For several years now the Boeing Unmanned Little Bird program has been examining various methods for executing a vertical takeoff and landing of an unmanned aerial system on to a moving vessel for launch and recovery operations. An engineering team describes the development and testing of a GNSS/inertial system that uses relative navigation techniques to do this successfully.
The Boeing Company initiated the Unmanned Littl e Bird (ULB) program in the fall of 2003 to create a developmental platform for an optionally manned, vertical takeoff and landing (VTOL) unmanned aerial vehicle (UAV). This article describes a recent Boeing sponsored flight test effort to integrate and demonstrate a novel and highly precise VTOL UAV navigation system for use in a maritime environment.
The result chronicled here portrays a method employing integrated GNSS and inertial navigation capabilities to autonomously guide a VTOL UAV-in this case, a Boeing H-6U helicopter-to a predetermined precision landing anywhere on a ship deck, regardless of deck dimensions. The purpose of this system development was to create a tool suitable for evaluating the performance of a non-GNSS-based terminal-area navigation system on a moving vessel.
RTK algorithms solve for the position-offset vector from the base to the rover receiver. The base receiver does not have to be stationary, and it does not need a highly accurate known coordinate if the only quantity of interest is the relative displacement of the rover with respect to the base.
An algorithm used with two GNSS receivers that do not move with respect to each other-a fixed-baseline RTK implementation-can solve for the heading and pitch of the fixed baseline. This algorithm can also be used with two receivers that are moving with respect to each other a moving baseline implementation. In this case, the base receiver obtains a single-point (autonomous) GNSS position solution and transmits code and carrier phase corrections to the rover based on that position. The rover then uses those corrections to compute the vector from the base to itself, resulting in an RTK-quality solution between the two receivers, even though the absolute position solutions for the two receivers are only of single-point quality.
The moving baseline RTK solution has the same benefits and drawbacks as a fixed baseline RTK solution. The main benefit is a very precise relative solution because the distance between the base and rover is quite short. The drawbacks are the usual challenges of requiring constant communication between the rover and the base, as well as maintaining enough common satellites in view during the landing maneuvers as the helicopter approaches the ship deck.
An inertial navigation system (INS) is typically added to a GNSS solution to address issues such as these. With a GNSS/INS system, the INS can “coast” through periods of GNSS signal blockage or degraded GNSS solution quality. An INS provides good relative accuracy over time, allowing it to “hang onto” a high-accuracy solution.