Augmentation Systems

Augmentation Systems

With the help of augmentation systems, the accuracy, continuity and integrity of global satellite navigation systems has increased enormously. One differentiates between two basic classes of augmentation systems. For one, there is the Space-Based-Systems and on the other hand the Ground-Based-Systems.

Space-Based-Augmentation-Systems (SBAS) just basically consist of any number of base stations that are distributed over a large area on the ground and generate with the help of master stations correction data for each region. These stations are then connected to one or several geostationary satellites, which provide the correction data available to users. This data transfer is usually held on the GPS L1 frequency and can thus be processed directly by the recipients.

Among the currently under construction Space-Based-Augmentation-Systems include the Wide Area Augmentation System in the U.S. (WAAS), the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-Function Transport Satellite Augmentation System (MSAS). All three systems cover after its scheduled completion various regions of the earth. The switching threshold between the systems or regions are firmly on specified longitude.

Briefly below the rough structure of the individual systems and their components will be explained:

•    WAAS
The full Wide Area Augmentation System will consist upon completion of 25 reference stations, two master stations and three uplink stations, which serve as connection to two geostationary INMARSAT III communication satellites. It will also provide a certifiable service for oceanic, en-route, non-precision and precision approach in the U.S., as well as in parts of Canada and Mexico. The target position accuracy should be about 7m vertical (LEP95) and horizontally (CEP95).

•    EGNOS
The European Geostationary Navigation Overlay Service when completed will consist of a total of 44 ground stations, most of which are located in Europe. He will also have three geostationary satellites and is compatible and interoperable with WAAS and MSAS. As with WAAS also a horizontal position accuracy of better than 7 m is required. In practice, however, a much higher accuracy is expected in the meter range. The experimental operation was begun in July 2005 and showed an outstanding performance with respect to the achieved accuracy (less than 2 m) and availability (99%). A certification and approval of the EGNOS system for use in safety-critical applications (Safety of Life) was held in March 2011.

An information portal on EGNOS and a current performence map of the system are available at EGNOS-Portal and EGNOS-Capabilities.

•    MSAS
The Japanese Multi-Function Transport Satellite Augmentaion system will have upon completion of one to two geostationary communications satellites and ground stations with a total of eight, six of which are located in Japan. In addition, one station will be placed in Australia and one in Hawaii.

As an important future element of satellite navigation, especially in air traffic, the method of Ground Based Augmentation Systems (GBAS) has established. In contrast to SBAS, GBAS has no geostationary satellites to send correction data to the users. Thereby the transmission takes place mainly over radio signals.

Functional Principle of GBAS (Source: Eurocontrol)

GBAS has an particular importance in the field of sattelite aided precision approaches and landings. It should serve as a future replacement for the currently used ILS (Instrument Landing System), because significant cost savings are expected with regard to acquisition, maintenance and flight inspection. In addition, the opportunity created by GBAS approaches to satellite and CATII and CATIII carried out (see table below). For this to be installed in the vicinity of airports on local ground monitor stations whose data are then transmitted by radio to the aircraft.

Singular flight phases of the precision approach, together with landing, place high demands on exactness, integrity, continuity and availability of navigation data. Besides the known ILS, GBAS is gaining more and more interest. Because of this, German air traffic control is testing precision approaches using GBAS at Bremen (CAT I, “ILS look-alike”). Compared to ILS, significant savings are expected, especially regarding purchase, maintenance and flight calibration. In this context, GNSS Landing System (GLS) describes the use of satellite navigation methods for determining the flight path during approach. The basic principle is shown in Fig. 5-16.

The category of Precision Approaches (PA) is divided into Categories of Operation (CAT). These are defined on the basis of the Decision Height (DH) and the visible range of the runway, Runway Visual Range (RVR). The Decision Height defines a specific height during Precision Approach at which a go-around has to be initiated, should the required visibility not be ensured.

Category of Operation

Decision Height (DH)

Runway Visual Range (RVR)


DH > 60m

RVR > 550m


30m < DH < 60m

RVR > 350m


0m < DH < 30m

RVR > 200m


0m < DH < 15m

50m < RVR < 200m


no DH

no RVR limitation

Decision Heights for CAT approach


The following figure shows the classification of methods of satellite navigation, regarding their availability and application to flight guidance.


Systems and Requirements