ENRI Int. Workshop on ATM/CNS. Tokyo, Japan. (EIWAC 2010)[EN­042] Development of GBAS Ionosphere Anomaly Monitor
Standards to Support Category III Operations+

M. Harris*, T. Murphy*, S. Saito***Boeing Commercial Airplanes
Airplane Systems/Air Traffic Management
Seattle, WA, USA**Electronic Navigation Research Institute (ENRI)
Tokyo, Japan

Abstract: This paper summarizes work done by Boeing and Electronic Navigation Research Institute (ENRI) to validate
proposed Ground Based Augmentation System (GBAS) requirements intended to mitigate the effects of ionosphericanomalies to support use of GBAS in Category III weather conditions. This paper describes the results from modeling andsimulation performed to explore the effectiveness of the ionospheric anomaly mitigations in the presence of multiple
ionospheric plasma bubbles that regularly occur over the geomagnetic equator. The work also deals in general with the
implications of multiple satellites being affected simultaneously by an ionospheric anomaly.
Keywords: GBAS, Category III, Ionospheric Anomaly, Standards.
additional publications [1], [9]. The philosophy is todetermine the largest errors that can persist after all the
mitigation measures (i.e. monitors, baseline siting
limitations and threat space limitations) have been applied.
Appendix A of that paper gives a detailed discussion of
the proposed mitigation elements and describes thesimulation program used to evaluate the monitor
performance in conjunction with siting constraints andthreat space assumptions. Although that work wascomprehensive in as far as it went, there were a number ofunanswered questions that needed to be addressed.

1. INTRODUCTION
Disturbances in the ionosphere can result in nondifferentially corrected errors in GBAS. Of particularconcern are disturbances that create large spatial gradientsin the delay experienced by GPS signals over a relatively
short baseline (i.e. a few kilometers). This problem has
been analyzed extensively in the course of implementing
GBAS ground subsystems intended to support Category I(CAT I) operations, i.e. Facility Approach Service Type C(FAST C). A means to mitigate the effects of suchionospheric anomalies has been proposed [1] andincorporated in the latest revision of airborne standards[3][4][5][6][7][8]. The concept is also the basis for somerequirements included in the revised airborne equipment
MOPS [3] and ICD [8]. The proposal for mitigating theeffects of the ionospheric anomalies relies on a
combination of:Recent work described in this paper addresses thequestions left unanswered by WP28 regarding theapplicability of the wedge model effects as a bound on
plasma bubble effects.
2. BACKGROUND

Monitoring in the ground subsystem,
Monitoring in the airborne segment,
Siting restriction on the ground subsystem, andA standard threat space which defines the range of
ionospheric anomalies to which the user will be
exposed.

The proposed ionospheric anomaly mitigation scheme for
GAST D has been described in detail in a number ofprevious papers [1][4].
Furthermore, specificrequirements have been added to the airborne and ground
requirements for GAST D [3][5]. The study described in[9] includes simulations of the performance of all the
mitigations taken together. The study consists of an
exhaustive computer simulation of the monitor
performance across the full threat space proposed in [10]and for all possible combinations of geometry between theuser, the runway and the ground station. Furthermore theWorking Paper 28 (Nov09/WP28) from the November
2009 ICAO Navigation Systems Panel (NSP) Category
II/III Subgroup (CSG) meeting discusses a testing
program that was done to evaluate the performance of
these standard mitigations, which was summarized in1

M. Harris, T. Murphy, S. Saito

analysis considers all possible relative motions between
the user and the ionospheric anomaly. The overall
philosophy for requirements allocation between the
ground, air and threat space for GAST D ionosphericanomalies was also presented in a companion paper
developed for the November 2009 meeting [1].

Since the November 2009 meeting, the CSG ionosphericanomaly ad-hoc group has been engaged in addressing theoutstanding questions described above.
This paper
discusses the work done and the conclusions that can be
drawn from that work.

