On Satellite Attitude Capturing by the Gravity Gradient Torque Condition and Passive Safe Hold Mode

摘要

The purpose of satellite Safe Hold Mode (SHM) isrnproviding some flight safe conditions in case ofrndetection of any life critical failure. Usually, in SHMrnall on-board nonessential electrical power consumersrnare powered down to minimize power consumptionrnand prevent any unexpected development into failure.rnIn many cases in SHM satellite Attitude ControlrnSystem (ACS) has to be powered down and satelliternangular motion is not controlled. However, veryrnoften random angular tumbling in space is notrnallowed. A certain orientation w.r.t. Sun and Earthrnshould be maintained to guarantee some safe powerrngeneration, thermal and radio communicationrnconditions. A kind of passive SHM should berndesigned to perform this task. This passive SHMrnusually is based on gravity gradient torque, satelliterninertia properties and satellite initial momentum. Tworndifferent concepts can be used for such a passivernSHM design: pendulum-like gravity stabilization andrnspinning gyro stabilization. In the first case, satelliternmomentum should be dumped and satellite isrncaptured by the gravity gradient torque. In the secondrncase, the gravity gradient torque should be overcomernto allow the satellite in a spinning process with arncertain momentum pointed in the desired direction.rnTo determine satellite behaviour in free angularrnmotion in gravity field (oscillation w.r.t. the gravity vertical orrnspinning) in dependence on initial conditions, one can usernwell-known mathematical condition of capturing ofrnsatellite attitude by the gravity gradient torque [1]-[4],rnwhich results from non-linear oscillation theory [7].rnHowever, these conditions presume that the rotationalrnkinetic energy, required to start spinning is transferred tornsatellite instantaneously (practically, fast enough).rnTypical case when these conditions can be used is therninitialization of satellite spin after separation from thernlauncher. The case of satellite transition into SHM fromrnthe nominal operation mode is different and requiresrncareful consideration of the capture conditions. In fact,rnfor the satellite with spinning reaction wheels, the processrnof slowing wheels down to a full halt always accompaniesrnthe transition into SHM. The duration of this process isrndetermined by whether the wheels are equipped withrnspecial brakes. In other words, a transient process ofrnmomentum transfer from the wheel to satellite body takesrnplace. Ignoring this fact in the satellite ACS designrnsometimes can lead to fatal consequences for satelliternoperation, when some non-benign angular motion occursrninstead of expected regular spinning aroundrnpredetermined axis. This article presents the investigationrnof the capture conditions of a satellite by the gravitationalrntorque. The results are illustrated by the analysis ofrnspinning passive SHM implemented in the ACS ofrnCanadian satellite RADARSAT-1.