Signal Processing - September 2017 - 98

processing techniques can be utilized. If a receiver loses the
hardware differences in the L1 and L2 signal paths cause intertracking lock, it needs to quickly reacquire the lost channel
frequency biases on both receiver (IFB) and satellite ^x gd h .
and reestablish tracking to reduce the impact of scintillation,
There also exists a code and carrier bias on each channel. Howwhich also requires appropriate signal processing strategies.
ever, the differences between the code and carrier bias on L1
This article aims to provide a broad and
and L2 are relatively small and therefore are
comprehensive understanding of ionosphernot included in (1). The parameter c is the
Signal
processing
is
ic impacts on GNSS-based safety-critical
speed of light in vacuum.
fundamental to design
systems and up-to-date signal processing
For GNSS augmentation applications, alalgorithms for monitoring
techniques for monitoring and mitigation
most all ionospheric delay error is removed
of ionospheric anomalies. We focus on two
when differential corrections broadcast by
both regional and global
error sources, ionospheric spatial decorrereference stations to nearby users (for exionospheric anomalies
lation and ionospheric scintillation, which
ample, within several tens of kilometers of
and also to develop novel
can cause potential threats to users of GNSS
a GBAS ground facility) are applied to user
mitigation techniques.
augmentation systems if undetected or unmeasurements because the reference station
mitigated. Signal processing is fundasees almost the same ionospheric delay as
mental to design algorithms for monitoring both regional
the users. However, residual correction errors remain due to ionand global ionospheric anomalies and also to develop novel
ospheric spatial and temporal decorrelation between reference
mitigation techniques.
and user. While differential GNSS user errors due to spatial and
temporal variations in ionospheric delay are almost negligible
Ionospheric anomalies
under nominal conditions, the reference station and user may
experience very different ionospheric delays under ionosphere
Ionospheric spatial decorrelation and its impact on GNSSs
storm conditions. When an ionospheric spatial decorrelation
The ionosphere is the upper atmosphere ionized by solar radia(or spatial gradient) is a few hundred times larger than a typical
tion, which has significant influence on transionospheric radio
one-sigma value (1-2 mm/km), residual ionospheric errors could
wave propagation. The ionosphere causes the velocity change
be unacceptably large even after differential corrections are apof transionospheric radio waves such as GNSS signals. This
plied. The worst ionospheric spatial gradients observed in midvelocity change is dependent on the frequency of the signal,
and low-latitude regions are discussed from the point of view of
which is the characteristic of a dispersive medium. Because
GBAS users as follows. Note that, while both spatial and temporal
the frequencies of the carrier ^z 1, z 2h and the modulated code
decorrelation could be potential threats to SBAS users as well, the
^ t 1, t 2 h of L1/L2 GNSS signals are different, they experience
section "Signal Processing for Ionospheric Spatial Decorrelation
different delays as expressed in (1) [11]
Monitoring and Mitigation" focuses on ionospheric decorrelation
monitoring and mitigation for GBASs.
Ionospheric spatial gradients in the slant domain (i.e.,
k
k
t L1 = r i + I i + f t 1
along
the actual path between satellite and receiver) of as
k
k
z L1 = r i - I i + m L1 N L1 + f z 1
large as 412 mm/km over baselines of 40-100 km have been
k
k
k
t L2 = r i + c I i + c (IFB i + x gd) + f t 2
observed in the United States during ionospheric storms since
k
k
k
April 2000 [13]. The largest gradients occurred when the
z L2 = r i - c I i - c (IFB i + x gd) + m L2 N L2 + f z 2
2
line-of-sight (LOS) of a GNSS signal from one station passed
f L1
c= 2 .
(1)
through a storm-enhanced density (SED) with a poleward
f L2
plume of ionization, while the LOS from the other station
stayed outside the SED. In GBASs, it was found that ionoThe common term, r ki , represents the sum of the true
spheric decorrelation could be modeled as a simplified linrange between the ith receiver and kth satellite, receiver/satear wedge parameterized by the gradient (or slope), width,
ellite clock biases and tropospheric error. The carrier-phase
maximum delay, and ground speed of the ramp, as shown in
observables ^z L1, z L2h contain the ambiguous integer numbers
Figure 2. An extreme gradient of 412 mm/km (the largest gra^ N L1, N L2 h of cycles of wavelengths ^m L1, m L2 h of the carrier
dient ever observed in midlatitude regions) can pose a potential
frequencies ^ fL1, fL2h, but have lower multipath and thermal
integrity threat to GBAS users because the residual range error
noise errors than the code measurements (i.e., f z % f t ). The
at the CAT I decision height can be as large as 8 m if undetected
ionospheric error, I, on the L1 signal is of equal magnitude but
by the GBAS ground facility. The hazard caused by ionospheric
opposite sign on the carrier phase relative to the code phase,
storms to aircraft performing precision approaches to CAT I
and this effect is usually expressed as the code delay and carweather minima (with the 10-m vertical alert limit) was unacrier advance. The ionospheric delay at the L2 frequency ^ fL2h
ceptable without further mitigation.
is proportional to the delay I at the L1 frequency ^ fL1h by the
Ionospheric activity in equatorial regions (within ±25° latisquared frequency ratio c. This frequency dependence in (1) is
tude of the geomagnetic equator) is known to be significantly
a first-order approximation, since the second-order effects can
more variable and intense than what is encountered (and has
be accounted for by a noise term in most applications [12]. The
been extensively studied) in midlatitude regions such as the
98

