# Improving GNSS Landslide Monitoring with the Use of Low-Cost MEMS Accelerometers
## 1 Introduction  
The measurement and monitoring of landslide or infrastructure tilt, together with position variation monitoring, is an important feature of deformation control [1] that is usually detected with GNSS or total station instruments. Displacements are also usually monitored with total stations and GNSS receivers [2], but other techniques, such as SAR, can be efficiently applied [3]. It may be useful to measure the variations in the inclination of a structure or rock mass or the support of a GNSS antenna with independent instruments. For example, if the GNSS antenna is placed on high poles to avoid surrounding obstacles, its inclination gives erroneous indications about the movement of the ground at the base. Complete information about a structure's movements can be obtained using both GNSS displacements and tilt measurements.  
Low-cost inclination sensors generally have very large systematic errors, with a very variable range from sensor to sensor. If these errors are stable over time, their individual calibration can improve the accuracy obtained by one or two orders of magnitude. A number of methods are possible for obtaining the tilt measurements--one of these is based on electronic levels. We can see one example inMicroplan levels [4]. These levels use a pendular device in an oil bath, which provides high accuracy and resolution, even up to one microradiant, but inside a modest operative range (a few arc minutes). The devices are mainly used in closed environments, as their use is unsuitable in open environments with strong temperature variations.  
Another tilt-measuring instrument is the gyroscope. The angle is obtained by integrating the angular velocity \(\omega\) over time. An uncompensated bias in the angular velocity causes a linear drift in the angular measurement. The solution cannot therefore be maintained for a long time with high accuracy [5; 6].  
A different approach to tilt measurements uses an accelerometer, which is able to measure acceleration through the forces acting on a known mass. In the absence of external accelerations, the only force measured is the acceleration due to gravity. Sensors based on micro-electro-mechanical systems (MEMSs) have recently been developed, which can provide a low-cost solution and are already commonly used in inertial navigation systems (INSs).  
In this paper, we analyse only the use of the accelerometer as a tilt measuring device. MEMSs are commonly produced using silicon components, which possesses significant electrical and mechanical advantages over other materials [5; 7]. Unfortunately, a variety of systematic errors affects the performance characteristics of these accelerometers. Section 2 describes the operating principles of MEMS accelerometers, Section 3 introduces calibration methods to reduce their systematic errors, and Section 4 describes the instruments used and the results of the calibration of eight MEMS accelerometers. Section 5 introduces an independent method to verify the tilt accuracy based on angular measurements using a total station. Section 6 shows an application installed integrally with the ground planes of six GNSS antennas. Finally, the conclusions follow in Section 7.