Ionosphere effects on GNSS positioning: data collection, models and analyses

João Francisgo Galera Monico, Paulo De Oliveira Camargo, Haroldo Antonio Marques, Heloisa Alves Da SilvaUNESP – FCT – Presidente Prudente, SP.

Bruno BourgardSeptentrio NV, Leuven.

Luca SpogliINGV, Rome.

Outline• Infra-structure available for GNSS research and

applications in Brazil• GNSS Services required in Brazil

• Brazilian Ionospheric Model– Mod_ION– Rinex_HO

• CIGALA Project– Objectives– Preliminary results

• Final Comments

Troposphere / GNSS Met

Precise Agriculture

Available Infra-structurein South America/Brazil

SIRGAS GNSS data

• SIRGAS-CON GNSS Network

Brazilian GNSS data (IBGE/INCRA)

• Brazilian Continuous GPS Network (RBMC). Some stations are operational since 1996

• ~100 stations

RBMC Real Time – RBMC_IP • Data of about 30 Brazilian

GNSS stations are distributed in real time, using NTRIP protocol.

•

GNSS/GPS Active Networkat São Paulo State – Real time data

Meteorological and GNSS stations

• Meteorological stations are required to be collocated with GNSS for GNSS/Met support– 18 are available at São Paulo State (all stations were

calibrated)

GNSS demands in Brazil

Off shore applicationsAir Navigation

Positioning in generalPrecision agriculture

Rural Cadastre (50 cm or better – 1 sigma)

….

PA in Brazil is demanding 24 hours RTK service

Concerning Air Navigation, Brazilian authorities decided to invest in GBAS

instead of SBAS.

A system from Honeywell Aerospace is under certification at Rio de Janeiro

Airport (Galeão). (Cosendey presentation on Nov 09).

Challenges for such GNSS applications

Ionospheric Scintillation!

São Paulo State Network RTK (VRS)

Preliminary results.

Local Base/RTK Initialization Sart End N. points collected

TUPÃ

VRS (GNSS) 1min 24 seg 13:07:01 as 13:18:17 205ARAC (GNSS) 84,13km 8 min 4 seg 13:24:52 as 13:43:41 205

VRS_S (GPS) 2 min 23 seg 13:47:55 as 14:08:33 205ARAC_S (GPS) 84,13 km 12 min 19 seg 14:18:35 as 14:44:30 205

Ionospheric Index (I95) based on São Paulo

State GNSS Network

Developments on GNSS/Ionosphere at FCT/UNESP

GNSS and Ionosphere• A Ion-model based on GNSS has been under

development at FCT/UNESP since 1997;– Mod_Ion (in-house iono model) generates Ionospheric

maps and coefficients for L1 users• Ionospheric Index (Fp)• Ionex files from Brazilian GNSS data • Real time ionosphere maps of TEC/ROT and of the

correspondent delays on L1 (Aguiar – presentation on Nov 9th).

n 4 m 4s s s s s s

1 2 j j 1 n*2 3 j j 1i 1 i 1j 2i 1 j 2i 10

VTEC a a B {a cos(i h ) a sin(i h )} a h {a cos(i B ) a sin(i B )}

2 2 2i 2i 1i 2iF f /(f f )

n 4s s s s s s s2 1 r r 1 2 j j 1 n*2 3

i 1j 2i 1

m 4s s s s

j j 1 P2 P1 r P2 P1 P21i 1j 2i 10

F(P P ) SF (a a B {a cos(i h ) a sin(i h )} a h

{a cos(i B ) a sin(i B )} F(R R ) F(S S ) F .

TEC s s TEC s s TECi 2r 1r i p2 p1 p2 p1 r i P21s

r

VTECF (P P ) + F [(S -S ) + (R -R ) ] + Fcos(z )

• • = > i = G, Rk

Ionospheric Regional Model (MOD_Ion) (GPS & GLONASS)

Mod_Ion with inequality equation

• Problem: at some situations, even with calibrated equipments, negative values of TEC are obtained.

