Typical Modes of Operation for a GNSS RTK Receiver:
- Traditional RTK using Fixed Base and Moving Rover
- Traditional RTK using Moving Base and Moving Rover
- Stand-alone RTK receiver outputting raw GPS / GNSS Data
- Stand-alone RTK receiver outputting Standard Position Solution (SPS)
- Use of Raw GNSS data in Post Processed Kinematic (PPK) mode
Let us take a look in detail on each of these modes of operation together with a real life application use case.
Traditional RTK using Fixed Base and Moving RoverThis is the most common use case for RTK and involves two RTK GNSS receivers - a base station and a rover. The base station is typically a stationary GNSS receiver that is configured to send out RTK corrections (often through a radio link). The rover GNSS receiver is configured (through the radio pair) to receive these RTK corrections sent by the base station, and applies these corrections to solve for an centimeter-accurate level vector between the units.
RTK requires two independent GPS / GNSS receiver modules to be linked with a robust communication link so that the primary (Base Station) unit can send RTK corrections to the secondary (Rover) unit. The rover receives these RTK corrections and solves for a ‘vector’ ΔX, ΔY ,ΔZ that is accurate to within 1-2 centimeters with reference to the base station unit. If one has accurately surveyed and entered the base station coordinates then the rover shall use these base station coordinates to solve for it’s position in the global reference frame.
Traditional RTK using Moving Base and Moving RoverThe major difference in this mode of RTK is the fact that the base station is now not stationary in the global coordinate frame and is moving relative to the rover receiver. The most common application for this mode of RTK is in heading or orientation determination for a moving platform. Let us take an example of a boat for which one needs to find out the boat’s yaw or heading with reference to True North. A moving base GNSS receiver and a rover RTK receiver can be mounted on the platform (boat) in a manner such that the relative distance (baseline distance) between the base station and rover will always be fixed on a given length.
However, in the global frame of reference, the base station is actually moving and not statically fixed. This mode of operation is called Moving Baseline RTK where both the base station and rover GNSS receivers are moving in space but fixed with reference to each other. This moving vector always creates an angle with respect to True North and is the heading of the platform as shown below.
Raw GPS / GNSS data refers to the raw or unsolved measurements that a receiver solves for by listening to the satellite signals from GPS satellites. Data includes raw code observations and carrier phase observations that are typically the core inputs to a Position-Velocity-Time Navigation Filter. The PVT Navigation filter uses these raw observations to output a final real time Position-Velocity-Time message for the receiver.
Stand Alone RTK receiver outputting raw GPS / GNSS Data
Typical use cases for an RTK receiver outputting raw GPS / GNSS data include some of the following:
- Use of GPS / GNSS raw observations in scientific research - e.g. universities or government labs indulging in research related to GPS navigation.
- Use of raw GPS / GNSS Observations for weather forecasting or monitoring of geophysical events such as Solar flares, Ionosphere, etc.
Stand-Alone RTK Receiver outputting SPS PositionThe use of a higher precision code + carrier phase GNSS receivers for standard (non-differential) positioning typically is the result of better (robust) positioning in difficult environments. While low cost (<$10) GNSS C/A code only modules (such as the ones used in cellphones) and higher precision receivers give roughly the same position accuracy when used in a very good open sky environment, the real difference starts to show once the receivers are compared in a difficult environment (around foliage, buildings, etc.) with high multipath. In these use cases, a code+carrier phase precise GNSS receiver fares much better and is less prone to GPS error sources such as multipath.
Use of Raw GNSS Data in Post-Processed Kinematic (PPK) Mode
- Use of raw data in seismic or geodetic monitoring applications - Raw data is often used in a post-processed environment (the PVT solution is solved using complex cumbersome algorithms at the expense of a real time solution). In such use cases, typically a solved position that is not real time but much more accurate is often needed for analysis and research work.
- Unmanned Aerial Vehicles (UAV) based Survey: UAV operators typically want to use RTK receivers in a Post Processed Kinematic (PPK) mode due to a variety of reasons:
- Drone based aerial photogrammetry typically is a non-real time requirement.
- The use of high quality raw GNSS data and features such as 1 PPS or Event Input can be used in a post processed environment to obtain much higher quality maps / outputs.
- It reduces the need to carry 2 way radio communication and can make use of a simple SD memory card for on-board data logging which can be processed later.