Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix H
DO-253D Change 1
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preamplifier, and the loss block has the same total loss as an installation. The loss block is
composed of a variable attenuator, a combiner, and associated cabling from the test
amplifier output to the receiver port for a total loss of LBlock. The loss from the combiner
input to the receiver port is Lcombiner.
This set-up uses a test amplifier with an adjustable noise figure in order to insert the
appropriate broadband noise at the input of the amplifier. An alternate set-up would
include a noise generator and a combiner prior to the test amplifier, and both the simulator
output and noise generator would have to be adjusted to achieve the desired signal and
broadband noise (specified as noise density in the test procedures) at the input to the test
amplifier. The amplifier also has adjustable gain, GAmp. An alternate test set-up could use
the active subassembly of the antenna, provided the subassembly was calibrated to have
known gain and noise figure and all signal and interference levels adjusted to compensate
for any difference with the specified characteristics of the integrated antenna including
The CW and pulse interference is inserted after the test amplifier. It is calibrated to output
a signal with power Cinterference as referenced to the input of the combiner. It could also be
inserted prior to the test amplifier, and the simulated power level would have to account
for any filtering in the amplifier (assuming the inserted power were within the operating
range of the amplifier).
In this example, the equipment under test is designed for use with a minimum standard
antenna. The installation criteria are as follows:
a) Lmax = 15dBandLmin = 5dB.
b) Nsky,antenna = -172.5 dBm/Hz.
c) Gain of the active portion of the antenna GAnt = 30 ± 3 dB.
d) Active antenna Selectivity per RTCA/DO-301, for example -50 dB at 1626 MHz.
The equipment is calibrated to determine noise levels and loss. In this example, the
following values are used:
a) GPS/SBAS Simulator: ISimulator = -174 dBm/Hz with an adjustable signal level of
CSimulator both referred to the input of the test amplifier.
b) Combiner with loss LCombiner = 5 dB from its input to the Receiver Port.
Use of the Test Set-up for the Accuracy Test (See Section 22.214.171.124.1)
As the broadband noise is generated by the simulator and the adjustable noise figure of the
amplifier, the appropriate noise figure must be determined for the accuracy tests. Two
levels must be computed for the minimum signal level tests since some tests include
external broadband interference and some do not. An additional level must be determined
for the test with all satellites at maximum power.
For the minimum signal level tests, the test amplifier gain (GAmp) is set to 27 dB (minimum
GAnt) and the total loss to 15 dB (LBlock = Lmax). A ten-channel simulator is used, with one
GPS satellite at maximum power (-121 dBm), seven minimum-power GPS satellites (-134
dBm) and two minimum-power SBAS satellites (-134 dBm).
Note: Noise due to SBAS satellites must be accounted for whether the GBAS receiver
tracks these satellites or not.
In order to determine the required GNSS test noise, the effective noise of the satellites
simulated in the test is subtracted from the total GNSS noise of -171.9 dBm/Hz.
Using the equation in 126.96.36.199.1:
ITest = 10log10[(NGL)10IGL/10 + (NGH)10IGH/10 + (NSL)10ISL/10 + (NSH)10ISH/10]
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