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.
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 protruded upwards, and a minimum of 10 feet lateral clearance between the main rotor blades and the closest ship structure.
A careful survey of the helipad yielded a zone of approximately five feet fore and aft in which the safety pilot could allow the H-6U to land and insure safe structural clearance. Simple but highly effective markers were installed to create a visual cue environment that could enhance the flight crew's judgment regarding a safe landing zone. The proximity of the helicopter rotors to the yacht structure, while fairly tight compared to dimensions generally found on DoD vessels, is common in the super yacht world.
The ULB team used survey instruments to measure the lever arms (offset vector from the IMU to the GNSS antenna) and point-of-interest offset vectors while the ship was docked. During the survey, it was exceptional windy, leading to ship motion and lower accuracy lever-arm determinations than desired.
The H-6U was equipped with the primary antenna on the “T” tail and a secondary antenna on the nose. A laser micrometer mounted on the belly center would measure absolute displacement of the belly above the heli-deck on initial touchdown and the final height after the landing gear had settled.
Data links transmitted differential correction data between the ship and the helicopter and also transmitted the real-time relative ship-to-helicopter solution, output in the log RELINSPVA, back to the “command center” via radio link. The GNSS/INS receivers on board the Allure Shadow logged raw inertial and GNSS data in order to be able to post-process the ship and helicopter conventional RTK trajectories. In post-processing data collected at the National Geodetic Survey's continuously operating reference station (CORS) “LAUD” located near Fort Lauderdale about 25 kilometres from the test area.
The accuracy of each post-processed trajectory was about three centimetres. For performance analysis, the real-time ship-to-helicopter relative position vector was compared to the post-processed ship-to-helicopter relative position vector.
The true test of the system's performance, however, came in the real-time testing as demonstrated in several successful autonomous landings. Tests were undertaken on July 4 and 5, 2012, at sea off the coast from Fort Lauderdale, Florida.
For most of the morning, the aircraft performed maneuvers behind the boat, following its movement. Although a sea state of 3 or 4 (wave height between 0.5 and 2.5 metres) would have been preferred during the trials, the water surface was essentially flat, sea state 0. The H-6U was also allowed to approach the landing pad and hover over the landing point to provide a sufficient confidence level that the system was functioning as expected. The aircraft then performed a single automated landing before returning to the airport for fuel. Figure 1 shows the trajectories of the boat (green) and the aircraft (red) during these operations.
The aircraft autonomously landed on the helipad at GPS time 316350-316772 seconds. The GNSS/INS system on the helicopter reported a real-time relative position to the helipad center of 0.024m North, -0.028m East, and 1.09m Up. The helicopter belly height measured was approximately 64 centimetres; so, the real-time results seem to have about 40 centimetres of vertical error, which matches the vertical error of the lever arm.
In post-processing, the new lever arm was used and the average relative position values of the helicopter on the landing pad were -0.383m North, -0.298 East, and 0.771 Up, which agrees much better to the known helicopter belly height. Figure 2 shows the difference between the real-time and post-processed relative position solutions as the helicopter was landed. Recall that the real-time solution shows about 35 centimetres of height error due to the lever arm used in real-time.
The nature of the test program did not allow for extensive tuning of the automated flight control system to respond in an optimal fashion to the navigation data input. Nevertheless, the results from the initial test program were impressive. Table 1 presents the difference between the H-6U position at 10 feet above the helipad and after landings to the helipad during one sortie.
|Landing||10' Over The Pad||On The Pad|
|Longitudinal (ft)||Lateral (ft)||Longitudinal (ft)||Lateral (ft)|
|1||0.5 Aft||0.1 Right||1.5 Fwd||0.1 Right|
|2||1.2 Aft||0.7 Right||0.5 Aft||0.8 Right|
|3||1.0 Aft||0.2 Right||0.3 Fwd||0.6 Left|
|4||0.7 Fwd||0.1 Left||2.6 Fwd||0.1 Left|
|5||0.2 Fwd||0.5 Left||0.5 Fwd||0.3 Right|
|6||1.0 Fwd||0.4 Right||1.5 Fwd||0.7 Left|
The radar altimeter output compared very favorably with the RTK solution, as shown in Figure 3.
Maritime flight tests during the summer of 2012 demonstrated the accuracy of the navigation solution, as well as the integration of the navigation solution with the automated flight control system on the Boeing H-6U Unmanned Little Bird.
A total of 16 fully autonomous landings and 13 fully autonomous takeoff/departures comprised this latest effort, with the flight crew closely monitoring the controls and the aircraft position when the aircraft was in close proximity to the deck.
In all, seven sequential days were required to accomplish the deck qualifications of two Boeing test pilots, integrate and debug all systems and software, and carry out maritime terminal operations until the operation became routine. These efforts demonstrated the value of the Unmanned Little Bird program's optionally manned system architecture.