美全球定位系统（GPS）Block IIF型卫星

    广泛地使用的全球定位系统（GPS）卫星群由27颗空间卫星组成-24颗各自在六条轨道位面上和三颗在-轨道上当做备用-包括Block II、Block IIR和Block IIR-M型号。在引进Block III之前美国空军（USAF）计划发射12颗下一种型号Block IIF，大约在2013年左右服役。

    在位于美国科罗拉多州施里弗空军基地（AFB）的美国空军第50空间空军联队的第二空间操作中队，负责监听和控制GPS卫星群。GPS操作控制环节由位于科罗拉多州施里弗空军基地的一座主控制站，连接到六个监控站（施里弗、卡纳维尔角、夏威夷、夸加林礁岛、迪亚哥贾西亚和阿森松岛）和位于夸贾林环礁、迪亚哥贾西亚、阿森松岛和卡纳维尔角的卫星上行链路组成的一个全球网络。

    当较早的卫星达到它们的寿命极限通过发射新的GPS星群已经保持提高期限。（许多GPS卫星已经保持运转中服役比较长的超过预期：在2005年10月，Block II SVN-15卫星成为第二颗Block II通过15年的限度，双倍它的设计寿命）。

    波音公司和洛克希德·马丁公司/通用公司正在竞争发展GPS Block III卫星，它的首次发射应该在2013财政年度（FY13）。二家公司在2004年1月接收到阶段 A（Phase A）概念设计和风险-减少合同和美国空军期待在FY2006中选择一个获胜承包商。

    在GPS III上的进步和新的能力将会包括将与欧洲伽利略兼容的新信号（L1C）和高数据传输速率交联（crosslinks）。一个高功率定点束（“战区”-尺寸） 预计去符合新的反-干扰要求用于+20 dB信号强度改善，优于对用户设备天线和处理技术安排实质上的改变。

    第一颗GPS III卫星的发射最初计划用于FY09和在2016/17年全面增加卫星；然而，前进迟延和在计划中其它地方资金不足已经延迟第一颗发射。

    附原文供对照参考：

GPS constellation

The widely used Global Positioning System (GPS) satellite constellation comprises 27 spacecraft - 24 in each of six orbital planes and three on-orbit spares - including Block II, Block IIR and Block IIR-M variants. The US Air Force (USAF) plans to launch 12 of the next version, Block IIF, before placing the first Block III in service around 2013.

The USAF 50th Space Wing's 2nd Space Operations Squadron at Schriever Air Force Base (AFB), Colorado, is responsible for monitoring and controlling the GPS satellite constellation. The GPS Operational Control Segment consists of a Master Control Station at Schriever AFB, Colorado, connected to a global network of six monitor stations (Schriever, Cape Canaveral, Hawaii, Kwajalein Atoll, Diego Garcia and Ascension Island) and uplink antennas at Kwajalein, Diego Garcia, Ascension Island and Cape Canaveral.

The GPS constellation has been kept up-to-date through the launch of new satellites as the earlier spacecraft reach the end of their lives. (Many GPS spacecraft have remained in service longer than expected: in October 2005, the Block II SVN-15 became the second Block II to pass the 15-year mark, double its design life).

Boeing and Lockheed Martin/General Dynamics are competing to develop the GPS Block III spacecraft, due for its first launch in Fiscal Year 2013 (FY13). The two companies received Phase A concept exploration and risk-reduction contracts in January 2004 and the USAF expects to select a winning contractor later in FY2006.

Improvements and new capabilities on GPS III will include a new signal (L1C) which will be compatible with Europe's Galileo, and high-data-rate crosslinks. A high power spot beam ("theater"-size) is intended to meet new anti-jam requirements for +20 dB signal strength improvement by providing additional power directly from the satellites, rather than making substantial changes to user equipment antennas and processing technology.

First launch for the GPS III satellite was originally planned for FY09 with a fully populated constellation by 2016/17; however, go-ahead delays and funding shortages elsewhere in the programme have delayed the first launch.

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