Puertos del Estado

Navigation Satellite Systems

Within the group of radioelectronic aids to navigation, the most significant innovation was the development of satellite-based systems that determine the position of a fixed or mobile point. These positioning systems can be classified into two groups. However, only the first group can be considered properly a navigation system:

• Positioning systems that consist of a group of stations emitting signals. These signals are analyzed by a receptor that allow the mariner to know his position (i.e., the position of his receptor antenna) as well as another helpful information.
  This group includes the following systems: NNSS (Navy Navigation Satellite System), also called TRANSIT, developed by the U.S. Army and already disappeared; NAVSTAR-GPS (Navigation System Time and Ranging-Global Positioning System), developed and used by the U.S. Defense Department; and GLONASS (Global Navigation Satellite System), developed by the former URSS and currently controlled and used by Russia.
• The second group includes the RDSS (Radio Determination Satellite Services). In this case, the stations ask the users and these ones reply through their receptors-transmitters. Next, the stations analyze the information received and inform the users about their position. These stations are based on both the mobile and radio communications.
  The following systems can be included in this category: GEOSTAR and STARFIX (both operated by U.S.A), the positioning service of INMARSAT-Standard C (U.S and U.K.), ARGOS (France) and EUTELTRACS (promoted by ALCATEL in various European countries).

The positioning systems were initially designed with military purposes. Later they began to be used in other fields, sometimes with some restrictions, and its current design has a worldwide perspective.

The users'equipments are quite complex since they include calculation devices to process a high volume of information as fast as possible. These equipments incur in high costs, not only during the implementation phases but also during its use since the satellites must be renewed periodically.
These systems consist of three subsystems or segments: special segment- that consists of satellites-, control segment responsible for following up the satellites, estimating the position and correcting the mistakes-, and user segment- that consists of the users equipments.
The satellite navigation systems are also GNSS (Global Navigation Satellite System). There exit two generations: GNSS-1 and GNSS-2. The first generation includes the current GPS and GLONASS as well as other differential systems based on them. These differential systems provide a more accurate position and have more coverage in wider areas than the local differential systems such the maritime DGPS. They are termed Navigation Overlay Systems. The second generation includes the new GPS and GLONASS, with better functionalities and services, and the European project GALILEO. All these systems have worldwide coverage.

The Navigation Overlay Systems are satellite navigation differential systems that use inland stations for the correction calculation and emit these calculation through the satellite communications. These systems are mainly used in the oceanic navigation; meanwhile the local coverage systems are used preferably in the coastal navigation or in restricted areas. Nowadays three systems of this type are being developed:
EGNOS (European Geostationary Navigation Overlay System) in Europe, WAAS (Wide&Area Augmentation System) in U.S.A and MSAS (Multi-function Satellite-based Augmentation System) in Japan. The last two systems are designed for the air traffic.