Signal Processing - September 2017 - 108

Conclusions and future research

scintillation, a traditional wide-band PLL is often sufficient
The ionospheric anomalies potentially hazardous to GNSSto maintain lock of the scintillation signals. Cycle slips and
based safety-critical systems and the appropriate mitigation
carrier-phase measurement errors tend to increase with wider
strategies depending on the characteristics of the anomalies are
bandwidth. Using optimized integration time (a few tens of
discussed in detail. As representative examples of safety-critimilliseconds), more sensitive discriminators such as atan2 and
cal systems, GBASs and SBASs are introduced, and the moniKFPs can help reduce cycle slips and phase estimation error.
toring and mitigation of ionospheric spatial decorrelation on
Equatorial scintillation is characterized by concurrent amGBAS and ionospheric scintillation on SBASs are explained.
plitude fading and random phase fluctuation. The optimization
However, note that the spatial decorrelation also needs to be
of filter design is an effective means to mitigate equatorial scinmitigated on SBASs, and the scintillation can be problematic
tillation in addition to adaptation of appropriate integration time
to GBASs and any other safety-critical systems as well.
and discriminator function. For a given filter design, there is a
To develop methods to detect and mitigate hazardously
functional bandwidth associated with a scintillation signal. The
large ionospheric spatial gradients, an ionospheric anomaly
functional bandwidth is defined by a dynamic limit as the lower
threat model is developed using GNSS observation data colbound and a noise limit as the upper bound. The dynamic limit
lected from a region where GBASs will be fielded. The ionois determined by the transient response of the filter to the phase
spheric anomaly threat model derived from a particular region
scintillation of the signal, while the noise limit is determined by
(e.g., U.S. CONUS) is not necessarily sufthe amplitude scintillation characteristics.
ficient to bound anomalies elsewhere in
For weak to moderate equatorial scintillaThe optimization of filter
the world, because abnormal ionospheric
tion, a filter bandwidth within the functional
design is an effective means range will ensure that a receiver can mainbehavior varies significantly with location.
to mitigate equatorial
This requires the development of new threat
tain lock on the signal.
models particularly in regions exposed to
For extremely violent equatorial scinscintillation in addition to
equatorial ionospheric behavior. As sumtillation events, the bandwidth functional
adaptation of appropriate
marized in this article, several threat modrange does not exist anymore as the noise
integration time and
els developed in each region have different
limit may be reduced to below the dynamic
discriminator function.
upper bounds on the worst ionospheric
limit. As a result, the tracking loop will not
gradient. Furthermore, GBASs should conmaintain lock on the scintillation signal
tinue to monitor ionospheric behavior in case future anomaregardless of the bandwidth values. For these types of challenglies generate larger gradients than those in the current threat
ing events, more sophisticated techniques have to be employed
model, for which LTIAM was developed and is being used by
to maintain lock on the signals. This article has highlighted
many states.
an adaptive gain Kalman filter, a VTL-A-STL, and a semioIonospheric spatial decorrelation mitigation has been a chalpen loop. The AKF simultaneously adjusts the Kalman gain
lenging problem to solve for GNSS safety-critical applications.
matrix and weights the measurement functions to minimize the
For CAT I operations, the CCD monitor within a GBAS ground
scintillation effects. The VTL-A-STL utilizes the energy from
facility cannot detect anomalous gradients under worst case
channels not affected by scintillation to aid tracking of scintilfront-user-satellite configurations. However, it was not practical
lation signals. The semi-open loop takes advantage of known
to modify GBAS avionics that had already been certified. The
receiver antenna location and precise satellite orbit information
PDGS method was developed as a solution, but with possible
to compute range and range rate during deep fading to provide
availability penalty. A key aspect of GAST-D (an upgraded
feedback to the carrier tracking loop. These techniques have
version of single-frequency GBASs) is the addition of airborne
been shown to be effective in combating extremely violent scinmonitoring to the existing ground monitoring. The mitigation
tillation events during the last two solar maxima.
strategies, currently being developed for GAST-D operations,
The study of both ionospheric spatial decorrelation and scincombine several monitors both airborne and ground to detect
tillation mitigation and their effects have been in large part motiionospheric anomalies within the threat model.
vated by the demands and needs of safety-critical applications.
Ionospheric scintillation mitigation has been an active and
Understanding GNSS signal characteristics under ionospheric
fruitful research area for more than two decades. This article
irregularities and developing effective mitigation techniques
has provided an overview of the most important findings of
are two mutually dependent tasks fundamental to safety-critical
these studies with emphasis on efforts having direct impact on
applications. A better understanding of both anomalously large
safety-critical applications. The objectives for these types of
ionospheric decorrelation and scintillation phenomena and their
scintillation-mitigation techniques are 1) maintaining lock of
impact on GNSS signals will lead to more effective monitoring
scintillation signals, 2) reducing the number of cycle slips, and
and mitigation techniques, while robust monitoring and mitiga3) minimizing carrier-phase measurement errors. Different
tion techniques will provide more opportunities to observe and
approaches have been developed to achieve these objectives
learn about ionospheric anomalies. The history of this subject
for different types of scintillation events.
has demonstrated this mutual dependence with ample examAt high-latitude phase scintillation is the dominant facples, starting with the very first ISM receiver development and
tor while amplitude scintillation is mild. For these types of
more recently with the development of LTIAM. We anticipate
108

IEEE SIGNAL PROCESSING MAGAZINE

September 2017

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