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little bird on the size and speed of the vessel, the dynamics observed aboard a ship can be very low, leading to degradation in the azimuth solution. The initial alignment poses another challenge as well. A stationary coarse alignment can be performed with tactical grade IMUs, but only when the system is truly stationary. A transfer alignment can be performed with the GNSS course-over-ground azimuth and pitch, but only when the vehicle's forward direction of travel is aligned to the IMU's forward axis (or there is a fixed, known offset between them). With a ship or helicopter, these alignment conditions cannot be assured due to crab angles, the angular difference between heading and actual ground path. A ship will often be moving enough to prevent a stationary alignment and its movement without any crab angle cannot be guaranteed. Even if an alignment is achieved, the dynamics will likely be too low for good GNSS/INS convergence. This will degrade the quality of the projected coordinate at the landing pad, which is where the helicopter is aiming. The helicopter system suffers a similar challenge in initial alignment. Helicopters are not an ideal platform to use a transfer alignment from GNSS course-over-ground measurements, due to their maneuverability. To solve the initial alignment problem (on ship and helicopter) and to address the attitude error convergence/observability problem (on the ship), the GNSS/INS was augmented with a second GNSS receiver and antenna, using the fixed baseline implementation of the relative RTK algorithm. The ship's GNSS/INS has two GNSS antennas associated with it, as does the helicopter's GNSS/INS. The offset vector from the IMU to both antennas must be measured and input. The pitch and heading of the baseline between the two antennas is used for the initial INS alignment. Because the roll angle cannot be observed with just two antennas, it is assumed to be zero in the initial alignment. After alignment, the GNSS azimuth is used as a heading update to the INS. This solution is critical for the ship system, be- For more Solutions visit http:/ / cause the ship will be experiencing low dynamics, making the attitude errors less observable. For the helicopter system, the GNSS azimuth updates are not as vital because the helicopter maneuvers much more and its attitude errors are generally observable via the vehicle dynamics. Ships at sea, however, exhibit the following helideck motions: pitch, roll, yaw, heave, sway, and surge. Ships also don't move across the Earth in the same direction as their heading due to local water currents, a factor that must be accommodated in the flight control laws. Moreover, conducting terminal flight operations in the intended operational environment must also deal with the wind turbulence generated by a ship's superstructure. These factors created a requirement for safety pilot training in a maritime environment. " Landing the H-6U helicopter safely on a moving yacht had everything to do Squeezing a Helicopter into a Moving Landing Zone with the Landing the H-6U helicopter safely on a moving yacht had everything to do with the relative dimensions of both and the adjacent physical structures. The Allure Shadow, a yacht based in Fort Lauderdale, Florida, is equipped with a helipad that measures 34 feet wide by 50 feet long and is surrounded on three sides by horizontal safety nets, which are raised about 5 inches above the helipad surface. At the forward edge of the helipad is an overhang from a pool deck located next to and above the landing zone. The pool deck overhang presents a contact hazard for the helicopter main rotor system while the helipad's safety net system presents a contact hazard for the helicopter's tail structure. An H-6U helicopter has the following dimensions: main rotor diameter, 27.5 feet; tail rotor diameter, 4.75 feet; total helicopter length, main rotor tip to rotor tip, 32.3 feet. The stinger, the lowest part of the H-6U's vertical stabilizer, is approximately 2.5 feet above the landing surface. Advisers recommended a minimum of 3 feet lateral clearance from the stinger to the edge of the helipad where the safety net frames protrud- relative dimensions of both and the adjacent physical structures. " aCKNOWleDGMeNT This article is a condensed version of a feature that was published in the May/June 2013 issue of Inside GNSS. 2013 velocity 17

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