Mitigating the Effects of Space Weather on the Canadian...

Geodetic Research Laboratory, Department of Geodesy and Geomatics Engineering, University of New Brunswick

MITIGATING THE EFFECTSOF SPACE WEATHER ONTHE CANADIAN WAAS

Richard B. Langley and Peter J. Stewart

Geodetic Research Laboratory

University of New Brunswick

Fredericton, N.B., Canada E3B 5A3

Session JSG28, IUGG 99, Birmingham, England, 27 July 1999


Introduction

• FAA sponsored Wide Area Augmentation System (WAAS) designedto provide en-route through precision approach navigation andintegrity information to suitably equipped aircraft

• UNB is currently working with Nav Canada to investigate atmosphericeffects on WAAS in Canadian airspace

• The airborne tropospheric model to be used in WAAS avionics wasdesigned and tested at UNB

• As we approach solar maximum, so the potential effect of theionosphere on GPS and WAAS intensifies; UNB have been chargedwith investigating ionospheric limitations on WAAS use in Canadianairspace

• CWAAS - Canadian WAAS


CWAAS


WAAS Architecture

Wide-Area Reference Sites (WRS) Wide-Area Master Site (WMS)

• All-in-View L1/L2 GPS Receiver• Carrier Smoothing• Fault Detection• Message Extraction• Vertical Iono Est

WRE

GPS/GEOS

GEOS

S,I

WRE for IV

Hot WRE for Redundancy

Same as Above

Corrected PRs, σPRsFault IndicatorsSat Message

Vertical Iono Est, σIsFault indicators Generate Grid of

Vertical Iono DelayEstimates & GIVEs

Estimate Sat Clock& Position Errors

& UDREs

GEOS Delays/GIVEs

IntegrityCheck

IndependentVerification

WAASMessage

& GEOS

Ranging

GPSGEOS Ground Link to GUS

WAAS Avionics All-in-View L1

GPS Receivers(2) • Carrier Smoothing

PR, Msg. data

WAAS Integrity• WAAS Flags• Protection Limits

Correct PR• Clock/Eph• iono/tropo

RAIM• Step Detect• FDE

Position Calculation

&Availability

StatusFlight

Control

(2 Independent Sets)

GUS

From JHU APL GPS Risk Assessment Study


WAAS Accuracy Requirements

• Accuracy requirements for WAAS are expressed in terms of thenavigation system error (NSE)

• In an operational system, the airborne GPS/WAAS receiver calculateshorizontal and vertical protection levels (HPLWAAS and VPLWAAS),which must be less than the allowed NSE with a probability of99.999% to ensure integrity

– The HPL and VPL values describe a region, centred on the trueposition, which is assured to include the indicated horizontal andvertical positions respectively

– The HPL and VPL values are computed as the sum of thevariances of the ionospheric, tropospheric, airborne receiver, clockand orbit errors


Ionospheric Delay Mitigation Techniques inWAAS

• A network of continuously operating reference receivers provides dualfrequency carrier phase and pseudorange measurements

• Line-of-sight ionospheric delay values estimated from each receiver toeach satellite– This involves estimating and removing the satellite and receiver hardware

biases

• Vertical ionospheric delay values at each of a series of ionospheric gridpoints (IGPs) are estimated along with an error bounding value (GIVE)– The surface described by these discrete grid points is at a height of 350km

– The spacing of these IGPs is latitude dependant, with a 5x5 degree grid atlatitudes less than 55N and S, a ten by ten degree grid spacing between 55and 75N and S, and 10 degrees of latitude by 90 degrees of longitudespacing above 75N and S

• Corrections for user line-of-sight delays, and a user error boundingvalue (UIVE) can then be created


Representing a Continuous Ionosphere witha Discrete Grid

• In order to minimise the WAAS link bandwidth and avionicscomputation requirements, a discrete set of IGP delays and errorbounds are broadcast to the user

