Satellite Based Augmentation SystemsSatellite Based Augmentation SystemsBrazilian Ionosphere GroupBrazilian Ionosphere Group

Training at Stanford UniversityTraining at Stanford UniversityOctober 27-30, 2003October 27-30, 2003

MODULE 1:

CONUS VERSUS GLOBAL IONOSPHERE

Part A: Midlatitudes

CONUS VERSUS GLOBAL IONOSPHERE CONUS VERSUS GLOBAL IONOSPHERE Part APart A

An introduction to the ionosphere

Understanding ionospheric basics so that the concerned ionospheric phenomena can be understood

Creation of the ionosphere, and the effects of solar zenith angle and ionospheric dynamics

This modulecovers:

Why?

Topics

Introduction to the IonosphereIntroduction to the Ionosphere

Introduction to the ionosphere Upper atmosphere Ion and electron production due to photoionization and particle

precipitation Chemical loss Ionospheric dynamics under the control of the intrinsic geomagnetic

field

References Introduction to Ionospheric Physics [Rishbeth and Garriott, 1969] Geophysical Handbook [Air Force Research Laboratory, …] The Earth’s Ionosphere [Kelley, 1989] Ionospheres [Schunk 2000]

Earth’s Upper AtmosphereEarth’s Upper Atmosphere

PhotoionizationPhotoionization

ehv

ehv

hv

22

22

2

O)1026(O

N)796(N

eN)911(OPhoton from the Sun

O+e-

Oatomic (or molecular) gas

electron ion

h

oxygenehv

ehv

NN)510(N

OO)662(O

2

2

Examples of Dissociative Ionozation

Examples of Photoionozation

Ionization Threshold Energies

ProductionProduction

Normalized Chapman production function versus reduced height z, parametric in solar zenith angle . [Rishbeth and Garriot, 1969]

h

h H

dhz

0

h0: reference height

h

a dhn sec

Optical Depth

sec1exp)(

)(exp)(),(

z

i

ezzeH

zznzq

Chapman Production Function

Layers of IonizationLayers of Ionization

• Electron production profiles by solar irradiances at the EUV band

• Radiation at different wavelengths contributes to the creation of E- and F-layers

• For SSN = 60– X(E), 8 – 140 Å– UV(E), 796 – 1027 Å– E = UV(E) + X(E)– F, 140 – 796 Å– E+F, 8 – 1027 Å

Ionization due to Aurora PrecipitationIonization due to Aurora Precipitation

Computed ionization rates for O+, O2+, and N2

+, respectively, due to precipitating charged particles at various energy levels

Loss of Ions and ElectronsLoss of Ions and Electrons

OHHO

NOON

NOON

NNOON

NNONO

OOOO

2222

22

2

2

22

f

r

k

k

ONNO

NNN

OOO

*

*2

*2

e

e

e

hve *** OOO

Charge Exchange Radiative Recombination (slow)

kf /kr 1.13

)nm630()P(O)D(O*O 31 hvAirglow Emission (red line)

k ~ 10-12 (250/Te)0.7

K ~ 10-7(300/Te)0.5

Dissociative Recombination

(k ~ 10-10)

ii nt

n

Ionospheric Dynamics - CIonospheric Dynamics - C

• Ionospheric plasma motions under the control of the geomagnetic field

– ExB drift– Neutral wind drag – Diffusion– Collision vs. gyro-rotation

Collision frequency Gyro-frequency

– F-region and E-region

iiiii nnPt

nv

Ionospheric DynamicsIonospheric DynamicsDynamo Electric FieldsDynamo Electric Fields

Vertical DriftZonal Drift

High Latitude Plasma ConvectionHigh Latitude Plasma Convection

Electric Potential

Plasma EPlasma EB DriftB Drift

• Motions of electron and ion under an external electric field (E)

– In the same or opposite direction of E

• Gyro-rotation– The direction of motion of a

charged particle is under control of magnetic field (B) so that the particle gyro-rotates around the B field

• Motions under both E and B fields– Both electrons and ions move in

the E B direction• In the ionosphere, ion-neutral

(mostly) and electron-neutral collisions also affect motions of charged particles

• The effects due to the collisions compete with gyro-rotations, and the superior determines the motion directions

562.1836

,,

e

p

ie

ie

ieie

m

m

m

m

m

eB

Gyro-frequencies:

