# Features
Advanced HIL (Hardware-in-the-Loop) Option
In GNSSGlobal navigation satellite system (GNSS): A general term describing any satellite constellation that provides positioning, navigation, and timing (PNT) services on a global or regional basis. See also simulation, the term HIL is generally used to indicate that the GNSS receiver is not tested as a standalone device but integrated with other simulators, equipment, and sensors.
In a HIL setup involving a Skydel GNSS simulator, we often have the following elements (or concepts).
There are many possible variations. For example, the Autopilot could be a human interacting with a virtualized dashboard, but the concept often remains the same. The key principle is that the loop is often closed, and the true position is not known in advance. Instead, the true position of the vehicle is determined as the scenario progresses and fed in real-time to the Skydel GNSS Simulator.
In this setup, the Skydel GNSS Simulator receives the true vehicle trajectory in real-time and generates the GNSS RF signal accordingly. A GNSS receiver tracks the signal and computes its position. This position is sent to a navigation system, such as the autopilot in this example. The autopilot analyzes the position and sends commands to correct the vehicle trajectory. Those commands are processed by the HIL simulator, which emulates the effect of such commands on the true position.
For this setup to work properly, the integrator must carefully consider the following questions:
- What is the acceptable latency between the HIL Simulator input (Autopilot Command) and the Skydel GNSS Simulator output (GNSS RF Signal)?
- How is this latency budget shared between the HIL Simulator, the Skydel GNSS Simulator, and the link connecting them?
- How are synchronization and the clock system handled?
- Can the HIL Simulator and the Skydel GNSS Simulator use a common clock source?
- At which rate can the HIL Simulator update the true position of the vehicle?
- What position data will be available (time, position, velocity, attitude, angular velocity, etc.)?
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