• In prevailing conditions at mid latitudes, this grid system has beenshown to adequately represent the ionosphere

• During ionospheric storms, the occurrence of which will increase withsolar activity, temporal and spatial gradients, especially in theequatorial, auroral and polar zones will require significant degradationof the broadcast IGP accuracy, typical forecasts being an increase of 2-3 times for mid latitudes


WAAS Ionospheric Modelling Concept

User iono correction =W(IGP1, IGP2, IGP3, IGP4)

IGP delay = W(IPP1, IPP2, IPP3, IPP4)


WAAS Ionospheric Modelling Concept

IGP delay = W(IPP1, IPP2, IPP3, IPP4)

User iono correction = W(IGP1, IGP2, IGP3, IGP4)

corrected pseudorange = measured pseudorange Ð (user iono correction * mapping function)


Ionospheric Grid Point Validity

(Mercator Projection)

70o W

10o N

15o N

20o N

25o N

30o N

35o N

60o W80o W

W A A S S pec . &R aytheon

(3 o f 4 boxesm ust containp ierce po ints )

S tan ford N STB(one or m orep ierce po intsinside circle)

Iono . g rid po intin question

W R S p iercepo ints

From JHU APL GPS Risk Assessment Study


The Concept of GIVE

• The Grid Ionospheric Vertical Error (GIVE) is designed to put a boundon the postcorrection ionospheric vertical error at each of the gridnodes

• The GIVE value should be less than 2m 99.9 % (3.29σ) of the time

• This corresponds to a requirement of ~60cm rms accuracy at each ofthe grid points


Issues

• Potential ionospheric limitations on WAAS use over theCanadian landmass

– Grid point density

– Range error:• anecdotal evidence suggests increases in range delays of up to 10

metres within a time interval of 2-3 minutes, and return within aboutthe same time at auroral and polar latitudes during disturbedconditions at solar maximum

– Scintillation:• magnitude and frequency of occurrence of “significant” scintillations

in the auroral and sub-auroral zone

• identification of potentially problematic periods for tracking of GPSand/or WAAS signals both by user and reference receivers

• prediction of effects of increasing solar activity


Research - Past, Present and Future

• Work completed to Date– Analysis software written to produce graphical representations of pierce

point density and grid point status

– Software written to produce grid ionospheric vertical delays (GIVD) andassociated GIVE values.

• Input data is RINEX format dual frequency GPS from IGS and NSTBsites

• Work in Progress– Evaluation of the model accuracy is done via a WAAS user simulation,

receiving the “broadcast” delays and GIVEs and applying these to theusers pseudorange values

– How far north will the current network of WAAS reference sites providereliable ionospheric corrections?


150˚W

120˚W90˚W

60˚W

60˚N

60˚N

150˚W

120˚W90˚W

60˚W

60˚N

60˚N

150˚W

120˚W90˚W

60˚W

60˚N

60˚N

NSTB and IGS Station Locations

IGS

NSTB


150˚W

120˚W

90˚W

60˚W

60˚N

60˚N

2550

7510

012

5

150˚W

120˚W

90˚W

60˚W

60˚N

60˚N

255075100

125

255075100

125

Ionospheric Pierce Point DensityIP

Ps

per

50 0

00 s

q. k

m


Ionospheric Grid Point Status

150˚W

120˚W90˚W

60˚W

60˚N60

˚N

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60˚N60

˚N

150˚W

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60˚W

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˚N

available

unavailable


Scintillations

• What are the effects of rapid fluctuations of amplitude and phase of the GPSsignal on the user and service provider?

• How can the occurrence of such scintillations be monitored?

• How can the effect of such scintillations be monitored?

• Previous work has shown a distinct correlation between enhanced ionosphericactivity and losses of lock of the L2 signal.

• Since estimation of the ionospheric delay with GPS relies on utilizing thedispersive nature of the ionosphere, loss of one frequency precludes suchmeasurement.

