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Space Segment
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The Space Segment consists of the DMSP spacecraft, the launch vehicle, and ground and factory support for the launch.
The spacecraft is placed into a near-circular, Sun-synchronous polar orbit at a nominal altitude of 450 nautical miles. Normally, two spacecraft comprise the constellation, traditionally with one early morning earth terminator orbit and one mid-morning orbit. The spacecraft is 3-axis stabilized and earth oriented. Sensing instruments are maintained in a continuous orientation toward Earth or space. The solar array rotates about the pitch axis to provide single-axis Sun orientation. |
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Satellites are known as F-xx when launched. Prior to launch, they are designated by S-xx. The 5D2 series of satellites encompasses F-6 through F-14, while the 5D3 series of spacecraft begins with F-15. On-orbit 5D2 satellites weigh approximately 1810 lbs, including 520 lbs of sensor payload. The 5D3 satellites weigh approximately 2684 lbs, including 677 lbs of payload. The sensor suite of 5D3 differs from that on 5D2. Though F-15 was the first of the 5D3 spacecraft, F-16 was the first to carry the new 5D3 sensors. The DMSP program has already procured all future DMSP spacecraft; the final DMSP satellite, S20, is scheduled to launch in late 2011. The current 2-ball primarily operational constellation consists of F16 and F17. Though DMSP is a 2-ball system, DMSP satellites with reduced capability continue to operate. The following DMSP satellites are currently on orbit:
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Figure: View of 5D-3 spacecraft. |
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In addition to the sensor suite, the spacecraft is divided into eight major subsystems. However, for this document, we will only address the major on-orbit functions, which are:
The spacecraft collects power by way of a deployable solar array. Two batteries onboard store excess energy for operation during peak periods and periods while the spacecraft is in darkness. Receipt of commands, downlink telemetry, and mission sensor data is accomplished via the Communications Subsystem. The Equipment Status Telemetry Subsystem provides health and welfare status, as well as command verification reporting to the ground system. Using a combination of stellar reference and inertial measurements made by gyros, a Sun-sensor unit, and an Earth-Sensing Assembly, attitude errors and satellite position are determined. Once determined, attitude corrections are made by Reaction Wheel Assemblies. The Command and Control Subsystem accepts real-time and stored commands for routing and processing. These commands may perform station keeping for the spacecraft or direct data collection by the sensor suite. |
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