Evolution to Modernized GNSS Ionospheric Scintillation and ...indico.ictp.it/event/a11159/session/44/contribution/28/material/0/0.pdf · Seminar on Development and Use of the Ionospheric

2333-1

Workshop on Science Applications of GNSS in Developing Countries (11-27 April), followed by the:

Seminar on Development and Use of the Ionospheric NeQuick Model (30 April-1 May)

A. J. Van Dierendonck

11 April - 1 May, 2012

AJ Systems/GPS Silicon Valley USA

Evolution to Modernized GNSS Ionospheric Scintillation and TEC Monitoring

4/18/2012 African Workshop 2012 1

Evolution to Modernized GNSS

Ionospheric Scintillation and TEC

Monitoring

Dr. A.J. Van Dierendonck, AJ Systems


Tutorial Outline

Short Review of GPS Receivers Emphasizing what functions are affected by scintillation Emphasizing modifications implemented for measuring scintillation effects

Amplitude and Phase Scintillation Measurements Measurement Limitations

How well does the receiver perform in a scintillation environment? How can a GNSS receiver be designed to better operate in a scintillation environment?

TEC Measurements Measuring TEC or satellite and/or receiver inter-frequency biases?

Example Measurements GPS Satellites SBAS Geostationary Satellites

Multiple Frequency GNSS Receiver

Functional Block Diagram


Pre-f ilter Low Noise

Amplif ier

Down

Conv erterA/D

Conv erter

Frequency

Sy nthesizer

Automatic

Gain Control

Digital

Receiv er

Channel

1

N2

Receiv er Processor,

Nav igation Unit

RF

RFRF

Analog

IFDigital

IF

Antenna

Oscillator

(Normally

TCXO)

Measurement

Output

Repeat for Each Frequency

AGC Control

Assumption: Receiver measures carrier phase and C/N0

GNSS Receiver Modifications

for Scintillation Monitoring


Pre-f ilter Low Noise

Amplif ier

Down

Conv erterA/D

Conv erter

Frequency

Sy nthesizer

Automatic

Gain Control

Digital

Receiv er

Channel

1

N2

Receiv er Processor,

Nav igation Unit

RF

RFRF

Analog

IFDigital

IF

Antenna

Oscillator

(Normally

TCXO)

Receiver Modifications to Measure

TEC and Scintillation


Add Scintillation and

TEC Measurement

Software AGC Control

Slave to Low

Noise OCXO

Choose

Appropriate

Antenna

Measuring Amplitude

Scintillation


Typical Receiver Channel for

Amplitude (Power) Measurements


Only use Prompt

I and Q Samples for

Power Measurements

202 2

1

Pk Pk

k

WBP I Q

2 2

20 20

1 1

Pk Pk

k k

NBP I Q

Digital

IF I

Q

Sin Cos

Carrier NCO

Applicable Code Generator

Early Prompt Late

Integrate &

Dump

Integrate &

Dump

Integrate &

Dump

Integrate &

Dump

Integrate &

Dump

Integrate &

Dump

I

I

I

Q

Q

Q

Ek

Pk

Lk

Ek

Pk

Lk

Signal Intensity Samples

Signal Intensity samples are based upon Narrowband (NBP) and Wideband (WBP) Power Measurements (50 samples/second)

Difference between NBP and WBP is proportional to received signal power

• Theoretically cancels noise power in the mean – Practically, it doesn’t completely – correction made later

Samples collected and stored over 60 seconds Thus, 3000 samples every minute These 50 sps samples are available as an output


k k kSI NBP WBP

Computing S4 (1)

Total S4 is standard deviation of normalized Signal Intensity

Scale factor of Signal Intensity is ambiguous, but this normalization with average value over 60 seconds takes care of that

Desirable to remove the effects of receiver noise, theoretically computed as

This is square root of expected value of S42, given noise only

is average measured signal-to-noise density over 60 second period – also an output, as well as the above noise contribution


22

24

k k

Total

k

SI SIS

SI

0

0 0

100 5004 1

ˆ ˆ19NS

S N S N

0S N

Computing S4 (2)

Noise contribution is removed as follows:

If square-root argument is negative, set to 0 (means noise dominates any amplitude scintillation)

This corrected value is computed off-line

Option also exists to compute average value of SIk as low-pass filtered value

This presents potentially unstable normalization because of filter delay – results in inflated S4 values


