How does it work?

Information on GPS accuracy, RTK, twin antenna, etc.

Real Time Kinematic (RTK)

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.


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.


Standalone GPS

2-5 m


(Code corrections)

1-2 m

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

(Code differential corrections)

0.3-0.8 m

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

(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.


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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Measuring attitude angles

Measuring attitude angles

You can measure a vehicle's roll or pitch and yaw (slip angle) using two GNSS antennas. Both the VBOX 3i and VBOX IISX data loggers are available with dual antennas, enabling you to measure attitude angles and understand the exact movements of the vehicle.

What is Slip Angle?

Slip angle is the difference between the direction a vehicle is travelling (course over ground measured from the primary GNSS antenna - known as heading), and the direction that the body of the vehicle is pointing (measured using the secondary GNSS antenna - known as the true heading.) 

Learn more

How to measure slip, pitch or roll?

By configuring two antennas on a vehicle at a set distance apart, you are creating a 'fixed baseline', which determines the relative position and height of each antenna to within a few millimetres.

Knowing the relative position of each antenna gives a very accurate ‘true heading’ of the straight line drawn between them. The normal GPS heading can then be compared against this 'true heading' to calculate the slip angle (or sometimes referred to as yaw angle).

Knowing the relative height gives a very accurate angle between the antennas and allows the unit to measure pitch angle or roll angle (depending on how the antennas are mounted).

Why measure slip, pitch and roll angles?

Knowing how much lateral or longitudinal movement there is in a vehicle when accelerating, braking or cornering, can be invaluable for engineers, particularly in high dynamic applications.

Measuring attitude with VBOX 3i

By using two GNSS (GPS/GLONASS) antennas, the VBOX 3i Dual Antenna (VB3iSL) measures all the normal VBOX GPS parameters, plus the azimuth and elevation between the antennas, i.e. the direction the antennas are pointing, and the angle between them measured from the horizontal. This allows the unit to measure slip angle, and also pitch angle (or roll angle depending on how the antennas are mounted), all at 100 Hz. This system is ideal for vehicle dynamics testing.

VB3i Dual Antenna

The VBOX 3i Dual Antenna also comes with VBOX Manager, a display enabling you to change the dynamic modes and filter settings, set up slip angle data and define antenna locations. An optional dual antenna mounting pole (max. width 2.5 m) for the roof of the vehicle ensures the most accurate attitude measurement.

Using an Inertial Measurement Unit (IMU) with a Dual Antenna setup

The VBOX 3i is our flagship data logger and offers support for an inertial measurement unit, such as the VBOX IMU, which features a Kalman Filter. This can be used to process the heading parameter and give a cleaner heading with lever arm compensation, which can be seen by comparing heading to raw heading. This ‘cleaner’ heading parameter is then used in the slip angle and translated slip. 

True heading is not processed by the Kalman Filter, however the VBOX 3i Dual Antenna will use the IMU’s yaw rate during translated slip angle calculations. The IMU yaw rate has a lower noise level than the dual antenna yaw rate, resulting in a cleaner translated slip angle measurement parameter set.

Note, this will not have any impact on the standard slip angle channel as it does not require yaw rate for the calculation. Also note that the Kalman filter does not need to be enabled to get this additional benefit.

Using an Inertial Measurement Unit (IMU) with the VBOX 3i Dual Antenna provides highly accurate measurements of speed, distance, heading and yaw rate, for applications such as testing Electronic Stability Control with a steering robot. 


If you do not require an IMU or 100 Hz logging, the VBOX IISX Dual Antenna could be the ideal choice. The two antennas are placed on the vehicle a set distance apart, and the distance is entered into the VBOX using the front panel and display. An algorithm then uses this 'fixed baseline' to determine the relative position and height of each antenna to a few millimetres. This can be done because errors received at one antenna can be used to cancel out errors at the other antenna, given the known distance between the two.

  • Antenna 1

    Knowing the relative position of each antenna gives a very accurate ‘true heading’ of the straight line drawn between them.
  • Antenna 2

    The normal GPS heading can then be compared against this 'true heading' to calculate the slip angle (or sometimes referred to as yaw angle).

Knowing the relative height gives a very accurate angle between them, and this gives the pitch angle if configured longitudinally (down the roof), and roll angle if configured laterally (across the roof). In both orientations, the slip angle can still be measured, the user just has to apply a 90 degree offset which is available through the front panel.

Note: The slip angle varies across the width of the car, so it is important to place the antenna directly above the point at which you wish to measure this angle. Ground plane antennas can be mounted outside the body of the vehicle using a suitable mount.

The accuracy of the slip angle and pitch angle depend on the separation of the antennas and the quality of the antennas which are used. The wider the separation of the antennas, the better the accuracy you will achieve.

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GPS Accuracy

GPS Accuracy

GPS systems have revolutionised the accuracy and convenience of testing, development, and validation within the automotive industry. Why and how is it so effective?


There is a common misconception that speed measured via GPS is done so as a function of position against time. If this were the case, GPS velocity would be just about unusable, because GPS position relies on precise measurements of the distance from the receiver to the satellite, and therefore suffers from numerous effects - such as atmospheric interference - which delays the signal.

Fortunately, velocity isn't measured like this: instead, the Doppler shift in the signals coming from the satellites is captured and this leads to an incredibly accurate measurement of speed.

The Doppler effect (for sound) is the increase (or decrease) in a sound wave frequency as the sound of an object moves away (or towards) an observer.

80712773 education chart of physice for doppler effect of sound diagram


If you are tracking seven satellites, then this is like having seven police radar guns aimed at you: the measurement is made by taking them all into account.

Other crucial factors involved in producing useful velocity measurements for vehicle testing are sample rate, frequency response, latency and noise.

