Saving Lives - Affordably

Precise monitoring of landslide-endangered areas can save many human lives, but it is expensive. But now reasonably priced early warning is becoming a real possibility thanks to the research work of Dr. Li Zhang, a scientifi c research assistant at the University of Stuttgart’s Institute of Engineering Geodesy (IIGS).

About one million cubic meters of rock thundered down into Southern Tirol’s Pragser Valley in August 2016 during a landslide. Fortunately, there was no damage to human life. But on a worldwide scale, landslides occur time and again and eradicate entire urban settlements under layers of mud and boulders. Such forces of nature are practically unstoppable: man is helpless when millions of tons of earth and rock go into motion. But early recognition of infinitesimal movements in such danger zones could help to perceive the threat of danger and allow settlements to be evacuated in time.

Li Zhang can only smile when asked whether it wouldn’t make sense in such danger zones to simply set up a network of GPS receivers like the ones pre-installed in smartphones or navigation devices. Li Zhang, 32 years old, has been working in her office on the sixth floor of the University of Stuttgart’s Institute of Engineering Geodesy since 2009 on precisely such early-warning systems. One result of her work was the finding that normal commercial GPS receivers like those in private use do not meet the requirements. ‘A ‘navi’ is relatively unconcerned about deviations of a few meters; it can still find the destination,’ says Zhang. ‘But we need a precision of millimeters in order to supervise deformations - the more accurate the better.’ If a slope moves only five millimeters, that could be a critical signal. The measurement must therefore be that much more accurate.

Navigation devices, on the other hand, function with a maximum precision much the same as visual evaluation, that is, about one to three meters. So-called ‘geodetic’ GPS receivers have long been used for professional purposes of this kind; they not only receive the same ‘code signals’ as navigation devices but also process much more accurate ‘phase signals’. The drawback: such receivers cost about 20,000 Euros apiece, and that is without the added cost of supply current from solar panels, backup batteries, and data transfer. The costs of investment needed to ensure extensive operations is thus very expensive for developing and emerging countries. When Li Zhang came to the IIGS in 2009, she therefore approached her institute director, Prof. Volker Schwieger, about the possibility of reasonably priced deformation monitoring - and that turned out to be the topic of her dissertation.

Li Zhang was born in 1984 in the East Chinese province of Zhejiang. She first studied teledetection and Geoinformatics at the University of Wuha. An exchange program brought the young researcher to Baden-Württemberg and the University of Stuttgart in 2003, where she first had to pass the German language test. ‘I must admit,’ she says with a laugh, ‘that I didn’t understand my class lectures completely, because not every teacher speaks as slowly as my language instructor!’ But now the language barrier has long been surmounted: Li Zhang herself now holds classes, for example about deformation analysis.

She studied Geodesy and Geoinformatics in Stuttgart from 2004 to 2009 and at the same time gathered practical experience at the Urban Surveying Office of Stuttgart and as a student worker at Robert Bosch GmbH. Zhang finds it easy to sum up the focal point of her research work: ‘I’m very interested in finding whether much less expensive equipment can yield high-precision results.’ Now that her dissertation is finished she can answer this question with a resounding ‘Yes!’ But she had to traverse a long gauntlet of challenges before reaching this point. At the beginning, she found it quite easy to find inexpensive receivers and suitable antennas for only a few hundred Euros. It was also not difficult to read out the integrated software results and analyze them.

As scientist Zhang says, ‘Signal reception involves a plethora of sources of error.’ For example, the signals from GPS Satellites 20,000 kilometers above the earth often do not reach the ground receiver directly but are reflected, for example from city building facades and other objects. And even when the signal is emitted from the ground and then arrives at the antenna, it has traveled a very long way. Such multi-path effects can result in inaccurate determination of positions.

‘I made models of these effects for my dissertation and then tried to minimize them,’ recalls Li Zhang. She now uses a ‘choke ring’ of her own design to reduce refl ective ground waves; it is a circular plate with metal rings of different diameters positioned below the antenna to stop signals from below. Zhang minimized other error sources by identifying their typical patterns and then correspondingly optimizing the receiver software with algorithms she developed herself. The inexpensive receivers, previously unsatisfactorily precise, now have an accuracy of only a few millimeters - quite comparable to that of high-priced geodetic receivers.

‘Our idea is to install such systems everywhere in endangered regions,’ says Zhang. Ideally, it might be possible to automate the monitoring process by transmitting the measurement data of the receivers automatically via WLAN in real time to a primary central location for assessment. ‘A few years ago we already built and tested a low-cost GPS monitoring system with an autonomous solar panel power supply and backup battery, automatic data transmission, and real-time evaluation. It functions fl awlessly.’

At least two receivers are required to monitor, for example, a slope which is in danger of sliding. To achieve millimeter precise accuracy, the distance between both receivers should be less than about ten kilometers. Area-wide monitoring in endangered regions is a cost issue, which is why Zhang’s’ research work in Stuttgart immediately drew worldwide attention. ‘I work in a team at the FIG, an international union of geodesic engineers. We also have a task for there for cost-effi cient monitoring, and this is very interesting above all for our colleagues from poorer countries,’ says Zhang. Admittedly, however, she has been unable up to now to develop a multipurpose monitoring device. ‘The equipment and the related software must be adjusted for the purposes of each individual task,’ says Zhang. Even in future, therefore, it will be impossible to purchase such high-precision GPS systems in the do-it-yourself store around the corner.

In future, it might be possible to employ the lowcost monitoring devices for other uses than early warning of landslides, as Zhang explains. Her system could also help in monitoring bridges or dams. It is well known that the gigantic concrete walls of dams bend according to the amount of water behind them, and every bridge has its own natural vibration frequency. Should such deformations exceed a specifi c level, this could serve as an alarm signal. Li Zhang and her colleagues at the IIGS have already formed plans for testing her system at a dam with an artifi cial lake.

Zhang spent a good five years working on her dissertation. Interested feelers have already come from industry. But she is still not finished with the issue of GPS monitoring, quite the contrary. She feels at home in Stuttgart, but can still envision working someday in China again. Until that happens, however, she intends to tackle other related challenges. ‘My interest lies in precisely tracking the movement of objects using reasonably-priced GPS devices.’ It can be assumed that there is a huge number of possible areas of application for this, especially in the era of Industry 4.0 and its ever-more digitalized production processes. Zhang also believes that the automotive industry with its current interest in ‘autonomous driving’ will be interested in such high-precision, low-cost GPS receivers. However, that will require finding a way to counter the many sources of interference of communication between satellites and receivers.

Zhang is therefore also experimenting with GLONASS, the Russian counterpart to the U.S. GPS system. ‘We have procured new receivers that can process the signals of GLONASS, GPS, and the Chinese satellite navigation system Bei Dou,’ says Zhang. It is important to be able to access more than one satellite precisely when moving objects are to be monitored. Since there are still only a few receivers for Bei Dou signals in Europe, some experts are already thinking about a cooperative venture with University of Wuhan. Jens Eber