22

2

0 0

100 5004 1

ˆ ˆ19

k k

Corrected

k

SI SIS

S N S NSI

Low-Pass Filtering Introduces

Delay in Normalization


• In low-passed version

(denominator) does not line

up with raw version,

increasing the variance

• Possible to correct for the

delay, but requires raw data

buffering that is not desirable


Measuring Amplitude Scintillation

Summary Amplitude Scintillation

Measure GNSS signal-plus-noise power

Remove, as well as one can, noise power

Relatively straight-forward • Some “detrending” issues separating scintillation

fades from multipath fading – a detrending bandwidth issue

• Detrending using averaging proves to be more stable than filtering, but results in higher S4 due to multipath fading

Measuring Phase Scintillation



Some History Relative to Measuring

Phase Scintillation Effects GPS Silicon Valley inherited commercialized scintillation monitoring technology from a US Air Force Small Business Innovation Research (SBIR) program

Toughest challenge on that program was measuring phase scintillation with standard GPS receivers using Temperature Compensated Crystal Oscillators (TCXOs)

• TCXO phase noise masked phase scintillation effects

• Problem solved using good Oven Controlled Oscillators (OCXOs)

These upgraded receivers provide good phase scintillation measurements

Even then, there are limitations to operation in a scintillation environment


Measuring Phase Scintillation Effects

To measure phase scintillation, GPS receiver must track signal phase using a phase lock loop (PLL)

Normally, weakest link in a GPS receiver Measurements include perturbations of receiver and satellite oscillators

• Mostly, these perturbations cannot be removed with “detrending”

Longer-term phase includes signal Doppler, multipath and ionosphere TEC (and oscillator frequency offset), mostly removed with “detrending”

Typically, measurement bandwidth is the PLL loop bandwidth Wide bandwidth makes loop more sensitive to amplitude fading, and thus, loss of lock Narrow bandwidth makes loop more robust, but filters out higher-frequency phase scintillation effects

Loop can be configured to have narrow loop bandwidth for robustness, but still provide wide bandwidth phase data


PLL Model with Wideband Phase

Estimator

Phase Discriminator measures current PLL

50-Hz phase error – added back onto phase estimate


Legacy Measurements of TEC

Measure difference of GPS PN code phase on L1 and L2, smoothed against negative L1/L2 difference in carrier phase

Legacy monitors use “semi-codeless” technique to measure on L2 Does not enhance ability to measure scintillation Semi-codeless L2 has 15 to 35 dB less signal power recovery than L1

• However, can use very low bandwidth PLL, aided with L1 Doppler phase, regaining 14 to 17 dB, depending upon C/N0

Limitations Typically not available if L1 C/N0 drops below 38 dB-Hz Must contend with L1/L2 biases

• Satellite biases (Tau_GD and C/A-to-P) and receiver and antenna L1/L2 biases

Real-time accuracies on the order of 1 – 2 TECU, after calibration Also, very much affected by multipath

Evolution to Modernized GNSS


Legacy GSV 4004B & Antenna


GSV4004B GPS IONOSPHERIC SCINTILLATION AND TEC MONITOR AND OPTIONAL GPS702GG

ANTENNA

Features of GPStation-6 GISTM


Features GISTM Receiver (Bold Red Indicates New Features)

Channel Configuration 120 independent channels

Signal Tracking

GPS (L1, semi-codeless L2P, L2C, L5)

GLONASS (L1, L2-C/A, L2P)

Galileo (E1, E5A/B, E5 Altboc)

SBAS (L1, L5), Compass (Upgradable)

Ionospheric Measurments 50 Hz phase and amplitude data (raw or detrended-raw)

Scintillation Indices GPS (L1 C/A, L2C, L5), GLONASS (L1, L2)

Galileo (E1, E5), SBAS (L1, L5)

TEC (Code and Carrier)

GPS (L1/L2P, L1/L2C, L1/L5), GLONASS (L1/L2)

Galileo (E1/E5A), SBAS (L1/L5)

(1 Hz raw and 4/minute smoothed)

Communication Interface USB/RS-232/RS-422 , I/O (PPS, Event, Position Valid)

Improvements by Adding L2C and L5


5 10 15 20 25 30 35 40 45 50 55 6034

36

38

40

42

44

46

48

50

52

Elevation Angle (deg)

Me

an

C/N

o (

dB

-Hz)

L1 C/A

L2 P(Y)

L2C

L5

0 5 10 15 206

8

10

12

14

16

18

20

22

24

Observation Period (Hrs)N

um

be

r o

f S

ate

llite

s

GSV4004B (GPS Only)

GPStation-6 (GPS + GLONASS)

