Establishment of control points using GNSS-RTK technique

. Nowadays, Global Navigation Satellite Systems (GNSS) methods are broadly utilized to determine positions of points. This study was conducted with the aim of determining the positioning of control points at Politeknik Kuching Sarawak Campus using GNSS-RTK Technique. For this purpose, twenty-four control points were established with an average baseline length between Base station and Rover less than 1 km were occupied with Topcon HiPer VR receivers for different time periods. In this study there are situation of positioning under satellite obstruction such as the control points situated in the near buildings. Multipath is a common challenge in GNSS-RTK especially in complex environments. The final result from the Horizontal and Height differences were determined and compared with those measured by both static and RTK surveys. The result of the study shows that GNSS-RTK positioning method ensure high accuracy GNSS position solution within few centimetres.


Introduction
The Global Navigation Satellite System (GNSS) consists of multiple satellite constellations (GPS, GLONASS, Galileo, Beidou, IRNSS, QZSS, SBAS) plays an important and essential role in the field of geodesy because it allows data to be obtained in a short period of time without interference in terms of distance, terrain, visibility and weather.Since the end of May 2022, more than 130 satellites are available for fully global operational satellite navigation systems [1].Civilian/non encrypted GNSSs, they are employed for proving positioning, navigation and timing (PNT) solutions across a wide range of industries [2].Global Navigation Satellite System (GNSS) can provide users a fast, cost-effective, and reliable positioning service [3].Different real-time kinematic GNSS-RTK technique have been used for determining the position of any point on the globe [4].GNSS-RTK is developing at a very fast rate and this a becoming very common and popular among the users.GNSS-RTK (Global Navigation Satellite System Real-Time Kinematic) is a satellite navigation technique that combines real-time satellite data with precise carrier phase measurements to achieve highly accurate and real-time positioning.To facilitate rapid and precise GNSS positioning, the integer carrier phase ambiguities need to be resolved [5].Real Time Kinematic (RTK) positioning makes use of carrier phase measurements in the differential positioning mode, multiple epochs or a short period of observations are involved to achieve reliable Ambiguity Resolution to determine the precise position, while the RTK solutions are derived from the current epoch [6].Nowadays it is frequently used in applications that require centimetre-level accuracy, such as land surveying, construction, precision agriculture, and autonomous vehicles.GNSS surveying in urban environments and areas with many obstructions, the signal obstructions and multipath effects is a common issue that can affect the accuracy of position measurements .These obstructions can lead to inaccuracies in the RTK solution due to the GNSS receiver receiving distorted or reflected signals.In such an environment, satellite signals are reflected, scattered or faded, and sometimes completely blocked by roofs and walls of high-rise buildings, flyover bridges, complex road structures, trees, mountains, and other obstacles etc. making positioning and navigation information inaccurate, unreliable, and largely unavailable [7].
GNSS-RTK is a technique used to achieve highly accurate and real-time positioning by combining signals from multiple Global Navigation Satellite System (GNSS) satellites .The technique relies on differential correction to mitigate errors and enhance the accuracy of position solutions.The positioning accuracy depends mostly on the baseline length due to the atmospheric errors [8].The baseline length and communication range are important factors that influence the accuracy and feasibility of differential GNSS techniques, such as Real-Time Kinematic (RTK).The baseline length between the base station and rover can impact accuracy.RTK accuracy decreases with increasing baseline length due to increased susceptibility to atmospheric errors.In general, shorter baselines result in higher accuracy.For short-range applications like precision agriculture or construction, baselines may be a few kilometres or less.In challenging environments, establishing a reliable data link over long distances may require the use of higher-power radios, taller antennas, or additional relay stations.Additionally, maintaining real-time communication between the base station and rover over long distances can be challenging.Urban canyon effects are a significant challenge for GNSS (Global Navigation Satellite System) positioning in urban environments [9].
GNSS-RTK is a differential GNSS technique that enhances the accuracy of positioning by using correction data from a nearby reference station.Consistent horizontal and vertical accuracy is crucial for precise positioning.GNSS-RTK positioning systems are susceptible to various sources of errors that can influence the accuracy and reliability of the positioning solution.These errors can be sorted into different types, including Atmospheric Errors and Ionospheric delay means GNSS signals pass through the ionosphere, a reg ion of the Earth's upper atmosphere and can cause changes in signal speed, leading to inaccuracies in the calculated positions.Meanwhile tropospheric delay means the troposphere, the lower atmosphere, can cause delays in GNSS signals due to moisture and atmospheric conditions.These delays can be significant, especially in humid or unstable weather.The main disadvantage of GNSS is the dependence on the openness of the horizon for the visibility of satellites [10].In ideal conditions with good satellite visibility, high-quality equipment, and real-time correction data from a nearby reference station, RTK GNSS can achieve centimetre-level horizontal and vertical accuracy.However, in challenging environments with signal obstructions, therefore in this study in such conditions, achieving centimetre-level horizontal and vertical accuracy can be more challenging.This can introduce errors and delay the receiver's ability to acquire a sufficient number of satellites for a fix.