• This presentation reviews recent work done at UNB to investigate varioussimple methods for the analysis of the spatial and temporal occurrence ofscintillation activity of sufficient strength to affect the L-band GPS signals.


FAA National Satellite Test Bed (NSTB)

• Network of dual frequency GPSreceivers, 5 of which are inAlaska.

• Two days (27 August and 13December 1998) of 1Hz dualfrequency data from Kotzebue,Fairbanks and Cold Bay wereused.

150˚W

60˚N 60˚N. Bethel

Sitka

’ Cold Bay

. Fairbanks

. Kotzebue

150˚W

60˚N 60˚N

150˚W

60˚N 60˚N


Geomagnetic Activity at College, AK

• The 1 minute mean of the total variation of the geomagnetic fieldmeasured at College, AK is shown for 25-29 August and 11-15December, 1998.

• Note the clear increase in geomagnetic activity during 27 August,compared to the surrounding days, and to the period 11-15 December.

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 1021081141205.6

5.65

5.7

5.75

5.8x 10

5

nT

Hours since midnight (UT), 25 August, 1998

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 1021081141205.6

5.65

5.7

5.75

5.8x 10

5

nT

Hours since midnight (UT), 11 December, 1998


Canadian Active Control System (CACS)

• Network of dual frequency GPS receivers using external atomic frequencystandards.

• Three days of 0.5Hz dual frequency data from CACS reference receivers atYellowknife, Churchill and Algonquin were made available, covering theperiod 17-19 February 1999.

150˚W

120˚W90˚W

60˚W

60˚N 60˚N

. OttawaAlgonquin

St John’s

Victoria . Penticton

Holberg . William’s LakeSchefferville Churchill

Yellowknife

150˚W

120˚W90˚W

60˚W

60˚N 60˚N

150˚W

120˚W90˚W

60˚W

60˚N 60˚N


Geomagnetic Activity at Yellowknife, NWT

• The 1 minute mean of the total variation of the geomagnetic fieldmeasured at Yellowknife is shown for the period 16-20 February 1999.

• Note the peak of geomagnetic activity during 18 February 1999.

0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 1021081141205.9

5.95

6

6.05

6.1

6.15x 10

5

nT

Hours since midnight (UT), 16 February, 1999


Losses of Lock on L2

• Losses of lock on the L2 carrier phase are proposed as a proxy formonitoring scintillation activity on an operational level.

• No other hardware would be required at sites already equipped withhigh quality dual frequency receivers.

– This is true of both the NSTB and CACS networks.

0 2 4 6 8 10 12 14 16 18 20 22 240

5

10

Num

ber

of S

atel

lites

SVs ExpectedL2 lost lock

0 2 4 6 8 10 12 14 16 18 20 22 240

5

10

Num

ber

of S

atel

lites

Time (UT)

SVs ExpectedL2 lost lock

Fairbanks, AK27 August 1998

Fairbanks, AK13 December 1998


0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 245.6

5.65

5.7

5.75

5.8x 10

5

nT

Time (UT)

Correlation of Losses of Lock on L2 withGeomagnetic Activity

• Taking the ratio of the number of epochs for which L2 is not tracked tothe number of observations expected in each 1 minute bin provides ameasure of the impact of ionospheric scintillations on the receiver.

27 August 1998 atFairbanks, AK


0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 245.6

5.65

5.7

5.75

5.8x 10

5

nT

Time (UT)

13 December 1998 atFairbanks, AK

Correlation of Losses of Lock on L2 withGeomagnetic Activity

• It is clear that the ratio of losses of lock to observations expected iscorrelated with the level of geomagnetic activity


Comparison with CORS Receiver at Fairbanks

• 30 second data from an8 channel TurboRogueSNR-8000 located inFairbanks was obtainedand used to provide acomparison with theNSTB Trimble datacollected on 27 August.

