Hello Blue Robotics community!
I’d like to reach out and start a conversation about the accuracy and consistency of coordinates recorded by ROV-mounted acoustic GPS tracking systems.
@m.williams and I have been using WaterLinked’s Underwater GPS system, 100m range, with the antenna and U1 locator for the past year or so as part of our ROV research conducting kelp forest surveys (you can learn more about our ongoing work here on the BR forums and here on GitHub).
To the best of my knowledge, we are using the hardware correctly. That is to say, we set the distance between the antenna and the topside GPS in the UGPS GUI (at 192.168.2.94), and we set the compass heading as well as the depth of the acoustic antenna (1m beneath the sea surface). The acoustic transmitter is mounted vertically on our BlueROV2 to maximize signal propagation. We typically operate on channel 5 (156 - 187 KHz). We’ve kept the topside hub’s software updated and currently run the latest 3.3.1 (400881892.d0af300). A network bridge enables coordinates to populate in our QGC .csv telemetry file.
There are two primary types of erroneous readings we experience with our UGPS system:
1. At times, the GPS location appears to oscillate between two attractors, with one being the correct position of the topside hub, and the other being just west of (in this example) . . . Africa (see the 1st image). These MASSIVE jumps in position have been observed both (1) with only the topside hub powered on (i.e., no U1 transmitter in the water), and also (2) with the U1 transmitter in the water.
2. Additionally, we appear to consistently have a large amount of error (or uncertainty?) associated with the ROV’s position (2nd & 3rd image). See the 2nd figure – these are the tracks from one of x4 ROV “transects”, where the ROV was flown straight (following a compass heading) for about 60m (the x4 transects are delineated by the x4 different colors in the 3rd figure). As you can see, what should be x4 approximately straight lines are x4 highly zig-zagged tracks. We plan to perform formal spatial analyses, thus accuracy in ROV position really does matter to us.
Specific questions and points of consideration:
- Are these two (presumably distinct? but perhaps not . . . ) behaviors characteristic of UGPS system performance based on other people’s experience using the same hardware? Note that the error 2. is fairly consistent, whereas 1. is more sporadic and intermittent.
- I’m aware that additionally incorporating a DVL would likely improve the accuracy and precision of the ROV’s predicted position. This is an option we are considering.
- We consistently observe a fair amount of antenna movement in the water in response to swell, vessel movement, etc. – could slight variation in antenna positioning affect the predicted ROV position to the extent that it produces the error noted above the latter two figures above?
- Following the above point, we do have the x4 individual D1 receivers (an alternative to the antenna), and we’ll try those out on a vessel this week (May 15) to see if they improve performance.
- Have others obtained better performance from other GPS systems?
Note that we contacted WaterLinked May 8th about these issues, and we await a response.
Lastly, for what it’s worth, we’re working with several other entities within the state of Washington who are evaluating the potential of obtaining a BlueROV2 and customizing it just like ours (e.g., the Port of Seattle has already done this . . . they now have an identically customized BlueROV2). This is desirable, as the more eyes/cameras we have on the seafloor conducting benthic surveys, the better we can track population and ecological health and inform conservation and management. However, at this time I am uncertain whether or not to recommend the WaterLinked UGPS system to our collaborators, and we’re actively evaluating other systems such as the Cerulean ROV locator bundle Mark III.
Note too that we’re working closely with @clyde on this project – Clyde, please feel free to chime in with any of your observations and thoughts!
I know this was a long post, so thanks to all that made it through.