关键词: 空间计量, 长度, SI 秒, SI 米, 相对论, 惯性系, 非惯性系, 原距离, 坐标距离, 时空度规, Sagnac效应

Abstract: The definition of length unit is found on the theory that light goes straight, light speed is constant in vac?uum, and light velocity is isotropy. The straight-line of 2-Dimension on sphere is looked like curve geodesic in space of 3-Dimension; and the straight-line in 3-D space is also curve geodesic in 4-D space-time. However, whether this definition is suitable for 4-D space-time or not? The definition of SI second and SI meter is the same with whole global space-time,but we must clear the difference between proper time and coordinate time when we use them. Einstein’s Special Relativity theory and the principle of light speed constant are only used on inertial-system, cannot be used on non-inertial-system.For example on non-inertial-system of rotation plate, we use coordinate transfer and calculation of space-time metric by GR to realize the phenomenon that space-time is curve and light goes non-straight path. For example to measure distance between satellite and earth-station, we calculate a special case of Sagnac Effect on non-inertial-system. Brief research show that range of distance extend from local to large area in space, these effects such as relative velocity, gravity red-shift and Sagnac, will become important influence factor of uncertainty of measuring distance. So the theory of Space Metrology must be found on the theory of General Relativity（GR）, and take the viewpoint of space-time on 4-Dimension to under?stand the problem of distance measurement.

Key words: Space metrology, Length, SI second, SI meter, General Relativity, Inertial system, Non-iner?tial system, Proper time, Coordinate time, Space-time metric, Sagnac effect