The ship is always in motion during these landings, making communication between the ship and the aircraft vital, Mogensen said. Waves can send the ship rolling, pitching and/or yawing even as the vessel is moving forward. This constant and complex movement makes it difficult to land an aircraft, and requires the high precision that NovAtel receivers provide.
“We need to communicate the location of the landing site between the ground station and the aircraft,” Mogensen said. “The landing site moves on a ship; so, you need a second set of high-precision GPS receivers onboard. The ground control station on the ship sends that information via a data link to the aircraft, allowing the aircraft to find the ship and to safely land in the correct location.”
GNSS standalone positioning could be off by many feet, which could be disastrous during one of these landings. A differential GNSS system on board the vessel tells the nearly 20 foot long Scion UAV exactly where the ship is in real-time, even as the ship’s location changes second–by–second. NovAtel’s differential GNSS guides the Jackal’s movement based on the location of the shipboard reference station, linking the movement of the two dynamic platforms.
“We can lock the two together because we can correct for where the ship is in relation to where the helicopter is,” Mogensen said, describing the need for accurate relative positioning of the two craft. “We need to know exactly where they are in reference to each other. We don’t need to know exactly where in 3D space that is, but the helicopter needs to know where the deck is so it can decrease the distance to the landing spot a few inches at a time and land exactly where you want it to land. Using the differential GPS system that NovAtel provides gives us that very high accuracy and enables us to land exactly where we want to.”
In addition to the two standard OEM615 receivers that Scion UAS uses on the ship and the aircraft, they also incorporate an additional receiver to enable the ALIGN® Heading feature that tells them the heading of both the ship and the aircraft, Mogensen said.
Traditionally a magnetometer, an instrument that measures the strength and direction of the Earth’s magnetic field, has been used to obtain heading measurements. However, these devices simply aren’t reliable in an environment filled with metal or magnetic fields, such as a ship with steel construction and electric motors, which is why Scion UAS uses NovAtel’s heading solution instead.
With the NovAtel solution, the Jackal can perform high-precision landing on a moving target, Mogensen said. The unmanned helicopter typically lands within six inches to a foot of the optimal landing spot, which Mogensen said “is pretty good for a large vehicle like the Jackal coming down in a moving scenario.”
While precise positioning is vital to successfully landing an aircraft like the Jackal on a moving platform, so is stability, Mogensen said.
“Helicopters don’t want to fly. They want to fall out of the sky,” Mogensen said. “Keeping a helicopter stable is difficult. For that reason we use the autopilot, but it needs input on not only where the helicopter is in 3D space, but what the attitude of the helicopter is at any one moment. We need to know if it is twisting or turning. We can use input from the GNSS receivers to see exactly how the aircraft is moving in space, and we can use that to correct the helicopter’s attitude and keep it stable.”
The team also uses other sensors to measure acceleration of the aircraft in all directions, and that information combined with the GNSS data tells them exactly what the helicopter is doing so that they can make any necessary corrections.