The most important parameter to measure in the quest for slip angle is the direction that the vehicle is pointing. This is measured by measuring the relative position of the primary antenna and the secondary antenna, and using this information to measure the angle between the antennas relative to true North.

In the picture on the right, the angle between the primary and secondary antennas is shown. This is calculated to within an accuracy of 0.1 degrees depending on the separation of the antennas.

The second measurement required to measure slip angle is the angle the vehicle is travelling. This is measured by the primary antenna, and is the normal GPS "heading" measurement or Course over Ground.

This is calculated from the direction of motion of the primary antenna, and is derived from the primary components of the doppler velocity (north velocity and east velocity). By subtracting the direction the vehicle is pointing from the direction it is travelling will give you a direct measurement of slip angle.

All measurements are made at the primary antenna. The primary antenna can be placed almost anywhere on the vehicle, and as long as the distance between the two antennas is fixed, they can be at any angle.

If the antennas are placed away from the roof, then an antenna with a built in ground plane must be used. This is usually the case when you are comparing the system with an optical sensor which cannot be mounted within the confines of the vehicle.

The Slip Angle Sensor can be used to provide measurements for lane change manoeuvres, steady state cornering tests, trailer angles relative to the towing vehicle and any test involving speed, distance, acceleration, slip, roll or pitch angles.

The speed and distance are as accurate as a normal VBOXII 20Hz, so you can still measure stopping distances down to 10cm, meaning you have one sensor which does many different kinds of test, and fits in seconds.

The relative positions of the primary and secondary antenna are calculated very accurately by using RTK techniques (RTK = Real Time Kinematic). This is possible by fixing the distance between the antennas, and telling the system what that distance is.

This constrains the RTK positioning routines within the GPS engines and gives a very accurate relative displacement between the primary and secondary antenna, including the relative height between them.

The graph below shows the output from the pitch channel during an acceleration in the first four gears. You can see the small changes in pitch between gear-changes is captured in fine detail by the pitch measurement. This was carried out using a separation of 1m between the antennas, and you can see the system is sensitive enough to measure a 0.6 degree change in pitch angle over a very short time period.

• 2 x Digital
  Output frequency proportional to Velocity, Slip angle, Pitch, Yaw Rate etc.
  
• 2 x Analogue
  Velocity -5v to +5v, scale adjustable, Velocity, Slip angle, Pitch, Yaw Rate etc.
• CAN
  All parameters - 1Mbit default, configurable baud rates.
• RS232
  All parameters - 115200 baud.

In competitive sailing, the slip angle or leeway is normally a very difficult parameter to measure, as it requires a precise measurement of the direction a boat is pointing relative to the direction of travel. In the past this has required expensive gyrocompasses or mangetometers, but by using the VBOX Slip angle sensor, the leeway or boat side slip is measured to a high degree of accuracy very easily.