At 180 km:i (O+) ~ 220 Hz, in ~ 10 Hz

Ionospheric Dynamics - BIonospheric Dynamics - B

• Thermospheric wind– Tidal forces: solar heating– HWM model

Um and Uz in 2-D Um and Uz in 1-D

Plasma Motions Controlled by B-FieldPlasma Motions Controlled by B-Field

• In regions where >> in (F region), plasma move in directions either perpendicular to magnetic field B (in the EB direction) driven by electric field E, or parallel to B driven by horizontal wind vn (gradient of pressure pi is not included)

B

vn

vi,∥

vi,up

I

)sin()cos(vv

)cos(vv

,

,

II

I

nupi

nIIi

Dynamical Effects Dynamical Effects

• Plasma move into different regions where the lifetime of the plasma changes due to the altitude-dependent chemical loss processes

• As the plasma move into a different region, dominant effects change

– Example: F-layer rises, plasma on the bottom side leave from a chemical-dominant region and enter a region where diffusion dominates

Fluid Dynamic Equations Fluid Dynamic Equations for the Ionospherefor the Ionosphere

Mass and Momentum Conservation

UvBvEgvv

UvBvEgvv

v

eeneeeeeeeeeeeee

iiniiiiiiiiiiiii

iiiii

nmqnnmkTnt

nm

nmqnnmkTnt

nm

LPnt

n

Ionization, chemical loss, and dynamicsIonization, chemical loss, and dynamics

e

i

ie

ie

ii

i

ii

i

ii nn

TTTT

TkT

m

kTnn

nD

B

kknL

ChHnP

iii

i

i

kkkk

ak

jj

ij

||||||||

||||||

2

2221

)/()(

1)(1

]O[]N[)O(),O()O()O(

)()(exp]O[)()()O(

gUv

BEv

Neutral, Ion, and Electron DensitiesNeutral, Ion, and Electron Densities

Middle Latitudes

Seasonal Variations at Mid-LatitudesSeasonal Variations at Mid-Latitudes

Seasonal Variations at Low-LatitudesSeasonal Variations at Low-Latitudes

11 Year Solar Cycle11 Year Solar Cycle

• Solar activity caries from minimum to maximum with a 11-year cycle

• During years of high sun-spot number years, solar radiation enhances at most of its spectrum, including solar flares and coronal mass ejections

• Increased solar activities directly affect ionospheric densities through photoionization and coupling of magnetosphere, ionosphere, and thermosphere, which gives rise to ionospheric disturbances

Global IonosphereGlobal Ionosphere

CONUS VERSUS GLOBAL IONOSPHERE CONUS VERSUS GLOBAL IONOSPHERE Part BPart B

The mid-latitude ionosphere and storms

Context for understanding ionospheric algorithms applied to WAAS

Understand why low-latitude algorithms will differ from WAAS algorithms

Ionospheric structure and behavior over US

Quiet versus storm-time behavior at mid-latitudes

This modulecovers:

Why?

Topics

Electron Density Profiles at Mid-LatitudesElectron Density Profiles at Mid-Latitudes

• Altitude profiles of the ion composition and ne measured using incoherent scatter radar at Arecibo

• Daytime: top panel

• Nighttime: bottom panel

• Peak at ~300 km

nnee Diurnal and Latitudinal Variations Diurnal and Latitudinal Variations

• ne profiles versus UT measured using incoherent scatter radar at Arecibo (Puteor Rico, LT = UT – 4 hrs) and Millstone Hill (Massachusetts: LT = UT – 4.7 hrs)

• Diurnal variations• Peak at ~300 km• Maximum in the

afternoon at ~ 2 LT• Minimum at dawn at ~5

LT• Latitudinal variations

TEC in CONUS: Nominal ConditionsTEC in CONUS: Nominal Conditions

• A snapshot of TEC derived from GPS dual-frequency observations using a ground-based GPS receiver network under nominal ionospheric conditions

• Small TEC spatial gradient allows a planar fit to represent its nominal behavior

TEC in CONUS: Storm ConditionsTEC in CONUS: Storm Conditions

• Under storm conditions, large gradient in ionospheric density and TEC can occur in the CONUS region• Storm-time ionosphere may not be well represented by a planar fit• A threat model must developed to provide warning and realistic error bound must be provided to WAAS

to protect the system from the increased errors

Corona Mass EjectionCorona Mass Ejection

Above: Helical structure in a CME observed with LASCO on June 2, 1998.

