As the Phoenix spacecraft approaches Mars, a process called "targeting" is used to determine the final aim point for the spacecraft's entry into the Martian atmosphere. This aim point is chosen by running a physics-based EDL simulator called DSENDS.
Phoenix Landing Points and Ellipse From DSENDS
DSENDS uses an initial guess at an aim point provided by an inter-planetary trajectory solution, and then propagates the spacecraft trajectory from entry through landing. Based upon the miss-distance to the desired landing site, DSENDS adjusts the aim point. The new aim point is then fed back to the inter-planetary trajectory tools and is used to determine the final trajectory correction maneuver (i.e., thruster firings) needed to achieve precision landing. Using the hazard map of the surface described above, a Monte-Carlo simulation of the EDL sequence is used to determine the dispersions of the landing point, and the associated risk of landing.