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Real Time Kinematic (RTK)

Real Time Kinematic is a technique used to increase the accuracy of GNSS positions using a fixed base station, that wirelessly sends out correctional data to a moving receiver.

How to receive RTK correctional data

The RTK correction source can either be from a local fixed base station or received over the internet from a third party correction service using NTRIP (Networked Transport of RTCM via Internet Protocol). Fixed base stations, such as the Racelogic RTK Base station, have a range of up to 10 kilometres (approx 6.2 miles). While NTRIP correction services will often require a per-user subscription and some form of internet access, they have the advantage of offering a wide coverage area that is not limited to a particular region; making it the ideal solution for open-road testing.

Fixed Base Station

By utilising corrections from a fixed base station, the GPS engine can fix the position of the antenna to within 1-2 cm. The technique involves the measurement of the carrier phase of the satellite signal, which is then subject to some sophisticated statistical methods to align the phase of these signals to eliminate the majority of normal GPS type errors.

This alignment process goes through three phases: acquisition, ambiguity ‘Float’ mode and ambiguity ‘Fixed’ mode. Accuracies in Float mode are in the region of 0.75-0.2 m and 0.01-0.02 m in Fixed mode. The correction signal is normally sent out at intervals of 1 second, but can be increased if needed in order to reduce the required data rate.

The Racelogic RTK Base Station can be used to improve the positional accuracy of VBOX GPS systems, by calculating and then transmitting differential correction data via radio to allow the roving GPS system to correct its position. Because a single base station can be used simultaneously by all receivers in range, fixed based stations are ideal for proving grounds or test tracks, where you are testing in a confined area of up to 10 kilometres radius.

NTRIP

It is also possible to receive correctional data via Network RTK, utilising a protocol called NTRIP (Networked Transport of RTCM via Internet Protocol).

This method requires a constant internet connection (via a GSM modem or smartphone) and a subscription to your local NTRIP service provider, who will have an infrastructure of fixed base stations forming a national or regional network. This enables the roving VBOX to send its position to the stations within the NTRIP network, and the NTRIP service provider can then calculate the appropriate corrections for the VBOX’s location based on the data from nearby reference stations. The correction information is then returned to the VBOX in the same RTCM format as is used for a single fixed base station.

One of the main advantages of NTRIP is that it uses a network of RTK base stations, that are already in place, and therefore has no range restriction. This is ideal for open-road testing where you will be more than 10 kilometres from a single, fixed base station. Using an NTRIP solution, you can expect to receive position accuracy to within 2 cm.

How accurate is RTK?

Standalone GPS engines typically offer 2 - 5 m accuracy, however using RTK as a source of correctional data can improve positional accuracy to within 2 cm, when used with an RTK enabled GNSS receiver.

DGPS (Differential GPS) offers enhanced position accuracy compared to standalone GPS, by making use of correction information broadcast from geostationary satellites, such as SBAS or EGNOS, which appear to stay in a fixed position in the sky when viewed from the ground. Using DGPS typically improves position accuracy to within 1 - 2 m.

In order to achieve centimetre level accuracy, you will need to use RTK corrections, either from a fixed based station or NTRIP modem.

SystemAccuracyRequirements

Standalone GPS

2-5 m

  -

SBAS
(Code corrections)

1-2 m

DGPS correction source, normally SBAS built in to the receiver.

DGPS
(Code differential corrections)

0.3-0.8 m

DGPS correction source from a DGPS Base Station (or NTRIP with code corrections).

RTK
(Carrier corrections)

1-5 cm

RTK corrections from a base station close to the rover

Moving Base

The base station can also be on a moving vehicle, in which case the corrections needs to be sent out at the sample rate of the receiving GPS engine, and the accuracy is slightly reduced to around 3-5 cm.

The Moving Base set up allows engineers to test ADAS applications on the open road, without having to connect to a base station or NTRIP modem. It works by linking two VBOX 3i RTK units, with the system in the subject vehicle transmitting corrections to the target vehicle at an update rate of 20 times per second.

The accuracy is enhanced by employing signals from multiple frequencies and constellations, as well as a method which uses refined delta positions obtained from carrier phase measurements. This reduces the noise levels of pseudo-range measurements (raw distances to each satellite) and removes positional jumps.

Which applications require RTK?

Any application that requires centimetre level accuracy is likely to benefit from having RTK as the source of correctional data. The VBOX 3i GNSS data logger with RTK offers 2 cm position accuracy, when used with the Racelogic RTK Base station or NTRIP modem. Typical applications include ADAS testing, autonomous vehicle validation, ground truth measurement, and path following.

Limitations

When using a fixed base station, the ‘Roving’ GPS engine has to be within communication range of the Base Station, in order to receive corrections. Using an NTRIP solution helps overcome this, but you will need to have a subscription with an NTRIP service provider, and they often charge per user.

The time taken to move through each of the acquisition phases and reach a Fixed mode position depends greatly upon the number of signals available to the receiver. A multi-frequency, multi-constellation GNSS receiver will reach the fixed mode significantly faster than a GPS only or L1 only system. In order for a signal to be used in the RTK solution it must not only be tracked by the rover but also be tracked by the base station and included in the correction messages that it is outputting.

Since both the base station and receiver need to have a clear line of sight to the sky, RTK does not always work very well in urban areas with tall buildings, with tree cover or under bridges. In these situations, it is best to use an inertial measurement system (e.g. the Racelogic IMU04) to smooth out the solution.

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