Low-cost vs. Geodetic-grade GNSS Instrumentation: Geomonitoring with High-rate and Real-time PPP

The low-cost GNSS market is still dominated by single-frequency devices. Since a few years, some manufacturers also produce low-cost dual-frequency receivers. Typically, these devices and instruments are much smaller and less expensive than geodetic dual-frequency equipment. However, this comes at the cost of a poorer usability, and a potentially poorer hardware and internal signal processing, resulting in observables with lower quality. Furthermore, GNSS low-cost antennas have worse performance than their geodetic-grade counterparts, especially in terms of multipath mitigation. In this contribution, the capability of such low-cost GNSS devices for real-time and high-rate geomonitoring purposes is investigated. For this purpose, we assembled the low-cost dual-frequency receiver u-blox ZED-F9P in a small and light-weight data logger with very low power consumption, which is thus very well suited for field campaigns in remote areas. Static measurements are performed to test the measurement accuracy and stability. To determine the influence of the antenna type on the measurement result, different antennas covering different price segments are connected to the low-cost receiver. Beside the static tests in kinematic mode, strong-motion dynamic ground movements are simulated with a robot, as they occur for large earthquakes. All experiments also involve measurements with a JAVAD SIGMA geodetic receiver, that have been carried out in parallel -- these are used for comparison purposes. For the processing of the measurements real-time precise point positioning (PPP) is used, since this is the primary method of choice for GNSS seismology. We focus on ambiguity float solutions which are determined under real-time conditions using the Centre National d’Etudes Spatiales (CNES) open-source PPP software and the CNES real-time products. This work demonstrates, that even with low-cost GNSS devices it is possible to obtain a precision of few centimeters (and even millimeters over short time intervals). Thus, these devices can be used to monitor dynamic ground movements at the centimeter level, which is a requirement for strong-motion seismology. However, it must be considered that the quality of the antenna used can have a significant influence on the results. In addition, ambiguity resolution has proven to be challenging when using low-cost antennas. Therefore, a more in-depth investigation is necessary to exploit the full potential of the ambiguity resolution, also with other software packages. However, we conclude that these devices provide an excellent basis for the densification of existing GNSS monitoring networks, as needed for strong-motion seismology, or atmospheric tomography methods.