for the timeliness of warnings, minimum object size, or the accuracy or uncertainty of predictions. In August 1997, NASA levied more detailed requirements on the SSN, but the SSN may be unable to meet these requirements.

In the absence of specific requirements, budget limitations have forced the DOD to reduce the number of ground-based sensors that supply most of the information about debris at orbiter operational altitudes. Since 1989, the number of radar sensors in the SSN has been reduced from 19 to 13, and no new radar sensors have been added, although upgrades have been made at a few sites. Plans for new or upgraded SSN sensors do not include requirements that would improve debris tracking. Unless action is taken, the SSN’s ability to provide collision warnings to the shuttle will probably diminish.

Finding. The capabilities of the SSN to provide collision warnings to NASA are eroding. Until recently, NASA had not issued requirements that might have helped to halt this erosion.

New Approach to Collision Warnings

The planned use of covariance data should help NASA make better decisions about collision avoidance maneuvers and reduce the number of unnecessary maneuvers. NASA is now waiting for the U.S. Air Force to complete development of covariance matrices. The covariance data will also be used for the ISS, for which it will be needed by late 1998.

NASA has recently specified the level of accuracy it requires for covariance or state vector uncertainties at conjunction. The method the SSN currently uses to compute a covariance does not represent uncertainty in the state vectors accurately enough to calculate Pc accurately at the orbiter’s operational altitude, where the major uncertainty is atmospheric drag. Inaccurate calculations of Pc could result in the orbiter performing unnecessary maneuvers or not performing necessary maneuvers. Current work (Barker, 1996, 1997) for the Air Force will greatly improve the computation of covariance but will not incorporate the uncertainty in atmospheric drag.

Finding. NASA plans to use a new probability-based approach to determine when a collision avoidance maneuver is necessary, but the collision avoidance data currently provided by the SSN is not accurate enough for the new approach to be effective

NASA Flight Rules for Collision Avoidance

In deciding whether to make a collision avoidance maneuver, NASA flight controllers assess whether the maneuver would compromise primary payload or mission objectives. The current wording of Flight Rule A4.1.3–6, suggests that a very close conjunction of the orbiter and a large, well tracked object could occur



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