In November 2015 off the coast of Toulon, Total, together with the French research institute, Ifremer, conducted a performance trial of Sonardyne’s Free Space Optical Communications technology, BlueComm. The objective was to characterise the performance of the BlueComm 200 variant collecting data on beam shape, maximum range and data transfer rate. The secondary objective was to control an ROV using only BlueComm confirming link stability and demonstrating real world practicality. Communications Application Engineer, Matt Kingsland, was aboard for Sonardyne.
BlueComm is Sonardyne’s innovative through-water wireless optical communication system that’s capable of transmitting data at very high speed. Unlike our traditional range of navigation and positioning technologies, BlueComm uses the electromagnetic spectrum rather than acoustic pressure waves to transmit high volumes of data.
Typically operating in the 450 nanometre Blue Light region of the spectrum, BlueComm can achieve data rates of greater than 500 Mbps. Optical data transmission is highly efficient, enabling 1Gb of data to be transmitted with the energy contained within a single lithium ‘D’ sized cell over distances greater than 150 metres.
If you think of it in the context of ‘underwater broadband,’ then the myriad of potential applications for the technology soon becomes apparent; tether-less vehicle control, real-time video streaming and well intervention using resident AUVs.
BlueComm 100, 200 and 5000 The BlueComm product family is now made up of three variants. BlueComm 100 is optimised for shallow water ‘high ambient light’ operating environments and offers a good balance between data rate and range.
At the other end of the model line-up is BlueComm 5000. Its dual laser configuration supports data transfer rates at an impressive 500 Mbps at ranges of up to seven metres – enough distance for passing AUVs to safely and efficiently harvest logged data from oil field infrastructure.
Under the spotlight at Ifremer was BlueComm 200, the 4,000 metre depth rated, long range (200 metre) model. It uses an array of high powered blue light emitting diodes (LEDs) that are rapidly modulated to transmit data. Its receiver uses photo multiplier tubes (PMT) that are sensitive to just a few photons. The unit is bi-directional with three data speeds selectable by the user.
For the trial, Ifremer scientists mounted one BlueComm 200 to a temporary overthe-side pole deployed from the stern of L’Europe, the institute’s 29 metre coastal research catamaran. A second BlueComm 200 unit was installed on Ifremer’s hybrid ROV Vortex. Initial tests used Vortex’s onboard camera to constantly stream video via BlueComm while later tests would also include command and control data for Vortex.
BlueComm 200 can be configured to operate using one of three data bandwidths depending on the user’s requirements; 2.5 Mbps, 6 Mbps or 12.5 Mbps.
Using a time division communication scheme, proportions of the bandwidth can be allocated to either the uplink or the downlink. For example, initial testing was spent streaming a 1.1 Mb/s video stream from Vortex to the ship as this was decidedas the minimum data rate for usable video.
To accommodate the video, the BlueComm was set to the 2.5 Mbps bandwidth with a 50:50 distribution uplink to downlink. Meaning 50% of the bandwidth 1.25 Mbps was dedicated to the uplink and downlink.
The trial was conducted at night time to simulate as far as practically possible the darkness of deep water. Environmental conditions were considered ‘good’ between98%-99% irradiance transmittance per metre. However, the shallow depth that the trial was conducted at (just 2.8 metres), light pollution from the full Moon and Toulon just a few miles away, were observable factors that affected range performance.
Testing began by manoeuvring Vortex to the limits of BlueComm’s ability to characterise the working beam shape. This can be seen in Figure 1 (right). A video range of 88 metres was achieved while the connection was capable of lower data rates up to 99 metres. The beam shape shows excellent omni-directional coverage with the maximum inline video range only reducing by 20% to 70 metres at the 90 degree point. Further testing showed the pair of BlueComms could reliably communicate with each other well beyond 90 degrees.
ROV command and control
The second phase of testing moved Vortex’s command and control over to the BlueComm link. For this testing, Vortex was fitted with an additional BlueComm white light emitter. This emitter is designed to provide lighting for the camera as conventional vehicle lighting can interfere with BlueComm’s sensitive receivers. The white emitter pulses in synchronisation with the transmissions so as not to cause any interference.
Vortex was successfully piloted remotely using the BlueComm for over 45 minutes. Meanwhile increasingly strenuous data transfer tests took place including HD video at 12.5 Mbps transfer speed. The 12.5 Mbps link showed a reduced maximum range of only 4% over the 2.5 Mbps link.
The remote control of Vortex showed as the link speed decreased with range, a way to prioritise data such as positioning command and control over less important data is needed. However, there are already Ethernet based standards in place such as QoS (Quality of Service) which can be implemented. This would allow an ROV to use the full range of BlueComm without risking a loss of connection.
In summary, the testing showed a representative maximum operational range of the BlueComm 200 for shallow water applications. The range, however, is still less than seen in deep water testing due to ambient light levels near the surface. The system did however cope with these conditions and provided a robust network link up to 12.5 Mbps between the ship and Vortex.
Efficient use of bandwidth such as a good video compression algorithm would allow the system to operate at lower speeds thus achieving greater range and lower power consumption. While a QoS system or reserving bandwidth would allow the ROV to operate without risk of losing comms.
This article was originally published in Baseline issue 15.