Real-time GNSS positioning, commonly known as RTK (Real-Time Kinematics), is probably the most widely used method in the national industry. Its efficiency and the possibility of obtaining coordinates on-site, coupled with its stakeout capability, make it the preferred technique today.

 

    But how does it work? 

    If we consider Figure 1, RTK is situated in differential positioning, for which the presence of a base and a rover is necessary. In this sense, the real-time characteristic is provided by a communication medium through which the differential correction goes from the base to the rover. Traditionally, radio communication, particularly UHF (Ultra High Frequency), addresses this characteristic of RTK. However, in recent years, the emergence of the internet, and communications through it, such as NTRIP and TCPiP, have competed hand-in-hand with UHF.

    While there is currently autonomous real-time positioning with centimeter precision, such as RTX, in this brief article we will focus on differential positioning.

       

      Fig 1. Basic GNSS positioning scheme

       

      Normally we find two typical configurations for RTK:

      • RTK System: The user has a base and a rover, therefore, installs the base, configures it to emit differential corrections, and then the rover "listens" (Figure 2).

       

      Fig 2. RTK System

       

      • Active Reference Station: Here, the base generally operates continuously, so it would only be necessary to go out with a rover and a data collector to "listen" to the continuous correction from the base. In this configuration, the user does not intervene in the base configuration. These installations are common in mining operations or large engineering projects. 
         

      The two methodologies presented below require the following:

       

      For RTK (UHF)

      • In the case of GNSS RTK (UHF) positioning, both the base and the rover must have a radio frequency module that allows communication (yes, some receivers are for post-processing only).
      • Know the frequency at which the base is emitting the differential correction (Figure 3). This value must also be configured in our rover.

       

      Fig 3. Base radio frequencies and mode

       

      • In Figure 3, the base radio mode is also key, which is normally given by "TRIMTALK v1 at 9600 bps".
      • Another important aspect is the differential correction format, which optimizes data flow by being CMRx.

       

         

        In the case of RTK (internet), it is necessary for the base to have the ability to emit corrections via the internet. For example, Trimble's R12i incorporates this capability, and obviously, there is a need for internet access. In addition to knowing the emission format, it is necessary to know the address of the NTRIP "caster", i.e., the address and port. In GEOCOM's case, we have:

          

           

          This way we can access the differential correction. Delve deeper into these and other configurations in our courses on the GEOCOM training platform.

           

          Limitations 

          One of the most common questions for RTK (UHF) positioning is: What is the range of an RTK radio? The answer is not strict; factors such as radio power, local topography (key), and good installation exist. Under this logic, the following should be considered:

           

          • Is my installation good for an RTK base?: Strategic positions where topography does not affect transmission should be prioritized.
          • What radio do I need?, it depends on the project. Generally, continuously operating stations have external radios (TDL), which have more power and also better infrastructure for installation.
          • Is it always good to have a lot of power?, it depends; if the installation is not adequate, regardless of the power, the correction will not have a constant flow.
          • It is important to know the performance of each GNSS receiver; this can be reviewed in the datasheet for each model.

            

          For RTK (internet), the distance between the base and the rover increases considerably, however, attention should be paid to the following aspects:

          • Lower precision, due to greater distance
          • Internet coverage
          • Ability to generate differential corrections and transmit them via the internet

           

          General Recommendations 

          While there are certain recommendations regarding the use of the base and rover receiver in an RTK survey, there are also general recommendations that can ensure proper operation and data storage when performing the work.

           

          • When turning on the equipment (base and rover), check the battery so that the measurement is not interrupted by running out of battery.
          • If working with UHF radio, the antenna must be connected to both devices.
          • If working with the Internet, ensure there is Internet coverage in the area.
          • When working with an RTK system (base and rover), install the base at a point with precise coordinates for proper georeferencing.
          • When making the connection, clearly identify which equipment will be the base and the rover, checking by serial number (information on the underside of the receiver).

           

           

          • Upon finishing the field measurement, the survey must be finalized from the controller to ensure proper saving of the information.

           

            

          • Turn off the equipment after finishing the survey and then remove the antenna from the receivers for better care.