• One solution: to apply inequality equation as follows:n 4

s s s1 2 j j 1

i 1j 2i 1

m 4s s s

n*2 3 j j 1 i 1j 2i 10

VTEC a a B {a cos(i h ) a sin(i h )}

a h {a cos(i B ) a sin(i B )} 0

GNSS Ionospheric Products

• TEC Maps

IONEX Files

2nd and 3rd order Ionosphere corrections• In-house software was developed (RINEX_HO)

• GPS Solutions, Online First: 21 April 2011, DOI: 10.1007/s10291-011-0220-1, "RINEX_HO: second- and third-order ionospheric corrections for RINEX observation files"  by H. A. Marques, J. F. G. Monico and M. Aquino

2nd and 3rd order Ionosphere corrections• The earth’s magnetic field

– Dipolar Approximation– International Geomagnetic Reference Field (IGRF) model

(IGRF11 model)– Corrected Geomagnetic Model from PIM (Parameterized

Ionospheric Model)

• TEC– From raw pseudoranges, from pseudoranges smoothed by

phase, or from Global Ionosphere Maps (GIM).

2nd order Ionosphere correctionsBipolar – IGRF and Differences

CIGALA Project“Concept for Ionospheric scintillation mitiGAtion for professional GNSS in Latin America”

Goal: Understand the cause and implication of IS disturbances at low latitudes, model their effects and develop mitigations through:– Research of the underlying causes of IS and the development of state-of-the-art models

capable of predicting signal propagation and tracking perturbations– Field measurement via the deployment in close collaboration with local academic and

industrial partners of multi-frequency multi-constellation Ionospheric Scintillation Monitoring (ISM) network

– Design and implementation of novel IS mitigation techniques in state-of-the-art GNSS receivers

– Field testing the mitigation techniques, leveraging the same partnership as during the measurement campaign.

CIGALA partners

• 8 ISM stations• Latitudinal and longitudinal

distribution over Brazil• Two stations at São José dos

Campos (crest of EIA) and Pres. Prudente

• Data stored locally and sent to repository at UNESP, Pres. Prudente

• Data mirrored at INGV, Rome

IS Monitoring Network in Brazil

CIGALA IS Monitoring Network in BrazilContinuous recording of :

• Amplitude scintillation index S4 : standard deviation of received power normalized by its mean value

• Phase scintillation index σΦ : standard deviation of de-trended carrier phase, with Phi60 its 60” version

• TEC (Total Electron Content)• Lock time• Code – Carrier Divergence• Spectral parameters of phase Power Spectral Density:

– Spectral slope p– Spectral strength T

• Raw high-rate I&Q correlation values (50Hz)

Septentrio PolaRxS ISM receiver is the base of the CIGALA network

(c) CIGALA Consortium

PolaRxS: facts Track GPS, GLONASS, GALILEO, COMPASS, SBAS L1, L2, L5, E5a, E5b signals, including GPS L2C, GLONASS

L2C and Galileo E5 AltBOC Very low phase noise OCXO 100Hz signal intensity and phase output for all signals Computation of S4, sf , TEC, spectral parameters,... for all

satellites and signals Interoperable ISMR file format Multiple Interfaces: 4 RS232, USB, Ethernet Rugged IP65 housing Temperature range: -40C to 60C Powering: 9-30V ; 6W

PolaRxS Phi60 Noise Floor <0.03rad

24-h Spirent simulation, Perfect GPS signal, L1

Receiver optimize for Maximum Tracking availability during Strong Scintillation

Optimized ISM receiver

0.40.6

0.81

5

10

15

200

10

20

30

40

50

60

S4 levelPLL bandwidth [Hz]

Loss

-of-l

ock

prob

abilit

y [%

]

0.40.6

0.81

5

10

15

200

2

4

6

8

10

S4 levelPLL bandwidth [Hz]

Loss

-of-l

ock

prob

abilit

y [%

]

Normal Receiver

Simulated with CSM on Spirent

Data bearing signals

Receiver optimize for Maximum Tracking availability during Strong Scintillation

0.40.6

0.81

5

10

15

200

10

20

30

40

50

60

S4 levelPLL bandwidth [Hz]

Loss

-of-l

ock

prob

abilit

y [%

]

0.40.6

0.81

5

10

15

200

2

4

6

8

10

S4 levelPLL bandwidth [Hz]

Loss

-of-l

ock

prob

abilit

y [%

]

Optimized ISM receiverNormal Receiver

Simulated with CSM on Spirent

Pilot Signal (L2C)

Comparison with currently deployed GSV equipment

• Scintillation free mid-latitude location (Nottingham)

