GPS alone does not always provide adequate performance, particularly in demanding environments or where a high level of integrity is required. The accuracy and integrity of GPS can be greatly enhanced by the use of information derived from observations from others sensor technologies The use of augmentation systems takes many forms but all share the same basic objective of improving GPS performance and/or trustworthiness.
Space Based Augmentation Systems (SBAS) are typically designed to improve GPS system integrity and accuracy for aircraft navigation and particularly landing. SBAS satellites broadcast correction messages back to Earth, where suitably enabled receivers use the corrections and integrity information to improve accuracy and integrity. The USA, Europe and Asia are developing their own SBAS systems. In Europe, the European Geostationary Navigation Overlay Service (EGNOS) system exists and is undergoing certification testing. In the USA there is the Wide Area Augmentation System (WAAS), in Japan the Multi-functional Satellite Augmentation System (MSAS) and India is focusing on the GPS Aided Geo Augmented Navigation (GAGAN) system.
Most of Spirent's GPS simulators include the capability to place satellites at geostationary positions and to allocate channels to provide WAAS, EGNOS and MSAS signals to allow testing of these SBAS receiver functionality.
An alternative approach to augmentation is to transmit correction messages from ground-based augmentation systems. An example is the Local Area Augmentation Systems (LAAS), which allows a suitably equipped receiver to derive enhanced accuracy and integrity information in a local area, at an airport for example, where there are stringent requirements necessary to be met to land a commercial aircraft. LAAS can be tested using Spirent's unique LAAS VHF data broadcast signal simulator.
For both SBAS and GBAS, simulation lends itself ideally to certification and type approval applications. The availability of controlled, repeatable signals ensures that specific test conditions can be readily replicated for different units under test. In addition, simulation enables testing of safety cases that include errors or anomalies in the transmitted signals. Testing under these circumstances ensures that the way the system under test deals with these unusual conditions can be understood and verified.
In Japan, in addition to MSAS, the Quasi-Zenith Satellite System (QZSS) is currently under development and scheduled for launch in 2010. QZSS will provide additional overhead ranging sources in the Japan area as an augmentation service to GPS. Highly elliptical orbits allow these satellites to dwell at high elevations in the sky allowing enhanced coverage in urban canyons. Spirent is the first simulator manufacturer to offer a QZSS simulation solution.
