Attitude – RTK only option
The SPEEDBOX-RTK uses moving-base real time kinematics technology to accurately measure the relative position between 2 antennas. This relative position can be expressed as 2 angles, which are the relative horizontal and vertical position of the antennas. Assuming that the antennas are mounted on the vehicle in a flat plane aligned with (but not necessarily on) the centerline of the vehicle, these 2 angles then express the yaw and pitch of the vehicle, the yaw being defined as the direction that the vehicle is pointing in the horizontal plane, and the pitch being the direction that the vehicle is pointing in the vertical direction. Neither direction will necessarily be co-incident with the direction of travel of the vehicle; the difference between the vehicle yaw and heading is defined as the vehicle body-slip angle, and the difference between the vehicle pitch and gradient is defined as the vehicle dive or squat angle.
Yaw measurement accuracy and noise
In order to assess the accuracy and noise of the SPEEDBOX-RTK angular measurement, data was logged continuously over an extended period of time from 2 antennas placed on the roof of the Race Technology offices. The 2 antennas used were the standard Race Technology GPS patch antennas, and they were placed on metal ground planes a distance of 1m apart, to simulate being placed on the metal roof of a vehicle. The antennas were both also aligned in the same direction in order to try and ensure consistency in the phase center position of the antennas, as recommended in the SPEEDBOX-RTK documentation.
Figure 1 - MB-RTK yaw angle during 1h20min of stationary testing.
Figure 2 - MB-RTK yaw deviation from mean during the 1h20min of stationary testing shown in figure 1.
Figure 1 shows the yaw angle from approximately 1h20min of stationary testing using the roof antennas, normalised to zero for clarity. It can be seen that the peak-to-peak variation of yaw is approximately 0.6°. This change happens over a fairly long time scale as the satellite constellation changes; the noise level over a shorter time scale is around 0.2° peak-to-peak.
Pitch measurement accuracy and noise
Figure 3 - MB-RTK pitch angle during 1h20min of stationary testing.
Figure 4 - MB-RTK pitch deviation from mean during the 1h20min of stationary testing shown in figure 3.
Figure 3 shows the pitch angle from the same 1h20min of stationary testing as shown for the yaw angle, once again normalised to zero degrees in order to improve clarity. Figure 4 shows the deviation from the mean for the same data. It can be seen that the accuracy of the pitch measurement is considerably worse than the accuracy of the yaw measurement, showing a variation in the region of 2 to 3° peak-to-peak. As with the degradation of vertical positional accuracy compared to horizontal positional accuracy, this is entirely due to the less favourable GPS geometry in the vertical axis, only magnified in the MB-RTK case since the increased error affects both antennas, and the error is hence effectively doubled in the calculation of pitch angle. The relatively lower quality of the pitch angle data compared to the yaw angle data should be kept in mind when assessing the suitability of the MB-RTK unit for any given measurement task.
Long term test of Yaw and pitch
Figure 5 - Distribution of error in yaw and pitch for data logged from the Race Technology office roof for 1 week, using a baseline of 1m.