IEEE SIGNAL PROCESSING MAGAZINE

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September 2017

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Table of Contents for the Digital Edition of Signal Processing - September 2017

Signal Processing - September 2017 - Cover1
Signal Processing - September 2017 - Cover2
Signal Processing - September 2017 - 1
Signal Processing - September 2017 - 2
Signal Processing - September 2017 - 3
Signal Processing - September 2017 - 4
Signal Processing - September 2017 - 5
Signal Processing - September 2017 - 6
Signal Processing - September 2017 - 7
Signal Processing - September 2017 - 8
Signal Processing - September 2017 - 9
Signal Processing - September 2017 - 10
Signal Processing - September 2017 - 11
Signal Processing - September 2017 - 12
Signal Processing - September 2017 - 13
Signal Processing - September 2017 - 14
Signal Processing - September 2017 - 15
Signal Processing - September 2017 - 16
Signal Processing - September 2017 - 17
Signal Processing - September 2017 - 18
Signal Processing - September 2017 - 19
Signal Processing - September 2017 - 20
Signal Processing - September 2017 - 21
Signal Processing - September 2017 - 22
Signal Processing - September 2017 - 23
Signal Processing - September 2017 - 24
Signal Processing - September 2017 - 25
Signal Processing - September 2017 - 26
Signal Processing - September 2017 - 27
Signal Processing - September 2017 - 28
Signal Processing - September 2017 - 29
Signal Processing - September 2017 - 30
Signal Processing - September 2017 - 31
Signal Processing - September 2017 - 32
Signal Processing - September 2017 - 33
Signal Processing - September 2017 - 34
Signal Processing - September 2017 - 35
Signal Processing - September 2017 - 36
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Signal Processing - September 2017 - 38
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Signal Processing - September 2017 - 42
Signal Processing - September 2017 - 43
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Signal Processing - September 2017 - 48
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Signal Processing - September 2017 - 51
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Signal Processing - September 2017 - 58
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Signal Processing - September 2017 - 60
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Signal Processing - September 2017 - 67
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Signal Processing - September 2017 - 69
Signal Processing - September 2017 - 70
Signal Processing - September 2017 - 71
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Signal Processing - September 2017 - 73
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Signal Processing - September 2017 - 76
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Signal Processing - September 2017 - 82
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Signal Processing - September 2017 - 85
Signal Processing - September 2017 - 86
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Signal Processing - September 2017 - 88
Signal Processing - September 2017 - 89
Signal Processing - September 2017 - 90
Signal Processing - September 2017 - 91
Signal Processing - September 2017 - 92
Signal Processing - September 2017 - 93
Signal Processing - September 2017 - 94
Signal Processing - September 2017 - 95
Signal Processing - September 2017 - 96
Signal Processing - September 2017 - 97
Signal Processing - September 2017 - 98
Signal Processing - September 2017 - 99
Signal Processing - September 2017 - 100
Signal Processing - September 2017 - 101
Signal Processing - September 2017 - 102
Signal Processing - September 2017 - 103
Signal Processing - September 2017 - 104
Signal Processing - September 2017 - 105
