CHAPTER 7 SYSTEMS: MOTION CONTROL SYSTEM—NAVIGATOR OF THE HEAVENS 122 control), whereas the GNC MDM and Terminal Computer determine attitude and position through the use of sensors, and physically fly the ISS with CMGs and small thrusters under guidance from Mission Control Center-Houston (MCC-H), Mission Control Center-Moscow (MCC-M), and occasionally the crew. Vectors and How NASA Uses Them An important concept to how space vehicles and Mission Control know their location in space and the relative location of other objects is that of the vector. The simplest kind of vector is a location within a coordinate system that is defined by a grid. In Figure 2, the point O is the origin of the system, and the point A is a point of interest at x coordinate 2 and y coordinate 3. Vector OA describes the location of this point in this simple coordinate system. 0 A 0A = (2, 3) x y Figure 2. A simple vector. Figure 3 shows a slightly more complex system, which extends into three dimensions to show the location of point A. x y z x, y, z Figure 3. Vector representation in three dimensions. A practical example of the use of a vector is shown in Figure 4. A position vector for the ISS can be described using kilometers in the X, Y, and Z axes by setting a coordinate frame in the center of the Earth. Mathematically, this is how the computers on the ISS and those in Mission Control store knowledge of the ISS position. Z X Y Figure 4. ISS position vector. The following paragraphs include references to several types of vectors, but all of them essentially describe the position (and sometimes velocity, as well) of one object relative to another object. Where Is the International Space Station? Fundamentally, the MCS senses and controls two elements—the orbit in which the ISS circles the Earth, and the attitude that the ISS holds relative to Earth during that orbit. The ISS flies a nearly circular orbit inclined 51.6 degrees to the equator and circles the Earth once every 90 minutes. Orbital altitude is typically