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Imaging planets about other stars with UMBRAS: target acquisition and station keeping

We present a novel coronagraphic imaging technique and design for space-based telescopes. The Umbral Mission 
Blocking Radiating Astronomical Sources (UMBRAS) is a space mission design consisting of a free flying occulter, the 
Solar Powered Ion Driven Eclipsing Rover (SPIDER), and possibly one or two metrology platforms. The UMBRAS 
spacecraft operate in conjunction with a space-based telescope. The size of the occulting SPIDER is dictated by 
the size of the telescope with which it will work. The goal of UMBRAS is to provide "paleolithic" (i.e., non-focal 
plane) coronagraphic capability to enable direct imaging of extrasolar Jovian planets and other bright substellar 
companions such as brown dwarfs. 
We discuss two aspects of the operation of a free flying occulter: acquisition of targets and station keeping. Target 
acquisition is modeled after the onboard schemes used by Hubble Space Telescope (HST) science instruments. For 
UMBRAS, the onboard commanding sequences would include imaging the field using instruments on the telescope, 
locating the target and the occulter in the field, and accurately positioning the occulter over the target. Station 
keeping consists of actively maintaining the occulter position in the telescope line of sight to the target. 
Velocity matching of the c)cculter with the space-based telescope is essential to mission performance. An appropriate 
combination of solar electric and cold gas thrusters provide the ability to match velocities using position 
information derived from communication and from ranging data between telescope, occulter and any metrology 
stations. 
The accuracy requirements for target acquisition and station keeping depend upon the science requirements, 
the occultation geometry, and the sensitivity of the science to changes in occultation geometry during an exposure 
sequence. Observing modes other than the ideal centered occultation of a target will be discussed.