When an SUAV avoidance maneuver was not terminated by the safety pilot, the Auto GCAS algorithm determined when the avoidance maneuver was no longer necessary and returned control to the ground control operator. In order to assess the timeliness of the avoidance maneuver termination, the flight-test results were compared with an ideal termination defined by generating a straight line tangent to the avoidance flightpath. A straight line tangent (including the current climb rate) was used as a way to identify that there was no obstructing terrain directly in front of the DROID. Although other methods could have been used, this was considered the simplest and would most directly correlate with the view of the pilot (in this case, the view provided by the forward-looking video camera). When the straight tangent line was projected to be clear of the DEM terrain for three consecutive time frames (approximately 0.2 s per frame), that third frame was considered the ideal termination.
The purpose for these termination logic cross-checks was to determine whether the Auto GCAS algorithm terminated the maneuver earlier than it should have, at about the right time, or later than necessary. As a general result, when the DROID maneuvered left to avoid terrain, the software logic caused the avoidance maneuver to terminate earlier than it should have. For a right avoidance, the Auto GCAS maneuver tended to terminate later than necessary. The overall results are described in the main body of this report; the methods used are described below.
Figure B27 illustrates how the Auto GCAS algorithm determined when to terminate the avoidance maneuver using an example from flight-testing. The black line in figure B27 represents the actual flightpath of the DROID prior to the avoidance maneuver. The navy blue line in figure B27 represents the trajectory prediction at initiation (in this case, the Auto GCAS algorithm determined that the DROID should avoid terrain by executing a left turn). The red line in figure B27 represents the actual flightpath during the avoidance maneuver. To determine when an avoidance maneuver should be terminated, the Auto GCAS
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algorithm computed a straight trajectory at every time frame throughout the maneuver. The labeled “1st blue line” in figure B27 represents the straight trajectory prediction one frame before the trajectory prediction was clear of the buffered digital terrain (green rectangular polygons). The labeled “2nd blue line” in figure B27 represents the straight trajectory prediction at the frame when the trajectory prediction was first clear of the buffered digital terrain. The straight trajectory predictions do not appear straight in figure B27 because of the P-factor, described in the main body of this report. The Auto GCAS algorithm terminated the avoidance maneuver when three consecutive frames were clear of terrain.
Figure B27. Example avoidance maneuver.
To accomplish termination logic cross-checks, the Auto GCAS team used MATLAB® to implement
the calculations combined with Google Earth as a visualization aid. Using Google Earth KML files, violet straight lines were drawn tangential to the red avoidance maneuver flightpath in figures B28 through B30. To avoid clutter, these violet lines were only drawn every fifth frame (there is roughly 1 s between each line).
119 Figure B28. Tangential lines (violet) drawn from the avoidance flight path (red).
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Figure B30. Left-turning avoidance terminated early.
Next, three arc-second buffered DEM terrain tiles (represented by the green polygons) were added. The buffered terrain added the TCB value to the DEM altitude but did not include any built-in buffers. Therefore, the results from any test run could be evaluated in the same manner as if the TCB were set to 0. The DEM terrain tiles were sized at three arc-seconds to be consistent with the resolution used by the Auto GCAS algorithm and to minimize computational time. This calculation being post-flight, the theoretical accuracy could have been improved using higher resolution DEM tiles (as fine as the one-third-arc-second resolution of the NED source data) but that increased accuracy was not considered necessary for this analysis.
The ideal termination heading was defined to be when the green DEM polygons did not block three consecutive tangential paths (as shown by the three lighter colored violet lines in figures B28 and B29). The determination of when the tangent lines no longer intersected with the DEM terrain was accomplished as a numerical calculation but is shown using Google Earth to help visualize the concepts.
Figures B28 and B29 illustrate termination calculations for similar right-turning avoidance maneuvers. Figure B28 shows the avoidance maneuver as viewed from almost directly above. Figure B29 shows the same maneuver from the perspective of a lower viewing angle. The darker violet lines represent the tangential paths that were blocked by terrain (drawn every five frames), and the three right-most lighter violet lines represent the three-frames-of-persistence clear of terrain (drawn every frame). In these cases, the right-turning avoidances illustrated in figures B28 and B29 indicate that the avoidance maneuver terminated approximately 26 to 29 deg later than necessary.
The short vertical lines along each tangential path represent a distance equivalent to one arc-second. Greater accuracy could have been achieved with smaller intervals for the tangential paths (that is, one-third arc second instead of one arc-second resolution) but computational time would have been increased significantly as a result.
121 Figure B30 illustrates termination calculations for a left-turning avoidance maneuver. The red flightpath in figure B30 once again represents the actual avoidance maneuver path up to the point at which the Auto GCAS algorithm terminated the maneuver. The violet straight lines (drawn every fifth frame) represent the tangential paths along the red actual avoidance path. The orange semicircle represents an extrapolated avoidance trajectory that the DROID would have taken had it continued the avoidance maneuver. The dark-orange straight lines (drawn every fifth frame) represent the tangential paths along the extrapolated orange semicircle. Finally, the three straight yellow lines (drawn at each frame) represent the consecutive tangential paths which do not intersect with the green DEM polygons.
To determine an extrapolated avoidance trajectory, several new elements were needed. Since the smartphone hosting the Auto GCAS algorithm did not use a consistent time interval for each frame, an average time interval was selected using the last five time increments in the red portion of the actual avoidance maneuver (see figure B31). Next, a constant radius was calculated for the extrapolated avoidance maneuver based on the arc between the last two points of the actual avoidance maneuver. This radius was calculated from equation (B2):
𝑠 = 𝜃 ∙ 𝑅 (B2)
where 𝜃 is the heading change calculated from the last two points in the red avoidance path in figure B30, and s is the arc distance between those two points. In this example case, the left-turning avoidance maneuver shown in figure B30 terminated approximately 32 deg earlier than it should have. Although this extrapolation method worked reasonably well on most runs, it was also susceptible to noise in the source data, causing some uncertainty in the results. An alternative method could use an extrapolated radius based on the average arc over the previous several frames.
Figure B31. Differential time element illustration.
For the extrapolated runs the delta time and delta heading were based on the new incremental elements described above.
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