1. Because of the complexity of the navigation constraints, it is impossible to define uniquely the most optimum algorithm for guidance and navigation of a lifting AOTV during its atmospheric flight. However, the general guidelines, ado ted in several of the proposed schemes,19920p2432T,38 can be stated, They are: 1) It is advantageous to fly with a negative L/D, that is, with the lift vector pointed toward the Earth, because this increases the perigee height (in exchange for a lengthened duration of the atmospheric flight) and thereby lowers the peak dynamic pressure and heattransfer rates. 2) The cross-range travel (orbital plane change) should be accomplished mostly during the descent phase; the ascent phase should be reserved for correcting for the errors caused by the fluctuation of the air density. 3) During the ascent phase, the vehicle should fly near its maximum -L/D, so that, if the atmospheric density is too large, the vehicle could roll 180" to produce a positive L/D which will raise the flight path and thereby shorten the flight duration and avoid catastrophic loss of velocity. navigational errors and fluctuations in atmospheric density are such that the vehicle cannot reach the destination orbit, effort should be made to insert the vehicle into the correct orbital plane, sacrificing accuracy in apogee height and phase angle (longitude). The vehicle should then execute in-plane rendezvous maneuvers propulsively to correct for the errors. By following these guidelines, a lifting AOTV gains two advantages over a nonlifting AOTV: a) It can fly at a higher altitude than a nonlifting AOTV of the same ballistic coefficient, thereby lowering dynamic pressures and heat-transfer rates, and b) it has a greater chance of being inserted into the correct orbital plane. 4) When the