Signal Processing - September 2017 - 97

IEEE SIGNAL PROCESSING MAGAZINE

September 2017

Signal Intensity

Ionospheric
Delay

Signal Intensity

and integrity information to users. While most ionospheric
cannot be accurately estimated using a single-frequency user
delays are removed after differential corrections are applied,
receiver. If a receiver tracks GNSS signals at two or more freresidual ionospheric errors still can exist due to spatial decorquencies, e.g., GPS L1 (1.575 GHz), L2 (1.227 GHz), and L5
relation between the ground facility and users as illustrated in
(1.176 GHz) signals, the ionospheric delay can be directly estiFigure 1. This error becomes threatening in the case of dismated utilizing the fact that the code delays vary at different freturbed ionospheric conditions, which vary significantly in time
quencies. However, note that the current GNSS avionics using
and at different locations.
GBASs or SBASs are single-frequency receivers, and dual-freSBASs also provide a safety-critical aviation service. For
quency GBAS and SBAS developments are still ongoing [6], [7].
example, the wide area augmentation system (WAAS), which
Thus, these receivers rely on differential corrections to remove
is the SBAS of the United States, can guide aircraft down to
code delays on single-frequency signals and compute conserva200 ft above the runway with the help of runway lighting. This
tive position bounds using ionospheric decorrelation parameters
approach procedure is referred to as localizer performance
provided by the augmentation systems as integrity information.
with vertical guidance (LPV)-200. In addition to the signal
The ionospheric scintillation remains a concern even for a
delay, the ionosphere can cause rapid amplitude and random
multifrequency GNSS receiver. Amplitude and phase fluctuaphase variations of transionospheric radio waves: this phenomtions due to scintillation can occur across the GNSS frequency
enon is known as ionospheric scintillation [4]. One of the main
band, and scintillation at GPS L2 and L5 frequencies tend to
challenges for the LPV service in the equatorial area, where
be more severe than at the L1 frequency [8]. If a receiver loses
strong scintillation is expected, is the ionospheric scintillation.
tracking on multiple frequencies simultaneously, the correFigure 1 conceptually illustrates the deep amplitude fading
sponding satellite cannot be used for position calculation. If
under strong ionospheric scintillation during GNSS-based aira multifrequency receiver maintains tracking on at least one
craft landing guidance. Random phase fluctuations that are not
frequency while losing the other frequencies, the impact of
illustrated in Figure 1 also degrade the navigation performance.
scintillation can be mitigated. Although the probability of
A generic GNSS receiver calculates its position based on the
simultaneous loss of multiple frequencies is expected to be
code and carrier-phase measurements from the signal tracking
low [9], the availability of a safety-critical system is reduced
loop. A GNSS aviation receiver performs additional processdepending on the probability level [10]. The probability of
ing based on the information provided by GBASs or SBASs to
simultaneous loss of multiple frequencies or multiple satellites
guarantee the safe use of its position solution. The carrier-phase
depends on the receiver's capability to maintain the tracking
measurement is the basis of high-precision GNSS applications
lock or to reacquire the lock once it is lost. It is challenging
that may require a centimeter-level accuracy. Although it is
for a conventional GNSS navigation receiver to maintain the
very precise, the carrier-phase measurement has an ambiguity
tracking lock under strong scintillation. To reduce the probproblem that should be avoided for safety-critical applications.
ability of loss of lock under scintillation, advanced signal
Tiberius et al. [5] discussed the possibility of single-epoch ambiguity resolution
with a very high success rate on a short
GNSS
baseline if future multiconstellation,
Satellites
Electron Density
Spatial
Irregularities
multifrequency GNSSs are available.
Decorrelation
However, an operational GNSS-based
safety-critical system with such an ambiguity resolution technique has not
yet been demonstrated. Thus, the code
measurement, which is relatively coarse
Ionosphere
but unambiguous, plays the main role
for safety-critical applications that reSpatial
quire a meter-level accuracy but high
Nominal Case
Scintillation Case
Decorrelation
integrity. The carrier-phase measurement is still useful for safety-critical applications in an assistant role to smooth
the noisy code measurements. This carrier-smoothing technique does not reTime
Time
Time
quire ambiguity resolution, but sudden
GBAS Antenna
jumps in carrier-phase measurements
under scintillation can be a problem for
this smoothing.
FIGURE 1. The amount of signal delay is determined by the total electron content (TEC) along the ray
Code delays on GNSS signals caused path. Different TECs along different ray paths cause the spatial decorrelation of the ionospheric delay.
by the ionosphere are a significant error In addition, electron density irregularities can cause rapid amplitude fading of GNSS signals. The ionosource for GNSS navigation, but these spheric scintillation results in rapid phase fluctuations as well, which are not illustrated in this figure.
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Table of Contents for the Digital Edition of Signal Processing - September 2017