The core of the navigation package is an inertial navigation system (INS), a standard sensor that works out where an aircraft is by measuring its accelerations. If left alone, an INS will gradually accumulate errors, “drift,” and so a GPS receiver is commonly added to update it, bringing it back on track. BAE has added SLAM as another source for updating the INS, one that will work if GPS is unavailable.
As the aircraft flies, sensors detect fixed features on the ground and the software works out their relative positions from their changing bearing, building up a map of them and determining the aircraft's position on the map. The sensors can be passive such as electro-optical, infrared or radio direction-finding, or active—millimetric radar has been tried—and BAE says synthetic aperture radar should work. For submarine navigation, sound has been tried with success.
Data from the INS and SLAM are combined to create a best estimate of position. While SLAM data is available, drift is limited to 1-5 meters (3-16 ft.), comparable with common GPS systems, but not the best military-grade GPS. In building its map, SLAM should have three objects to watch at a time, in case one turns out to be moving relative to the others. With many objects and enough sensor fidelity, SLAM can achieve positional precision of less than 1 meter, says BAE.
If an object is lost—say, because of cloud cover—it can be reacquired. If SLAM cannot find any features, then the INS just carries on alone for as long as necessary. There are no fixed visible features at sea, and mainly for that reason BAE incorporated radio direction-finding into its SLAM system. So, instead of watching a tree visually, the system might watch focus on transmitter on a cell phone tower or the radar of a distant airfield. BAE has previously referred to its work on such techniques, which are called navigation with signals of opportunity.
One expert in autonomous navigation questions whether suitable radio signals would be available far out to sea. On the other hand, SLAM may not be needed far from land, since an enemy's distance would increase its challenge in interfering with GPS.
SLAM is distinct from georegistration, in which the navigation system looks for objects of known location to fix its position, and from Tercom, a method of working out the position of the aircraft from radar measurements of the shape of the ground.
BAE says its SLAM system can also be used for tracking targets, noting that it was a considerable challenge to develop a system that could do the three necessary tasks at once: making a map of fixed features from a moving position (the aircraft), simultaneously locating that moving position on the map and at the same time tracking another moving object on the map.
Notably, the SLAM map can be shared between several vehicles, which then navigate and localize themselves and targets on a common grid. “This is extremely difficult and can only be achieved by combining our decentralized data fusion technology with our SLAM technology,” says Yelland.
The guidance package need not use the traditional waypoint approach, directing the aircraft from one designated point to another. Instead it can be given a moving 3-D block of space which it must stay in but within which it can freely fly the aircraft toward the set target. “The benefits of this approach are reduction in mission-planning workload, optimization of mission execution and the ability to manage airspace deconfliction for integrating UAVs into shared airspace,” says Yelland. In designing the blocks, mission planners incorporate terrain avoidance and masking. The guidance system, working with the mission system, can choose to execute a low-priority mission before a high-priority mission if its track happens to suit changes in the sequence.