Right: The August 11, 1999, eclipse.

Sun-Earth Connection and Sun-Earth Connection and Living With a StarLiving With a Star

InteractingInteracting• Magnetic fields, plasma, energetic particles• Ionosphere and Atmosphere

VaryingVarying• Radiation, Energetic particles• Solar wind

Magnetosphere-Ionosphere CouplingMagnetosphere-Ionosphere Coupling

Geomagnetic Storms During April 2002Geomagnetic Storms During April 2002

Storm EffectsStorm Effects

• Charged particle precipitation in the auroral zone• Significant enhanced plasma convection at high latitudes• Penetration of electric fields into middle and low

latitudes– Steepened mid-latitude ionospheric trough– Storm-time Enhanced Density (SED)– Ionospheric Undulation and irregularities at subauroral

latitudes– Enhanced equatorial anomaly– Triggering of equatorial “bubbles” or irregularities and

causing scintillation• Auroral electron jet

– Joule heating and friction heating• Heating in the high-latitude thermosphere

– Traveling ionospheric disturbances (TID): positive storm effects

– Enhanced equatorward wind Positive storm effects Possibly suppressing equatorial irregularities

– Global thermospheric circulation change– Thermospheric composition change: negative storm effects

• Erosion of the plasmasphere

SEDSED

Storm EffectsStorm Effects

Negative Storm EffectsNegative Storm Effects

Positive and Negative Storm EffectsPositive and Negative Storm Effects

• Storm-time positive and negative TEC changes as well as large TEC gradient at mid-latitudes present a great challenge to WAAS

Mid-Latitude Irregularities during a StormMid-Latitude Irregularities during a Storm

MODULE 1:

CONUS VERSUS GLOBAL IONOSPHERE

Part B: Low Latitudes

CONUS VERSUS GLOBAL IONOSPHERE, CONUS VERSUS GLOBAL IONOSPHERE, Part BPart B

The low latitude ionosphere

Understand why low-latitude SBAS is challenging

The Equatorial Ionization Anomaly (EIA)

Local time behavior of the EIA

Plasma depletions (bubbles)

Scintillation

Storm versus quiet time behavior

This modulecovers:

Why?

Topics

[Placeholders][Placeholders]

• Global TEC map pointing out equatorial feature• Overlay geomag equator if possible

• Classic picture of EIA formation with arrows• TOPEX plot showing anomaly• Statistics relative to planar fit• Picture of E with pre-reversal enhancement• Post-sunset plasma instability• Picture of depletion size/scale• TEC plots of depletions –– Dehel• Depletions and scintillation• Plot of amplitude scintillation• Some statistics of scintillation• Attila storm versus quiet statistics• Summary

Equatorial Ionization Anomaly(EIA)

TOPEX AltimeterTOPEX Altimeter

• TOPEX/Poseidon satellite carries a dual-frequency radar measuring the height of sea level

• Ionospheric vertical TEC is derived from the differential delay of the signals• Vertical TEC is measured above oceans at mid- and low-latitudes for many

years

Equatorial Anomaly Shown in TECEquatorial Anomaly Shown in TEC

Low latitude ionospheric structures under nominal conditions• Large gradient and curvature: Equatorial anomaly

Dynamical Effects at Low-LatitudesDynamical Effects at Low-Latitudes

Dynamical Processes Dynamical Processes in the Equatorial Ionospherein the Equatorial Ionosphere

Ionospheric Plasma Vertical DriftIonospheric Plasma Vertical DriftIn the Equatorial RegionIn the Equatorial Region

• Averaged patterns of vertical plasma drift in the equatorial region• Plasma move upward during daytime and downward at nighttime• A pre-reversal enhancement occurs around dusk

Equatorial Anomaly Shown in Equatorial Anomaly Shown in nnee

Calculated electron contours (log10 ne) as a function of altitude and latitude at 2015 LT for equinox conditions

Equatorial Anomaly Shown in TECEquatorial Anomaly Shown in TEC

• EIA primarily appear in daytime and evening

• The peak-to-trough ratio becomes large around the dusk due to the pre-reversal enhancement in the plasma vertical drift

