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Antenna

Apparatus for transmitting and receiving electromagnetic waves. In our project we use high-end GNSS antennas and receivers that can determine the position with very high precision of a few centimeters or even less.

Azimuth

One of the coordinates in the horizontal system. Azimuth is the angle, measured positive towards the east, between north and the projection on the horizon of the direction in which an object is observed.

BeiDou

A regional and global navigation satellite system developed and operated by the Peoples Republic of China. The name refers to the constellation (Big Dipper or Plough) used to find the north direction in stellar navigation.

Elevation

The angle measured from the observer’s horizontal plane with respect to the station-satellite line-of-sight.

Equator

An imaginary great circle on the Earth body, which is perpendicular to the Earth’s axis of rotation and midway between its geographical poles. The equator separates the northern and southern hemispheres, and defines the reference plane for the system of geographic coordinates at which latitude is defined to be 0°.

GALILEO is the European Global Navigation Satellite System (GNSS)

Geodesy is the Earth science of accurately measuring and understanding Earth’s geometric shape, orientation in space and gravitational field.

GLONASS (Global’naya Navigatsionnaya Sputnikova Sistema)

Global navigation satellite system of the Russian Federation, providing global permanent positioning, navigation and timing service for land, air and space users worldwide. GLONASS is a dual-use system providing both authorized and open access services.

GNSS (Global Navigation Satellite System) refers to a constellation of satellites providing signals from space that transmit positioning and timing data to GNSS receivers. The receivers then use this data to determine position and its time variation (navigation). Examples of GNSS include Europe’s GALILEO, the USA’s NAVSTAR Global Positioning System (GPS), Russia’s Global’naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) and China’s COMPASS-BeiDou Navigation Satellite System. The term is currently used collectively for GPS, Galileo, GLONASS, and BeiDou.

GPS (Global Positioning System)

A global navigation satellite system owned and operated by the United States Air Force.

Gradient

The gradient of a physical quantity indicates how much its values changes in space and in which direction the change is greatest. In our case the value of the PVW depends on the observation site. The associated gradient is a vector that points in the direction of the strongest PVW increase. The magnitude of this vector is a measure of how large the increase is.

Horizon

The imaginary line of intersection between a plane tangent to the surface of the Earth at the observer’s position and the celestial sphere. The sun (and also an artificial satellite) disappears behind the horizon when it gets out of view.

Integrated Water Vapor (IWV)

Total amount of water vapor present in a vertical atmospheric column. Taking into account the density of liquid water, the IWV expressed in kg/m² is equivalent to the Total Precipitable Water (TPW) expressed in mm of liquid water. Strictly speaking, however, not all the water vapor is actually precipitable.

Ionosphere

The ionized part of the upper atmosphere from 50 km up to about 1000 km height.

Ionosphere-free Combination

A linear combination of two GNSS signals having different frequencies, which eliminates the dominant contributions of ionospheric path delays that vary with the inverse square of the signal frequency.

Multipath

The phenomenon whereby a transmitted GNSS signal reaches the receiver via multiple paths due to reflection and diffraction in addition to the direct-path signal. The superposition of direct and non-direct-path signals results in errors in the positioning.

Noise

Random fluctuations in a measured signal.

Numerical Weather Prediction

It is a part of meteorology for the purpose of weather-forecasting. From the state of the atmosphere at a given time (data-set of measurements) its future state is calculated by solving numerically (i.e. approximately) the relevant physical equations with the help of supercomputers.

Orbit

The trajectory of a natural or artificial satellite around a central body – the Sun in the case of planets in the solar system, or the Earth in the case of artificial Earth-orbiting satellites.

Precipitable Water Vapor (PWV)

In a vertical column of air extending from the ground to the top of the atmosphere with a base of 1 m², the TPW (Total Precipitable Water Vapor) content of this column equals the amount of water if all water vapor was condensed and accumulated on the bottom of the column; it is expressed in mm of liquid water.

Pseudorange

A distance-like measurement obtained from the time difference between transmission and reception of a radio signal and the known speed-of-light. Due to time offsets between the local clocks measuring the two times, the measurement differs from the true distance and includes a contribution related to these clock offsets. It is hence called a pseudorange.

RINEX (Receiver Independent Exchange format)

Is a data interchange ASCII format for raw GNSS data. Independent means that it does not depend on the manufacturer of the receiver acquiring the raw data in binary format. This allows users to collect and post-process the received data for a number of applications.

Slant Total Delay (STD) and Zenith Total Delay (ZTD)

The extra time needed for a signal propagating through the troposphere in a given (slant) direction compared to the propagation time in vacuum. It is often expressed in units of length, using the speed of light in vacuum for the conversion. For practical reasons, the slant total delay (STD = SHD+SWD) is divided into a slant hydrostatic delay (SHD) and a slant wet delay (SWD). A special case is the delay in the zenith direction. This zenith total delay (ZTD) is also divided into a zenith hydrostatic delay (ZHD) and a zenith wet delay (ZWD). The ZHD is approximately 2 − 3m at sea level and proportional to the Atmospheric Pressure. The ZWD can be anything between 0 − 40 cm, depending on the climate zone and the specific weather conditions.

Stratosphere

Is the layer in the Earth’s atmosphere above the troposphere. It starts in about 8 − 13 km, but the actual value depends on the weather conditions and varies systematically with the latitude and the season. The top is at around 50 km height.

Troposphere

Is the lowest layer of the Earth’s atmosphere, where the temperature on the average decreases with height. It ends at the tropopause, which is located in the range from 8 − 13 km, depending on the weather conditions, and varies systematically with latitude and season. The troposphere contains the weather, e.g., clouds and precipitation.

Tropospheric Refraction

Describes the signal propagation delay and bending induced by the electromagnetic neutral part of the atmosphere (slant total delay). The wet delay and hydrostatic (dry) delay components are typically separately accounted for in GNSS measurement processing.

Zenith

An imaginary point on the celestial sphere that is the projection of a local vertical direction. The Sun reaches the observer’s zenith when it is 90° above the horizon, and this only happens in proximity to the equator between the Tropic of Cancer and the Tropic of Capricorn.

Zenith Total Delay (ZTD)

See Slant Total Delay (STD)

References

• Erdbeobachtung mit Navigationssatelliten, GFZ-Journal (OER in German)
  
• Ahrens C.D. Meteorology Today: An Introduction to Weather, Climate, and the Environment (9th Edition). Brooks/Cole-Belmont, CA 94002 USA. ISBN-13: 978-0-495-55573-5. Pp. 620.
• OSCAR (Observing Systems Capability Analysis and Review Tool), World Meteorological Organization.
• Teunissen, P.J.G.,Montenbruck, O., 2017. Springer Handbook of Global Navigation Satellite Systems. Springer International Publishing, Cham https://doi.org/10.1007/978-3-319-42928-1.