possibilities than a standard GPS receiver,
• There must be the possibility of simultaneous control/supervision of a number of SISNet users in order to guarantee the efficiency of the system.

System SISNet (figure 3.) consists of four basic elements:

Base Station – it is a computer connected to the GPS/EGNOS receiver via series port. It acquires EGNOS messages and relays them to the Data Server.

Data Server – it is a computer with high processing power, optimised for work with large number of connected users. Its main task is the relay of EGNOS messages to the system users in real time.

User Application Software – it is a set of software enabling the current reception of EGNOS messages (one message per second or 250 bits/s) from the Data Server. Each SISNet application is characterised by work stages and calculation processes that ensure proper functionality.

Web Server – it serves to store the received data sent from the Data Server via FTP protocol.

Fig. 3. The structure of SISNet system.Source: author’s work.

REQUIREMENTS FACING CURRENT NAVIGATIONAL SYSTEMS
The contemporary development of transport enforces the creation of systems of navigation that, given the present speeds of travelling vehicles and the heavy traffic, can ensure safety of all users of transport networks. Therefore, an ideal system of navigation should take into consideration the needs of its particular users regardless of their position; it should be infallible and highly reliable. The type of means of transport and the environment in which the vehicle is currently moving influence the quality of navigational data indispensable in assuring the correct execution of a task and therefore its safety. The users of navigational systems have been divided into three groups: air transport, sea transport, and land transport. Principal criteria making EGNOS and other systems of navigation suitable for particular users include the following: accuracy, reliability, accessibility, infallibility and continuity. Detailed formulas of calculating the parameters for particular criteria see: D. Iwen, Navigational characteristics of EGNOS system, thesis work written under the guidance of Commodore Lieutenant Cezary Specht PhD Eng, Navy Academy, Gdynia 2004, copied typescript, unpublished

Table 1. Sea transport requirements.

Table 1. Sea transport requirements.

1) Continuity is not so important for ocean and inshore navigation2) More rigorous performance may be required for units moving at the speed of over 30 knots.Source: author’s work based on the data of European Space Agency

Considering the subject included in the article we are dealing mainly with sea transport, (table 1.). Users of this form of transport use all aspects of navigational infrastructure and positioning, e.g. autonomous navigational appliances, VTS (Vessel Traffic Services). The presence of some elements is imposed by the international regulations that consider them indispensable for navigation (compass, logbook, radar and others). Present tendency to integrate all those appliances in order to automate the process of navigation (e.g. ECDIS – Electronic Chart Display and Information System) require high quality navigational data at all times and in all conditions. The requirements posed by sea transport greatly depend on the type and size of the vessel and the region of its operation. Depending on the type of environment and traffic in various water regions, the navigational requirements of the sea navigation and transport refer to five zones: the ocean, the inshore zone, inland waters, harbour basins.
The assessment of the possibility of using EGNOS system in sea transport and other users of the system

Considering the measurements in accordance with various criteria, it should be stated that the discussed system guarantees effectiveness in all types of transport. By applying corrections included in EGNOS messages, the errors affecting the correctness of positioning, in comparison with errors in the distance measurement by GPS system, are greatly reduced (table 2).

Table 2. Comparison of GPS errors with EGNOS errors

Egnos Table 2

Source: author’s work based on the data of European Space Agency

Table 3. Possibilities of using EGNOS system.

Source: author’s work based on the data of European Space Agency

Considering the zones in sea navigation and transport it should be stated that, as regards its accuracy and reliability, the system is not applicable only in ports/harbours. In all other zones, the main criteria as accuracy, accessibility and reliability are met (table 3). Other criteria: continuity and infallibility are connected with accessibility and characterise the work of EGNOS system. The biggest influence on their values is made by such factors as excessive multitracking, satellite movements, GPS failures and receiver noise of RIMS stations. Continuity as well as infallibility is ensured at the level of 99.97 in the whole territory of Europe.

CONCLUSIONS
1. EGNOS system is the European contribution into the creation of the first generation Global Satellite Navigation System. It should be noted that this is the first European system of satellite navigation constructed by civil institutions.
2. The experiences collected during its creation and exploitation are extremely helpful during the construction of GALILEO, the Global European Satellite Navigation System. Test version of EGNOS system has been in operation almost since its beginning and thanks to the test version there has been an ongoing introduction of changes resulting from the use of the system.
3. The advantage of the system is the format of the radio signal that can be received by GPS receivers (following modifications) – this does not force potential users of the system to incur additional costs connected with the purchase of a new receiver.
4. EGNOS signal is compliant with SBAS standards included in RTCA DO-229, which enables the cooperation with similar systems created all over the world. Another interesting aspect of EGNOS system is an original project of SISNet – access to EGNOS messages in the Internet via GSM/GPRS links, which makes it possible to use navigational services in case of limited transmissions from geostationary satellites.
5. As the result of the assessment of the possibility of using EGNOS in navigation it has been stated that the system is applicable for all groups of users. In the case of sea navigation and transport it is useful in ocean and inshore navigation, in narrow passages and in inland water navigation. The system also meets the needs of vessel traffic management.
6. An infallible system in this field means favourable development of sea transport. Ships create added value carrying cargo and passengers. Transported mass of goods is incomparably larger than in other means of transport. This is therefore much more significant to apply the systems that increase safety in such area of economy.

BIBLIOGRAPHY

1. Spagnulo M., Giubilei R., Bardelli L., Leggeri G., Badessi S., An Innovative Satellite Payload for Advanced Navigation Systems. Proceedings of the 6th International Conference on Integrated Navigation Systems, S. Petersburg 1999.
2. Benedicto J., EGNOS - The first European Implementation of GNSS Project status overview Proceedings of the GNSS’99″, Genoa 1999.
3. Giubilei R., Bardelli L, Proceedings of the 2nd EGNOS System Test Bed Workshop, Nice 2001.
4. Toran F., Ventura-Traveset J. SISNET User Interface Document. ESA document. Ref. E-RD-SYS-E31-010, Toulose 2003.
5. Iwen D., Nawigacyjna charakterystyka systemu EGNOS, [Navigational characteristics of EGNOS system], AMW [Navy Academy], Gdynia 2005.
6. Iwen D., Wielokryterialne porównanie systemów nawigacji satelitarnej [Multi-criteria comparison of satellite navigation systems], AMW [Navy Academy], Gdynia 2004.