Accurate Modbus positioning and time sensing with GPS

When a remote cattle station needed its pivot sprinkler to operate in a safe manner, Ocean Controls provided a great solution.

The KTA-291 GPS Modbus gateway is a versatile tool to provide accurate values of time and position including:

  • Universal Time Coordinated time (accurate to 60 nanoseconds);
  • latitude and longitude (accurate to 2.5 metres);
  • altitude (accurate to 0.1 m/s);
  • velocity (accurate to 0.1 m/s); and
  • direction of travel (0.1-degree resolution).

All data is updated four times a second and made available on Modbus RTU (RS-485) at user-configurable slave address, baud rate and parity. GPS quality, horizontal precision and the number of satellites currently in use can be monitored to indicate the accuracy of received readings. Two outputs are provided – one to monitor the GPS lock, and the other a pulse output (PPS) for synchronising external clocks.

Case Study: Pivot Irrigator Position

Ocean Controls was approached by a client operating a remote cattle farm in rural Western Australia. The paddock was irrigated using a pivot sprinkler operating in a 270-degree arc, which was instructed to run remotely from headquarters in Perth.

To ensure safe operation, the angular position of the irrigator needed to be known by the control system. Traditional magnetic compasses were not accurate enough in this application.

Instead, a KTA-291 was mounted on the pivot head. Waypoints were taken for the extent of travel in each direction.

The control system then compared the current latitude and longitude with the saved waypoints to make sure that the pivot stopped at each end of the 270-degree arc.

In addition, logs were taken of the position and velocity data for later analysis of irrigator performance. The GPS UTC was used as a source of accurate and drift-free timestamps