Velocity

2014

NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

Issue link: http://velocitymagazine.epubxp.com/i/380251

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Kerkyra velocity 2014 For more Solutions visit http://www.novatel.com 34 of the imperceptible breezes to coax the boat to each leg's finish line—and on many legs that didn't happen until at least 10 minutes after the time limit expired. "We had some decent periods of wind but the most remarkable thing was how calm it was overall. We had very flat water," Thistle says. "That was disappointing in a few ways. We were out there to measure motion and there wasn't much. And we were out there to com- plete race legs, which was hard to do with those light winds." All this equipment and nothing to measure? Well, not quite. Even in the lightest breeze and stillest waters a boat experiences motions de- tectable by IMU measurements. "We had one night of rough water on the outside coast, and for me it was interesting to look at the measurements of what the boat was actually doing compared to 'gut' feel," Thistle recalls. "I will not forget my three-hour 'off' shift when I was trying to sleep up in the bow that night. It was the closest I've come to los- ing my lunch since I was about five years old. I was sharing the bed with two sail bags and a bunch of emergency gear, and I felt like a to- mato being tossed with the other ingredients in a salad—sometimes finding myself on top of the gear, sometimes underneath.Tired as I was, I couldn't wait to get out of bed." Figure 2 shows what some of the true boat motion looked like that night. Nonetheless, the long intervals with very little acceleration encountered during the 2013 Van Isle 360 race presented a real challenge for gyro and accelerometer technology. A key dif- ference between the entry-level and high-per- formance IMUs carried on board the Kerkyra is the drift in gyro bias in low-dynamic condi- tions. High dynamics allow for alignment of the IMU output, but long periods with rela- tively small accelerations cause measurement errors to accumulate and create a bias in the output. The sailboat provided good examples of this effect (see Figure 3). What They Learned Despite the calm seas, the NovAtel crew not only had a chance to learn about the gear they brought on board, they had to make sure it was running properly and collecting data through- out the race. The SPAN receivers logged inertial and GNSS measurements that could provide post- race insights for enhancing future performance. "When you make an adjustment on the boat to improve performance, you're going by feel," Thistle says. "Part of my interest in it was to see if you could bear out in the data what you felt you were doing to improve the performance of the boat. This made me realize some of these fine tunings may have made improvements, but that's very hard to measure with all the mo- tion going on in boat and all the variables. It's hard to separate those different factors." The position data they obtained gave an ac- curate record of their route, providing the type of detail that Thistle and the rest of the crew agree would be valuable training data if pre- sented in near real-time. "I had some ideas about [real-time tracking in this race] but had enough to prepare with- out it," Thistle says. "Even now, after looking at the data, I can see it would take more thought to figure out what is the best information to provide in real time and what is the best way to present it." The team at NovAtel considers the experi- ment a success. "There is a wealth of good measurement data from this equipment," Thistle says. "I can see how it might be com- bined with wind and sail trim information and 'mined' to draw more conclusions about how best to sail a boat. This would take a lot more work, but some racing programs spend Edging out the competition When it comes to high- end competitive yacht racing, fne tuning boat performance and trim can be the difference between a win and a loss. A closely coupled GNSS/IMU system can provide further advancements, such as: Improved smoothing algorithms for velocity measurement Correction of boat velocity by removing the rotational components of pitch, roll and yaw Sea-state metrics to factor into performance benchmarks Gyro Bias (deg/hr) Roll Accuracy (degrees) Pitch Accuracy (degrees) Heading Accuracy (degrees) LCI-1 <1.0 0.007 0.007 0.018 IMU-CPT 20.0 0.020 0.020 0.060 tAble 1: Specifcations for the inertial measurement units used on board the Kerkyra.

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