REXDESKREXGEN AirManual

REXGEN Air: GNSS, dead reckoning and geofencing

Updated

Record position with REXGEN Air, keep the track going through tunnels with dead reckoning and flag when a vehicle enters or leaves a circular geofence.

How GNSS works on REXGEN Air

REXGEN Air has an internal u-blox GNSS module. It takes an external active antenna with a 3 V supply on its blue FAKRA Code C port. The receiver tracks GPS and QZSS, GLONASS, BeiDou and Galileo satellites. Keep the antenna connected and tight whenever the logger is powered. If it comes loose, position data is lost.

Dead reckoning and geofencing need the u-blox M8U receiver. For the receivers and their rates, see REXGEN Air: technical specifications.

Set up GNSS in REXDESK

  1. Open the GNSS channel. In the configuration window, select the GNSS icon under Channels.
  2. Set the Sampling Rate (Hz). This is how often the logger records a position. The maximum depends on the receiver fitted.
  3. Choose the platform. The Platform is the receiver's dynamic model. Select the one closest to how the logger moves.
  4. Choose the GNSS type. Select the satellite systems to track.
  5. Enable dead reckoning, if you need it. Align the IMU first. Then tick Dead reckoning so the track continues when satellites are lost.
  6. Choose the items to log. Under Select Items to log, tick Standard and Extended together (REXDESK needs both enabled at the same time), then any of Attitude orientation, IMU, Geofence and Accuracy.
  7. Send the configuration. Select OK and send the configuration to the logger.
REXDESK GNSS settings window with Sampling Rate, Platform, Dead reckoning, GNSS type, the Geofence fields and the Select Items to log checkboxes
The GNSS settings window. Your REXDESK version may arrange the fields slightly differently.
Platform
Portable, Stationary (no movement), Pedestrian, Automotive, Sea, Airborne up to 1 g, 2 g or 4 g, Wrist, Motorbike, Robotic Lawn Mower and Electric Kick Scooter.
GNSS type
Default, GPS, Galileo, GLONASS, BeiDou, GPS + Galileo, GPS + Galileo + GLONASS, GPS + Galileo + BeiDou, GPS + BeiDou, Galileo + GLONASS, Galileo + BeiDou and GLONASS + BeiDou.

For every GNSS and IMU setting, see REXDESK: set up GNSS and the IMU.

Watch: Set up GNSS and its advanced options using REXDESK (2:17)

Dead reckoning

GNSS gives a precise position in open sky, but its signal weakens or disappears in tunnels, urban canyons, dense forest, underground car parks and subways. Dead reckoning fills those gaps. It keeps estimating the position from the logger's own movement data, such as speed and direction, until the GNSS signal returns.

Dead reckoning bridges a GNSS gap GNSS · DEAD RECKONING Dead reckoning bridges a GNSS gap Tunnel no GNSS signal GNSS only the track stops no position With dead reckoning the gap is estimated Signal lost Fix returns GNSS fix Estimate from the logger's own movement data, such as speed and direction Not to scale. The same gap opens in urban canyons, dense forest, underground car parks and subways. Dead reckoning is only as good as the IMU alignment, so align the IMU before you enable it.

Dead reckoning is only as good as the IMU alignment. Align the IMU before you enable it.

Align the IMU

The vehicle frame has X pointing forward (longitudinal), Y pointing to the left (lateral) and Z pointing up (vertical). In the reference installation, the logger is mounted with its bottom facing the sky, so its IMU axes line up with the vehicle's.

