Line of sight antenna coverage Model (Shado)
- Introduction
The SHADO program is a line-of-sight antenna coverage model, which can handle up to two antennas. It generates a plot of a specified area and will shade the areas that are within the line-of-sight of either antenna or both antennas. SHADO uses terrain/topographic database.
2. Analysis
Terrain profiles are generated for each antenna site. Profile analysis is then performed to determine whether the end point of each profile is in the radio line-of-sight (LOS) or it lies within the earth’s shadow (Figure 1). In Figure 1, qn is the radio horizon elevation angle to nth profile point. Primary objective is to determine the elevation that constitutes the limiting LOS along each profile.
Figure 1. A profile path from an antenna site
Two factors must be taken into consideration: the bending of radio waves as they propagate through the atmosphere due to refractivity and the effects of earth’s curvature with respect to the elevations. The program uses the user specified refractivity value to compute the amount of ray bending. If the default refractivity of 301 is used, a 4/3 radius earth is assumed to be correct for ray bending. Radio horizon angles are then calculated from the beginning of the profile to the end-point, which represents a grid point coordinate. The rays representing waves from the antenna site along the profile to the end point are represented as straight lines for a 4/3 earth’s radius.
If the elevation angle of the profile end-point, with respect to the antenna site, is less than the calculated radio horizon angle, then the grid point coordinate lies within the earth’s shadow (Figure 2).
Figure 2.
Profile path with an end-point within earth’s shadow
For an end-point within the LOS, the radio horizon angle is that formed by a direct ray from the antenna site to the profile end-point (Figure 3).
Figure 3.
Profile path with an end-point within LOS
Plots are generated where each grid point is shaded to indicate whether it is within the LOS with respect to each antenna site (Figures 4 and 5). The program generates 3721 (61 X 61) profiles along a number of equally spaced radials emanating from each antenna site to each grid point on the plot as an extension of the PROFILE program. These sites do not have to be within the area of the overlay.
Figure 4. Path 1 shows a radial with Figure 5. Path 1 end-point within
end-point within LOS, Path 2 with LOS and is clear at bin 3,7, path 2
end-point within earth’s shadow. End-point within earth’s shadow
and is black at bin 5,9.
Analysis of the profiles makes it possible to determine the grid coordinates at which signals approaching or emanating from the site will be detected on a LOS basis. This information is consolidated to produce LOS shading contours around the site, which provide a composite of the sites’ coverage, as determined by terrain features. The engineer can determine a site location with the best use of the terrain characteristics by locating the proposed antenna at each point on the grid.
3. Input Parameters
The input parameters are the latitudes and longitudes of the southwest and northeast corners of the plot area, the latitude(s) and longitude(s) of antenna(s), surface refractivity, the antenna height(s), and optionally the antenna site elevation(s). The latitudes and longitudes are expressed in degrees North and West respectively. The default site elevation is calculated from the terrain profile. The antenna heights and the site elevation can be in English or Metric units.
4.Output
The output from the program SHADO is a map, which shows the location(s) of one or two antennas and the LOS coverage by the antenna(s)
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