Global Positioning System Reference
In-Depth Information
without additional ILS systems. For CAT III approaches (zero visibility), even the SBAS will
not suffice, and ILS is still required.
EGNOS provides a European-wide, standardized and quality-assured augmentation service
suitable for different fields of applications. Integrity is a key quality and safety parameter,
and it alerts users when the system exceeds tolerance limits. EGNOS broadcasts wide-area
differential corrections to improve accuracy, and alerts users within six seconds if something
goes wrong (integrity).
The receiver combines satellite/user geometry information, with EGNOS-corrected pseudo-
ranges, and internal estimates of the tropospheric delay to compute the user position.
Ideally, the user would like to have the difference between the computed position and the
true position - the true position error (PE) - to be less than the AL. However, since the true
position is not known, the PE cannot be determined, and an alternative approach is
required.
In fact, the receiver continuously estimates a predicted position error, known as the
protection level (PL), for each position solution. The PL can be estimated using the UDRE
and GIVE parameters and other local error-bound estimates. It is scaled for compatibility
with the probability of non-integrity detection so that the PL should always be larger than
PE.
Integrity assessments are based on PL and AL. A new PL is estimated for each computed
position solution, then it is compared with the required AL, and an integrity alert is
triggered if PL>AL. There is an underlying assumption, that PL>PE, when assessing
integrity, and this corresponds to the “safe” zone to the left of the leading diagonal in Figure
3. In the nominal operation case, PL<AL and the system is available. If PL>AL for a
particular operation, the EGNOS integrity cannot support the operation, and the system is
unavailable.
There is also an “unsafe” zone to the right of the leading diagonal where PL<PE and the
integrity assessment provide misleading information (Figure 3). The case at the bottom left
corner of the diagram (PL<PE<AL) is also “safe,” theoretically, because the AL has not been
exceeded, but it should be noted that EGNOS also protects against these out-of-tolerance
situations (ESA, 2005).
Different parameters, used in the XPL computation, must be elaborated by the ground
segment (Roturier et al., 2001):
•
the variance σ
2
UDRE,i
of a zero-mean normal distribution that describes the user
differential range error (UDRE) for each ranging source after the application of fast and
long-term corrections and excluding atmospheric effects and receiver errors;
•
the variance σ
2
UIRE,i
of a zero-mean normal distribution that describes the L1 residual
user ionospheric range error (UIRE) for each ranging source after ionospheric
corrections have been applied. This variance is determined from the variance (
2
σ
)
GIVE i
,
of an ionospheric model based on the broadcast grid ionospheric vertical error (GIVE).
2
•
the variance
local
σ of a zero-mean normal distribution that relates the pseudo range
error due to local receiver noise and multipath;
,
2
•
the variance
tropo
σ of a zero-mean normal distribution that defines the residual pseudo
range error of a tropospheric correction model.
,
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