# Feasibility of Using Low-Cost Dual-Frequency GNSS Receivers for Land Surveying
## 1 Introduction  
Global Navigation Satellite Systems (GNSS) are widely used in a variety of geoscience applications [1], including tectonic plate motion monitoring [2], surface deformations [3], landslide monitoring [4], seismology [5], as well as ionosphere [6] and troposphere [7] remote sensing. A GNSS receiver remains the core sensor in navigation systems of unmanned aerial vehicles [8] and autonomous vehicles [9]. Yet the GNSS have revolutionized not only geosciences but also land surveying [10], as they allow for determining position coordinates with centimeter-level precision in real-time or up to sub-millimeter accuracy in post-processing solutions. Achieving precise and accurate position coordinates requires a combination of the three following aspects: availability of a well-defined reference frame; processing of carrier-phase measurements, use of an appropriate measurement model. Two general approaches can be distinguished, namely absolute and relative positioning. In absolute positioning, the reference frame for measurements is provided by satellite coordinates, while in relative positioning, a reference station has well-known coordinates. Therefore, the positioning accuracy of a GNSS directly depends on the accuracy of the coordinates of either the satellites or the reference station. The International GNSS Service (IGS, [[https://www.igs.org/](https://www.igs.org/)]([https://www.igs.org/](https://www.igs.org/))) provides satellite orbits, clocks, and established control point networks with centimeter-level accuracy, in order to maintain national reference frames all over the world. The relative positioning is commonly used in land surveying applications, in real-time post-processing. Real-Time Kinematic (RTK) or Network RTK (NRTK) modes allow for achieving centimeter-level accuracy in real-time [11] if there is a base station or a network of reference stations near the mobile receiver. A millimeter level accuracy can be achieved with a differential approach for long static surveys by means of networks of GNSS receivers separated by tens to hundreds of kilometers [12]. The dominant technique in absolute positioning is the Precise Point Positioning (PPP) [13], which works worldwide, even if high-precision results are still limited to static surveys and dual-frequency receivers [14]. Therefore, the choice of the measurement model should depend on the required positioning accuracy and availability of a local GNSS frame. Even if both the well-defined reference frame and the chosen measurement model are available, the acquisition of carrier phase measurements remains critical. For many years, such a feature was limited to dual-frequency geodetic-grade receivers, whose high cost was the limiting factor for many applications [15; 16]. Since low-cost single-frequency receivers capable of logging carrier phase measurements have become available, several studies aimed at investigating their positioning accuracy [17; 18] and demonstrations of low-cost monitoring applications were performed [19; 20]. Two main advantages of using low-cost receivers are the possibility of deployment in hazardous areas and logging a higher amount of position data [21]. Millimeter [15] to centimeter-level [22] accuracy can be achieved under favorable conditions [23], as the antenna is the key factor limiting the positioning accuracy [18]. Moreover, the ionosphere delay remains a major error source affecting single-frequency PPP [17] and long-baseline RTK [15]. Most of the low-cost single-frequency receivers available on the market do not meet their nominal performance in urban areas characterized by the multipath effect [24]. After the development of low-cost multi-frequency receivers, the ionosphere delay is no longer an issue and can be removed by combining the measurements obtained at two frequencies.