Signal Processing - September 2017 - 101
SNR, and other useful parameters. However, those tools do not
focus the signal quality parameters that affect the quality of ionospheric products the most.
To remedy this weakness in the freeware tools, Kim et al. [25]
developed a comprehensive algorithm to compute key GNSS
signal quality parameters that affect the quality of ionospheric
parameter estimation and monitoring. The algorithm extracts
information not provided by the freeware tools, including small
cycle slips, short arcs, and outliers that typically degrade the
accuracy of ionospheric delay estimates. A methodology to select
high-quality GNSS observations was further developed by using
the signal quality parameters having a strong correlation with the
quality of the ionospheric parameters [26]. WAAS uses a different approach to control GNSS signal quality when generating its
postprocessed high-precision ionospheric delay data (known as
supertruth). This different approach is a voting algorithm that
selects one of three measurements from three colocated receivers/antennas with the same firmware versions [27].
Using high-quality GNSS L1/L2 measurements, slant ionospheric delay on the L1 signal is calculated in three ways. The
L1/L2 code-derived ^ I t_meas h, L1/L2 carrier-derived ^ I z_meas h,
and L1 code-minus-carrier (CMC) ^ I CMC_meas h measurements
are computed by differentiating the raw dual-frequency code
^ t L1, t L2 h and carrier ^z L1, z L2 h measurements, as shown in
I t_meas =
t L2 - t L1
= I + c ^ IFB i + x kgd h + f t
c-1
c-1
I z_meas =
z L1 - z L2
= I + c ^ IFB i + x kgd h
c-1
c-1
+ m L1 N L1 - m L2 N L2 + f z
c-1
I CMC_meas =
where c =
t L1 - z L1
2
f 2L1
.
f 2L2
= I - m L1 N L1 + f CMC
2
(2)
The dual-frequency code-derived measurement ^ I t_meas h
is noisier than the carrier-derived measurement ^ I z_meas h
because the carrier-phase measurements have lower multipath and noise errors than the code-phase measurements
(i.e., f z % f t ). Thus, I z_meas is used to obtain precise estimates of ionospheric delays. I z_meas contains integer ambiguities ^ N L1, N L2 h from both L1 and L2 frequencies. To
remove these ambiguities, I z_meas is fitted to I t_meas, introducing a level parameter, Level z [27]. The level is computed
for each continuous arc by averaging the difference between
I t_meas and I z_meas over the epoch t i ^i = 1, 2, f, N h using
an elevation ^el h -dependent weighting, as shown in (3). The
dual-frequency carrier-derived ionospheric delay estimate,
I z_est, is then obtained by removing the receiver and satellite hardware biases ^ IFB and x gd h from the leveled carrierderived estimates, I z_leveled . While utilizing x gd provided by
the international GNSS service (IGS), IFBs are estimated
using a method given by [28]. This method determines the
best estimate of each IFB by searching for the estimate that
minimizes the cumulative standard deviation of vertical ionospheric delays to their mean on a given day.
N
/ ^I _meas (t i) - I
t
Level z =
z_meas
(t i)h sin 2 el i
i= 1
N
/ sin 2 el i
i= 1
I z_leveled = I z_meas + Level z = I +
I z_est = I z_leveled -
c
c- 1
c
c- 1
(IFB + x gd)
(IFB + x gd) .
(3)
The ionospheric spatial decorrelation is obtained using the
precise ionospheric delay estimates, I z_est, and the well-known
station-pair method [13]. The slant spatial gradient of ionospheric
delay, dI, is estimated by dividing the difference in slant ionospheric delay between two stations i and j viewing the same satellite k by the baseline distance d at each epoch t.
dI (t) =
I kz_est_i (t) - I kz_est_j (t)
.
d
(4)
This parameter was thoroughly examined by processing
GNSS observations collected from the GNSS reference stations in CONUS during the 2000-2004 solar maximum period to develop the local area augmentation system (LAAS) (the
GBAS of the United States) CAT-I ionospheric threat model.
However, even within CONUS, this threat model cannot be
trusted indefinitely because it was built based upon a small
number of severe ionospheric events during a single solar
cycle. This limitation motivated development of the long-term
ionospheric anomaly monitoring (LTIAM) tool. This tool
continuously monitors ionospheric behavior over the life cycle
of GBASs, evaluates the validity of the current threat model,
and updates it if necessary [29].
The LTIAM software tool automatically gathers GNSSs
and external data from public space weather websites. This
external information is used to select potential periods of
anomalous ionospheric events. High-quality data from subsets of GNSS reference stations with short separations is
chosen and processed to compute ionospheric delays and gradients. A simplified algorithm to estimate precise ionospheric parameters was implemented to perform the estimation
procedure previously described. The tool then automatically
searches for any anomalous gradients that are large enough to
be potentially hazardous to users. A significant portion of outputs among the resulting gradients can be faulty candidates
caused by receiver-instigated measurement errors. Thus, the
selected anomaly candidates are manually validated before
reported as real anomalies. While multiple approaches are
possible depending on the details of the anomaly candidates,
the most typical method is to compare the L1/L2 carrierderived estimates (prone to semicodeless tracking errors on
L2 measurements) with the L1 CMC estimates (derived from
IEEE SIGNAL PROCESSING MAGAZINE
|
September 2017
|
101
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
Signal Processing - September 2017 - 37
Signal Processing - September 2017 - 38
Signal Processing - September 2017 - 39
Signal Processing - September 2017 - 40
Signal Processing - September 2017 - 41
Signal Processing - September 2017 - 42
Signal Processing - September 2017 - 43
Signal Processing - September 2017 - 44
Signal Processing - September 2017 - 45
Signal Processing - September 2017 - 46
Signal Processing - September 2017 - 47
Signal Processing - September 2017 - 48
Signal Processing - September 2017 - 49
Signal Processing - September 2017 - 50
Signal Processing - September 2017 - 51
Signal Processing - September 2017 - 52
Signal Processing - September 2017 - 53
Signal Processing - September 2017 - 54
Signal Processing - September 2017 - 55
Signal Processing - September 2017 - 56
Signal Processing - September 2017 - 57
Signal Processing - September 2017 - 58
Signal Processing - September 2017 - 59
Signal Processing - September 2017 - 60
Signal Processing - September 2017 - 61
Signal Processing - September 2017 - 62
Signal Processing - September 2017 - 63
Signal Processing - September 2017 - 64
Signal Processing - September 2017 - 65
Signal Processing - September 2017 - 66
Signal Processing - September 2017 - 67
Signal Processing - September 2017 - 68
Signal Processing - September 2017 - 69
Signal Processing - September 2017 - 70
Signal Processing - September 2017 - 71
Signal Processing - September 2017 - 72
Signal Processing - September 2017 - 73
Signal Processing - September 2017 - 74
Signal Processing - September 2017 - 75
Signal Processing - September 2017 - 76
Signal Processing - September 2017 - 77
Signal Processing - September 2017 - 78
Signal Processing - September 2017 - 79
Signal Processing - September 2017 - 80
Signal Processing - September 2017 - 81
Signal Processing - September 2017 - 82
Signal Processing - September 2017 - 83
Signal Processing - September 2017 - 84
Signal Processing - September 2017 - 85
Signal Processing - September 2017 - 86
Signal Processing - September 2017 - 87
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
Signal Processing - September 2017 - 158
Signal Processing - September 2017 - 159
Signal Processing - September 2017 - 160
Signal Processing - September 2017 - 161
Signal Processing - September 2017 - 162
Signal Processing - September 2017 - 163
Signal Processing - September 2017 - 164
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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