Hi,
When doing measurements with SAR sensor to identify movements on the ground, we can resolve only one surface displacement vector. What the satellite measures is the distance between the satellite sensor and the ground but we cannot say that the ground was moved horizontally or vertically using only one satellite orbit (ascending or descending). If you want, we can make the assumption that the ground was moved vertically (we cannot be certain for that). SAR is much more sensitive to vertical changes rather than to horizontal ones. We need to combine ascending and descending orbits to resolve for the ‘true’ ground displacement.