The study in [9] shows the following:

3. VALIDATION OF IONOSPHERIC ANOMALYMONITORING FOR GAST DThe study shows that the absolute largest error thatcan persist after all the mitigations have been
applied is on the order of 2.75 meters in thepseudorange domain when considering a missed
detection probability of 1x10-9.
 The simulations show that different monitors are
effective in different parts of the threat space. The
monitors must be evaluated in concert with thesiting limitations and for a defined threat space.
 The maximum errors appear to be insensitive to theexact nature of the speed profile of the approaching
airplane. However, a speed change from typicalcruising speeds to approach speeds is assumed.
The CSG discussed Nov09/WP 28 and agreed that there
are a number of issues that required further work

A high-fidelity modeling and simulation effort wasundertaken to answer some of the issues requiring further
work after the last meeting. Since a very simplified range
domain model of the plasma bubble was used previously,
and since that model maintained some wedge model
specific assumptions, a three-dimensional ionosphere andplasma bubble model was coupled with a GPS
constellation model to gather much higher fidelity resultsthat could include the impact of more than two satellites
impacted simultaneously by multiple plasma bubbles.
This section describes the plasma bubble effect, the highfidelity modeling of the bubble and its impact on the
GAST D system, and results of the modeling andsimulation.

Real performance of the combination of thesemonitors hinges on feasibility of a very small
threshold on the ground gradient monitor. Morework is required to determine ground monitor
feasibility and perhaps to better quantify RAIM FD
performance.

3.1 Overview of Plasma Bubble Effect
The plasma bubble is a local ionospheric plasma depletionthat occurs in the low latitude region [1]. It accompanies
sharp gradients in the ionospheric delay. The outstanding
characteristics of the plasma bubble compared with theionospheric front (or storm enhanced density: SED)
observed over the United States are existence of a pair of
gradients with opposite sings at the east and west edges ofa plasma bubble and multiple plasma bubble appearance inseries with several hundred kilometers separation as wellas its very frequent occurrence in high solar activity.
Therefore, it is often the case that there are multiple (twoor more) gradients associated with plasma bubbles andmultiple satellites can be impacted at the same time.

The study described in Nov 09/WP 28 had been
done assessing the maximum error on a single
pseudorange. It has been proposed that translation
to position domain be accounted for by a simple
assumption on projection into the position domain
given geometry screening.
This is a very
conservative way to handle the effect of multiple
satellite impacts.
Furthermore, with plasma
bubbles, the probability of multiple satellites being
affected is non-trivial. Hence additional work is
required to determine how the impact on multiple
satellites can be addressed.

The general characteristics of a plasma bubble areillustrated in Figure 1. Since the plasma bubbles develop
upward over the magnetic equator (and hence poleward inlatitude), they are narrow in longitude, elongated inlatitude, and the east and west edges are like steep walls,
in general. It is expected that satellite-receiver paths along
the walls of plasma bubbles at low elevation angles would
be impacted the most, and those across the walls would be
less. If this speculation was justified, then the impact ofplasma bubbles on multiple satellites could be reduced (wewould not have to give the maximum errors thosesatellites).
For validating this, a two-dimensional
ionospheric delay model is not sufficient, and a threedimensional model is necessary.

Higher fidelity plasma bubble modeling is needed
to assess the impact of ionospheric anomalies that
affect multiple (e.g. 3+) satellites simultaneously.

Also at the November 2009 meeting, the CSG discussed
NSP Nov 09 WGW/WP31 [1] which showed that somesmall disturbances in the ionosphere had been observedthat appear to have a speed of propagation in excess of thelimits currently proposed as a standardized threat model[10]. The group agreed that these fast moving anomalies
should be considered and, if appropriate, the standard
threat model should be modified accordingly.

