Smartphones, smartwatches, navigators: these are just some of the devices equipped with GPS. We often use them to determine our position and get directions but few know that they are also very useful in geophysics. As? To find out, we interviewed Grazia Pietrantonio, a technologist at the INGV National Earthquake Observatory who has always been involved in geodesy.
Grace, what kind of tool is GPS?
The acronym GPS stands for Global Positioning System; when we speak of GPS we refer to a global positioning system of points on the earth's surface, based on the reception of radio signals emitted by a constellation of artificial satellites in orbit around the earth at a height of approximately 20000 km. What is usually called GPS in common parlance is the instrument that allows the reception of these signals, i.e. the GPS receiver, which is in turn connected to a GPS antenna.
Today we are all familiar with this tool because there are so many devices in common use that have a GPS receiver inside them: car navigators, smartphones, tablets, watches.
The GPS system, of US origin, was the first and is still the most used satellite navigation system in the world, but by now speaking only of GPS is rather limiting, as over the years new systems similar to it have seen the light and are now operational, such as the Russian GLONASS, the Chinese BEIDOU and the European GALILEO; India is also developing its own satellite navigation system, called IRNSS. All of these satellite navigation systems collectively make up GNSS (Global Navigation Satellite System).
What is the working principle of GPS?
Simplifying as much as possible, the operating principle of the GPS can be described by saying that the signal emitted by the satellites, a rather complex signal, made up of various components, is received on the ground by antennas connected to suitable receivers. Since the position of the satellites is known and by measuring the time taken by the signal to complete the satellite-receiver path, it is possible to determine the position of the point on the ground.
Over the years, the improvement of measurement techniques and data processing algorithms has made it possible to achieve sub-millimeter precision in the estimation of the 3D coordinates (horizontal components and altitude) of points on the surface of the Earth.
When was he born"?
GPS was "born" in the USA about 50 years ago for military purposes. The project was originally launched by the United States Department of Defense in 1973 to provide an answer to the need of its armed forces to be able to locate military assets anywhere and in any weather condition. Only about twenty years later, the use of GPS was opened to civil purposes and since then its diffusion has grown more and more.
What are the uses of GPS in geophysics?
GPS today is a key tool in several areas of geophysics. The possibility of measuring the position and relative movement of points on the surface of the Earth with such high precision has opened the field to many uses. To mention just a few areas: the study of tectonic and volcanic processes, the estimation of earthquake failure parameters and post-seismic phenomena, as well as hydrological and atmospheric studies, estimation of sea level variations.
Personally, I mainly deal with the study of tectonic deformations, making extensive use of the large amount of GNSS data available. In the last 20-25 years, the so-called permanent networks have developed and spread, consisting of a large number of stations (receivers connected to antennas) which record GNSS signals 24 hours a day, 24 days a year. In Italy and in the world there are many permanent networks, set up by universities, research institutes, local authorities, private companies; nationally one of the most important is our RING network (), consisting of about 200 permanent stations. This large number of stations allows us to study in detail how the earth's crust is deforming, through the analysis of the daily coordinate variations of the stations themselves and the estimation of their displacement speeds.
What are space geodesy techniques?
They are all those techniques that make extensive use of artificial satellites and signals from space. In addition to GPS, which is certainly the best known, the main ones are: DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellites), SLR-LLR (Satellite Laser Ranging - Lunar Laser Ranging), VLBI (Very Long Baseline Interferometry).
The advent of these techniques and the consequent ability to determine with high precision the relative position of points, even very distant from each other and belonging to different plates, have made it possible to make enormous strides in determining the motion of tectonic plates. The latter has been one of the main problems in Earth Sciences since, in the second half of the last century, the theory of plate tectonics took hold. As is known, according to this theory, the outermost envelope of the Earth (the lithosphere, about 100 km thick) is divided into rigid plates floating on a less rigid layer immediately below, called the asthenosphere.
How is the use of GPS applied to earthquakes?
In the field of geophysics, the deformations that occur in correspondence with an earthquake are of particular interest, called "co-seismic" deformations, and also after it, in this case we speak of "post-seismic" deformations. In this regard, GPS data play a fundamental role, being different and complementary to those provided by seismology and geology. By measuring the displacements of the points due to the earthquake, i.e. the difference between the position after the event and the position before the event, using suitable algorithms, it is possible to create a model of the fault "responsible" for the earthquake, i.e. to trace its position, extension and to the distribution of movement on the fault itself.
In Italy, the strong seismic events of the last 20 years, starting with the Colfiorito sequence of 1997, but above all with the Aquila earthquake of 2009, passing through the Emilia earthquake of 2012 and up to the most recent seismic sequence that began in 2016 in central Italy, have repeatedly shown the great potential of the technique in the study of earthquakes.
The earthquake that hit L'Aquila in 2009 can be considered the first strong Italian seismic event that occurred in the middle of the "GPS era"; it is also the first earthquake in Italy to have been recorded by high-frequency GPS receivers, i.e. instruments that record the signal with a very small sampling interval (1, 10, even 100 data per second), instead of the classic sampling interval of 30 seconds. High-frequency GPS data allowed us to observe for the first time the dynamic movement of the ground during the passage of seismic waves, during the main shock of April 6, 2009.
Finally, what are the next steps in using this technology in geophysics and not?
Numerous scientific works testify how GNSS data have become an indispensable tool for the study of earthquakes; in particular, today great attention is paid to high-frequency data which, in the event of strong seismic events, are able to provide a rapid estimate of the magnitude and fault parameters, which is the basis of an effective tsunami warning system, which in turn can save lives.
In general, all possible applications real-time of high-frequency data, offer great prospects, in the geophysical field but also in more purely technological fields.
Further important developments in the near future could come both from the joint use of signals from the various GNSS satellite constellations, which is already in part a reality and which will bring operational advantages and further improvements in positioning accuracy, and from the development of low-cost GNSS instruments cost, which is making ever wider applications possible in various sectors, in particular in the field of landslide and infrastructure monitoring.