Aligning the IMU: reference mounting and three misalignments REXGEN AIR · IMU ALIGNMENT Aligning the IMU: reference mounting and three misalignments Vehicle axes, seen from above Front X forward Y left Z up, towards you Reference mounting, seen from the left Vehicle, front to the left X, X′ Z, Z′ Y, Y′ towards you Bottom faces the sky Mounted bottom up, the IMU axes line up with the vehicle axes. Rolled 45° about X seen from behind Y Z Y′ Z′ 45° <MountRoll>45</MountRoll> Z and Y misaligned Yawed 45° about Z seen from above Y X Y′ X′ 45° <MountYaw>315</MountYaw> 360° − 45°: X and Y misaligned Pitched 45° about Y seen from the left X Z X′ Z′ 45° <MountPitch>-45</MountPitch> Z and X misaligned. Sign Vehicle axis Logger IMU axis (′) Logger Axis towards you Axis away from you Correct Z first, then Y, then X. The arcs show the size of each misalignment, not its direction. The pitch sign convention is (decision 3.30): the original drawing reads 90° − 45° = 45°, the XML -45.
  • Mount the logger so it cannot shift and disturb the IMU orientation.
  • If it cannot sit in the reference position, try to keep it misaligned about one axis only.

If the logger is misaligned, enter the mounting angles in the XML configuration. Correct the Z axis first, then Y, then X.

IMU mounting angles
XML element Rotation about Range Default
MountYaw Z, the vertical axis 0° to 360° 0
MountPitch Y, the lateral axis -90° to 90° 0
MountRoll X, the longitudinal axis -180° to 180° 0

To find each value, measure the angle by which the logger is misaligned and subtract it from the axis's actual position.

Rolled logger

A logger rolled by 45° has its Z and Y axes misaligned.

<MountYaw>0</MountYaw>
<MountPitch>0</MountPitch>
<MountRoll>45</MountRoll>

Yawed logger

A logger turned by 45° about the vertical axis has its X and Y axes misaligned. The yaw value is 360° - 45° = 315°.

<MountYaw>315</MountYaw>
<MountPitch>0</MountPitch>
<MountRoll>0</MountRoll>

Pitched logger

A logger pitched by 45° has its Z and X axes misaligned.

<MountYaw>0</MountYaw>
<MountPitch>-45</MountPitch>
<MountRoll>0</MountRoll>

For the XML elements, see REXGEN XML reference: analogue, digital, IMU and GNSS inputs.

Watch: Set up the IMU using REXDESK (0:58)

Geofencing

A geofence is a circle around a location of interest. REXGEN Air supports one circular geofence and logs whether the vehicle is inside or outside it. Geofencing works only while dead reckoning is active.

One circular geofence and the value the logger records REXGEN AIR · GEOFENCE One circular geofence and the value the logger records Centre: Latitude, Longitude Radius, metres 0 2 2 1 1 1 1 2 2 Vehicle track Logged geofence value 0 Unknown: not enough satellites to decide 1 Inside the geofence 2 Outside the geofence One circular geofence per logger. Geofencing works only while dead reckoning is active. Not to scale; the track and its samples are illustrative. Set the geofence in the GNSS settings window (Use Geofence) or in the XML configuration (Geofence_LIST), and tick Geofence under Select Items to log.
  1. Enable the geofence. In the GNSS settings window, tick Use Geofence.
  2. Enter the centre. Enter the Latitude and Longitude of the location.
  3. Set the size. Enter the Radius in metres.
  4. Log the geofence state. Under Select Items to log, tick Geofence, then select OK.
Geofence output values
Value Meaning
0 Unknown: not enough satellites to decide
1 Inside the geofence
2 Outside the geofence

You can also set the geofence in the XML configuration:

<Geofence_LIST>
  <Geofence>
    <Latitude>17</Latitude>
    <Longitude>20</Longitude>
    <Radius>5</Radius>
  </Geofence>
</Geofence_LIST>

This is the last guide in the REXGEN Air series. To upload logs over LTE, continue with the cloud upload series.

Next: REXGEN Air: choosing and setting up cloud upload.

Products in this guide

REXDESK

Logger setup & data export

View REXDESK Ask an engineer

REXGEN Air

CAN to cloud

View REXGEN Air Ask an engineer

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