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8.4.2 Within this range, threat Model A generates the dead zones described above. (Waveforms with lead need not be
tested, because their correlation functions are simply advances of the correlation functions for lag; hence, the MI threat is
identical.)
8.5 Threat Model B introduces amplitude modulation and models degradations in the analog section of the GPS or
GLONASS satellite. More specifically, it consists of the output from a second order system when the nominal C/A code
baseband signal is the input. Threat Model B assumes that the degraded satellite subsystem can be described as a linear
system dominated by a pair of complex conjugate poles. These poles are located at σ ± j2πfd, where σ is the damping factor in
nepers/second and fd is the resonant frequency with units of cycles/second.
ATT D-43 23/11/06
Annex 10 — Aeronautical Communications Volume I
8.5.1 The unit step response of a second order system is given by:
0 t
e(t)
1 exp( t) cos dt sin dt t 0
d
⎧ ≤ 0⎫
=⎩⎪⎪⎨− −σ ⎢⎣⎡ ω +ωσ ω ⎥⎦⎤ ≥⎪⎭⎪⎬
where ωd = 2πfd.
8.5.2 Threat Model B for GPS corresponding to second order anomalies uses the following ranges for the parameters Δ,
fd and σ:
Δ = 0; 4 ≤ fd ≤ 17; and 0.8 ≤ σ ≤ 8.8.
Threat Model B for GLONASS corresponding to second order anomalies uses the following ranges for the parameters
defined above:
Δ = 0; 10 ≤ fd ≤ 20; and 2 ≤ σ ≤ 8.
8.5.3 Within these parameter ranges, threat Model B generates distortions of the correlation peak as well as false peaks.
8.6 Threat Model C introduces both lead/lag and amplitude modulation. Specifically, it consists of outputs from a
second order system when the C/A code signal at the input suffers from lead or lag. This waveform is a combination of the
two effects described above.
8.6.1 Threat Model C for GPS includes parameters Δ, fd and σ with the following ranges:
–0.12 ≤ Δ ≤ 0.12; 7.3 ≤ fd ≤ 13; and 0.8 ≤ σ ≤ 8.8.
Threat Model C for GLONASS includes parameters Δ, fd and σ with the following ranges:
–0.11 ≤ Δ ≤ 0.11; 10 ≤ fd ≤ 20; and 2 ≤ σ ≤ 8.
8.6.2 Within these parameter ranges, threat Model C generates dead zones, distortions of the correlation peak and false
peaks.
8.7 Unlike GPS and GLONASS, the SBAS signal is commissioned and controlled by the service provider. Moreover,
the service provider also monitors the quality of the signal from the SBAS. To this end, the threat model will be specified and
published by the service provider for each SBAS satellite. The SBAS SQM will be designed to protect all avionics that
comply with Table D-12. Publication of the threat model is required for those cases where a service provider chooses to
allow the SBAS ranging signal from a neighbouring service provider to be used for precision approach by SBAS or GBAS.
In these cases, the service provider will monitor the SBAS ranging signal from the neighbouring satellite.
8.8 In order to analyse the performance of a particular monitor design, the monitor limit must be defined and set to
protect individual satellite pseudo-range error relative to the protection level, with an allocation of the ground subsystem
integrity risk. The maximum tolerable error (denoted as MERR) for each ranging source i can be defined in GBAS as:
MERR = Kffmdσpr_gnd,i and
2 {2 }
MERR=KV,PA σi,UDRE+min σi,UIRE
23/11/06 ATT D-44
Attachment D Annex 10 — Aeronautical Communications
for SBAS APV and precision approach where min { 2 }
σi,UIRE is the minimum possible value for any user. MERR is
evaluated at the output of a fault-free user receiver and varies with satellite elevation angle and ground subsystem
performance.
8.9 The SQM is designed to limit the UDRE to values below the MERR in the case of a satellite anomaly. Typically,
the SQM measures various correlation peak values and generates spacing and ratio metrics that characterize correlation peak
distortion. Figure D-9 illustrates typical points at the top of a fault-free, unfiltered correlation peak.
8.9.1 A correlator pair is used for tracking. All other correlator values are measured with respect to this tracking pair.
8.9.2 Two types of test metrics are formed: early-minus-late metrics (D) that are indicative of tracking errors caused by
peak distortion, and amplitude ratio metrics (R) that measure slope and are indicative of peak flatness or close-in, multiple
peaks.
8.9.3 It is necessary that the SQM has a precorrelation bandwidth that is sufficiently wide to measure the narrow
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