The control of stationary earth satellites is one of the most important near-future applications for low-thrust space propulsion systems. The function of the propulsion system in this application is twofold, attitude control and station keeping. In the attitude control mode, the system must hold the orientation of the satellite about three axes to a specified angular accuracy for a period of years. This is accomplished by imparting small but frequent angular impulses to counteract disturbance torques which arise from such natural effects as solar radiation pressure and micrometeorite impact, and such internal effects as gas leakage and moving parts. In the station keeping mode, the control system must maintain the satellite in a stationary position relative to the surface of the earth. The two major sources of orbit perturbation (position change) of a 24 hour satellite are the triaxiality of the earth (elliptical equator) and the gravitational attraction of the sun and moon. The triaxiality effect causes the satellite to oscillate in an east-west direction with amplitude of up to 90 degrees longitude and with a period of about one year. A velocity increment of 17 ft / sec / yr is necessary to counteract this perturbation. The solar-lunar perturbations tend to pull the satellite in a north-south direction resulting in maximum change in orgit inclination (latitude of 0.948 deg / year. A velocity increment of 167 ft / sec / yr is required to correct for this latter effect. Therefore, a total velocity increment of 184 ft / sec / yr must be provided by the propulsion system each year to maintain the satellite longitude and latitude.
The Hughes Research Laboratories has recently been awarded a contract by NASA to develop an ion propulsion attitude control and station keeping system for the purpose of controlling 24 hour satellites. The system will employ cesium surface-contact ion engines to provide thrust in the nine directions necessary for complete satellite control. The ion thrustors used for station keeping will produce a thrust of 1.5 mlb whereas 0.5 mlb is the thrust level of the attitude control thrust devices.
This system, when used to control a typical 500 lb. satellite, would operate in the following manner. The station keeping engines would thrust about 10 minutes each day for the east-west orbit correction and a total of about 1 ½ hours for the north-south correction. The attitude control engines will operate only a few seconds approximately every quarter hour. The propellant required by the ion engines to hold the satellite to the desired orientation and to maintain proper station longitude and latitude for a period of three years is only 2.25 lbs.
A comparison of the weight of the ion propulsion system with equivalent cold gas systems shows that as the total impulse required increases (i.e.. as mission time or satellite weight increase), the ion propulsion system becomes more and more attractive. Most of the weight of the low specific impulse cold gas systems is in the propellant, whereas, for the ion engine most of the weight is in the power source. The advantage of the ion engine is that its over-all weight will increase very slowly with required operating time (less than 1 lb. per year). For satellite lifetimes of two or more years, ion propulsion will provide the lightest control system.