A Focus on Data Interfaces
The transmission of data between nodes and between subsystems in the UAV is possibly the most demanding engineering problem to address. For example, the navigation subsystem requires data, such as airspeed and directional gyro, from various instruments as well as the control surfaces.

The navigation system must make adjustments to course, altitude and speed and direct the control surfaces to respond in a way that implements those adjustments. Of course, data must be available in real time so that the movement of control surfaces is contiguous with speed, direction and altitude data.

Likewise, each of the major subsystems also requires multiple connections. For example, the navigation subsystem on board the vehicle must be able to provide data directly to ground-station instruments so the pilots remotely controlling the device have real-time feedback on aircraft position. In addition, a pilot's movement of controls in the ground station must result in immediate responses by airborne control surfaces.

The most common approach is to implement connectivity between processing components as a series of point-to-point, dedicated connections between those subsystems requiring predictable real-time communication.

A point-to-point connection guarantees a fast and predictable data-transfer time, which is a key component of overall response time. This architecture results in good overall performance. It is seemingly simplistic in concept and implementation.

However, the use of dedicated, point-to-point connections has two serious failings. First, though the connections look simple, when there are N different subsystems, establishing direct data connections between each of them results in N times N connections.

Though it is likely that direct connections between all subsystems are not required, such connections would still increase the complexity of highly interconnected systems to the point where cost and maintainability would be problematic.

For example, what level of engineering work would have to be accomplished if a new subsystem dependent on point-to-point connections had to be introduced at a later time?

Consider the later introduction of a logging subsystem that captures all real-time data traffic for off-line mission replay or analysis. The new logging subsystem would have to be connected to all or most existing subsystems.

Second, point-to-point connections do not take into account the need for redundancy in case of failure.

If the connection between the remote pilot on the ground and the control surfaces in the air fails, there could be a complete system failure -and subsequent UAV crash. Therefore, it is necessary to build redundancy into the architecture, which requires multiple direct communications pathways or routing from other subsystems.

Figure 1: Point-to-point architecture is suitable for simple component connections, but it becomes overly complex and brittle when supporting many components.

Figure 2: In complex systems, it is most efficient to provide for a

In either case, redundancy further adds to the complexity of the system. Much of this complexity would fall upon the software, which would have to handle failure detection, data recovery and rerouting.

It is certainly possible to design and build a UAV with multiple direct connections between processing and control subsystems and to implement failover and other Quality-of-Service (QoS) features that are distinct and customized to that vehicle.

However, the cost of doing so can be significant, and a custom design using these characteristics will almost certainly negatively impact maintainability throughout the life of the vehicle.