Tuesday, January 20, 2015

Ka-band Return Channel Maximum PSD

Summary for Fixed 24"x24" Square and Circular ESA plus Gimbaled 30x6 Horn Array
Emissions in the Ka Band from a terminal towards a GSO satellite operating under FSS rules are strictly regulated to prevent interference to adjacent satellites.

Limitations are levied in the form of a "spectral mask", in that the PSD (dBW/40 kHz) is limited based on the angular separation from the target satellite.

It is common for an operator to coordinate their operation with all other operators at least six degrees away, and for this coordination to become the basis of approval, even if in conflict with regulation.

Routine approval is easier if following the spectral mask levied by the regulator.

47 CFR 25.138 (Ka band)  
Licensing requirements for GSO FSS Earth Stations in the 18.3-18.8 GHz (space-to-Earth), 19.7-20.2 GHz (space-to-Earth), 28.35-28.6 GHz (Earth-to-space), and 29.25-30.0 GHz (Earth-to-space) bands.

The most stringent restrictions are emissions along the GSO, as there may exists a neighboring satellite that is using the same frequencies.
Co-Polarization - Along the GSO

O3B operates a non-geostationary constellation of satellites that utilize Ka band.  Frequency assignments prevent most issues with FSS satellites along the GSO (which are primary).  Generally, a boundary of about ten degrees above and below the GSO is drawn whereby O3B will utilize alternative space vehicles to prevent any overlap.

By and large, aviation terminals have claimed relief from any restrictions across the GSO, as their antenna aperture has a high axial ratio resulting in a desire to spill considerable excess energy above and below the GSO.  To my knowledge, no aviation terminal has been approved without an accommodation for the across GSO emissions having no detriment.  In this context, they are effectively ignored.

Co-Polarization - Across the GSO

Cross-polarization suppression is levied effectively ten dB down from co-polarization, which is usually easily achieved.  The use of circular polarization removes a degree from beam steering as the polarization does not need any adjustment being either left-hand or right-hand.

Cross-Polarization - Along the GSO

Antenna Modeling

I have modeled three antennas to reveal how the spectral mask impacts available PSD for a given scenario.
  1. Multi-Gimbaled 30"x6" Horn Array
  2. 24"x24" square Electronically Steerable Array
  3. 24"x24" circular Electronically Steerable Array
The beam patterns are fabricated and somewhat idealized.

The point of the analysis is to focus on the physical aperture as it changes orientation relative to the servicing satellite transponder.

Measured antenna patterns must be examined for proper application of the spectral mask.

Gain estimates are assuming 70% antenna efficiency based on idealized performance of on an equivalent reflector (with equal area).

There is 0.2 degree beam steering error built into the analysis.

Skew angle rotates the antenna beam pattern as inscribed along the GSO from most discriminating at zero degrees to least discriminating at 90 degrees.

Elevation angle assumes zero degrees at the horizon and 90 degrees at zenith.

Multi-Gimbaled 30"x6" Horn Array

The first antenna is a multi-gimbaled horn array 34" wide by 7" tall.

Beam steering accomplished by physically pointing the boresite in both azimuth and elevation.

There is no RF gain change at any steering angle (azimuth or elevation), given the mechanical pointing.

Elevation (relative to the aperture) is always 90 degrees.

Gain does not change as a function of elevation angle, since the aperture remains square to the illumination.

Skew angle rotates the beam pattern between the most discriminating 34" aperture to the least discriminating 7" aperture.

Gain does not change as a function of skew angle.

Because beamwidth changes along the GSO as a function of skew angle, maximum PSD is a function of skew angle.

The following plots are run at 0, 30, 45, 60 and 90 degree skew angle.

24"x24" Rectangular Electronically Steerable Array

The second antenna is an electronically steerable array (ESA), square in shape 24"x24".

The ESA is mounted on the top of the airplane and has no moving parts.

The ESA effective area is a function of elevation angle.

At 90 degrees elevation (zenith), the full extent of the ESA is illuminated.

At zero degrees elevation (horizon), the ESA illumination is effectively zero.

The square array presents an elevation and azimuth axis when considering what part of the aperture is illuminated.

The azimuth axis GSO extent does not change in size with elevation angle.

The azimuth axis GSO extent of a rectangular/square array does change as a function of skew angle.  A square array azimuth GSO extent can increase 44% at 45 degrees, for example.

The azimuth axis GSO extent of a circular array does not change as a function of skew angle.

The elevation axis GSO extent does change with as a (sine) function of elevation angle.

Because the elevation axis GSO extent always changes with elevation, the flat ESA, whether circular, rectangular, or square; changes with skew angle.

The beamwidth along the GSO is an independent function of both skew angle and elevation angle.

The illumination area of a flat panel varies as a (sine) function of elevation angle and so therefore the gain of the flat panel antenna is also a (sine) function of elevation angle.  

Gain does not change with skew angle for any antenna; beamwidth (PSD) always changes with skew angle.

Both gain and beamwidth are a (sine) function of elevation angle.

Because beamwidth changes along the GSO as a function of both elevation and skew angle, maximum PSD is a function of both elevation and skew angle.

The following plots are run at 

  0 skew angle - 90, 60, 30, 20, 10 elevation angles
30 skew angle - 90, 60, 30, 20, 10 elevation angles
45 skew angle - 90, 60, 30, 20, 10 elevation angles
60 skew angle - 90, 60, 30, 20, 10 elevation angles
90 skew angle - 90, 60, 30, 20 elevation angles

24"x24" Circular Electronically Steerable Array

I have also modeled a circular array.

A circular array has slightly lower gain than a rectangular array with the same extent in aperture (about one dB).

A circular array GSO extent is at a maximum at zero deg. skew angle, and decreases with increasing skew angle.

The skew angle effect is due to the shrinking elevation extent due to elevation angle illumination effect, noted earlier with rectangular ESA.

Gain varies only as a function of elevation angle.

I just added the circular results to the above results and created this summary table of data.

NOTE: I have changed the frequency down to 29 MHz, representing widest beamwidth in Tx band.

Peter Lemme

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