Materials and methods
In this study, the study area and location of control points is around the Politeknik Kuching Sarawak as shown in Figure 1.The GNSS equipment used are Topcon Hyper VR GNSS receivers.Table 1 show the receiver specifications for tracking of signals.The Topcon HiPer VR GNSS receivers is multi-GNS S Positioning where the GNSS receivers enables the determination of position coordinates using satellites of at least one system.The Hiper VR GNSS receivers is designed to provide comprehensive satellite tracking capabilities by supporting multiple GNSS constellations and signals.This multi-constellation and multi-frequency tracking capability enhances the receiver's accuracy, reliability, and availability.This receiver receives signals transmitted by multiple satellites in orbit around the Earth.These signals contain information about the satellite's position, time, and satellite health.There are two GNSS receivers Topcon Hyper VR such as Base Station and Rover and Field Communicator or data collector provides a user interface for monitoring and controlling the GNSS receiver used in this study as shown in figure 3. The Software Applications Magnet Field are used to configure and monitor the RTK session.This involves communication such as establish real-time communication between the base station and the rover.Fig. 2. A dual-frequency GNSS-RTK rover receiver, data collector and GNSS software.

Data acquisition
The practical surveys were conducted using Topcon Hiper VR GNSS receivers using RTK technique for determination of Control Points coordinates.The concept of RTK positioning by utilize two GNSS Receivers, a Base and Rover in order to observe high accuracy positioning.During the measurement one GNSS receiver is set up over a known control point with known coordinates such as a base station while another GNSS receiver or GNSS rover is set up at locations where control points are to be made.We set up Base station and Rover with baseline length below 1000m.Where the radio signals are strong and used to establish real-time communication between the base station and the rover so that the differential correction data from the base station to the rover, enabling the rover to achieve highly accurate positioning in real time.
The GNSS-RTK measurements are taken at different time periods, in this study there were measured three time periods.The first observation period is conducted in the morning, the second observation period is conducted in the afternoon and the third observation period is conducted in evening.The control points survey monument was surveyed with the GNSS rover.The rover receivers also track GNSS satellite signals and collects raw measurements.The rover receivers are setup on 24 control point such as PKS801, PKS802, PKS803, PKS804, PKS805, PKS806, PKS807, PKS808, PKS809, PKS810, PKS811, PKS812, PKS813, PKS814, PKS815, PKS816, PKS817, PKS818, PKS819, PKS820, PKS821, PKS822, PKS823 and PKS824 in areas with many obstacles.
This GNSS receiver tracking technology for all satellites and constellations and multiple frequencies such as L1, L2 and L5.This increases the number of available signals, improving the chances of maintaining a fix in obstructed areas.RTK requires fast ambiguity resolution for initialization at the start of the survey, or when losing satellite tracking [11].RTK-fixed solutions mean that the rover using corrections from the base resolved ambiguities in its positional calculation and achieve the solution with sentimental level accuracy but if it's get RTK-float solutions that means that the rover receives corrections from the base but cannot resolve all ambiguities and in this case the accuracy is usually at the sub-meter level.When the rover is not receiving correction that means it does not receive any data from the database that can help to calculate the solution.that mean status was RTK single solutions or Autonomous, the accuracy is usually in meter level accuracy.The single solution or Autonomous means that the rover has found a solution based solely on the signal from the satellites.Meanwhile the GNSS campaign for GNSS static surveys was started by set up a GNSS receiver of Topcon Hiper VR, dual frequency receiver at a Reference Control Point.A second GNSS of the same Topcon model was setup at 24 control points and occupation durations of 1 hour, and the base line were less than 1km.Selecting a stable GNSS reference point is crucial in ensuring the accuracy and reliability of GNSS surveys, particularly in applications like land surveying, construction, and geodetic control.In this study a location of Control Point that is geodetically stable and stability is essential for long-term accuracy.In this study the software to process the GNSS data, for static surveys was using Topcon Tools Software.The software can compute baselines using simultaneous measurement data from two or more GNSS receivers.These baselines represent the vectors between the positions of the GNSS receivers, and they are essential for a variety of applications in surveying, geodesy, and precise positioning.By computing baselines and analysing the relative positions of GNSS receivers, users can determine accurate and precise spatial information for a wide range of purposes.Baselines in the context of GNSS represent three- dimensional lines that connect the positions of two GNSS receivers.These baselines provide essential information about the relative spatial separation between the receivers .