2

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan. (EIWAC 2010)

Figure 2 - Example Plot of Vertical TEC Based on NeQuick

This time and location is the general region that waschosen for the user location in these simulations.
A plasma bubble model is defined in the vertical plane in
the magnetic equator given as relative depletion comparedwith background. By defining the plasma bubble shape inthe magnetic equatorial plane and utilizing high magnetic
conjugacy of the plasma bubble, the plasma bubble shape
can be modeled in the 3-D space. So far, a very simple
plasma bubble shape, a rectangle in the magnetic
equatorial vertical plane, is used as illustrated in Figure 3.
The plasma bubble moves zonally at a constant velocity.
During the drift it is assumed to keep the same shape. A
tilted-dipole magnetic field model is used to trace
magnetic field line from an arbitrary point to the magnetic
equatorial vertical plane. Parameters that have been
implemented to characterize plasma bubbles are as
follows:

Figure 1 Equatorial Plasma Bubble Ionospheric Anomaly Visualization(North-South and East-West Perspectives)

According to the discussion above, the plasma bubble may
impact multiple satellites, while impacts on some satellites
may be potentially less significant. Therefore, we
conducted exhaustive simulations with a threedimensional ionospheric delay model to evaluate how
many satellites can be impacted at the same time and how
significant the impact is.



3.2 Three Dimensional Modeling of Plasma Bubbles
The three-dimensional ionosphere delay model consists ofa background ionospheric model and a plasma bubble
model. NeQuick was used as a background ionosphericmodel, which is an ionosphere model that is used tocompute plasma density distribution between two arbitrary
points, namely the GPS receiver and GPS satellites[12],[13],[14]. The earth's magnetic field is approximatedby a dipole model. The background ionosphere ischaracterized by the following parameters:









Number of plasma bubbles
Positions at the magnetic equator
Maximum depletion level relative tobackground
Height at the magnetic equator
Zonal width
Scale length of the plasma bubble boundary
Zonal drift velocity

theIonospheric delays are calculated for a ground station
fixed on the ground and an aircraft flying toward theground station at a constant velocity. In our analysis,
aircraft velocity of 80 meters per second is adopted. The
output parameters from the model are as follows:

Geographic latitude, longitude, and altitude
Date and time in UT
Solar activity in terms of the solar radio flux index(F10.7) or the sunspot number indexFigure 2 shows an example global vertical Total ElectronCount result from the NeQuick model that is used torepresent the background ionosphere for this analysis [15].
Note that the largest TEC values happen at a particulartime of day at the magnetic equator.


3Satellites’ index in the field of view of the groundstationSatellite position in the ECEF coordinate
Non-carrier-smoothed delays for the ground stationand the aircraft
Difference between carrier-smoothed delays
between the ground station and the aircraft.

M. Harris, T. Murphy, S. Saito

measurements into the vertical and lateral components ofthe position solution. DV and DL refer to the difference
between the 30-second and 100-second smoothed position
solutions in the vertical and lateral.
Airborne and Reference Ionosphere Monitors:

Airborne satellite geometry screeningoSvert, Slat, Svert2, Slat2, DV, DL, VPLHO, HPLHOAirborne RAIM fault detection

Airborne code-carrier divergence monitor

Reference code-carrier divergence monitor

Reference absolute gradient monitor

3.4 Results of Plasma Bubbles Analysis
This section describes the range of parameters considered
to develop simulation scenarios, assumptions made, andfinally the simulation results from the scenarios.
3.4.1 Scenarios and Assumptions
A range of scenarios were simulated to exhaustively
search relative airplane, bubble, and satellite motion tofind the worst case GAST D position errors. Figure 4
illustrates the model parameters that were varied during
the study. A 100km wide, 1500km tall depletion region
with 20km transition regions was used in a highly active
ionosphere environment since this combination of thesmallest observed plasma bubble dimensions among thelargest background ionosphere delays was believed tohave the potential to produce the largest differential
position errors. Three plasma bubbles were included in
the simulation with the second and third plasma bubbles atfour degrees longitude East and West of the center bubble,
and having the same dimensions and speed. The airplane
speed was assumed to be a constant 80m/s. A constant
airplane speed was expected to produce the worst casescenario since no benefit could be derived from the dual
solution monitor during airplane speed changes. The
airplane approach direction was varied in 90 degreeincrements, and three latitudes were chosen for thereference location: 0, 22, and 30 degrees magnetic
latitude.

