#  Radar Basics Figure 1: Coordinate systems for a space-based radar Figure 1: Coordinate systems for a space-based radar

The geometry for space-based radar is characterized by three Cartesian coordinate systems.

One of them is an initial coordinate system (xI; yI; zI ), whose origin is at the center of Earth, and whose zI axis is along the Earth's rotation axis. In this coordinate system, the Earth rotates and the radar satellite describes an elliptical orbit around it according to Kepler's laws. Its orbital period is T.

A second coordinate system (xE; yE; zE ) uses the same origin. The orientation of the zE axis is identical to zI. However, this coordinate system is bound to the rotation of the Earth and rotates with the angular velocity ωE. Each point on the Earth's surface occupies a fixed position in this coordinate system, which, with direction and distance from the origin, forms a vector pE. The magnitude of the distance from the center is ρE. In this coordinate system, the radar satellite describes a wave-shaped orbit (see Figure 2). Only after a large number of orbits, the satellite is again above the same point of the Earth. Figure 2: Wave-shaped satellite orbit above the Earth's surface Figure 2: Wave-shaped satellite orbit above the Earth's surface

The third coordinate system (xA; yA; zA ) is bound with its origin to the phase center of the radar antenna. In this coordinate system the radar measures the direction and the distance of the reflecting objects. The coordinate zA points in the same direction as the instantaneous vector RE(T). The coordinate xA is perpendicular to the vector of the tangential velocity of the satellite. In this direction the radar measures a runtime of the echo signals and calculates a range from it. The coordinate yA lies on the tangent of motion and a measurement result in this direction is called azimuth (or cross-range, see Side-Looking-Airborne Radar (SLAR).