Handbook of Propagation Effects for Vehicular and ... - Courses

10-2
10.2 General Methodology
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
The method consists of the following steps: (1) Images seen through a fisheye lens are
photographed at potential user locations. (2) Clear, shadowed, or blocked path states are
extracted as a function of look angle. (3) Path states are combined for single or multiple
satellite scenarios with frequency-appropriate statistical fade models to predict fade
distributions and diversity performance as a function of elevation angle. Measurements
of optical brightness along the line-of-sight path have demonstrated a statistical link
between fading and optical intensity [Vogel and Hong, 1988]. The images are acquired
with a 35-mm camera with a fisheye lens having a full 90° field of view in elevation and
360° in azimuth. The camera may be mounted on a vehicle, hand carried, or fixed on a
tripod. A compass is used to align the top of each frame towards north. Resulting slides
are scanned into a personal computer and subsequently analyzed.
Figure 10-1 shows an example of a fish-eye lens image in an urban location in
Austin, Texas where the zenith is at the center [Akturan and Vogel, 1995]. The 24-bit
color image is reduced to an 8 bit gray-scale and it is unwrapped from a circular image
(zenith at center) to a rectangular one with the zenith at the top. An image histogram is
constructed based on the gray scale value of 32,400 pixels (360° azimuth times 90°
elevation, where each pixel size is 1° by 1°. The second derivative of the image
histogram, after filtering, is used to estimate the gray-level threshold value separating sky
and obstacles [Bovik, 1991]. A binary ski/no-sky image is subsequently produced and
used to calculate the elevation angle for each azimuth at which the sky becomes visible
(e.g., the skyline). The corresponding skyline of the unwrapped image of Figure 10-1 is
shown in Figure 10-2 with the abscissa representing the azimuth (relative to north at 0°)
and the ordinate is the corresponding elevation angle.
Figure 10-1: Example of fisheye lens image in Austin, Texas