Figure 3 Simple Plasma Bubble Model

3.3 Ionospheric Anomaly Monitoring Algorithms
As described in the previous section, outputs of interest
from the three-dimensional plasma bubble simulations are
the airplane, reference, and satellite locations, and the noncarrier-smoothed ionosphere delay for each visible
satellite at both the airplane and the reference locations.
The ionosphere code phase delay for each satellite and an
opposite carrier phase advance are input into the carriersmoothing algorithm to produce a carrier-smoothed-code(CSC) error due to the ionosphere only. This is done foreach satellite at the airplane and reference locations using
both 100-second and 30-second smoothing time constants.
Differential range error (DRE) due to the changing
ionosphere is generated as the difference between the CSC
errors at the airplane and reference receivers for bothsmoothing time constants.
The differential range error is saved for each satellite, andthen the search for worst case scenarios begins. First therange domain ionosphere monitor outputs are determined,
then a search is performed of all possible subset satellite
geometries and their position domain geometry screensand ionosphere anomaly monitors. For the purposes ofsimulation, the ionosphere anomaly monitors are declared
to detect after the ionosphere errors exceed the monitor
threshold and an additional margin for measurement noisethat is dependent on a desired missed detection probability
for analysis. The ionosphere anomaly monitors are listed
below; please see [9] for more detail on theimplementation of these monitors in the simulation. The
parameters Svert and Slat refer to the projections of range

Simulation parameter search ranges:






4

Approach direction (degrees heading)90, 180, 270, 360 (80m/s)
Reference location (degrees latitude)0, 22, 35 (135 East Longitude)
Plasma bubble speed (East meters per second)-100 : 10 : 200
Plasma bubble phase (t = 0 latitude, center bubble)
132.6 : 0.1 : 136.9 ( f(speed) )Satellite constellation phase (simulation start time):
24hrs at 10-minute intervals

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan. (EIWAC 2010)(additional plasma bubbles +/- 4 degrees
longitude relative to the center bubble)

Vertical Position Error (m)VAL

100m/s

W=100km
T=20km80m/s
HE=1500km+133.62E+134.6E

Reference

Magnetic Equator+135E

depletion

background ionosphere(highly active)

GPS Time (s)Simulated GPS Satellite Geometry

Figure 5 - Vertical Position Error versus Time for All Subset Geometries
for an Example Plasma Bubble Scenario with No Geometry Screening
and No Ionospheric Anomaly Monitoring Compared with Vertical Alert
Limit

Figure 4 - Visualization of the Plasma Bubble Simulation Scenario

3.4.2 Simulation Results
Figure 5 through Figure 7 show example time histories ofvertical ionospheric errors for each possible subset
geometry for a single scenario. Figure 5 illustrates thevertical position errors that could result from the plasma
bubbles for all possible geometries with four or moresatellites out of the full set of satellites in view versus time
and the VAL given that no geometry screening or
ionospheric anomaly monitoring is performed. It is clearthat the plasma bubble can impose a risk for some satellite
geometries.

Vertical Position Error (m)VAL

Next, Figure 6 illustrates the impact of simply screeningout geometries using modest alert limits and projection
limits, although this is merely an example, so the overall
effectiveness of geometry screening will be shown later inthis paper. Finally, Figure 7 shows that, for this example,
airborne and reference CCD monitoring easily mitigates
the effect of the plasma bubble. The maximum error thatremains unmitigated at the threshold or touchdown at a
distance, X, from the reference was chosen as a metric that
could be collected for each scenario in an ‘exhaustive’
search of the trade space. The example chosen and shownin Figure 5 through Figure 7 leaves approximately two
meters of vertical error unmitigated. Next, results for allof the scenarios are summarized.

