Tuesday, December 8, 2015

VICTS (2Ku) is Mechanically Steered & Other Antenna Characteristics

Classic Ku/Ka  (Multi-Gimbal)

Classic Ku/Ka antennas are multi-gimbal.  A horn array, or other fixed aperture, is mechanically oriented to face the satellite squarely in azimuth and elevation, while polarization is managed by mixing the x and y antenna feeds proportionally.  Two axis or two gimbals are needed from the pedestal.  These antennas are small in size, but are always optimally aligned to the satellite. Multi-gimbal antennas are in service from AeroSat/Astronics, TECOM, Panasonic, and ViaSat.

A novel solution from QEST and Global Eagle Entertainment adds a third axis - Tilt.  This axis provides relief to skew angle effects, making this antenna perform better than other two-axis antennas of similar size.

Flat Antenna

All other technologies leave the array aligned to the aircraft body (a "flat antenna"), and suffer scan loss.  When the satellite is at 20 degrees above the horizon, a flat antenna like 2Ku has shrunk to the effective size of a typical multi-gimbal array.   Everywhere else, 2Ku is bigger, and more effective.

Skew angle is a function of azimuth and elevation beamwidth.  A multi-gimbal array has a fixed beamwidth, wherein the elevation beamwidth is quite broad, making skew effects a problem at any satellite elevation.  A flat antenna elevation beamwidth is a function of satellite elevation - the skew effects are only a factor when the satellite is low to the horizon.

A feature available to flat antennas is multi-beam receive.  It is possible to create two receive beams from the same phased array, wherein each beam has the full benefit of the aperture (no loss).

By their nature, flat antennas tend to be discrete: one transmit aperture and one receive aperture. This creates challenges in beam steering and alignment. It also makes the installation more complex compared to a multi-gimbal array. Phasor, Kymeta, and even ThinKom have hinted that an overlapping design, one aperture assembly for transmit and receive, is being developed. This is a very important feature for smaller aircraft. From a drag perspective, a longer radome is more efficient. One aperture assembly should be lighter.

VICTS - 2Ku Flat Antenna (Single Gimbal)

Variable inclination continuous transverse stub array (VICTS) is the core technology in the ThinKom 2Ku array fielded by Gogo.

The use of phased array in describing VICTS leads to confusion.

VICTS is not steered electronically.  VICTS is steered mechanically.

VICTS uses differential motion between stacked plates to steer in elevation and to align linear polarity.  VICTS uses common motion of all plates to steer in azimuth. The motion of each platter is along a single axis, or gimbal.

Here is an excerpt from the Falcon 3030 ThinKom Ku antenna:
  • Azimuth Coverage: 360° continuous
  • Elevation Coverage: (+10°) +15° to +85°
  • Agility (ARINC 429 NAV): >100°/sec, >100°/sec2 
  • Tracking Accuracy: < 0.2°
There are limits to the tracking rates (albeit, quite impressive), which are an artifact of the mechanical limitations.

Elevation coverage is limited to 85 to zenith.  This is due to "the keyhole" that is a fundamental characteristic of mechanically steerable arrays (where-in, the antenna has to swing rapidly back and forth 180 degrees of azimuth to track the satellite).

Electronically Steerable Flat Antenna (no gimbal)

Electronically steering involves managing the phase shift of a signal across an array of elements.  Classic electronically steerable arrays use a phase-shift command to each element to focus the antenna.  Panasonic is working on such an array.

Phasor Solutions lets each element manage "tracking phase" autonomously.

Kymeta uses optical arrays to focus the beam.   This technology has no moving parts, relying instead on liquid crystal displays to issue the phase shifts.

None of the electronically steerable arrays have mass or momentum to slow them down, and no difficulty in steering to zenith.

Antenna Characteristics

When looking at an antenna, consider (as a function of azimuth, elevation, skew)
  • Figure of Merit (G/T) = receive sensitivity and spectral efficiency
  • EIRP for transmit power = transmit maximum data rate
  • Instantaneous Bandwidth = receive maximum symbol rate, two carrier receive, cross-polar receive.
  • Receive antenna plots = sensitivity to adjacent satellites - Carrier to Interference
  • Transmit antenna plots = Maximum power spectral density or spectral flux density - transmit maximum symbol rate and spectral efficiency

Peter Lemme

Copyright 2015
All Rights Reserved.