The UAV VTOL application requires continuously precise and accurate relative positioning of the helicopter and the ship. The solution implemented on the Little Bird uses GNSS positioning and inertial navigation and is a modernization of a system previously demonstrated in 2005.
Both the ship and the helicopter were outfitted with SPAN-SE-dual antenna GNSS/INS receivers. The relative RTK solutions was provided by NovAtel's ALIGN® algorithm. Postprocessing of the CORS data with the shipboard and helicopter GNSS/INS data used the Inertial Explorer® post-processing software from NovAtel's Waypoint® Products Group. The accompanying sidebar describes the evolution of the integrated navigation solution to achieve the relative navigation used in the Little Bird tests.
In landing a helicopter aboard a moving ship, the quality of the attitude solution on the ship's system plays the most significant role in determining the overall relative accuracy. The ship's GNSS/INS system is mounted in a convenient location away from the landing pad, but the landing pad is the true point of interest. Similarly, the landing gear is the point of interest on the helicopter, not the location of the inertial measurement unit (IMU).
Both GNSS/INS systems must project their solutions from the IMU to the point of interest. To implement this coordinate projection, the offset vector from the IMU must be measured in the IMU frame, and the rotation matrix between the IMU reference frame and the GNSS's Earth-centered Earth-fixed (ECEF) frame must be known. The accuracy of the solution at the point of interest therefore depends on the quality of the measured offset as well as the quality of the rotation matrix from the IMU frame to the ECEF frame.
This rotation matrix is maintained as part of the INS solution. The quality of the rotation matrix is very dependent on the quality of the initial INS alignment (i.e., finding the IMU's orientation with respect to gravity and north), and the overall convergence of the GNSS/INS solution. The longer the offset vector is to the landing pad, the larger the effect of the rotation matrix errors (i.e., a classic pointing error in survey terminology).
Attitude errors in GNSS/INS are best observed with vehicle dynamics. In particular, horizontal accelerations allow the azimuth error to be observed and controlled. Depending 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, because 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.