GPS Time (s)Figure 6 - Vertical Position Error versus Time for Only Those Subset
Geometries Passing Geometry Screening Tests for the Example Plasma
Bubble Scenario

Vertical Position Error (m)VAL

Worst Errors Remaining at Threshold(X km from Reference)

GPS Time (s)Figure 7 - Vertical Position Error versus Time for Only Those Subset
Geometries Passing Geometry Screening Tests and Airborne andReference CCD Monitors with 10-9 Pmd for the Example Plasma Bubble
Scenario5

M. Harris, T. Murphy, S. Saito

As described in the previous section, four approachdirections were considered, along with multiple missed
detection probabilities (Pmd) and geometry screening.
Figure 8 summarizes the simulation results that have beencollected to date for the 090 approach direction at a
distance of 5.6 kilometers from the reference location.
The results have been narrowed to those vertical position
errors with Pmd = 10-9 and for the worst case user location
at each plasma bubble speed for three different
assumptions about geometry screening. Three plots show
the effect of increasingly strict limits on the total verticalprojections of the two most influential satellites in thegeometry (e.g. Svert2 < 6, 5, and 4).

Figure 9 shows the same results as Figure 8 except that
scenarios with gradients larger than 500 mm/km are notincluded. If larger gradients are considered feasible, thenfurther work could be done by the service provider todetermine if additional constraints on the GBAS
installation are necessary, such as additional ionospheremonitoring, satellite geometry screening, or siting
restrictions, for example.

The results in red represent the scenario with a reference
location at 30 degrees North latitude. Errors up to 16
meters in these cases result from the fact that a very
extreme background ionosphere was assumed. This
results in up to 50 meters of slant range ionosphere delaythat could transition to nearly zero ionosphere delay over a
horizontal distance of 20 kilometers, which translates into
a 2500 mm/km ionosphere gradient, which is well above
the 500mm/km that was assumed feasible by the draftchanges to the ICAO SARPs.

Figure 9 - Maximum Vertical Position Error at D=5.6km from Reference
versus Reference Station Magnetic Latitude (0=blue, 22N=green,
30N=red), Plasma Bubble Ground Speed and Geometry Screening after
Air CCD (dot) and Reference CCD Monitoring (circle), Dual Solution
Monitoring, and Absolute Gradient Monitoring at the Reference Station,
All Cases With Max Gradient < 500mm/km

4. CONCLUSIONS
The analysis presented above, in conjunction with therange domain analysis based on the Ionospheric Wedge
model presented in [9] supports the following conclusions:1.

Figure 8 - Maximum Vertical Position Error at D=5.6km from Reference
versus Reference Station Magnetic Latitude (0=blue, 22N=green,
30N=red), Plasma Bubble Ground Speed and Geometry Screening after
Air CCD (dot) and Reference CCD Monitoring (circle), Dual Solution
Monitoring, and Absolute Gradient Monitoring at the Reference Station

6

The maximum vertical or horizontal position errors
induced by an ionospheric anomaly that will persist(with a probability of greater than 10-9) after all theionospheric anomaly mitigations have been applied
can be limited to less than 10 meters. Errors on theorder of 10 meters or less have been shown to resultin an airplane still landing in the safe landing box

ENRI Int. Workshop on ATM/CNS. Tokyo, Japan. (EIWAC 2010)

2.

3.

4.

5.

[5] ICAO NSP/CSG, “Flimsy 9: GAST D GBAS SARPS[1]. The maximum error can be reduced somewhat
by using more aggressive geometry screening.
The ground gradient monitor plays an importantrole in limiting the error magnitudes when anomaly
speeds are near the aircraft pierce point speeds.
A 5 km baseline siting restriction appears toprovide adequate performance. Although somepossibility to relax this restraint still exists, it isrecommended that 5 km be adopted as the baseline
and that future work be undertaken during theoperational validation phase to determine if thissiting restriction can be relaxed or if additional
siting flexibility can be achieved in some other way.
The general rule that the maximum error in thepseudorange domain of 2.75 meters (postulated in[9]) appears to hold for plasma bubbles as well as
This maximum error
the wedge model.
characterization can be used in the formulation of a
fault model for use in airworthiness assessments.
The conservative approach of accounting for
multiple satellite by geometry limiting based on
Svert2 or Slat2 appears valid since no more severe
effects have been found using the high fidelity 3-D
plasma bubble in conjunction with a satellite
geometry simulation than were found with thepseudorange domain wedge model as scaled by
Svert2 limits.