• Significantly bettertracking performanceappears to be the casefor the Rogue

0 2 4 6 8 10 12 14 16 18 20 22 240

5

10 CORS

Num

ber

of S

atel

lites

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 240

5

10 NSTB

Num

ber

of S

atel

lites

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Rat

io

Time (UT)


Losses of Lock on L2 at CACS Receivers

• Three consecutive days of dual frequency data from the Yellowknifereceiver were analysed for losses of lock on L2.

• The approximate repetition of the pattern of losses of lock suggeststhat these are multipath- and/or signal blockage-related rather than aresult of ionospheric activity.

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

17 FebruaryR

atio

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

18 February

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

19 February

Rat

io

Time (UT)


Rate of Change of Carrier Phase

• Given the apparent relative imperviousness of the TurboRoguereceivers used at the CACS and CORS sites to rapid fluctuations in thephase of the incoming signal, some other method of quantifying theinfluence of ionospheric scintillation activity is required.

• Differencing the L1 and L2 carrier phase removes all systematiceffects common to both frequencies:– satellite motion, satellite clocks, selective availability, troposphere

• High pass filtering removed any remaining constant and long periodeffects, and the standard deviation of 60 second bins of the data wastaken.


Sixty Second Standard Deviation of L1-L2Phase Difference

• The elevation angle cut-off was set to 15 degrees in order to partially mitigatemultipath effects.

• A comparison with the plots of geomagnetic field variation indicates that thestandard deviation of the phase difference mirrors the ionospheric activity.

• Note the residual multipath at approximately 2010 UT

15 16 17 18 19 20 210

0.01

0.0217 February

met

res

15 16 17 18 19 20 210

0.01

0.0218 February

met

res

15 16 17 18 19 20 210

0.01

0.0219 February

met

res

Time (UT)


Spatial Distribution of Scintillation Activity

• Plotted below is the L2 loss ratio for 27 August 1998 at Fairbanks, Kotzebueand Cold Bay.

• Note the following:– The correlation between the L2 loss patterns at Fairbanks and Kotzebue– The striking difference between the number of losses of lock reported at

Cold Bay compared to that at Fairbanks and Kotzebue

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Fairbanks

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Kotzebue

Rat

io

0 2 4 6 8 10 12 14 16 18 20 22 240

0.5

1

Cold Bay

Rat

io

Time (UT)


Geographical Distribution of L2 Lossesof Lock

• Maps were produced of the pierce point locations of losses of lock of L2 at analtitude of 300km.

– This height was chosen to reflect the assumed location of F-regiondisturbances which are thought to be the main source of phase fluctuationsat the GPS frequencies.

• Also plotted is the location of the Holzworth and Meng mathematical model ofthe auroral oval.

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

170˚W160˚W 150˚W 140˚W

130˚W120˚W

50˚N50˚N

60˚N60˚N

70˚N 70˚N

Poleward andequatorwardboundaries ofauroral oval

L2 tracked

L2 not tracked


180˚150˚W

120˚W90˚W

60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W90˚W

60˚W

30˚W

0˚

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60˚E

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120˚E

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E

180˚180˚

150˚W

120˚W90˚W

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0˚

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60˚E

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E

180˚180˚

150˚W

120˚W90˚W

60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚

IGS and CORS Stations

• International GPSService (IGS) andContinuously OperatingReference System(CORS) dual frequencyGPS data availablefreely over the internet

• Data distribution islimited by the locationsof these receivers, andthere are still large“holes” in coverage

IGS

CORS

IPP


Variation of Ionospheric Delay from DualFrequency Phase Observations

• Differencing the L1 and L2 phaseobservations provides a precise butambiguous measure of ionosphericdelay.

• Assuming that no cycles slips occur,the ambiguity is removed, and anaccurate measure of the rate ofchange of ionospheric delay can beobtained.

