October 23, 2021

Biology Reporter

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More accurate measurements of distance to satellites and the moon

More accurate measurements of distance to satellites and the moon

Scientists from the University of Life Sciences in Wroclaw have proposed a new method of laser distance measurement, thanks to which more accurate observations of the shape of the Earth, the melting of glaciers and changes in ocean water levels will be proposed.

Researchers from the Institute of Geodesy and Geographical Information at UPWr have demonstrated that the current approach to correct errors caused by laser beam delay in the atmosphere was flawed. They propose a measurement that takes into account the thickness of the layers of the atmosphere through which the laser passes.

Laser measurements are based on recording the time difference between the moment a laser pulse is sent at the station and the time the same pulse returns after being reflected by a retroreflector on a satellite or the moon. During the measurement, the laser beam passes through the Earth’s atmosphere twice, where it is deflected and delayed. The technology of laser detectors allows to obtain an accuracy of less than a millimeter. However, errors in determining the delay of the laser beam in the atmosphere are many times greater and are the main source of errors in laser measurements of satellites and the Moon.

Innovative way to correct errors

The station’s meteorological readings are used to determine the laser delay value. For these measurements, the correction is calculated depending on the height of the satellite above the horizon and the initial value of the delay of the laser beam. Scientists from Wroclaw propose to analyze all distances measured at all stations. Then, corrections can be calculated for each station, which is directly proportional to the delay of the laser beam caused by direct meteorological measurements and the thickness of the atmosphere.

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According to scientists from the group led by Prof. Krzysztof Sośnica, it is enough to calculate such a meteorological correction once a week for each laser station. As a result, the calculations remain stable even for stations with a small number of recorded laser measurements of the satellites, and at the same time the error caused by the delay in the atmosphere is almost completely removed. The method developed by the Polish team allows for 75 percent effective disposal. Up to 90 percent systematic errors in laser measurements caused by air delay errors.

According to the authors, the method for reducing meteorological errors has a chance to become a standard in laser distance measurements for satellites. Due to its simplicity and universality, it will increase the accuracy of even historical observations of the moon and satellites. It will also allow the detection of erroneous barometer readings that previously negatively affected satellite observations of the Earth and the Moon. This will translate into improved future and past observations of the shape of the Earth, the so-called Earth bodies, changes in the Earth’s center of mass and irregular circulation, and monitoring of glacier melting and changes in ocean water levels.

Why are distances measured to satellites?

Thanks to laser measurements of artificial and natural Earth satellites, scientists have learned what the gravitational constant and Earth’s mass are, by how much the Earth’s level has changed over time. They can correct and calculate position corrections for the Galileo and GLONASS satellites. They determined where the Earth’s center of mass is and how it is traveling in time due to melting glaciers in Greenland. Laser measurements of the moon revealed that the moon is moving away from the Earth by 3.8 cm per year. They made it possible to accurately describe the fluctuations in the movement of the Moon, that is, librations and review the origin of the silver globe.

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A research group led by Prof. Sośnica deals with the development of laser and microwave technologies in satellite geodesy, as well as with determining the exact orbits of satellites and parameters describing the Earth. At UPWr’s Institute of Geodesy and Geographical Information, the Analysis Center associated with the International Laser Range Locating Service (ILRS) has been operating since 2017. The Center is responsible for monitoring the quality of satellite orbits of the Global Navigation Satellite Systems (GNSS): Galileo, GLONASS, BeiDou and QZSS using Orbits based on microwave observations and direct laser measurements. As the only one in the world, the center in Wroclaw specializes in a combination of two satellite observation technologies: laser and microwave GNSS.

PAP – Science in Poland

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