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11-May-00 | Delta flight 278 – NAVSTAR IIR-4

Another perfect flight for Delta II as Air Force controllers have made contact with NAVSTAR 2R-4, the latest addition to the Global Positioning System (GPS) constellation, and have determined it to be in its proper orbit. The three-stage 7925 model lifted off from Cape Canaveral’s Space Launch Complex 17, Pad A, on Wednesday, 10 May, at 21:48 EDT, the opening of a 29-minute window. Spacecraft separation occurred 25 minutes, 19 seconds later.

NAVSTAR (Navigation Signal Timing and Ranging) 2R-4 is the fourth upgraded model to fly and the 31st satellite in the Block 2 (operational) constellation. All have flown aboard Delta II’s (the experimental Block 1 GPS sats flew on Atlases), and 17 additional Block 2R’s are manifested over the next few years. This satellite replaces NAVSTAR 2-1, the original Block 2, which was launched aboard the very first Delta II on 14 February, 1989. NAVSTAR 2-1, having exceeded its 7-year design lifetime, suffered shutdown of its reaction wheels on 26 March and was decommissioned on 14 April.

In additional GPS news, on 1 May the Air Force ceased the use of selective availability (SA), a system that intentionally degrades the GPS signal available to civilian users. SA was arguably meant to prevent potential enemies from using off-the-shelf receivers in missile targeting systems, but the White House and the Department of Defense agreed “that worldwide transportation safety, scientific, and commercial interests could best be served by discontinuation of SA.” (USAF release) SA can be (and still is) used on a regional basis in those parts of the world considered to contain threats to U.S. interests, and the U.S. mililtary continues to use an encrypted portion of the ranging signal for maximum precision. However, the discontinuance of SA means that improvement in accuracy for civilian receivers is thought to have increased up to tenfold, and much of the benefit of more expensive differential (DGPS) receivers is moot.

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