Required Antenna Height

Option on Calculate, Intervisibility menu of a map window with associated DEM.

Pick the location of a fixed antenna (use point selection verification, so that you get a very accurate placement) and its height.

Set these options. Antenna height--height of the fixed antenna at the center of the diagram Antenna range--range of the sensor.  Radials will drawn this distance, or to the edge of the DEM. Minimum vertical standoff--used with required flying height Choose to compute Map of required antenna height--this will be in meters above the ground surface. Map of required flying height--if an aircraft must fly a specified height above the terrain, the minimum vertical standoff will be included in the computations.  This will be height above sea level. Map of grazing angle  Map of earth curvature LOS Profile: creates Line of sight/Topographic Profile at sensor location with correct sensor/target heights; you can easily change the azimuth. Vertical earth curve: pick vertical earth curvature algorithm to use.

Required antenna heights. This map requires that the location of the fixed antenna, and its height, be specified. The computation then determines how high a mobile antenna would have to be if it were located at each grid node in the DEM. The resulting surface can be displayed like a DEM. The split map at the left shows a reflectance view of the resulting surface, which retains a recognizable imprint from the landscape, on the top, and a colored height field on the bottom. Both depictions have a radial pattern emanating from the location of the fixed antenna, caused by dead zones behind hills which project outward. These hills have the greatest impact when they occur close to the fixed antenna. The map on right combines elevation and reflectance shading, and probably provides the best tool for analysis. These two diagrams assume a sensor range that can cover the entire DEM.  If the range is less than the DEM size, regions outside the visible range will be missing data. This can also be used to calculate the permissible flying heights that will be visible or masked to a fixed radar.

Map of grazing angle for a sensor. Positive angles face the sensor, and negative angles slope away from it. Missing areas are masked.

Earth curvature (with 3d shading from the DEM).

Technical Details  The algorithm creates a box from the antenna location, going out to the range requested, or the edge of the DEM, whichever comes first.  It then draws radials from the location to each DEM grid location on the edges of the box, and fills in the location of the LOS as each grid point along the way. This is the minimum number of radials to be sure that a radial passes over every grid posting in the DEM.  It uses a spacing along the radial of half the grid posting.  Points close to the fixed antenna will be computed on a number of radials, and the last one computed will be the value retained.  Earth curvature will always be a floating point grid; the others will have the same precision as the input DEM.

Applications.  This could be used in at least two frames of reference:

1. You are sneaking across the border into a country, and pick your location.  Then the resulting map shows how high a sensor (think tower or UAV or even manned aircraft) would have to be to see you.
2. You are the tower or UAV or manned aircraft, and want to see where you can see the sneaking person.  This is like a viewshed, but it shows how much cushion/safety margin there is, which you can factor into how much you trust the results based on the quality of the DEM.

Last revision 7/25/2014