1.0 Introduction


1.1 Mission Motivation and Objectives

The nature of aerospace funding has passed through a crucible since the end of the Cold War, for all parties involved.   Funding of current and future space exploration will be provided by government and private agencies with an eye on economy.   Out of a feeling of despair over shrinking budgets has come the burgeoning understanding that more can be accomplished than ever before for less money.  

The recent trend in the industry has been to reduce the need for custom design and hardware; changing the entire way the business of space exploration and development is done.   Already, hardware miniaturization and new approaches to management have reduced the costs of designing and realizing spacecraft by as much as a factor of ten.   This new approach has already displayed great success in such missions as the Ballistic Missile Defense Organization's Clementine Mission which used off the shelf technology and design to reduce the cost of a semi-autonomous Lunar mapping mission.   However, the operational costs of some recent missions threaten to rival the total budget of the booster and upper stage production and assembly.   The operations budgets have failed to follow the reduction trend of production costs because of:

  1. The limited availability of Earth based deep space tracking stations.
  2. And the need to retain 'standing armies' of engineers to monitor the navigation of probes in their passive cruise phases (often measured in years).

P-STAR seeks to address the reduction of these costs by establishing a constellation of GPS-like solar satellites at approximately 5 Astronomical Units (AU) in various inclinations.   These satellites will become the focus of a single dedicated Earthside tracking organization and will, in turn, provide passive navigation and tracking resources to any spacecraft operating within and possibly slightly beyond the orbit of Jupiter.   The economic benefits of such a system will only be realized in the event of a much higher traffic density between the inner planets, i.e. a series of Martian exploratory probes.   Hence, the primary motivation of this project comes from the following two assumptions:

  1. There will a global increase in deep space exploration in the next fifty years.
  2. That the majority of this intrasolar traffic will occur in the inner solar system with only a minority of probes being sent to the outer planets.

The P-STAR program intends to provide passive tracking of spacecraft operating in the inner Solar system for a period of no less than 100 years.   The program must be achieved with fewer than 10 satellites orbiting the Sun within 5-6 AU.   Furthermore, the launch systems for P-STAR must be selected from the most efficient technology available today that will be able to place, at a minimum, one satellite into orbit per launch.   Thus, no more than 10 launches will be required to achieve nominal operations or P-STAR.   The launch schedule must be such that the entire system can be in various stages of delivery from Earth within 5 years and completely in place within 15 years.

The operation of P-STAR's tracking transmissions will be such that spacecraft in the inner solar system will require only medium reception capabilities.   The concept is such that comm equipment will not dominate the mass budget of any autonomously navigating craft.   It will not be the primary concern of P-STAR to provide attitude determination capability to spacecraft in general, although the data provided may be used as such if it is designed into the GNC subsystem of a specific spacecraft.

It is beyond the scope of this feasibility study to determine what specific technology will be placed in the P-STAR transponders.   However, it is strongly recommended that a heavy emphasis be placed on pushing the limit of the mission technology freeze by utilizing cutting edge computational and data handling hardware to allow for remote software and capability upgrading.   Operating at the orbital distances of 5-6 AU from the Sun, there will not be the possibility of repair missions in the foreseeable future.


1.2 Comparison of Systems for Solar Positioning and Global Positioning

The basic concepts of passive navigation are the same for global and solar positioning.   A constellation of transponders with well defined orbits transmits time signals which are picked up by a navigation receiver.   This receiver then calculates a range between it and the various transponders by noting the elapsed time between transmission of the signal and its reception.   At least three transponders must be visible to the navigation unit to define two possible points of intersection (one of which can often be discarded).   Four or more are recommended to filter out error associated with range tracking; providing a more accurate position measurement.   A similar concept is applied today in deep space tracking, except that the Earth stations receive only one range signal from a spacecraft, providing only range measurements separated only by the radius of the Earth the distance traveled by the spacecraft between tracking segments.

The GPS system relies upon a constellation of 24 satellites in 12 hour period orbits to achieve Earthside navigation.   The large number of satellites is a direct consequence of the Earth's proximity, curvature, and opacity...we can't transmit signals through it!   A benefit of such a large number of transponders; however, is that lower transmission power is required from each satellite since several are nearby any given location.  

A constellation directed at solar positioning, and operating at 6 AU, would not be eclipsed by massive bodies, i.e. planets and the sun, but for the briefest of periods and would require fewer transponders.   However, the disadvantage of such a widely displaced constellation is that the time signals must carry over greater distances to be received by a passive navigation unit.  

With this problem in mind it was determined to realize P-STAR through a constellation of 6 transponders transmitting by means of a sweeping signal system.   Six transponders will provide four units for minimal functionality plus a redundant unit each for mechanical failure and solar eclipse.   Each transponder will "sweep" the inner solar system with a tightly focused signal rather than try to broadcast over the entire 10+ AU encompassed within the constellation.   The advantage of this system is that it lowers the power requirement of each transponder without seriously affecting the navigation of an inner solar system spacecraft.   In fact, the navigation unit on a spacecraft utilizing P-STAR will now serve as a miniature tracking station by recording range signals as they sweep over its position in the solar system.   Each time a signal is received it will be incorporated into an onboard sequential tracking filter to update the state estimate of the spacecraft.

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Tim Crain
Graduate Aerospace Engineering
The University of Texas at Austin
crain@csr.utexas.edu
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