• GPS L1CA• 24h recording

• S4: correlation coefficient = 0.9

• Phi60:– PxS: 0.0292– GSV: 0.0547

PRN19

Field Validation (C/N)

• CIGALA receivers PRU1 and PRU2 at Presidente Prudente• February to April 2011

L1 L2

Field Validation (CCSTDDEV)

• CIGALA receivers PRU1 and PRU2 at Presidente Prudente• February to April 2011

L1 L2

Using GLONASS for IS monitoring

• GPS and GLONASS orbits are complementary to increase spatial and temporal observability of the ionosphere

• GLONASS provides open signals on both L1 and L2 in all SV

Moderate Scintillation Occurrence (S4) observed using GPS vs. GLONASS

• INGV GBSC software is used to draw maps of rate of occurrence of S4>0.25 as a function of lat/long or lat/time

• Maps plotted for L1 observations between Feb and April 2011• Increased probability of scintillation clearly observable in EIA post-sunset• Very good match between GPS and GLONASS observation => data can be merged

GPS GLONASS

EIA EIA

Moderate Scintillation Occurrence (Phi60) observed using GPS vs. GLONASS

GPS GLONASS

• INGV GBSC software is used to draw maps of rate of occurrence of Phi60>0.25 as a function of lat/long or lat/time

• Maps plotted for L1 observations between Feb and April 2011• EIA observable for GPS• No match GPS and GLONASS observations

Understanding lack of Phi60 observability when using GLONASS signal

• Short term stability of the GLONASS satellite clock lower than GPS

• Small scale phase scintillation cannot be measured from single frequency observation

• Solution: Using differenced L1/L2 measurement to cancel the satellite clock effect

Strong Scintillation Event on Sept 25, 2011

S4 During Scintillation

0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1

1.2

1.4

UTC time [hours]

S4 fr

om L

1CA

(elev

atio

n m

ask

of 2

0 de

g)PRU2, Sep-25, 2011

0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1

1.2

1.4

UTC time [hours]S4

from

L2C

(ele

vatio

n m

ask

of 2

0 de

g)

PRU2, Sep-25, 2011

PRN2PRN15PRN26

PRN15

• S4 reported continuously during scintillation• S4 in L2 reported thanks to PRN15 (L2C) pass

L1CA L2C

SigmaPhi during Scintillation

0 0.5 1 1.5 20

0.5

1

1.5

UTC time [hours]

Phi6

0-L1

CA [r

ad] (

elev

atio

n m

ask

of 2

0 de

g)

PRU2, Sep-25, 2011

0 0.5 1 1.5 20

0.5

1

1.5

UTC time [hours]Ph

i60-L

2C [r

ad] (

elev

atio

n m

ask

of 2

0 de

g)

PRU2, Sep-25, 2011

PRN2PRN15PRN26

PRN15

L1CA L2C

• sphi reported continuously on ISM optimized receiver

Tracking robustness (Cycle Slips)

1500 2000 2500 3000

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

UTC time [hours]

Detre

nded

L1

carri

er p

hase

[cyc

les]

PRU2, PRN15, Sep-25, 2011

carrier phasenav bit error

Phase tracking continuous during the whole event despites the very high S4 level

3 cycles slips seen on L1CA (PRN15) No cycles slips on L2C!

Effect on Real Time Precise Point Positioning

0 0.5 1 1.5 2 2.5 30

0.2

0.4

0.6

0.8

1

1.2

1.4

UTC time [hours]

S4 (e

leva

tion

mas

k of

20

deg)

PRU2, Sep-25, 2011

PRN2PRN15PRN26

0 0.5 1 1.5 2 2.5 3432.6

432.8

433

433.2

433.4

433.6

433.8

434

UTC time [hours]

PPP

Heig

ht [m

]

PPP service continuous during the whole eventUp to 40cm error during event(service specification is 12cm 95%)

• Brazil is a very challenge place for GNSS applications, mainly due to the Ionosphere behavior in the equatorial region;

• Several applications are already suffering the effects of such problem (IS) and will increase in the next two years;

• In the PA and aviation there is a need for more developments and tests;

• CIGALA network will continue collecting data after the final of the project (March 2012) and may provide data for scientific purpose.

Final comments

More information?http://gege.fct.unesp.br