Home' RTCA Documents for Review : MOPS GNSS Aviation Antenna-FRAC Release Contents 11
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Passive Element Gain
The passive radiating element gain for the active antenna unit is specified in terms of the
absolute gain (dBic) for the passive element and any associated passive feed network and
radome with respect to an ideal circular isotropic antenna. The passive radiating element
gain at 1575.42 MHz and 1176.45 MHz and 5° elevation shall be at least -4.5 dBic over all
azimuth angles. The maximum passive element gain above 75 degrees elevation shall be
limited to +4 dBic.
Note: In their installation instructions, the manufacturer should identify the minimum
passive element gain at 5 degrees and the maximum passive element gain above
75 degrees elevation for their equipment. Receiver manufacturers can take
advantage of performance better than the minimum requirement, and documenting
improved performance facilitates the installation and integration of the antenna
Polarization and Axial Ratio
The antenna radiation pattern polarization shall be nominally right-hand (clockwise)
circularly polarized and its axial ratio shall not exceed 3.0 dB over the operating frequency
range as measured in a region extending from boresight down to 40 degrees of elevation
off boresight across all azimuth.
Antenna Sensitivity: The G/T Ratio
The antenna, irrespective of its implementation, shall ensure delivery of a minimum
acceptable C/No GNSS signal to the receiver. The quantity for ensuring this (as used
traditionally with other satellite communication systems) is the G/T.
In the L1/E1 band, the GNSS antenna has to demonstrate that its G/T value shall be >= -
30.6 dB/K for all elevation angles θ (where: 5° <= θ <= 90°) , across all azimuth and for
all frequencies within fC±8MHz; with fc set to 1575.42 MHz.
In the L5/E5a band, The GNSS antenna has to demonstrate that its G/T value shall be >= -
32.5 dB/K for all elevation angles θ (where: 5° <= θ <= 90°), across all azimuth and for all
frequencies within fC±10.23 MHz; with fc set to 1176.45 MHz.
The antenna gain-to-noise temperature ratio G/T is usually computed at the interface
between the passive antenna and the input of the antenna front-end (low noise amplifier).
However, the G/T value is the same at any interface in the antenna system.
The G/T is computed by dividing the antenna gain at the input of the antenna front -end (i.e.
the gain of the passive antenna element) by the antenna system noise temperature at the
input of the antenna front-end:
G/T = Gpassive antenna / Tantenna system
The antenna system noise temperature is the sum of the sky noise temperature and the
equivalent noise temperature at the input of the antenna front-end electronics:
Tantenna system = Tsky + Tequivalent noise temperature
Tsky is computed by evaluating the sky noise temperature taking into account the radiation
pattern of the antenna. For an antenna with a hemispherical coverage in L-band this
temperature (Tsky) is no more than 100 K.
The gain of the antenna decreases with the elevation angle. Since the G/T requirement is
also applicable down to 5o elevation angle, the maximum equivalent noise temperature can
be determined by using the absolute gain at this elevation. A minimum figure for the
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