Global Navigation Satellite Systems
Signals from global navigation satellite systems (GNSS, e.g., the American GPS, the Russian GLONASS, and the coming European Galileo system) can be used to tackle a large variety of scientific questions. Onsala Space Observatory houses several GNSS receivers used for different purposes: accurate positioning, atmospheric remote sensing, and measurements of the sea level.
The GNSS receiver in the SWEPOS network at Onsala Space Observatory.
Copyright: Magnus Thomasson
The position (and the time) of a GNSS receiver is determined by measuring the travel time of signals transmitted from at least four GNSS satellites. By having many satellites in the sky and many receivers on the ground favourable geometries are obtained and very accuarate positioning (and timing) can be realized.
Onsala Space Observatory is one of the fundamental reference station in the SWEPOS network (see information about SWEPOS at Lantmäteriet, or the SWEPOS web site (only in Swedish)).
Atmospheric remote sensing from the ground
GNSS signals travelling through the atmosphere are delayed (compared to the speed of light an vacuum). The ionosphere (from roughly 80 kilometres to 1000 kilometers height, contains free electrons. The number of free electrons can be measured by using GNSS signals at two frequencies. In the neutral atmosphere, ranging from the ground up to the ionosphere, the amount of water vapour can be measured when satellite signals are received from satellites at many different elevation angles, thereby travelling through the layers of water vapour along paths of different lenghts.
The traditional way to observe sea level is with tide gauges, which results in measurements relative to the Earth’s
crust. The Earth’s crust is, however, continuously moving and in order to fully understand sea level change processes,
measurements of sea level in relation to the Earth’s centre of gravity are necessary. One way to achieve this is by using GNSS receivers. Different techniques exist. In Onsala, an experimental station using two GNSS receivers and two antennas has been installed close to the sea. One of the antennas is up-looking for direct signals, and one is down-looking for signals reflected from the sea surface. See also Tide gauges.
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