• Measured at Calgary, AB, Canada

• GPS Modernization improves Signal Quality – C/N0

• Adding Constellations increases Number of Ionospheric Pierce Points

Comparison of L1C/A - L2C and L1C/A

- L2P(Y) for Measuring TEC


• Negative TEC because receiver is not delay calibrated

L2P(Y)/L2C TEC Performance

Differences Not much difference in displayed performance

3 dB loss in L2C I/Q multiplexing

Wider tracking loop bandwidth on L2C

Multipath errors dominate – lower chipping rate on L2C

However, L2C tracking much more robust and less dependent on L1 aiding

Larger TEC bias using L2P(Y) More filter delay of wideband signal


GPS Scintillation Measurement Comparisons


21:00 22:00 23:00 00:00 01:00 02:000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Local Time (Hrs)

Am

plitu

de

Scin

tilla

tio

n In

de

x (

S 4)

GSV4004B

GPStation-6

21:00 22:00 23:00 00:00 01:00 02:000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Local Time (Hrs)

Ph

ase

Scin

tilla

tio

n In

de

x (

)

(60

-se

co

nd

s)

(ra

d)

GSV4004B

GPStation-6

• Calama, Chile • S4 – Legacy vs

Modernized

• – Legacy vs

Modernized

• Comparison shows excellent backward compatibility

Modernized Monitor Includes GLONASS


21:00 22:00 22:00 23:00 00:000

0.2

0.4

0.6

0.8

1

1.2

1.4

Local Time (Hrs)

Am

plitu

de

Scin

tilla

tio

n In

de

x (

S 4)

PRN 1

PRN 2

PRN 23

21:00 22:00 22:00 23:00 00:000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Local Time (Hrs)

Ph

ase

Scin

tilla

tio

n In

de

x (

)

(60

-se

co

nd

s)

(ra

d)

PRN 1

PRN 2

PRN 23

• S4 •

SBAS GEO Measurements


Legacy SBAS S4 Measurements in

Non-Scintillating Environment


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 4 8 12 16 20 24

Sig

ma C

od

e/C

arr

ier

Div

erg

en

ce

-m

ete

rs

GPS TOW - Hours

• Standing wave

multipath

detrends out

very well

• Code/carrier

divergence

due to crossing

Doppler of 2

GEOs

Easy to Distinguish between Multipath and

Amplitude Scintillation from GEOs


0

0.5

1

1.5

2

2.5

3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Sig

ma

_C

od

e_

Ca

rrie

r D

ive

rge

nc

e -

me

ters

Corrected S4

Everything above the line is likely multipath fading plus noise

Sigma_PR < 2.9412 X Corrected_S4 - .4412

• No scintillation

• Slow varying

standing wave

multipath

Modernized SBAS Measurements –

Same Performance as Legacy Receiver


21:00 22:00 23:00 00:00 01:00 02:00 03:000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Local Time (Hrs)

Ph

ase

Scin

tilla

tio

n In

de

x (

)

(6

0-s

eco

nd

s)

(ra

d)

PRN 133

PRN 138

21:00 22:00 23:00 00:00 01:00 02:00 03:000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Local Time (Hrs)

Am

plitu

de

Scin

tilla

tio

n In

de

x (

S 4)

PRN 133

PRN 138

• S4 •

• Time difference is due to pierce point

location difference

• Noise is due to GEO payload

transponder phase noise

• Some phase scintillation observable

between 9 and 10 pm

Scintillation Monitoring

Limitations That Apply to Both

Legacy and Modernized

Monitors



General GNSS Receiver Limitations in

Scintillation Environment Phase Scintillation

Generally, not a problem at L1 or L5, or on L2C • Unless a very narrow tracking bandwidth is used • No worse than low-grade TCXO typically found in GPS Receivers • Requires relative wide bandwidth PLL for phase tracking

Larger problem for “semi-codeless P(Y)” on L2 • Very narrow bandwidth PLL coupled with erroneous (required) aiding with

L1 phase (doesn’t agree with Doppler aiding)

Amplitude Scintillation Primary culprit for loss of phase lock

• Deep and long fades steal signal from PLL • Narrower bandwidth is better, but could require a better oscillator, and

may lose lock due to strong phase scintillation • False alarms from lock detectors during fades (apparent loss of lock)

Loss of data (symbols) from SBAS signals

Phase Scintillation Limitations



GNSS Scintillation Monitor Limitations

in Phase Scintillation Environment Can’t measure scintillation at “semi-codeless” L2 P(Y) – Loop bandwidths too narrow Measurement limitations on coded signals (L1, L2C and L5) dominated by receiver oscillator

Typical receiver oscillator phase noise masks phase scintillation (See PSDs and plots in next charts) Thermal Noise limitation is about 0.1 radian @ 30 dB-Hz OCXO phase noise typically better than 0.05 radians

