RTK GPS
Demonstration of a Trimble RTK GPS system
GPS is now commonly used by most people since the advent of smart phone technology. GPS integration on mobile phones offers many capabilities such as navigation and mobile data capture. One drawback however, is the accuracy which is limited to approx 2-4 meters. For many surveying applications this level of accuracy is not appropriate, particularly in regard to detailed topographic surveys.
Real-Time Kinematic (RTK) differential positioning improves the accuracy of standalone GNNS receivers. Sub 3cm accuracy in X, Y and Z can be achieved using RTK dGPS.
RTK dGPS makes use of two receivers - a base station and a rover. The rover is the mobile unit recording the features of interest. The base station remains stationary over a fixed known point often referred to as a benchmark.
Example of a temporary benchmark
The base station computes its error compared it to its precise known location. These errors are then transmitted via the radio in real time to the rover, which uses this error data to improve on its location and can achieve cm accuracy.
A benchmark's accurate position is determined by placing the base station over it for a few hours collecting raw data for that position by employing the RTK and Infill method.
Base station capturing raw data for benchmark postprocessing
This raw data, saved on the receiver, is later post processed in Trimble Business Centre software using OS rinex data for that time period to give a corrected precise x, y, z location, thus establishing an accurate benchmark.
The image to the right shows how the benchmark is post processed using OSI active GNNS stations (rinex data). In this example, 7 active stations (as marked by the triangles) are used to correct the benchmark (is this instance labelled 101). Recording GPS points using a post processed corrected benchmark means all recorded data is referenced to that datum. For example in Northern Ireland that may mean data is recorded using Irish National grid and heights referenced to OD Belfast. Thus any recorded GPS points should be positionally accurate both horizontally and vertically when overlaying on other OSNI layers such as orthophotos.
It should also be noted that RTK networks can be accessed via paid subscription. Such access to RTK networks (such as Trimble VRS Now) eliminates the need for a base station unit in the field as a network of base stations can be accessed via signals received by a SIM card within the rover. Therefore error corrections are sent directly to the rover via the SIM without the need for setting up a base station on site. Surveying on remote locations with poor mobile phone signals however can be problematic. More info on Trimble VRS Now Network can be found here
The video to the right, demonstrates how the base station is set up in the field. It is important the base station is level and set up exactly over the same point, particularly if doing repeat surveys. The laser tribrach ensures this is possible and is attached to the top of the tripod
Tribrach
The base station antenna is attached to the tribrach using a pole and connected to the R9 receiver using a cable.
GNNS Antenna
The Trimble R9 base unit is fixed to the side of the tripod.
Trimble R9 base receiver
A radio and an antenna (used for transmitting between the base and rover) are connected to the R9 receiver using a cable.
Radio
It is very important to accurately measure the height of the base antenna from the benchmark. Any error here, persists within the your final data.
A connection is made between the base station R9 receiver and the TSC7 controller/tablet via blue tooth.
TSC7 with Trimble Access software
The base station coordinates are set and base station antenna height is set.
The rover is usually attached to a pole at a set height. This is usually 1.8m. A whip antenna is connected to the underside of the rover to ensure connection with the base via the radio signal.
R10 rover with whip antenna
The pole also has a bubble level, to ensure points are captured accurately. Some new rover models have the ability to account and correct for tilt of the pole when capturing points
Rover survey pole with TSC7 controller
Rovers can also be mounted on quad bikes for example. Vehicles such as quads can speed up the data capture process, however it is again important to measure exactly the height the rover antenna is off the ground to get true readings.
Quad bike with mount rover and TSC7 controller
The video on the right shows how data is exported and imported into GIS software for further analysis. The Job file is transferred from the TSC7 field tablet to the office computer and opened and exported using Trimble Business Centre software. All recorded points radiate from the Base station.
All points radiate (blue lines) from the base station
Points can be exported in various different formats such as csv, .txt or .shp to name a few. These can then be imported into other software such as ArcGIS Pro for further analysis.
RTK GPS has many applications within construction, utilities, OS mapping, archaeology and environmental monitoring to name a few. RTK GPS continues to complement many new technologies. For example checkerboards can be used as GCPS (Ground Control Points) during drone surveys to georeference drone outputs to a certain coordinate system.
Example of a checkerboard used for GCP
These checkerboards can be placed throughout the area of interest prior to a drone survey. The image below shows a RTK GPS point being captured in the center of the checkerboard prior to a drone survey of woodland. This GPS data is then used georeference the outputs during processing of the imagery.
Capturing checkerboard GCPs
The video to the right shows an RTK GPS sensefly ebee drone. This drone makes use of RTK by communicating to a base station directly thus improving it positional accuracy removing the need for checkerboards (GCPs) on the ground. Thus RTK GPS in conjunction with other technologies can improve accuracy of mapping and also improves efficiency.
