Wide area augmentation system

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Wide Area Augmentation System ( WAAS ) is part of the Satellite Based Augmentation System (SBAS ) to improve the existing US GPS . WAAS signals are sent by individual additional satellites on the same frequencies as GPS and are used to disseminate correction data for the GPS signals distorted by the signal propagation in the ionosphere .

Reception area (footprint) from WAAS over North America

WAAS is broadcast over the United States by two to four geostationary satellites and is inaccessible to all other regions of the world. In Europe and Japan, systems compatible with EGNOS and MSAS are replacing WAAS. Some systems are currently still under construction.

Since navigation by means of civil GPS is usually only based on one transmission frequency (C / A code on the so-called L1 frequency), atmospheric effects cannot be corrected with sufficient accuracy. Many inexpensive GPS receivers also have only one receiving part for only one receiving frequency.

After the artificial errors in the civilly usable C / A code of GPS were switched off on May 1, 2000, the largest remaining errors are atmospheric runtime effects. This means that the radio signal does not always need exactly the same length of time to pass through the atmosphere, and therefore the time measurement, which is important for determining the position, is not exactly correct.

With systems such as WAAS and the fully compatible EGNOS , additional correction signals are transmitted on the L1 transmission frequency from geostationary satellites such as Inmarsat . So that simple single-channel receivers can also benefit from this correction signal, the correction data are transmitted on the same frequency as the GPS data - but with different GPS identifiers (satellite number 33 and up).

One of WAAS's 30+ ground stations to determine correction data in Barrow , Alaska

The correction data is obtained from a large number of stationary ground stations that record atmospheric errors in their reception area and then calculate correction factors for different geographical regions. From this, WAAS / EGNOS-compatible GPS receivers can determine the additional runtime effects of the signals in the atmosphere more precisely, whereby accuracies of 0.3 to 1 m can be achieved. In comparison, without this correction data, only maximum spatial resolutions of 5 to 15 m are possible with civil GPS.

The use of signal improvement techniques often results in a higher energy requirement for the receiver. However, users of mobile receivers on the ground often have the problem of not having a sufficiently clear view of the geostationary satellite orbits. The reception of WAAS / EGNOS signals is usually only possible on hills or in the open sea.

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