The Environment Agency use a range of Acoustic Doppler Current Profilers (ADCPs) to monitor river flow. This is used to aid flood models used as part of flood defence planning. The ADCP can generate a relative trajectory using the reflections from the river bed, however this is not georeferenced. This trajectory can also be unreliable in deep waters, or waters where the river bed is moving, or has high silt levels. The requirement was to provide a compact GNSS antenna located over the ADCP, with power and a wireless link to the shore where the data was being collected by the ADCP software.
The initial requirement was for a range of ~100m, however the Environment Agency later had an aspiration to roll this out over larger bodies of water to take advantage of their remote control boats, and make them much more flexible.
Position accuracy was to be sub-metre, with the facility to enable centimetre RTK in the future if needed.
Our ReACT antenna was an ideal starting point for this project. Incorporating the precision Antcom GNSS antenna and NovAtel OEM615 receiver into a single environmentally robust package. What the ReACT does not feature is internal battery or wireless communications. For this, we implemented a simple plug-in module featuring a removable, and rechargeable battery pack and a Bluetooth adapter. The ReACT was configured to track the EGNOS service to achieve sub-metre SBAS positioning, however river banks, trees, buildings etc. can often obscure the EGNOS satellites from visibility. After some field testing, the addition of GLONASS, and utilisation of the NovAtel OEM615’s PDPFilter feature, position availability and accuracy was maximised without the need to add additional sensors such as Inertial Measurement Units.
The ReACT gives the flexibility to enable RTK positioning if needed, via a firmware unlock, meaning there is no need to change the hardware.
Following the successful deployment of the Bluetooth system, the range requirement was increased. To keep the system compact, but to increase the transmission range to 1km or more, it was decided to move away from Bluetooth, and explore the use of 869MHz xBee radios. In tests, with basic antennas, and no amplification, we observed transmission ranges in excess of 2km line of sight, as well as impressive performance when there were obstructions in the way, such as trees.