38.8 Speed and velocity

38.8.2 State the frame of reference for speed measurement provided by a GNSS and inertial systems.

The frame of reference for speed measurement provided by GNSS (Global Navigation Satellite System) and inertial systems is typically defined as follows:

1. GNSS:

The GNSS provides speed measurements relative to the Earth’s surface.

It calculates the speed by determining the change in position over time based on satellite signals.

The reference frame is geodetic, meaning it takes into account the curvature of the Earth and is aligned with the Earth’s center of mass.

2. Inertial Systems:

Inertial measurement units (IMUs) use accelerometers and gyroscopes to calculate speed based on changes in motion.

The frame of reference for inertial systems is typically a local frame that is aligned with the aircraft's body.

This means that speed is measured relative to the aircraft's current position and orientation, without external references.

Combining both systems can provide a more accurate and reliable measurement of speed, as GNSS offers a global reference while inertial systems provide real-time data that can help in maintaining accuracy during periods when GNSS signals may be weak or unavailable.