Seasonal Variations at Low-LatitudesSeasonal Variations at Low-Latitudes

• Plasma Bubbles and Plumes

• Electron Density and TEC Depletion

Low-Latitude Ionospheric IrregularitiesLow-Latitude Ionospheric Irregularities

Plasma Plumes at the EquatorPlasma Plumes at the Equator

Coherent scatter echoes recorded in a range-time-intensity map using the Jicamarca ISRSignals are backscattered by 3-meter ionospheric density irregularities

Fluid Rayleigh-Taylor InstabilityFluid Rayleigh-Taylor Instability

nnee Bubbles and Depletion Bubbles and Depletion at Low Latitudesat Low Latitudes

• Back scattered UHF ISR signal power indicates plasma irregularities

• The AE satellite flew through the plasma bubbles – depleted region – in the Pacific low-latitude ionosphere

• Bubbles and depletions shown in satellite ni profiles

TEC Depletion at Low LatitudesTEC Depletion at Low Latitudes

• Plasma depletion or “bubbles” were captured in GPS dual-frequency phase measurements at a equatorial site in a solar maximum year

• Large values of the rate of TEC (ROT) and rate of TEC index (ROTI, standard deviation of ROT over a time interval), derived from the same GPS phase data, indicate ionospheric irregularities

• The measurements show that the irregularities are closely associated with the plasma depletion

• The irregularities cause scintillation in GPS signals

Curtsey: FAAPlasma depletion or bubbles

Random fluctuations in rate of TEC change

Longitudinal Extension of Plasma BubblesLongitudinal Extension of Plasma Bubbles

• Incoherent scatter radar measurements of electron densities and coherent scatter echoes due to irregularities• Multiple bubbles can occur on a single night, separated by a few hundreds of kilometers in the E-W direction

Low-Latitude Low-Latitude nnee Depletion DepletionShown in Airglow EmissionShown in Airglow Emission

• Depleted region is elongated along the magnetic flux tubes

• The extension of the depleted region in the north-south direction is in the order of 103 of kilometers

• The width (in longitude direction of depleted region can be in a few hundreds of kilometers

• There can be multiple depletion strips in longitude dimension on a single night

• Depleted regions move eastward in a speed of ~100 m/s

Low-Latitude Magnetic Field ConfigurationLow-Latitude Magnetic Field Configuration

Latitude extension between 400 and 800 km

Plasma R-T InstabilityPlasma R-T Instability

• g gravitational acceleration

• L-1 gradient parameter or

scale length in ion-neutral collision

frequency F-region loss

coefficient

• vp plasma vertical drift

10

1

10

1

Lv

nn

L

Lg

p

ehe

in

Linear growth rate of plasma R-T instability

Rise of the Rise of the FF-Layer at Dusk-Layer at Dusk

• Altitude profiles of the ne measured using ALTAIR incoherent scatter radar at Kwajalein

• The ne profiles were measured during evening hours and showed rise of the F-layer

• The F-layer peak now is at ~470 km (instead of ~300 km)

• The rise of the layer continues for some time

Electron Density Profiles at Low LatitudesElectron Density Profiles at Low Latitudes

• Measured ne profiles at Kwajalein were used to the rise of the F-layer which approximately indicates the plasma vertical drift

• Top panel: ISR measurements

• Middle and bottom panels: ionosonde data

Plasma R-T Instability Growth RatePlasma R-T Instability Growth Rate

The growth rates of plasma R-T instability were computed using the ALTAIR ISR measurements of ne profiles and MSIS neutral atmospheric model

Ionospheric Scintillation

GPS Lab Tests without ScintillationGPS Lab Tests without Scintillation

• March of 1999 at JPL. t = 50-Hz T = 5-min

• S4 = 0.027 ~ 0.035 = 0.11 ~ 0.13 radians (1 cycle = 2 radians)

GPS L1 Scintillation in an Equatorial RegionGPS L1 Scintillation in an Equatorial Region

• October 26, 2000, at Arequipa (Peru) t = 50-Hz T = 5-min

• S4 = 0.18 ~ 0.45 = 0.22 ~ 0.45 radians (1 cycle = 2 radians)

Scintillation MorphologyScintillation Morphology

(Geophysical Handbook, 19xx)

Scintillation IndicesScintillation Indices

S4 I2 I 2

I 2

2 2

S4 I2 I 2

I 2 100

S N1

500

19 S N

L1-C/A • L1-C/A sampled at 20-ms• Detrended phase and intensity• Signal-to-noise ratio• 30-sec indices

