SPS Feasability Study SD76SA0239-2

Second, the triaxial mass distribution of the earth produces gravity perturbations that cause a geosynchronous satellite to drift east-west. The maximum AV required to maintain the satellite in position is 1.83 meters/second/ year. For a satellite located at a stable point, no velocity control is required. Hence, for a complete constellation, the annual velocity requirement for each satellite will vary from 0 to 1.83 m/sec/yr. Microwave Pressure Effects To produce a net of 5 Gw of power on the ground, each satellite must transmit about 6.5 Gw of power. As a result, a total recoil force of about 22 newtons will act on the vehicle. If the rectenna is on the same longitude .. as the satellite, the recoil will have a radial component and a cross track component. The cross track component will displace the satellite slightly south of the equator but will not affect the satellite position within the orbit. The radial component has a more severe effect. If left uncorrected, it effectively tends to cancel out part of the gravitational force thereby increasing orbital period. For small forces, the change in period is given by For the reference configuration, the change of period is 0.185 sec. After 3.6 years of uncorrected drift, the satellites will have drifted 1° out of position. The perturbation can be passively controlled by biasing the satellites to a slightly lower altitude. The change in semimajor axis to restore the original period is given by For the reference configuration, the orbital altitude must be reduced by 60.4 m. Velocity Error Corrections When a satellite is initially placed in the SPS constellation, small residual velocity errors will remain. These errors cause the satellite to drift out of its nominal position within the constellation. If left uncorrected for sufficient duration, a satellite could drift over and collide with an adjacent satellite. For example, a satellite with a velocity error of 1 m/sec would collide with its neighbor after 8.5 days of drift. To prevent such an occurrence, the stationkeeping system will have to go through an initial transient cycle to eliminate even very minute initial velocity errors. After the initial transient, only a very low level of effort will be required to counteract differences in accelerations from such sources as venting, solar pressure, or microwave recoil. The important point is that these differences will be small and will require only a small expenditure of propellant to overcome.