Issue in satellite communications as they pass overhead

[senmeis]

Hi,The data sheet of an antenna system can be found at http://www.tcs.la/Portals/0/TCSXY.pdf. This description is taken from it:

Our XY pedestal is ideal for tracking satellites in Low-Earth Orbits (LEOS) because it completely eliminates the problem of the overhead keyhole (lost data effect that occurs when satellites pass directly overhead) more reliably and less expensively than a three-axis system.My question is, if this issue is eliminated just because the elevation is greater than 90 degree?Senmeis

 

[.Scott]

They are comparing an X/Y system to an azimuth/elevation system. With an azimuth/elevation system, the azimuth mechanics need to swivel rapidly as the satellite crosses overhead.

The problem with the azimuth/elevation system is dependent on the speed of the satellite, how close to 90 degrees of elevation it gets, and how fast the azimuth mechanics can move.

 

[Baluncore]

There are really three satellite tracking antenna mount geometries. They share features with astronomical telescope mounts.

Altazimuth or Az-El mount is best for very large heavy antennas because the azimuth axis can be supported on a circular rail, while the elevation axis can be counterbalanced. Az-El is least affected by the wind but has velocity problems tracking near the zenith. https://en.wikipedia.org/wiki/Altazimuth_mount Includes a short discussion of the zenith blind spot or gimbal lock. The azimuth axis is used alone for marine RADAR.

Equatorial is really only used for small antennas and telescopes that track astronomical objects as the Earth rotates. They require a minimum of feedback and control systems. https://en.wikipedia.org/wiki/Equatorial_mount An equatorial mount without a significant declination axis is called polar mount. It is used only for selecting geostationary satellites. https://en.wikipedia.org/wiki/Polar_mount

The lesser known X-Y mount avoids the high tracking velocities as satellites pass close to alignment with a fixed axis such as the zenith or pole. One problem with X-Y mounts is that counterbalancing becomes more difficult and the antenna becomes more susceptible to high winds. The antenna shown in your link looks like a sunflower with no obvious counterbalance weights.
There is an example of a counter-balanced ex-NASA satellite tracking antenna with an X-Y mount here.
https://en.wikipedia.org/wiki/Mount_Pleasant_Radio_Observatory

Google images 'radio astronomy antenna' to see more examples of all types.