Tackling Skew for Reliable SATCOM to Airplanes

Have you ever wondered why you can get a satellite signal almost anywhere on earth, but not typically in an airplane? Many professionals today can’t afford to be out of touch for hours due to air travel. What are the issues, and why hasn’t someone figured them out by now? Well, Intelsat General has and here’s a bit of Satellite 101 to explain how. 

One of the challenges with satellite communications is known as skew. This term refers to the mismatch between the satellite’s horizontal plane and an antenna’s local horizon.  Anywhere on Earth, a geosynchronous satellite’s horizontal plane is the plane of the equator. An antenna’s horizontal plane, however, is the flat plane that touches the earth only at the antenna’s location; i.e. touches the antenna location and is tangential to the earth in all directions.

If one uses a low profile, elliptical antenna its long dimension should be skewed so it aligns to the equatorial plane, not the local horizon. Many airplanes wishing to have satellite communications prefer to install low-profile antennas because of their typically smaller SWaP – size, weight, and power. Such antennas are mounted to present the lowest profile and least drag.  On an airplane, however, one cannot skew the antenna to match the equatorial plane.  Azimuth and elevation can be adjusted but not reflector skew. Here lies a critical challenge unique to airplane communications.

Let’s illustrate with an example.

If I am just west of Chicago looking at the Galaxy 28 (G-28) satellite, it is directly south of me at an elevation of 42 degrees above my horizon.  In addition, the satellite’s horizontal plane is parallel to my local horizon.

My antenna’s azimuth is due south because G-28 is positioned 22,300 miles above the spot on the equator that is due south of my location (just east of the Galapagos Island).  This is called the sub-satellite point.

Now, if I am near Sacramento, California and looking at the same G-28 satellite, I must now position my antenna to point southeast – toward that same sub-satellite point east of the Galapagos Island – and at an elevation of 34 degrees. From this location, the equatorial plane is no longer parallel to my horizon.  Instead, it is skewed by 33 degrees.

The wider section of a low-profile, elliptical antenna more tightly focuses the RF energy transmitted and received.  At each location one wants the wide section to align perfectly with the satellite’s horizontal plane (i.e. with the plane of the equator).  By doing this, the antenna’s tighter focus is aligned with neighboring satellites which minimizes the amount of RF energy our low profile antenna transmits towards and receives from those adjacent satellites.  In other words, this positioning minimizes ASI.  

Now let’s look at the same example when airborne.  

Without skewing the reflector, as an airplane flies from Chicago to California using its low-profile antenna, performance to G-28 is unaffected.  Peak antenna performance is at the antenna’s boresite and the boresite is kept accurately pointing to G-28 throughout the flight. Consequently, the EIRP transmitted toward the satellite and the G/T of the airplane antenna towards G-28 do not change during the flight.

What does change is the amount of energy transmitted towards and received from adjacent satellites.  The antenna reflector remains aligned to the local horizon – we are assuming level flight for this example – while the plane of adjacent satellites skews.  As skew increases, the low-profile antenna presents less and less effective area (be it reflector, or radiating elements in a phased array antenna) in the direction of adjacent satellites.  With less effective area, there is less focusing of the RF signal and, consequently, an increase in both received and transmitted ASI. 

Fortunately, the values of these performance changes are precisely measured and known.  During end-to-end network design and FCC licensing, system engineering accounts for these variations. The coverage areas of aeronautical networks are set to ensure that ASI limits are not exceeded.  In addition to coverage limits, an airplane terminal constantly knows its instantaneous skew value and will automatically cease transmission if the limit is exceeded due to, for example, airplane banking.

That’s how the skew challenge can be managed today. Having sophisticated analysis, design tools, and hardware allows Intelsat General to deliver top quality comms-on-the-move solutions to our aero customers. End users need not worry about SWaP, ASI or any other acronym; they can simply enjoy maximum coverage and high throughputs.

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