How does it work?

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

GPS Accuracy

Posted in How does it work?

Real Time Kinematic (RTK)

Posted in How does it work?

Real Time Kinematic is a technique used to increase the accuracy of GPS signals by using a fixed base station which wirelessly sends out corrections to a moving receiver.

Base-Station-Setup

24-hr-scatter-plot
24hr scatter plot - RTK plot in blue, normal GPS in red

By utilising these corrections, the GPS engine can fix the position of the antenna to within 1 - 2cm.

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 - 0.2m and 0.01 - 0.02m in Fixed mode.

The correction signal is sent out at intervals of 1 second.

 

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 - 5cm.

Moving-Base

Automotive applications

RTK is particularly suited for measuring the relative position of a number of moving and stationary objects, making the technique very useful in the test and verification of ADAS systems.

Limitations

The ‘Roving’ GPS engine has to be within communication range of the Base Station in order to receive corrections. Dropouts of around 10s can be tolerated without any undue loss of accuracy.

In order to work quickly, the GPS receiver has to see two sets of satellite frequencies, and the best way to achieve this is to track GPS and the Russian Glonass system. Therefore the Base Station and receiver need to track both constellations, and require antennas which can receive both frequencies.

IMU04Both the base station and receiver need to have a clear line of sight to the sky, RTK does not 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.

Measuring attitude angles

Posted in How does it work?

Slip, pitch/roll and yaw angles with
VBOX IISX Dual Antenna

The two antennas are placed on the vehicle a set distance apart. This distance is then 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.

slip-antennas1 Knowing the relative position of each antenna gives a very accurate ‘true heading’ of the straight line drawn between them.
slip-antennas2 The normal GPS heading can then be compared against this 'true heading' to calculate the slip angle (or sometimes referred to as yaw angle).

slip-antennasKnowing 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 of the body of the vehicle using a suitable mount (see picture on the right).

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.

Slip, pitch, roll and yaw angles with
VBOX IISX Triple Antenna

The triple antenna unit uses the same methods to measure pitch, roll and yaw angles as the VBOX IISX Dual antenna.

3-AntennaeThe exact distances between the primary antenna and the two additional antennas are entered into the unit, which then uses these ‘fixed baselines’ to enable an RTK technique which can measure the relative height and position of each antenna to a remarkable accuracy. 

This is possible because similar errors are present at each antenna, so a sophisticated algorithm is able to eliminate these errors, resulting in a very precise relative height and position measurement.

3-ant roof annotated sThe picture on the right demonstrates a typical setup, where antenna A is the primary antenna. In this instance, antennas A & C are measuring roll angle, whilst antennas A & B are measuring both yaw angle and pitch.

The exact distances between A & B and A & C have been measured and entered into the VBOX via the front panel. Note that the slip angle depends on where the measurement is taken, imagine turning a sharp corner very slowly, the front of the car will be showing a large negative slip angle value due to the steering input, and the rear of the car will show very little slip angle.

Antenna-extensionThe slip angle also 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 of the body of the vehicle using a suitable mount.

The VBOX has been configured with the distance between the antennae, and thanks to this layout is able to record all the measurements as shown in the table below.

Antenna Combinations True
Heading
Slip
Angle
Pitch
Angle
Yaw
Rate
Lateral
Velocity
Roll
Angle
A no no no no no no
A + B yes yes yes yes yes no
A + C no no no no no yes
A + B + C yes yes yes yes yes yes

We supply propriety, magnetic mount, ground plane antennas which have been optimised for use in this application to give the best results.

To obtain these accuracies, the distance between the antennas needs to be accurate to within 0.5cm, and the system used in a clear, open area without any trees, tall masts, fences or buildings nearby.

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