ATP-3-09-02 Field Artillery Survey Download
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Chapter 9 9-2 ATP 3-09.02 16 February 2016 Figure 9-2. Global positioning system SPACE SEGMENT 9-3. The space segment is made up of a 24-satellite constellation that orbits the earth once every 12 hours. These satellites are deployed in six orbital planes and are configured so that four or more satellites will be in view at all times. This arrangement allows for 24-hour, three-dimensional, worldwide coverage. As with the stars, the satellites rise above the horizon about 4 minutes earlier than the previous day. CONTROL SEGMENT 9-4. The control segment consists of five passive-tracking monitoring stations, active-tracking ground antennas, and the master control center. These tracking stations, located around the world, are capable of monitoring the satellite navigation messages and time signals better than 90 percent of the time. This information is relayed to the master control center, which has the capability to effect any needed corrections to the satellite timing and navigation messages. USER SEGMENT 9-5. The user segment consists of navigation receivers designed for marine, aircraft, and man pack or vehicle use. The receivers must have electrical line of site with the satellites to receive and decode the satellite signals. The internal computer uses these satellite data to generate a precise time, velocity, and 3D position data. The receiver must track four satellites to obtain a 3D position, and three satellites will yield a 2D position. Current position coordinates and height are obtained from a 3D position and only current coordinates are obtained from a 2D position. The receiver needs only one satellite for precise time. STANDARD POSITIONING SYSTEM AND PRECISE POSITIONING SYSTEM 9-6. The standard position system (SPS) is available to all GPS receivers worldwide, both military and civilian. When a receiver is in the SPS mode, almanac, navigation, and timing information are received on the non-encrypted course acquisition (CA) code satellite signal. To deny unauthorized users the full accuracy of GPS, the Department of Defense (DOD) intentionally places errors in the navigation and timing signal. This process is called selective availability (SA). The SA errors are unpredictable and can produce significant horizontal and elevation errors. This is one reason why SPS receivers are not authorized for combat operations.