Draft - Baseline Post Seattle Meeting,” presented by
Tim Murphy, Seattle, WA, July 2009.
[6] NSP/CSG/Toulouse, July 2008, WP-12 “Proposed

SARPS Changes for GAST-D”.
[7] NSP/WGW/Montreal, Oct 2008, WP-XX “Proposed

SARPS Changes for GAST-D”.
[8] RTCA DO-246C – "GNSS Based Precision Approach

Local Area Augmentation System (LAAS) - Signalin-Space Interface Control Document (ICD)".
[9] ICAO NSP WGW/WP 28, " Validation of GAST D

Mitigation of Errors Induced by Ionospheric
Decorrelation ", Montreal, November 2009.
[10] ICAO NSP WGW/WP 29, " Standard Threat Model

Used in GAST D Ionospheric Monitoring Validation", Montreal, November 2009.
[11] ICAO NSP Nov 09 WGW/WP31, " GBAS

Ionosphere Threat Model Validation in Germany",Montreal, November 2009.
[12] S. Saito, T. Yoshihara, and N. Fujii, “Study of Effectsof the Plasma Bubbles on GBAS by a ThreeDimensional Ionospheric Delay Model,” Proceedingsof ION GNSS 2009, Savannah, GA, USA, Sept. 2225, 2009.
[13] G. di Giovanni, G. and S. R. Radicella, “An analyticalmodel of the electron density profile in theionosphere”, Adv. Space Res., 10, 27-30, 1990.

As mentioned above, as a part of the operational validation
phase, the full complement of planned simulations will becompleted to verify that no unexpected large errors result
from scenarios not yet modeled.

[14] S. M. Radicella, and M. L. Zhang, “The improved

5. ACKNOWLEDGMENTS[15] Nava, B., et. al., “A new version of the NeQuick

DGR analytical model of electron density heightprofile and total electron content in the ionosphere”,
Annali di Geofisica, 38, 35-41, 1995.
ionosphere electron density model”, Journal of
Atmospheric and Solar-Terrestrial Physics, 70, 18561862, 2008.

The authors would like to thank the ICAO Navigation
Systems Panel Category II/III Sub Group for their review
and inputs regarding this work, particularly all of those
who participated in the ionospheric anomaly ad-hoc group.

[16] Murphy, T.; Anderson, L.; Harris, Matt; Tang, N.

“CAT III Simulated Landing Performance for ILS and
GLS”, Proceedings of the ION GPS 2001, Sept 2001,
Salt Lake City.

6. REFERENCES[1] ICAO NSP/WG1 WP 26, "Mitigation of Iono

Gradient Threat for GSL D", New Delhi, March 6th
2007

7. COPYRIGHT[2] Harris, Matt; Murphy, Tim; “Putting the Standardized

GBAS Ionospheric Anomaly Monitors to the Test”,
Proceedings of the ION GPS 2009, Dallas, TX, USA,
Sept 2009.

The authors confirm that they, and/or their company or
institution, hold copyright of all original material includedin their paper. They also confirm they have obtained
permission, from the copyright holder of any third party
material included in their paper, to publish it as part oftheir paper. The authors grant full permission for the
publication and distribution of their paper as part of the
EIAC2008 proceedings or as individual off-prints from theproceedings.

[3] RTCA DO-253C "Minimum Operational

Performance Standards for GPS Local Area
Augmentation System Airborne Equipment", Dated
Dec 16 2008.
[4] ICAO NSP WGW/WP29 " Ionospheric Anomaly

Detection Requirements for GAST D" March 09.
7