Saturday, February 21, 2015

Skew Angle and Effective Aperture of an Airborne Antenna

Skew angle is a difficult concept to grasp, but the effect can be profound.

Low-profile antennas installed on the top of an airplane fuselage introduce special challenges.

Second generation antennas just entering the market sit flat, creating a change in gain and beamwidth as a function of the relative elevation of the target satellite.

Skew angle rotates the beam-pattern as shown above for various antenna types, at both zero and 90 degree skew.

Fixed Satellite Services (FSS)
The defining feature of the FSS is supporting antennas that can be "bolted" down; that do not track a moving target.

Very Small Aperture Terminals (VSAT) were defined around Ku-band service and a 48" (1.2m) diameter dish (parabolic reflector) antenna.

Aviation applications are particularly sensitive to the size of the aperture with a result that first generation antennas are typically less performing than an 18" (0.45m) dish antenna and second generation antennas are now approaching a 30" (0.76m) dish antenna.

Equivalent reflector size for various airborne antennas,
which can change with satellite elevation

Skew Angle
Skew angle is defined to represent the angle of the radial directed towards/from the mobile terminal on the airplane, assuming the wings are level to the local horizon.

Zero degrees skew angle would apply to any point along the same longitude as the point on the equator directly below the geostationary satellite (in the picture below, 80 degrees West longitude).

90 degrees skew angle applies to any point along the equator.

Skew angle can also change if the airplane rolls the wings so that one side is lower than the other.

If the airplane were flying due south along the 80 deg W longitude (while being in the northern hemisphere), then the local skew angle is equal to any airplane roll angle.  The issue can occur on any radial from the satellite on a heading along the radial.  If the same airplane were crossing the 80 deg W longitude while flying due west, then airplane roll angle would change elevation steering, but have no effect on skew angle.

The roll contribution to elevation steering can become overwhelming, whereby the roll angle puts the satellite below the antenna coverage range.  This effect can be pronounced when operating near the edge of satellite coverage (low elevation anyway) and flying on a heading normal to the direction to the satellite.

Airplane heading itself has no effect to skew angle or elevation angle

Airplane pitch angle has a similar effect to skew angle, except its contribution is with headings normal to the radial to the satellite.  Pitch angle changes are generally modest except during some maneuvers.

The issue with skew angle is two-fold: linear polarization compensation and the airplane antenna beam pattern along the geostationary arc (GSO).

Linear polarization compensation is a factor for Ku band only.  Compensation is applied using a variety of techniques, but none of them are particularly different in intent and overall performance.

The beam pattern along the GSO defines the discrimination towards adjacent satellites, which are expected to be as near as two degrees away.

At zero degree skew angle the antenna has the most favorable axis, the wide azimuth axis, aligned along the GSO.  This results in the tightest beam pattern, or most discrimination towards adjacent satellites.

Here are three antennas for comparison
  1. 18" (0.45m) parabolic reflector
  2. 30" (0.76m) circular single-gimbal antenna (SGA) at both 20 deg and 90 deg elevation angle
  3. 35"x6" (0.89m x 0.15m) multi-gimbal array (MGA)
The circular beam pattern does not change with either elevation or with skew.

The SGA beam pattern changes with elevation and skew, to be fully described below.  Of note, the skew angle effect is minimal at high elevation and moderate at low elevation angles (compared to MGA).

The MGA beam pattern is only a function of skew.

The dashed lines in the figures below are a scalable reference to the zero-skew angle GSO beamwidth.

Zero skew angle - GSO beamwidth

At 90 degrees skew angle, with the wings level, the antenna has the least favorable axis, the narrow elevation axis, aligned along the GSO.  The results in the widest beam pattern, or least discrimination toward adjacent satellites.

90 degree skew angle - GSO beamwidth

At 90 degree skew angle, the SGA low elevation GSO beamwidth has more than doubled, whereas the MGA beamwidth has grown more than five-fold.

At intermediate skew angles, the beam pattern is rotated to ascribe an intermediate level along the GSO compared to zero and 90 degrees skew angle.

The figure below represents 45 degrees skew angle.

45 degree skew angle - GSO beamwidth
The forward channel issues with skew angle are the result of adjacent satellites operating with over-lapping beams and using the same frequency and polarization.  In some cases, the adjacent channels can involve significant carriers in comparison to the signal of interest.   In other cases, there may be no adjacent satellites with interfering carriers.

The return channel issues with skew angle are the result of regulation and coordination.  It is possible to coordinate emissions beyond what is normally approved by regulation, but even if approved today this coordination may be revoked by the adjacent satellite operator at their convenience.  Hence, it is expected that the return channel will be compliant to the spectral mask.

Antenna Types
Antennas installed on top of the fuselage are a compromise (as is everything on an airplane).

Weight, drag, size, power, cooling compete with each other and with other airplane systems.

Fuselage mounted antennas try to minimize their height, then their width, and finally maximize their length and shape.

Other equipment sits on the fuselage limiting the footprint of the antenna.

The space underneath the antenna can become very compact and crowded.

Ku-band and Ka-band antennas share common desires for high gain and a beam pattern with the most discrimination towards adjacent satellites.

Antenna boresite beamwidth turns out to be a dominant factor with small aviation antennas.

Side-lobe issues resulting from antenna design features and certain orientations are ignored on the basis of excellent engineering, but examples are evident where there can be problems.

