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To: Don Lloyd who wrote (7741)8/26/2001 3:51:11 PM
From: Ilaine  Read Replies (2) of 74559
 
Sorry, Don, but I am correct that satellites need to use thrusters to correct for gravitational pull and provide momentum control. Here is a description of Boeing's XIPS system - I will cut and paste the relevant parts, and you can read the whole thing at the URL which I will provide.

>>The on-board XIPS system is used primarily for spacecraft stationkeeping. Small thrusts are required to correct for the tug of solar or lunar gravity and to reposition the satellite in its proper orbit and altitude. A satellite's lifetime as well as its launch weight is thus determined by the amount of fuel aboard for its thruster system. . . . A typical satellite will use up to four XIPS thrusters (two primary, two redundant) for stationkeeping, all connected to the same xenon supply. Each primary device will be switched on and off by a smart power unit that monitors and diagnoses operations automatically. In normal operation, each Boeing 601HP ion thruster will operate for approximately 5 hours per day. Each Boeing 702 ion thruster will operate for approximately 30 minutes per day. The Boeing 601HP satellite uses the 13-centimeter XIPS to perform all north-south stationkeeping and spacecraft momentum control in two axes. The 13-centimeter thruster operates at a specific impulse rate (ISP) of 2568 seconds with 18 millinewtons (mN) of thrust. The satellite flies four 13-centimeter xenon thrusters and two power processor units. Orbit and momentum control are accomplished through a series of two burns on each day of the stationkeeping cycle. Only two of the four thrusters are required to perform a complete mission of on-orbit maneuvers. The Boeing 702 uses its high-power capacity to take full advantage of XIPS technology with the previously developed higher-power 25-centimeter thrusters. The 25-centimeter thruster operates at an ISP of 3800 seconds with 165 mN of thrust. The satellite flies four 25-centimeter thrusters and two XIPS power processors. The HS 702 uses the XIPS to perform all stationkeeping and spacecraft momentum control. Like the HS 601HP, only two of the four thrusters are required to perform the entire on-orbit mission maneuvers. These functions are accomplished autonomously with a series of four daily burns providing precise orbit control. This strategy maintains a ±0.005 degree stationkeeping box, allowing for collocation of many satellites in a single orbital slot.<<

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