Sample Rate

The number of samples produced each second is of primary importance in vehicle testing. The most usual update rate for GPS is once a second, which is fine for navigation, but it is not much use if you are trying to measure speed, braking distance, acceleration, lap time or track position. The minimum for these sorts of tests is 5 Hz to see any level of detail, and 20 Hz is normally considered the minimum entry point for a decent GPS vehicle testing solution. The top end GPS systems can update 100 times a second, and these are suitable for specialised applications such as brake testing and high speed dynamic manoeuvres.


GPS latency is the time between the measurement of velocity, and when the VBOX reports this velocity. The latency is only important if you are measuring other parameters at the same time, or if you want to use the output of the GPS engine to control something.

GPS latecy

Velocity comparison between a consumer 1 Hz GPS engine (blue) and a survey grade 20 Hz engine (red)


The faster you sample the velocity, the more noise will appear on the signal. There is a very careful balance between filtering the velocity to remove noise, and introducing a poor frequency response. By incorporating an IMU and Kalman filter, you get the best of both worlds, an improved frequency response with less noise.

VBOX 3i connected to an IMU04

Frequency Response

This is how fast the velocity measurement can react to a change in vehicle motion, for example, during the onset of an ABS assisted stop, the rate of change of acceleration (jerk rate) is very high, and to measure distance and speed accurately, the GPS has to be able to track these changes very accurately.

To show an example, here are three brake stops which were carried out using a low, medium and high frequency response setting on the GPS engine. The blue trace is the reference (IMU-corrected) vehicle speed, and the red trace is the GPS measured speed. You can see as the frequency response gets higher, the over/undershoot of the velocity gets smaller.

Low frequency response
Medium frequency response
High frequency response

Commercially available GPS engines

The standard type of GPS engine used in sat nav devices and mobile phones is very small and very cheap. Coupled with a very simple and small antenna, these devices update their speed and position once a second and have an accuracy of around 3-5 m (95% CEP*). However, even at this low end of the market, the velocity is usually still fairly accurate at around 0.2-0.5 km/h. Whilst an update rate of once a second (1 Hz) is not enough for any kind of high speed vehicle analysis, we have optimized the settings in one of the best commercial grade receivers to output data at 10 Hz.

Survey Grade

Survey grade GPS engines use much higher quality components than commercial GPS engines, employ very powerful processors and use patented techniques to get higher positional accuracy and high speed updates. They are a lot larger than the commercially available GPS engines, and can cost up to 100 times more.

You may ask what is special about the GPS engines we use in our VBOX products? The answer is that we have not only selected the best GPS engines for different products, but we have also worked with the manufacturers of these GPS engines to optimize them for our particular requirements, so you can be sure that you are getting the very best performance that is available.

*95% CEP (Circle of Error Probable). This means 95% of the time the position readings will fall within a circle of the stated radius.
The 24hr position scatter plot on the left shows a commercial engine in red, an un-aided survey grade in blue, aided by SBAS corrections in green, aided by a 20 cm base station in purple, and you can just make out an RTK 2cm aided system in yellow!


GPS satellites are equipped with an atomic clock, which ensures timing stability to less than one-millionth of a second. By integrating Doppler-derived speed with this level of time signal reliability, an extraordinarily accurate distance measurement is achieved.

There are a number of tests which we have carried out over the years to verify and improve our measurement algorithms for applications in vehicle testing.

One such test is to place two reflective strips on the road at a known distance apart. Using a laser sensor connected to the trigger input of a VBOX, the vehicle is then driven between these two points a number of times, and the distances compared. In such tests, the VBOX 3i will always be within 3 cm in 1000 m, which is about the same as the measurement uncertainty due to the slight deviation of the vehicle during the driving.


GPS positional accuracy is subject to ionospheric interference, which causes a satellite signal to change in length as it travels towards the Earth's surface. This means that, depending on the quality of the receiver, accuracy can vary by several metres.

VBOX data loggers are fitted with superior GPS engines that give better positional accuracy than those found in mobile phones and satellite navigation devices. By using additional signals other than those from the standard US GPS satellites, this can be enhanced further. The free SBAS services augment the positional capabilities of a survey-grade receiver to as good as 1m without the use of a Base Station.

However, by adding another GPS engine at a fixed position, the majority of errors can be calculated and removed from the final result. This is the function of a Base Station: because it remains stationary, it is able to broadcast correction messages to a roving VBOX, resulting in much improved positional accuracy.

By incorporating signals from the Russian GLONASS satellite constellation and employing a Real-Time Kinematic (RTK) algorithm, the VBOX 3i Dual Antenna RTK with suitable Base Station achieves a positional accuracy of +/-2cm.

The advantage of using both sets of constellations is that there are almost twice as many measurements available to use, and they are on two slightly different frequencies. This allows sophisticated algorithms inside the GPS engines to eliminate all of the major errors normally associated with position estimation, leading to the highest accuracy GPS engine on the market today.

See this video for a demonstration.

The image shows a scatter plot of samples taken over a period of 24 hours and demonstrates just how accurate the RTK + GLONASS really is: Normal GPS in red, RTK in blue.


By employing multi-antenna systems, it becomes possible to measure vehicle body attitude parameters. When placed in line down the length of the vehicle roof, a dual antenna setup can be used to capture slip angle and pitch; when placed across the width of the roof it can record roll.

This works by configuring the VBOX with the antenna separation distances, and with an RTK lock operating between them, a very accurate angle measurement is produced, achieved by comparing the heading taken from the primary antenna against the relative position of the others.

Multi-antenna VBOX data loggers are used not just in the automotive sector but also in Marine applications, where they are perfect for measuring trim, heel, and lee of the vessel.

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