Signal Processing - September 2017 - 106
Signal Processing - September 2017 - 107
Signal Processing - September 2017 - 108
Signal Processing - September 2017 - 109
Signal Processing - September 2017 - 110
Signal Processing - September 2017 - 111
Signal Processing - September 2017 - 112
Signal Processing - September 2017 - 113
Signal Processing - September 2017 - 114
Signal Processing - September 2017 - 115
Signal Processing - September 2017 - 116
Signal Processing - September 2017 - 117
Signal Processing - September 2017 - 118
Signal Processing - September 2017 - 119
Signal Processing - September 2017 - 120
Signal Processing - September 2017 - 121
Signal Processing - September 2017 - 122
Signal Processing - September 2017 - 123
Signal Processing - September 2017 - 124
Signal Processing - September 2017 - 125
Signal Processing - September 2017 - 126
Signal Processing - September 2017 - 127
Signal Processing - September 2017 - 128
Signal Processing - September 2017 - 129
Signal Processing - September 2017 - 130
Signal Processing - September 2017 - 131
Signal Processing - September 2017 - 132
Signal Processing - September 2017 - 133
Signal Processing - September 2017 - 134
Signal Processing - September 2017 - 135
Signal Processing - September 2017 - 136
Signal Processing - September 2017 - 137
Signal Processing - September 2017 - 138
Signal Processing - September 2017 - 139
Signal Processing - September 2017 - 140
Signal Processing - September 2017 - 141
Signal Processing - September 2017 - 142
Signal Processing - September 2017 - 143
Signal Processing - September 2017 - 144
Signal Processing - September 2017 - 145
Signal Processing - September 2017 - 146
Signal Processing - September 2017 - 147
Signal Processing - September 2017 - 148
Signal Processing - September 2017 - 149
Signal Processing - September 2017 - 150
Signal Processing - September 2017 - 151
Signal Processing - September 2017 - 152
Signal Processing - September 2017 - 153
Signal Processing - September 2017 - 154
Signal Processing - September 2017 - 155
Signal Processing - September 2017 - 156
Signal Processing - September 2017 - 157
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Signal Processing - September 2017 - 159
Signal Processing - September 2017 - 160
Signal Processing - September 2017 - 161
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Signal Processing - September 2017 - 165
Signal Processing - September 2017 - 166
Signal Processing - September 2017 - 167
Signal Processing - September 2017 - 168
Signal Processing - September 2017 - 169
Signal Processing - September 2017 - 170
Signal Processing - September 2017 - 171
Signal Processing - September 2017 - 172
Signal Processing - September 2017 - 173
Signal Processing - September 2017 - 174
Signal Processing - September 2017 - 175
Signal Processing - September 2017 - 176
Signal Processing - September 2017 - 177
Signal Processing - September 2017 - 178
Signal Processing - September 2017 - 179
Signal Processing - September 2017 - 180
Signal Processing - September 2017 - 181
Signal Processing - September 2017 - 182
Signal Processing - September 2017 - 183
Signal Processing - September 2017 - 184
Signal Processing - September 2017 - 185
Signal Processing - September 2017 - 186
Signal Processing - September 2017 - 187
Signal Processing - September 2017 - 188
Signal Processing - September 2017 - 189
Signal Processing - September 2017 - 190
Signal Processing - September 2017 - 191
Signal Processing - September 2017 - 192
Signal Processing - September 2017 - 193
Signal Processing - September 2017 - 194
Signal Processing - September 2017 - 195
Signal Processing - September 2017 - 196
Signal Processing - September 2017 - Cover3
Signal Processing - September 2017 - Cover4
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