• Large variations in ionospheric delayindicate large spatial and temporalgradients

• Large spatial and temporal gradientssuggest that the satellite to receiverline of sight is passing through theauroral oval

13.5 14 14.5 15 15.5 16 16.5−2

−1

0

met

res

Yellowknife

13.5 14 14.5 15 15.5 16 16.5−4

−3

−2

met

res

Churchill

13.5 14 14.5 15 15.5 16 16.5−2

−1

0

met

res

Hours (UT)

Algonquin


Rate of Change of Ionospheric Delay

• Differencing successive epochsremoves the influence of theunknown ambiguity and the inter-frequency biases

• Data binned into ten minutesections

• Standard deviation taken in eachbin

• This parameter is then used as theinput to surface fit routine, fromwhich maps of the auroral zone arecreated

14 14.5 15 15.5 16 16.5

−0.2

0

0.2

met

res/

epoc

h

Yellowknife

14 14.5 15 15.5 16 16.5

−0.2

0

0.2

met

res/

epoc

h

Churchill

14 14.5 15 15.5 16 16.5

−0.2

0

0.2

met

res/

epoc

hHours (UT)

Algonquin


Geomagnetic Field Variation as an Indicator ofAuroral Activity

0 2 4 6 8 10 12 14 16 18 20 22 245.94

5.96

5.98

6x 10

5

18 May

nT

Time (UT)

0 2 4 6 8 10 12 14 16 18 20 22 245.94

5.96

5.98

6x 10

5

21 June

nT

Time (UT)

• Fluctuations in the local geomagnetic field occur as a result ofenhanced electric currents flowing in the auroral ionization.

• Heightened geomagnetic variability can therefore be seen as a reliableindicator of increased auroral activity.


Location of Auroral Oval from GPSObservations: 18 May 1999

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E15

0˚E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

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0˚E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

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60˚E

90˚E

120˚E15

0˚E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W

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30˚W

0˚

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150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

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0˚

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60˚E

90˚E

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E

180˚

0.0000

0.0001

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0.0010

m/s

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

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60˚E

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E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

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E

180˚180˚

150˚W

120˚W

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0˚

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E

180˚

0.0000

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m/s

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

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150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

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0˚

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60˚E

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E

180˚180˚

150˚W

120˚W

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0˚

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90˚E

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E

180˚

0.0000

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0.0010

m/s

00 00 UT

18 00 UT12 00 UT

06 00 UT


Location of Auroral Oval from GPSObservations: 21 June 1999

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E15

0˚E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E15

0˚E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E15

0˚E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s

180˚150˚W

120˚W

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30˚W

0˚

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E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

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E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

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60˚E

90˚E

120˚E

150˚

E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

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60˚E

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120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

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0˚

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60˚E

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120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s

00 00 UT

18 00 UT12 00 UT

06 00 UT


Comparison with NOAA Statistical Auroral Oval

Image provided courtesy of the U.S. Department ofCommerce, NOAA, Space Environment Center.

180˚150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚180˚

150˚W

120˚W

90˚W60˚W

30˚W

0˚

30˚E

60˚E

90˚E

120˚E

150˚

E

180˚

0.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

m/s


Conclusions

• Monitoring the rate of change of ionospheric delay with GPS showspromise as a method of locating the auroral oval

• Large spatial and temporal gradients in the auroral ionosphere canhave an effect on GPS and WAAS in two ways:– any grid model is unlikely to have high enough spatial resolution to

adequately represent an active auroral zone

– scintillation activity in the auroral zone is a potential problem, and hasbeen shown to cause losses of lock of the L2 signal

• It is therefore important to understand the spatial extent of areas whichare likely to have an effect on GPS

• Due to the higher inclination of satellites, GLONASS data could beused to augment any GPS based monitoring of the auroral zone.


Summary and Future Work

• Implementation of WAAS in Canada requires careful consideration ofionospheric effects

• Validation of WAAS ionospheric grid model a primary task

• Outline system and methodology to monitor operationalWAAS/CWAAS ionospheric modelling performance

• Contingency plan if current WAAS model proves to be insufficient

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Mitigating the Effects of Space Weather on the Canadian...

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