Wind Forecast Accuracy and PADS Performance Assessment

If the wind is known and the navigation and guidance logic of a parachute system leads it close to the drop zone (DZ) for final approach, a good guidance and control scheme can make it land a few yards from the IP. Yet when winds change and the system's flight software is not robust enough, however fine the guidance and control part of the algorithm, the airdrop error can become very large. Weather forecast agencies can give at best a good evaluation of an average wind field in a given area for a given period of time. Wind is seldom stationary, and its fluctuations are such that two wind profiles measured 30 minutes apart can show a difference of roughly 4 m/s over a height span of 1000 m (observed July 6, 2005 during a Precision Airdrop Capability Demonstration (PACD) in Biscarosse, FR). The direct consequence is that even with the best forecast system and drop sondes used just prior to the airdrop, Precision Air Delivery Systems (PADS) will face winds different from what was used in the mission planing. Therefore a class of models was developed in ASTRAL software that simulates accurately generic navigation logic with simplified guidance and control models. They are valid as long as the navigation logic plans trajectories with few turns and lots of straight branches, or smooth curves that allow the system to make turns in almost stabilised turn configurations. Therefore they can be simulated with only speed and turn rate performance data on the systems (1st order dynamics with 3 DoF, as opposed to the 2nd order 6 DoF dynamics model also available in ASTRAL), and are highly representative of the main part of the descent, the homing-phase, and can be correctly representative of some final approach logic, except in cases where highly dynamic manoeuvres are implied (by extension, those model cannot determine if a self guided system is stable in turbulent wind conditions). Presented at the AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar (19th) held in Williamsburg, VA on 21-24 May 2007.