Limitation can be overcome by differencing phase between satellites

Creates a requirement for high-rate data collection and substantial post processing


Phase Noise PSD Comparisons

-80

-70

-60

-50

-40

-30

-20

-10

0

10

0.001 0.010 0.100 1.000 10.000 100.000

Frequency Offset - Hz

Po

we

r S

pe

ctr

al D

en

sit

y -

dB

-ra

d2/H

z

TCXO Spectral DensityOCXO Spectral DensityWeak VHF Scintillation Scaled to L1Stronger Antofagosta ScintillationTCXO Differenced Spectral DensitySBAS GEO Signal


Antofagosto Phase Scintillation vs.

TCXO Phase Noise

= 0.396 radians = 0.46 radians


Tradeoffs Regarding Using Low-Noise

Oscillators (OCXOs) Cost of low-noise OCXOs has diminished somewhat over recent years

The cost driver is their packaging with the receiver (low-volume quantities)

• This packaging must also meeting international radiation and conductive emission (CE) requirements

As stated, TCXO noise can be eliminated by differencing phase across satellites

Creates a data storage and post-processing burden Receiver tracking bandwidth must be kept high, preventing tracking in noisy conditions and during deep fades

Amplitude Scintillation

Limitations



Scintillation Monitor Limitations in

Amplitude Scintillation Environment Amplitude Scintillation

High S4 can cause loss of phase lock

• S4 is still usually valid – it is based upon non-coherent power measurements, at least for short to medium length fades

• See state diagram

Multipath fading limits minimum S4 capability

• Longer duration, but shallow fades

• Can be detected and eliminated because multipath also causes code/carrier phase divergence – scintillation does not

Fade Depths and Widths Using 50

Hz Amplitude Samples


0 5 10 15 20 25 30 35 40 4510

20

30

40

5050Hz c/no for 8 November 2004 01-02 UT Tromso GSV4004

23.3 23.35 23.4 23.45 23.5 23.55 23.6 23.65 23.725

30

35

40

45

50

time (minutes since start of hour)

c/n

o (

dB

Hz)

Distinguishing Between Amplitude

Scintillation and Multipath Fading


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Sig

ma

_C

od

e_

Ca

rrie

r D

ive

rge

nc

e -

me

ters

Corrected S4


Sigma_PR < 0.625 X Corrected_S4 -

• No Scintillation

• Varying Multipath

• All GPS Satellites

Distinguishing Between Amplitude

Scintillation and Multipath Fading


0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6

Uncorrected S4

Sig

ma C

CD

iv -

me

ters

PRN_30

PRN_22

PRN_14

PRN_29

PRN_6

PRN_24

PRN_5

PRN_9

PRN_28

PRN_10

PRN_3

Amplitude Scintillation

MultipathNoise/Multipath

• Moderate

Scintillation

• Varying Multipath

• All GPS Satellites

Multipath Fading Tracking SBAS

Signals


No Scintillation,

Slow Varying Multipath

2 SBAS Geostationary

Satellites

0

0.5

1

1.5

2

2.5

3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Sig

ma

_C

od

e_

Ca

rrie

r D

ive

rge

nc

e -

me

ters

Corrected S4


Sigma_PR < 2.9412 X Corrected_S4 - .4412


Signal Tracking State Diagram

• Not necessarily implemented in all receivers, but is in Scintillation

Monitors described here

Example Phase Measurements

Collected in San Francisco Area

Non-Scintillation Environment


Typical Plot of 1, 3 and 10 Second

Sigma-Phi from All Satellites in View


0

10

20

30

40

50

60

70

80

90

100

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 4 8 12 16 20 24

C/N

0 -

dB

-Hz,

Ele

v A

ng

le -

Deg

rees

Sig

_P

hi -

Rad

ian

s

GPS TOW - Hours

Sig_Phi_1

Sig_Phi_3

Sig_Phi_10

ElevAngle

C/N0

SBAS GEO Phase Measurements


0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 4 8 12 16 20 24

Lo

ck T

ime -

Sec

Sig

_P

hi -

Rad

ian

s

GPS TOW - Hours

Sig_Phi_1

Sig_Phi_3

Sig_Phi_10

Lock Time

• Phase Degraded by

GEO Transponder

Code/Carrier Control

• However, constant

45 degree elevation –

no multipath effects

Documents

Evolution to Modernized GNSS Ionospheric Scintillation and ...indico.ictp.it/event/a11159/session/44/contribution/28/material/0/0.pdf · Seminar on Development and Use of the Ionospheric