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approach was already planned, this advantage would not be available.  If it is possible to take advantage of the other end of the runway, the ability for energy correction could be as large as 22 miles.  This would be a case where the approach azimuth brought the Orbiter in along the path useful for low energy downmoding, but the trajectory is targeted to use a nearly 360-degree turn on the far end and approach the runway from the opposite direction.  This would usually be the case for this approach azimuth.  At EAFB, the runway peculiar options with wet lakebeds are the same as at KSC. If the lakebed runways are available, other runways can be selected to find the lowest energy approach.  The result is that, for all general approach azimuths at EAFB, the available downmode correction is generally in the 25 to 30 nautical mile range while at KSC it is limited based on approach azimuth with optimal benefit approaching the general capability of EAFB.

In addition to runway selection, two other energy conservation aids are available.  The flight software will automatically shrink the size (radius) of the alignment cone so that the circumferential travel around the cone is reduced.  Another option selectable by the pilot is to allow guidance to bring him to the outer glideslope at an altitude of about 6000 feet rather than about 12 000 feet.  These items can account for about 6 miles without changing runways.

Flight performance in entry energy management and navigation to date has been excellent.  Navigation sources include the inertial measurement units, tactical air navigation equipment and ground stations, air data probes and their equipment, and the MLS.  In addition, the ground uses S-band and C-band tracking data to derive an independent navigation state which can then be uplinked to the Orbiter to correct significant errors.  For all error sources, a series of Monte Carlo runs defines a 3σ error envelope.  This error is handled easily by the entry guidance - possibly three times the 3σ error could be tolerated.  Flight experience has shown that the navigated state error has always been less than the expected 1σ performance.  Virtually all navigation data sources are triply redundant with ground uplink as an additional backup.  The MLS is dual string; however, the Flight Rules provide visibility requirements which when coupled with TACAN accuracy allows the flight crew to use the PAPI and Ball Bar landing aids to achieve MLS equivalent accuracy.

Navigation accuracy alone is not a sufficient argument for selecting the EAFB landing area over KSC.  There may be some other multiple failure scenarios that could result in a rapid energy drop.  Certainly, if an unlikely failure (stuck open speedbrake or very early landing gear deploy) occurs at just the right time, a lakebed runway would offer an additional advantage.  These failures are considered to be highly unlikely and do not represent a credible argument for avoiding KSC landings.

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