Continuous Monitoring with Real Time Kinematics GPS

Matheus Alves Laranjeira Neri, summer semester 2017

The continuous monitoring is a surveying method that enables structures to be constantly monitored or inspected without the need to be taken out of service. The Continuous Monitoring with Real Time Kinematics GPS is a technique that uses the Global Satellite Navigation Systems to enhance the precision of the monitoring. The resulting data can be analyzed and used to identify structural modifications. This technique is usually applied for the Structure Health Monitoring (SHM) of systems exposed to adverse conditions because of its versatility. A continuous monitoring system can cover miscellaneous fields, such as temperature, acceleration, speed, humidity, location, etc. over time, dependent exclusively of the set of sensor and receivers installed on the structure to be measured.^[1]

Global Satellite Navigation Systems

One unusual way to monitore structure is with the help of satellites. There are nowadays various fully operating global satellite navigation systems and others in development providing time information and location around the Earth. The Global Position System (GPS), created, owned and maintained by the USA Govern with 32 satellites is the most known system.^[2] However there are other systems like the Russian Global Navigation Satellite System (GLONASS), China's BeiDou Navigation Satellite System, India's NAVIC and Japan's Quasi-Zenith Satellite System and the European Union Galileo positioning system (expected to be ready in 2020). ^[3] Although for the common usage of these systems the accuracy is good enough, varying from 15 m until 5 m, for some specific applications in the Engineering it is not sufficient.

The satellite signal sends two information: Time and Carrier Wave (a modulated waveform with an input signal to be electromagnetically transmitted). The distance between a receiver and a satellite is normally calculated based in the time it takes for a signal to travel from the satellite to the receiver. Since the satellite signal takes time to reach the receiver, the satellite's signal sequence is delayed in relation to the receiver's sequence. It is necessary to digitally delay the receiver's sequence, so the two sequences are henceforth aligned.

Real Time Kinematic GPS

The Real Time Kinematic (RTK) is very flexible surveying method to improve the measurement of points using as reference navigation systems granting high positioning performance. This method uses the same principle to calculate the distance, but with some particularities.

For a single receiver as with a personal satellite navigation system, estimating the location is subject to errors that depend on modification to signal transit time due to atmospheric and other effects. If a fixed base or reference station is used nearby, given the known fixed location, the ‘differential’ errors can be identified and used to adjust the receiver position estimate.^[4]

Another difference is that the information in the satellite signal (time and Carrier Wave) are completely used for the location measurement. The satellite signal's carrier wave uses a much higher frequency, where the pulses are closer together. The distance to the satellite is calculated based on the number of wavelengths and the angle of the wave, achieving a more accurate position.

These two circumstances allow an accuracy of 1 to 2 cm in the measured position. For that it is needed an undisturbed reception of the signals from at least five GPS satellites and a total of at least two GPS antennas, as shown in the Figures 2 and 3. One antenna is fixed and works as the reference station, the other is the rover, whose position is determined by three-dimensional polar attachment to the reference station according to the baseline method.^[5]

The resulted accuracy of the measurement is dependent of the quality of the receiver's electronics when accurately processing the signals from the satellites, and additionally the error sources from delays in the ionosphere and troposphere, satellite clock and ephemeris errors, etc.^[6]

Applications in the Engineering

There are various cases in that Continuous Monitoring with Real Time Kinematics GPS can be used in the engineering fields. To illustrate the applications of it, the monitoring of tall buildings and wind towers could be cited as good examples.

High Building Monitoring

As reported by Khoo, Tor and Ong^[7], a real-time monitoring scheme based on GPS RTK was installed on the roof-top of Republic Plaza (building in Singapore) with dedicated internet connection to a monitoring control centre located off-site. The system was capable for a real-time detection of tremors from distance earthquake events, like a 7.6-magnitude earthquake that occurred in the southern coast of Sumatra at 10:16:09 (UTC Time) on 30th September 2009 with the epicenter of the earthquake about 497km from Singapore. The exactness of this technique can also be used to estimate displacement of a point and detecting long-term deformation due to ground movements and how it affects the whole structure.

Wind Turbines Monitoring

In the field of the SHM, it was conducted by Mostböck and Petryna^[5] a comparison between two different techniques for monitoring wind turbines. A traditional one is based on strain measurements in critical cross-sections. And a second one based on use of the GPS sensors for displacement measurements of the tower. The obtained results show that the GPS measurement techniques in combination with validated finite element models are able to provide sufficiently accurate information on the arbitrary displacements and strains of the structure. This presumes, however, that the structural model describes the global and local behavior of the real structure sufficiently well. Under such conditions, GPS techniques could generally be applied in SHM systems. Their success in practical applications depends, however, on the previous mentioned circumstances.

At the beginning, the price of the GPS technique was too high for wide-spread applications^[5], but nowadays there are affordable solutions in the market. Comparable with strain gauges or accelerometers, RTK GPS presents itself as an alternative solution for the problems in the continuous monitoring of structures.

Literature

1. Carden, E..; Fanning, P.: Vibration based condition monitoring: a review, Volume: 3, Issue: 4, Pages: 355-377, Structural Health Monitoring, Dublin 2004.
2. Official U.S. government information about the Global Positioning System (GPS). www.gps.gov/systems/gps, 2017.
3. Official Website from the European Global Navigation Satellite System (GNSS). www.galileognss.eu/about, 2017.
4. Brownjohn, J.M.W. et al.: Experience with RTK-GPS system for monitoring wind and seismic effects on a tall building. Structural Health Monitoring and Intelligent Infrastructure, London 2006.
5. Mostböck, A.; Petryna, Y.: Structural vibration monitoring of wind turbines. 9th International Conference on Structural Dynamics. Porto 2014.
6. Radzeviciute, K. et al.: A Comparative Evaluation of Various RTK Systems' Performance Under Operational Conditions. 16th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland 2003.
7. Khoo, V.H.S.; Tor, Y.K.; Ong, G.: Monitoring of High Rise Building using Real-Time Differential GPS. FIG Congress, Sydney 2010.