Effect of clouds on optical imaging of the Space Shuttle during the ascent phase: A statistical analysis based on a 3D model

Clouds are highly effective in obscuring optical images of the Space Shuttle taken by ground-based and airborne tracking cameras. Because the imagery is used for quick-look and post-flight engineering analysis, the Columbia Accident Investigation Board (CAIB) report recommended that the return-to-flight effort include an upgrade of the imaging system to be able to obtain at least three useful views of the shuttle from lift-off to at least solid rocket booster (SRB) separation.

Because the lifetimes of individual cloud elements capable of obscuring optical views of the shuttle are typically 20 minutes or less, accurately observing and forecasting cloud obscuration over an extended network of cameras poses an unprecedented challenge for the current state of observational and modeling techniques. In addition, even the best numerical simulations based on real observations will never reach “truth.” In order to quantify the risk that clouds would obscure optical imagery of the shuttle, a tool to calculate probabilistic risk was developed. The tool was a computer simulation model which was used to estimate the ability of a network of optical imaging cameras to obtain at least N simultaneous views of the shuttle from lift-off to SRB separation in the presence of clouds. The model generated line-of-site (LOS) data for a prescribed camera network and vehicle ascent trajectory. The camera network and ascent trajectory were embedded in a 3-dimensional field of randomly distributed clouds. The LOS from each camera to the shuttle was computed along the shuttle trajectory and cloud obscuration was noted as a binary variable, either obscured or clear. The obscuration data was then analyzed to determine the percent of time from liftoff to SRB separation that at least N simultaneous views of the shuttle were obtained by the camera network, where N ranged from 2 to 6. A total of 1000 trials with randomly distributed clouds were analyzed for each of approximately 19 different cloud scenarios. The cloud scenarios had prescribed cloud bases, tops and sizes, with cloud coverage ranging from clear to overcast by 1/8s.

The model output was analyzed to determine the effect of changes in network and cloud parameters on viewing conditions. The parameters were as follows: an upgrade to the camera network including the addition of airborne cameras, and cloud characteristics such as coverage, height of bases, thickness and horizontal extent of individual clouds. NASA is currently using the results of the analysis to determine the best path to follow in responding to the CAIB’s recommendation.

In addition to the results of the study, another potentially useful outgrowth of the statistical modeling study was an ability to map out the geographical boundaries of the domain where clouds with specified bases and tops would have the potential for obscuring views of the shuttle sometime during the ascent phase. Implementation of an operational mapping tool on the Meteorological Information and Data Display System in the Range Weather Operations facility would enable the Launch Weather Officer to quickly determine if existing cloud cover has the potential to obscure optical imaging of the shuttle.