pituussuuntaa

pituussuunta is a term used primarily in geodesy and oceanography to describe the direction that is aligned with the Earth's gravitational field or the radial line from the Earth's center of mass. It is often translated as “longitudinal direction” or “vertical direction” in English. The concept is important for interpreting measurements in a global reference frame, particularly when converting between different coordinate systems such as ECEF (Earth-Centered, Earth-Fixed) and local geodetic coordinates.

In a local ellipsoidal coordinate system, the pituussuunta is orthogonal to the ellipsoidal surfaces of constant altitude. When a geographic location is expressed in latitude, longitude and height, the pituussuunta lies directly downward (or upward) along the radius that passes through that point. For many practical applications, such as navigation, satellite attitude control, or sonar mapping, the pituussuunta provides the “down” vector needed to align sensors and instruments with the Earth’s gravity.

Satellite navigation systems, including GPS, GLONASS, and Galileo, utilize the pituussuunta when transforming satellite position and velocity vectors into the local vertical-tangent frame. This transformation is essential for precise orbit determination, estimation of atmospheric drag, and error modeling. In oceanography, the pituussuunta is employed when interpreting current direction data collected by moorings and gliders, as it helps differentiate vertical motions from horizontal flows that align with the Earth's normal vector.

The term is also relevant in the context of Earth observation missions such as the Gravity Recovery and Climate Experiment (GRACE) satellite pair, where variations in the Earth’s gravitational field are mapped relative to the pituussuunta. It plays a role in geophysical studies, including studies of polar motion and the vertical component of tectonic crustal deformations.