Simon Dawson

Autonomous orbit maintenance system

Abstract Orbit maintenance is a major cost factor for Earth satellites in specialized orbits, such as a repeating ground track, or in formations. While autonomous attitude control is well established, the spacecrafts orbit is usually uncontrolled or maintained by ground station commands. For small, lower cost satellites, operations costs can be a dominant element of both cost and risk. This implies a need for low-cost autonomous orbit maintenance in order to allow such systems to be economically viable, particularly in todays constrained budget environment. In addition, if the position of the spacecraft is controlled, it is therefore known in advance. Thus, mission planning can be done as far in advance as desired, without the need for replanning and frequent updating due to unpredictable orbit decay. An interesting characteristic of autonomous orbit maintenance is that it typically requires less software, and less complex software, than does orbit control from the ground. In many cases, an onboard orbit propagator is not needed.

Autonomous Constellation Maintenance

Creating and maintaining the long term structure of a constellation can be a major element of cost and risk. Both can be substantially reduced by the use of straightforward, low-cost autonomous orbit maintenance. In most cases, this can be done with the computer, sensing, and thruster hardware already required onboard the spacecraft.

Reducing planetary mission cost by a modified launch mode

Abstract Major cost reductions in planetary missions, especially those with high launch energy requirements, are made possible by a modified launch mode † in which part of the total launch energy is generated by onboard propulsion rather than by the launch vehicle. Spacecraft separation from the launch vehicle occurs at or near the point of reaching escape velocity. The net effect is a large increase in net payload mass, since the dry mass of the launch vehicle upper stage does not get accelerated to the same final velocity. In missions that require large deep-space maneuvers en route or at destination the modified launch mode is particularly attractive since the spacecraft has to carry an onboard propulsion system, anyway. The paper describes this launch technique in some detail and examines the various cost saving categories it offers. These include selecting a smaller, less costly launch vehicle for the mission, given a specific payload mass; increasing the payload mass if desired; avoiding complex and time consuming detours for planetary gravity assist purposes to increase payload capability; and avoiding costly miniaturization of design elements. The paper compares the different mission and system requirements associated with the conventional and the modified launch mode, discusses the inherent cost differences and indicates relevant implementation factors.

LOW-COST AUTONOMOUS ORBIT CONTROL ABOUT MARS : INITIAL SIMULATION RESULTS

Abstract Interest in studying the possibility of extraterrestrial life has led to the re-emergence of the Red Planet as a major target of planetary exploration. Currently proposed missions in the post-2000 period are routinely calling for rendezvous with ascent craft, long-term orbiting of, and sample-return from Mars. Such missions would benefit greatly from autonomous orbit control as a means to reduce operations costs and enable contact with Mars ground stations out of view of the Earth. This paper present results from initial simulations of autonomously controlled orbits around Mars, and points out possible uses of the technology and areas of routine Mars operations where such cost-conscious and robust autonomy could prove most effective. These simulations have validated the approach and control philosophies used in the development of this autonomous orbit controller. Future work will refine the controller, accounting for systematic and random errors in the navigation of the spacecraft from the sensor suite, and will produce prototype flight code for inclusion on future missions. A modified version of Microcosms commercially available High Precision Orbit Propagator (HPOP) was used in the preparation of these results due to its high accuracy and speed of operation. Control laws were developed to allow an autonomously controlled spacecraft to continuously control to a pre-defined orbit about Mars with near-optimal propellant usage. The control laws were implemented as an adjunct to HPOP. The GSFC-produced 50 × 50 field model of the Martian gravitational potential was used in all simulations. The Martian atmospheric drag was modeled using an exponentially decaying atmosphere based on data from the Mars-GRAM NASA Ames model. It is hoped that the simple atmosphere model that was implemented can be significantly improved in the future so as to approach the fidelity of the Mars-GRAM model in its predictions of atmospheric density at orbital altitudes. Such additional work would take the form of solar flux (F10.7) and diurnal density dependencies. The autonomous controller is a-derivative of the proprietary and patented Microcosm Earth-orbiting control methodology which will be implemented on the upcoming Surrey Satellite Technology (SSTL) UoSAT-12 and the NASA EO-1 spacecraft missions. This work was funded by the NASA Jet Propulsion Laboratory under a Phase I SBIR (96.1 07.02 9444) and by internal Microcosm R&D funds as well as earlier supporting work done under a variety of USAF Research Laboratory-sponsored contracts [1, 2, 4, 12].

Solar-Electric Planetary Missions with an Initial Out-of-Ecliptic Thrust Phase

A solar‐electric propulsion mission proe le with Earth gravity assist is considered that differs from more conventional mission designs by letting the thrust phase preceding the Earth swingby be inclined with respect to the ecliptic.Thethrustacceleration isoriented normal to theorbitplanethusadding inclination without increasing the 1-astronomical unit solar distance. Theout-of-ecliptic mission phase serves to accumulatea majorrelative velocity increase, directed normal to thespacecraft’ sorbit planeasthrust timeprogresses. This effect isessential to gaining greater effectiveness of the subsequent Earth swingby maneuver that converts this relative velocity component to the desired outbound direction, for example, in missions to Jupiter or other outer planets. This mission proe le considered hereoffersconsiderablee exibilityin theselectionoflaunch datesand simplie esa tradebetweenmission duration and solar ‐electric power. An initial inbound thrust phase in outbound missions is avoided, and thereby, the spacecraft design is simplie ed. The various characteristics of the modie ed mission proe le that differ from the conventional proe le are described, and the respective advantages that are due to these modie cations are shown. It is shown that the out-of-ecliptic thrust phase, in combination with Earth swingby, offers considerably greater e exibility in the overall mission proe le design, which in turn relieves launch date constraints and simplie es thrust vector control requirements. These factors have not been sufe ciently explored as yet in previous contributions to the electric ‐propulsion mission and system design literature.