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2.5.2 MLS measurement methodology
2.5.2.1 The PFE, PFN and CMN are evaluated by using the filters defined in Figure G-11. The filter characteristics
are based on a wide range of existing aircraft response properties and are considered adequate for foreseeable aircraft
designs as well.
2.5.2.2 While the term “PFE” suggests the difference between a desired flight path and the actual flight path taken by
an aircraft following the guidance signal, in practice, this error is evaluated by instructing the flight inspection pilot to fly a
desired MLS azimuth and recording the difference between the airborne equipment output guidance indication from the PFE
filter and the corresponding aircraft position measurement as determined by a suitable position reference. A similar technique
using the appropriate filter determines the CMN.
2.5.2.3 Error evaluation. The PFE estimates are obtained at the output of the PFE filter (test point A in Figure G-11).
The CMN estimates are obtained at the output of the CMN filter (test point B in Figure G-11). Filter corner frequencies are
shown in Figure G-11.
2.5.2.3.1 The PFE and CMN for approach azimuth or for back azimuth are evaluated over any 40-second interval of
the flight error record taken within the coverage limits (i.e. T = 40 in Figure G-12). The PFE and CMN for approach
elevation are evaluated over any 10-second interval of the flight error record taken within the coverage limits (i.e. T = 10 in
Figure G-12).
2.5.2.3.2 The 95 per cent probability requirement is interpreted to be met if the PFE or CMN does not exceed the
specified error limits for more than 5 per cent of the evaluation interval (see Figure G-12).
2.5.2.3.3 An alternative flight inspection procedure can be used which does not rely on an absolute reference. In this
procedure, only the fluctuating components of the flight record produced at the output of the PFE filter are measured and
compared with the PFN standard. The average value of the PFE is assumed to not exceed the mean course alignment
specified during the flight inspection period. Therefore, the mean course alignment is added to the PFN measurement for
comparison with the specified system PFE. The CMN may be similarly evaluated without accounting for the mean course
alignment.
ATT G-5 23/11/06
Annex 10 — Aeronautical Communications Volume I
2.5.2.4 Ground and airborne instrumentation errors. The instrumentation error induced by the ground and airborne
equipment may be determined by measurements taken in an environment which is free from reflected signals or other
propagation anomalies which can cause beam envelope perturbations.
2.5.2.4.1 First, the instrumentation errors associated with the standard airborne receiver are determined using a bench
test instrument, and the centring error is adjusted to zero. Airborne equipment errors can be measured by recording 40
seconds of data using a standard bench test set. The data can then be divided into four 10-second intervals. The average of
each interval is considered to be the PFE while twice the square root of its associated variance is the CMN.
Note.— The receiver output may be evaluated using the PFE and CMN filters, if desired.
2.5.2.4.2 Second, this standard receiver is used to measure the total system instrumentation error by operating the
ground equipment on an antenna range or in some other reflection-free environment. Since the receiver centring error has
been made negligible, the measured PFE can be attributed to the ground equipment. The ground equipment CMN is obtained
by subtracting the known standard receiver CMN variance from the CMN variance of the measurement. The average error
over a 10-second measurement interval is considered to be the PFE, while twice the square root of the differential variances is
considered to be the instrumental CMN.
2.6 Power density
2.6.1 General
2.6.1.1 Three criteria establish the angle power budgets:
a) angle single-scan acquisition requires a 14-dB signal-to-noise ratio (SNR) as measured at the beam envelope filter
(i.e. the video SNR);
b) the angle CMN must be maintained within specified limits;
c) the DPSK transmissions must have a detection probability at the extremes of coverage of at least 72 per cent.
2.6.1.2 The source of CMN at 37 km (20 NM) is primarily internal receiver thermal noise. The noise induced error (dθ)
can be estimated by:
d ( BW )
2 SNR g
θ
θ =
Function sample rate
g =
2 (Filter noise bandwidth right)
where θBW is the antenna beamwidth in degrees and g is the ratio of the function sample rate to the noise bandwidth of the
receiver output filter. For a single pole filter, the noise bandwidth is π/2 times the 3 dB bandwidth. This expression reflects
　
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