GPS Scintillation in the Equatorial RegionGPS Scintillation in the Equatorial Region

L1 Amplitude Phase

Occurrence Rate

GPS Scintillation At the Equatorial AnomalyGPS Scintillation At the Equatorial Anomaly

L1 Amplitude Phase

Occurrence Rate

GPS Scintillation At High latitudesGPS Scintillation At High latitudes

L1 Amplitude Phase

Occurrence Rate

Simultaneous Strong Scintillation Simultaneous Strong Scintillation on Multiple Satellite Links on Multiple Satellite Links

• Statistics of strong scintillation events observed at an equatorial anomaly site

• Bars show how many events were observed in which strong scintillation occurred simultaneously to multiple radio links (satellites) from a single receiver

• The information provides a reference to the possibility that the number of satellite links may be lost simultaneously to a receiver under scintillation conditions

GPS L1 Signal Power Fading GPS L1 Signal Power Fading Under Scintillation ConditionsUnder Scintillation Conditions

• Recording of GPS L1 signals under ionospheric scintillation conditions has been made at low latitudes since 2000

• Signal power fading and associated duration are obtained by processing the L1 amplitude data

• The deepest fading from an equatorial anomaly region (Santiago) reaches ~ -30 dB

• Power fading at -10 dB can last longer than 1 second

• Such data sets are a useful reference to innovative design of GPS receivers

Effects of Scintillation on GPSEffects of Scintillation on GPS

• Strong amplitude and phase scintillations were measured at an equatorial anomaly site

• S4, , and ROTI characterize the scintillation activity

• The receiver lost lost at least L2 tracking of certain number of satellites

• Positioning using phase data is affected

MODULE 2:

IONOSPHERE ESTIMATION USING GPS

Part A: Measurements

IONOSPHERE ESTIMATION USING GPS, IONOSPHERE ESTIMATION USING GPS, Part APart A

Using GPS signals to measure the ionosphere

Understand purpose and operation of SBAS reference stations

Understand how ionospheric corrections are formed

Forming ionospheric measurements from GPS observables

Data quality and editing

Calibration of GPS data

This modulecovers:

Why?

Topics

[Placeholders][Placeholders]

• Material from Attila presentation on supertruth – leveling, editing, etc.• Some plots of supertruth-based data for three levels• Something showing phase vs range• How biases are removed• Leads naturally to GIM• GIM algorithm and plots

MODULE 3:

IONOSPHERIC THREAT MODEL

IONOSPHERIC THREAT MODELIONOSPHERIC THREAT MODEL

Design of a “threat model” for the ionosphere

The threat model is used to prove the system is safe under all conditions, including when the ionosphere is disturbed

Deals with the critical issue of “undersampling”

The spatial threat model – augmenting GIVE because the ionosphere is not always nominal, and reference station sampling is limited

Temporal threat model – augmenting GIVE due to ionospheric variability between transmitted updates

This modulecovers:

Why?

Topics

[Placeholders][Placeholders]

• See Larry’s material• Defer depletion discussion to Module 4

MODULE 4:

RECENT WORK ON THE EQUATORIAL IONOSPHERE

RECENT WORK ON THE EQUATORIAL RECENT WORK ON THE EQUATORIAL IONOSPHEREIONOSPHERE

Recent algorithm development and research needed to deploy a low-latitude SBAS

New algorithms are needed for vertical guidance in a low-latitude SBAS

New standards (EGOPS) must be proposed

Low-latitude data sets

New algorithms for estimating user ionospheric corrections

Current understanding of plasma depletions and expected impact

This modulecovers:

Why?

Topics

[Placeholders][Placeholders]

• Review issue of equatorial spatial gradients• Show planar fit residuals, a map and other information (e.g. Raytheon?) that

demonstrates low-latitude challenges• Show some GIVE or vertical guidance numbers

• Review challenges associated with applying thin-shell planar fit to equatorial environment

• Descrive Conical domain method• Review existence of plasma depletions• Discuss characteristics of depletions with respect to solar cycle, local time• Discuss characteristics of depletions, what is known and not known

• Discuss depletion characteristics: growth rates, lat/lon extent, bunching, etc.• Mitigation by just setting large GIVEs, or possibly in-situ detection.

• USE DEHEL PRESENTATION• Relationship of depletions and scintillation• Recent progress on scintillation

• Spacing of GEOs• Amplitude depth verus duration