RTK : Getting deeper into this GNSS technologyFebruary 12, 2019 12:39 pm
The position given by a GPS alone gives a good idea of your location to a few meters. But this location is not precise enough to allow precision work. It requires a correction so that its precision get increased and usable in industrial work applications.
RTK : what does it mean?
First, let’s understand what RTK really stands for! Real Time Kinematics is a GNSS technology that allows to partially remove signal errors due to propagation in the atmosphere.
These errors are numerous:
• Atomic clocks biases
• Receiver’s noise
• Antenna’s phase center variation
• Troposphere propagation
• Ionosphere propagation
Ionosphere propagation is the most important effect. The ionization of the propagating medium causes reflections and refractions of the electromagnetic waves. The propagation time measurement time performed by the receiver is therefore false. Another important effect that cannot be modeled is multipath. It corresponds to wave reflection on obstacles near the receiver (trees, buildings…) that retard or duplicate signals. It can be strongly attenuated with a good hardware.
What about real life application?
RTK requires consequently two GNSS receivers, a “base” station, generally motionless and whose position is perfectly known, and a “rover” mobile receiver. The base sends correction data to the rover (raw data) so that the rover can compute the double-difference RTK algorithm. This means that pseudoranges and carrier phases from the base will be “substracted” (it is a bit more complicated though) from those from the rover.
Getting deeper into the RTK topic…
To get deeper into details, RTK uses carrier measurements in order to get centimetric. GPS signal wavelength is about 20 centimeters, so if you are able to measure the phase of this signal, that you know satellite’s phase center and that you calculate the integer number of periods between the satellite and you, this is where you get to centimetric precision. We call ambiguity the previous integer number of periods. If the algorithm finds an integer solution, then the solution is called a “fix”. This is when when maximum accuracy is reached. Otherwise, if the solution is a float, the solution is called “float”. This solution is less precise.
We will refer as “single” for the positions neither “fix” nor “float” (happens if no correction data is received, or if signals are not good enough). Something really important about RTK is that if your antennas do not provide good enough signals, the computed position will be erroneous!
Drotek RTK positioning systems
We have developed a complete range around RTK positioning technology, including SIRIUS RTK GNSS Base (M8P). The NEO-M8P, with centimeter-level accuracy, uses Real Time Kinematic (RTK) technology to achieve the GNSS performance needed for unmanned vehicles and other applications requiring accurate guidance.
The last version of RTK technology features the u-blox ZED-F9P, a multi-band module with higher performances compared to the previous NEO-M8P module.
Our latest product featuring this technology is the RTK ZED-F9P GNSS. This module enables precise navigation and automation of moving machinery in industrial and consumer grade products in a compact surface mounted form factor. The module includes Moving Base support, allowing both Base and Rover to move while computing a centimeter-level accurate position between them.