Flood risks: More accurate data due to COVID-19
Study investigates the influence of lockdown on geodetic measurements


Date:
September 23, 2020
Source:
University of Bonn
Summary:
Emerging use of Global Navigation Satellite System (GNSS) makes
it possible to continuously measure shallow changes in elevation
of Earth surface. A study now shows that the quality of these
measurements may have improved significantly during the pandemic,
at least at some stations. The results show which factors should
be considered in the future when installing GPS antennas.



FULL STORY ========================================================================== Emerging use of Global Navigation Satellite System (GNSS) makes it
possible to continuously measure shallow changes in elevation of Earth
surface. A study by the University of Bonn now shows that the quality of
these measurements may have improved significantly during the pandemic,
at least at some stations. The results show which factors should be
considered in the future when installing GPS antennas. More precise
geodetic data are important for assessing flood risks and for improving earthquake early warning systems. The journal Geophysical Research
Letters now reports on this.


==========================================================================
A number of countries went into politically decreed late hibernation
at the onset of the Covid-19 pandemic. Many of those affected by the
lockdown suffered negative economic and social consequences. Geodesy,
a branch of the Earth Science to study Earth's gravity field and its
shape, on the other hand, has benefited from the drastic reduction in
human activity. At least that is what the study now published in the Geophysical Research Letters shows. The study, which was carried out by geodesists from the University of Bonn, investigated the location of a
precise GNSS antenna in Boston (Massachusetts) as an example.

GNSS receivers can determine their positions to an accuracy of a
few mm. They do this using the US GPS satellites and their Russian counterparts, GLONASS.

For some years now, it has also been possible to measure the distance
between the antenna and the ground surface using a new method. "This
has recently allowed our research group to measure elevation changes
in the uppermost of soil layers, without installing additional
equipment," explains Dr. Makan Karegar from the Institute of Geodesy
and Geoinformation at the University of Bonn. Researchers, for instance,
can measure the wave-like propagation of an earthquake and the rise or
fall of a coastal area.

The measuring method is based on the fact that the antenna does not only
pick up the direct satellite signal. Part of the signal is reflected
by the nearby environment and objects and reaches the GNSS antenna with
some delays. This reflected part therefore travels a longer path to the antenna. When superimposed on the directly received signal, it forms
certain patterns called interference. The can be used to calculate the
distance between the antenna and the ground surface which can change over
time. To calculate the risk of flooding in low-elevation coastal areas,
it is important to know this change - - and thus the subsidence of the
Earth surface -- precisely.

This method works well if the surrounding ground is flat, like the surface
of a mirror. "But many GNSS receivers are mounted on buildings in cities
or in industrial zones," explains Prof. Dr. Ju"rgen Kusche. "And they are
often surrounded by large parking lots -- as is the case with the antenna
we investigated in Boston." Cars cause disturbance In their analysis,
the researchers were able to show that parked cars significantly reduce
the quality of the elevation data: Parked vehicles scatter the satellite
signal and cause it to be reflected several times before it reaches
the antenna, like a cracked mirror. This not only reduces the signal
intensity, but also the information that can be extracted from it: It's "noisy." In addition, because the "pattern" of parked cars changes from
day to day, these data can not be easily corrected.

"Before the pandemic, measurements of antenna height had an average
accuracy of about four centimeters due to the higher level of noise," says Karegar. "During the lockdown, however, there were almost no vehicles
parked in the vicinity of the antenna; this improved the accuracy to
about two centimeters." A decisive leap: The more reliable the values,
the smaller the elevation fluctuations that can be detected in the upper
soil layers.

In the past, GNSS stations were preferably installed in sparsely
populated regions, but this has changed in recent years. "Precise
GNSS sensors are often installed in urban areas to support positioning
services for engineering and surveying applications, and eventually for scientific applications such as deformation studies and natural hazards assessment," says Karegar. "Our study recommends that we should try to
avoid installation of GNNS sensors next to parking lots."

========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Makan A. Karegar, Ju"rgen Kusche. Imprints of COVID‐19
lockdown on
GNSS observations: An initial demonstration using GNSS
interferometric reflectometry. Geophysical Research Letters, 2020;
DOI: 10.1029/ 2020GL089647 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200923124736.htm

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