Globalstar Product Management – Technical Support

Benchmarking Product Performance — Clear Line-Of-Sight for LEO Satellite Systems

Question:  What is “clear line of sight” and why do we need to do this when choosing a location for benchmark testing a Globalstar Satellite Terminal?  Provide the rationale and examples.

 Answer:    The Globalstar communications system utilizes low earth orbit (LEO) satellites, some of which pass directly overhead and some of which trace an arc as low as 10 degrees above the horizon.   Since they are in low earth orbit, the earth rotates beneath the orbital paths, and the view of the satellite constellation from any point on earth is constantly changing.
For benchmark testing, the satellite terminal must be able to communicate with all the satellites that could provide service to the terminal.  This requires the satellite terminal antenna to have a clear unencumbered line of sight to all points in the sky in all directions down to 10 degrees above the horizon.  A unit with clear line of sight is able to see every point in the sky from zenith (directly overhead) down to 10 degrees above the horizon, and through all 360 degrees of azimuth with no obstructions above 10 degrees in any direction (N-S-E-W).

Easy Rule:  It is possible to use simple geometry to calculate the allowable height of buildings, hills and other nearby obstructions.  Since the tangent of 10 degrees (0.176) is a little less than 0.20, a good rule to remember is that the height of the obstruction above the height of the antenna must be no greater than 20% of the horizontal distance between the satellite antenna and the obstruction.

For example:

  1. If a transmitter is 6ft from a building, the satellite terminal antenna would have to be no more than 12 inches below the highest point of the structure. (perhaps the roofline, an air conditioner or a facade)
    ==>   6ft x tan(10deg)  ==>  6ft x 0.2  ==>  12 inches
  2. For a transmitter on top of a car (height 5ft) in an open parking lot, a building that is 100ft distant can only be 20ft higher than the car’s roof, or a total of 25ft high
    ==>  100 x 0.2 + 5ft   ==>  20 + 5  ==>  25ft)
In actual practice, the satellite terminal will often be used in environments that are less than perfect.  However, when benchmarking the product performance, it is important to have the terminal in an ideal environment.  This will provide the product developer with the best information regarding inherent product performance.
 Field Installations:  Field Installations are different than Benchmark Locations.  For a benchmark test, you want to make sure that there is no sky blockage above 10 degrees.  For a field installation, you recognize that you may have to live with some non-ideal blockage, but you want to choose the site that has the lowest amount of blockage.  For Field Installations, the best approach is to use an Abney Rule (see Abney Rule – Easy Way to Select Satellite Installation Site).
Additional Keywords:  product performance, simplex, duplex, sky view

Written by Joseph Crowley

March 12, 2013 at 6:23 pm

Posted in Uncategorized

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