Result and discussion
After the completion of GNSS-RTK field data acquisition, then export the collected GNSS-RTK data from the data collector to a computer using USB pen drive to move the data.Real-Time Kinematic (RTK) positioning is faster than static GNSS positioning.Static GNSS surveys involve longer observation periods and post-processing to achieve the highest accuracy.GNSS static measurements were made at 24 control points and these measurement values were accepted as exact measurement values and reference values.The differences of the static measurement values and the measurement values obtained by GNSS-RT K measurement methods were taken respectively and to assess the accuracy and reliability of the two measurement methods.The coordinates of GNSS-RTK were compared with the coordinates of the 24-control point obtained from the GNSS Static surveys.These measurement differences were analysed and compared in various aspects.This approach to understanding the performance and reliability of these measurement methods.Table 2 shows the experimental results of horizontal position and elevation of the points measured by GNSS-RTK in the morning can reach less than 2cm and 3cm precision level.Also, the results of these table are illustrated in figure 5.The results indicate that there was a height differences were bigger due to obstructed environment such as high building and tree shading in value less than 5cm at PKS803, PKS804, PKS808, PKS809, PKS810, PKS811 and PKS813.Table 3 show that experimental results of horizontal position and elevation of the points measured by RTK in the afternoon can reach 2cm and 3cm precision level.Also, the results of these table are illustrated in figure 6.While in obstructed environment, such as high building and tree shading at PKS803, PKS804, PKS808, PKS809, PKS810, PKS811 and PKS813 the height differences are less than 5cm.GNSS receivers, relies on receiving unobstructed signals from satellites to determine precise positions.Obstructions like trees, buildings, and other structures can significantly impact the propagation of GNSS signals, leading to reduced accuracy or even signal loss.Table 4 shows the observation values horizontal position and elevation in Control Points give good and acceptable height value in 2cm and 3cm except height value in PKS803, PKS804, PKS808, PKS809, PKS810, PKS811 and PKS813, less than 5cm where the control points situated in the near buildings.These obstructions can lead to inaccuracies in the RTK solution due to the GNSS receiver receiving distorted or reflected signals .

Conclusion
The result show that GNSS-RTK can achieve under 2 cm level accuracy of horizontal position and under 5cm level accuracy of elevation of the points measured by RTK in high building, tree shading environment.meanwhile GNSS-RTK can achieve horizontal accuracy typically in the range of 1 centimetre (cm) to 2 centimetre (cm) in ideal conditions.Vertica l accuracy with GNSS-RTK closely matches horizontal accuracy, ranging from 1 cm to 3 cm.Multipath is a common challenge in GNSS-RTK especially in urban or complex environments.The increase in the number of satellites in GNSS constellations can have significant benefits such as Improved Signal Availability.With more satellites in the constellation, users on the ground have a higher probability of having a clear line of sight to multiple satellites, even in challenging environments with obstacles like buildings and trees.The GNSS technology enhanced accuracy with a larger number of satellites provides redundancy in measurements, allowing for more accurate positioning.Redundancy enables the receiver to identify and mitigate errors more effectively, leading to improved accuracy, especially in difficult conditions.The horizontal position and elevation of the points can be measured quickly by GNSS-RTK technology and become so fast and efficient which means we can now get centimetre accuracy.An increased number of satellites can reduce Faster Time to First Fix (TTFF), allowing for quicker and more efficient positioning.More satellites in view at any given time led to better geometric configurations.Improved satellite geometry minimizes dilution of precision (DOP) and enhances the accuracy and reliability of positioning solutions The result implies that, GNSS-RTK can determine their position fast, easily, and cost-effectively at any time in real-time within a few cm-level of accuracy.In the obstructed areas where centimetre accuracy is required, the GNSS-RTK technique is suitable.

Fig. 5 .
Fig. 5. Graf shows data observation differences at Control Point in Politeknik Kuching Sarawak in the morning.

Fig. 6 .
Fig. 6.Graf shows data observation differences at Control Point in Politeknik Kuching Sarawak in the afternoon.

Fig. 7 .
Fig. 7. Graf shows data observation differences at Control Point in Politeknik Kuching Sarawak in the evening.

Table 1 .
Receiver specifications for tracking of signals.

Table 2 .
The Differences Between The GNSS-RTK Observation Values and The GNSS Static Observation Values in the morning.

Table 3 .
The Differences Between The GNSS-RTK Observation Values and The GNSS Static Observation Values in the afternoon

Table 4 .
The Differences Between The GNSS-RTK Observation Values and The GNSS Static Observation Values in the evening