On the 4th of May 2020 Xsens announced the launch of a new RTK-compatible motion tracker, the MTi-680G. The MTi-680G is a GNSS/Inertial Navigation System (INS) module that features an integrated RTK GNSS receiver, as well as provides synchronized 3D orientation and heading outputs.
What is RTK (real-time kinematic)?
Real-time kinematic (RTK) positioning is a satellite navigation technology used to increase the precision of position data obtained from satellite positioning systems.
Traditional GNSS receivers measure how long it takes for a signal to travel from satellites to the receiver. Using 4 or more satellite signals and trilateration mathematics will enable the receiver to calculate its position. These signals, however, can get distorted when passing through the ionosphere and atmosphere affecting the calculated position accuracy (2 to 10 meters). RTK solves this issue by using real-time corrections from a base station. The base station knows its fixed position and therefore is able to estimate the errors of the received signals.
A base station consists of a GNSS receiver with known and fixed coordinates. The base station tracks the same satellites at the same time as the rover receiver. The base station is monitoring the GNSS errors and computes the position corrections. The position corrections are sent via a radio link or internet services to the rover receiver, which uses these messages to correct the real time position.
RTK in a nutshell
- Differencing done in real-time
- Static base-station
- Known position
- Sends "RTCM" correction messages to rover
- Mobile rover
- RTCM messages contain:
- Code- and carrier measurements
- For both L1 and L2, all in-view satellites
Source: GPS for Land Surveyors
Standard GNSS Errors Corrected by RTK
Using RTK allows the position estimate of the GNSS receiver to be significantly more accurate by compensating for inaccuracies derived from how position is estimated using standard GNSS. Standard GNSS uses the estimation of the pseudorange between satellites and the receiver to determine the receiver’s 3-dimensional position on Earth. In this estimation, a variety of error sources can cause significant inaccuracies in the estimated position of the receiver. Clock errors between the GNSS satellite and GNSS receiver as small as one microsecond will produce an error of 300 meters. By establishing a GNSS fix with four or more satellites at the same time, the receiver can solve for four unknowns: its 3-D position and its clock error with respect to the global time.
Source: GPS for Land Surveyors
Another source of error in standard GNSS position estimation is atmospheric delays and inconsistencies that distort the radio signals between the receiver and satellite. Inconsistencies can vary depending on the location of the satellite particularly in the ionosphere causing refraction and diffraction of the signals. Atmospheric pressure and humidity in lower layers of the atmosphere will also influence the timing of the signal.
Source: GPS for Land Surveyors
Because the satellite’s position is required to determine the position of the GNSS receiver, distortions in the shape of the satellite’s orbit can cause errors in the estimated position. The GNSS ground control system does send correction messages to the satellite, but small inaccuracies can still cause significant positioning errors.
The advantage that RTK GNSS has is that further correction information is available to mitigate the errors inherent in satellite communication. By receiving RTK correction information from a known base location relatively nearby the receiver, the atmospheric errors in the pseudorange estimation can be calculated by the base station and transmitted to the rover. By using RTK, positional data can be improved from meter-level accuracy to centimeter-level accuracy.
Advantages and disadvantages of RTK
The key benefit of using RTK is the high positioning(centimeter) accuracy in real-time.
Other advantages of using RTK:
- Precise navigation
- Improved position estimation
- Improved heading at low velocity
- Reduce positioning jitter when static
Disadvantages of RTK:
- Requires a base-station with known coordinates
- Higher infrastructure cost
Traditional RTK vs RTK Networks and NTRIP Services
The traditional RTK infrastructure consists of two components: the base station and the rover. The rover receives corrections from the base station via radio modems (UHF/VHF/Spread). This infrastructure can be expensive, complex, and has limitations. The RTK corrections are limited to the transfer distances of the radio signal (5-10km) and the atmospheric and environmental conditions. Radio signals can be blocked by hills, buildings, trees, etc.
The main purpose of an RTK Network is to eliminate the errors caused by the atmosphere and environmental conditions and send the corrections over long distances (100km) in real time via GSM modems. The RTK infrastructure is much more complex than the traditional RTK. This system consists of 3 or more reference stations that make continuous observations. Reference station data is processed by one or more central servers before being distributed to the rover. The servers are using correction computing methods to calculate the corrections.
NTRIP (Networked Transport of RTCM via Internet Protocol) is a protocol that enables streaming of RTK correction data via the internet over common TCP/IP methods. The system consists of two parts which communicate via the internet: the server side and the rover side.
The server is responsible for receiving data from the base station and rebroadcasting them to the rover via TCP/IP. The NTRIP services give access to RTK Network to users who do not own an RTK base station.
The benefits of using NTRIP over Traditional RTK:
- No need to own an RTK base station
- Minimize the chance of radio interference
- No limitation on communication range. UHF/VHF/Spread radios have a limited range
The limits of using NTRIP:
- Requires cellular service for receiving RTK corrections data via the Internet
- Some NTRIP Services require paid subscription