Multi-Gimbal Array
The first fuselage-mounted antenna to gain wide-spread acceptance is the multi-gimbaled array (MGA) (horn or otherwise).

The MGA uses an aperture with a fixed boresite, and then rotates and pivots the aperture for azimuth and elevation, while applying polarization corrections based on the skew angle.

The MGA aperture, being low profile, is about 30"-36" (0.61m-0.89m) wide and about 6"-8" (0.15m-0.20m) tall.

There is a variant of the MGA, using a second or even a third aperture mounted in tandem, where the rear aperture is shadowed at low elevation angles and is most beneficial at high elevation angles.

The azimuth axis is associated with zero skew angle and the elevation axis is associated with 90 degrees skew angle.

The gain of an MGA does not change with azimuth or elevation pointing angle, nor with skew angle.

The full extent of an MGA is illuminated due to mechanical pointing.

The beam pattern does not change with azimuth or elevation pointing, but it rotates with skew angle.

Forward channel performance is impacted by the airplane receiver picking up stronger interference from adjacent satellites.

Return channel performance is impacted by regulatory application of a spectral mask based on the GSO beam pattern (primarily the boresite beamwidth).

The power spectral density (PSD) also known as EIRP spectral density spectral mask limitations are quite sever at even 35 degrees skew angle.

In the figure below, the red "bow-tie" represents severe limitations to the return channel starting at about 45 degrees skew angle.

The bow-tie regions are continental in size.

Performance in the bow-tie region is only possible by sacrificing spectral efficiency and limitations in available data rates; in high skew angle regions it may not be possible to establish a link.

Single-Gimbal Array (SGA) and Electronically-Steerable Array (ESA)
Second-generation antennas sacrifice any attempt to "square" the aperture to the target satellite, where the antenna remains fixed flat to the airplane top (curved) surface.

The first and fore-most achievement is accommodating a much larger aperture than possible with a MGA.

Instead of one small panel (combined for transmit and receive), there are now two large (24"-30", 0.61m-0.76m) panels, one for transmit and one for receive.

Beam-steering is accomplished through proprietary measures, with the SGA having a single-axis of rotation, and the ESA having no moving parts.

As the satellite moves in elevation, the SGA/ESA apparent aperture varies.

If elevation is zero when the satellite is on the horizon and 90 degrees when the satellite is overhead, than the apparent aperture (and elevation extent) is scaled by sin (elevation).

The azimuth extent is constant regardless of beam steering or skew angle.

The illumination is maximized at 90 degree elevation.

The illumination is reduced as the satellite moves down in elevation.

The illumination is lost at zero degrees elevation.

The changing illumination is modeled as a change in the size of the apparent aperture.

At 20 degree elevation, the aperture has shrunk to one-third of the size at 90 degree elevation.

The figure below shows the gain as a function of elevation, noting it is about a 5 dB loss from zenith to 20 degrees elevation.

Performance at ten degrees elevation is another 50% reduced from 20 degree (or 3 dB further down).

It is possible to continue to operate at ten degrees elevation with the challenge in lower spectral efficiency due to lower gain (forward channel G/T).

In the figure below, both the elevation and azimuth beamwidth are narrowest at 90 degree elevation angle.

In the figure below, the elevation beamwidth has expanded when pointing at 45 degree elevation angle.

In the figure below, the elevation beamwidth is significantly expanded when pointed at low elevation angles.

The beam pattern along the GSO is a combination of a constant azimuth beamwidth and an elevation beamwidth varying as a function of elevation angle.

The following figure represents the beam pattern from a 30" (0.76m) SGA at varying elevation angles.

Accounting for the SGA/ESA beamwidth highlights limitations as elevation angle sets below about 20 degrees (the red ovals) and also when elevation sets below about 30 degrees elevation at high skew angles (the red ear-muffs).

Taking each of the SGA/ESA beam patterns into a stack, and at both zero and 90 degree skew, plus adding an MGA at both skew angles, and an 11.5" (0.3m) and 18 (0.45m) reflector into the following composite figure.

Zero and 90 degree skew angle beam patterns
White - SGA/ESA at 20-90 degree elevation angle
Yellow - MGA
Blue - 11.5" (0.3m) reflector
Green - 18" (0.45m) reflector

In the figure below, the GSO arc is shown along with positions for the target satellite and first and second adjacent satellites on each side, assuming two degree spacing.

The zero degree skew patterns for both SGA and MGA are very discriminating towards the adjacent satellites.

The 90 degree SGA skew pattern picks up the first adjacent satellite only when elevation dips to about 30 degrees.

For elevation above about 30 degrees, the SGA remains discriminating even at 90 degree skew.

The 90 degree MGA skew pattern picks up all four adjacent satellites within the half-power beamwidth.

The following figure puts the MGA and SGA coverage areas together.

The red bow tie regions are most challenging to MGA.

The area circled in yellow are most challenging to SGA.

The central bow tie regions now readily served by SGA are continental in size, substantial in benefit.

Peter Lemme

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All rights reserved

Satcom Engineering Blog Series
Link Budget Background, Definitions, and Assumptions
Spot beams Vs. Wide beams Ku band

Forward Channel Considerations
Transponder Downlink Contours
Forward Channel Downlink Regulations
Ku-band Airborne Antenna Figure of Merit (G/T)
Ka-band Figure of Merit (G/T)

Return Channel Link Budget
Ku-band Return Channel Maximum PSD
Ka-band Return Channel Maximum PSD

Rain Fade

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