GPS - Stanford University...Energy in Ionospheric Spectra (IRI90) SSN = 150 SSN = 60 Bandwidth Constraint Model Residuals AJHansen 6/30/98. National Satellite T est Bed (NSTB) Net

WAAS Ionospheric Research at Stanford

Andrew Hansen

30 June 1998

Abstract

This presentation summarizes the WAAS ionospheric research conducted by the WADGPS Laboratory at Stanford

University including primarily tomographic estimation, ionospheric data collection, and high �delity WAAS simulation. The

fundamental physical resource for the work is the National Satellite Test Bed (NSTB) reference network. The coupling of live

data from the NSTB and the theoretical algorithms developed at Stanford has realized a working WAAS prototype with the

capability of exploring operational di�culties and the sensitivities to natural variations such as the ionosphere. We present

performance results for both live and synthetically disturbed conditions.

Electronic copy is available under \Presentations" at http://www.stanford.edu/�ahansen/ on the Web.

Purview

� Real-time tomographic estimation

{ real-time estimation algorithm

{ ionospheric reconstructions

{ hardware bias calibration

{ navigation performance

� Ionospheric data collection

{ continuous real-time ionospheric parameters (DDai)

{ post processing of 1Hz NSTB data (Nusawardhana)

� High-�delity WAAS simulation facility (EPeterson)

{ ionospheric disturbances

{ clock/SA modelling

{ orbit disturbances

{ measurement noise

AJHansen 6/30/98

Wide Area Di�erential GPS Navigation (WAAS)

MS

r

ionoclockephemeris

GPS

GEO

RS

ethernet

1 2

3 N

ionosphere

δ

AJHansen 6/30/98

Wide area Ionospheric Corrections

Reference observations spatially correlated by the ionosphere areassembled into continental scale estimates

-130 -110 -90 -7010

20

30

40

50

60

70

Correlated user ranging errors are corrected by projecting theestimate onto the local SV geometry

AJHansen 6/30/98

Tomography, Solution to an Inverse Problem

Medical Imaging (CAT scan) Ionospheric Imaging (GPS)

AJHansen 6/30/98

Real-time Ionosphere Estimation Methodologies

3D Tomography 2D Thin Shell

AJHansen 6/30/98

Vertical Basis of EOF's

heig

ht (

km)

0 0.05 0.10

100

200

400

500

600EOF #1

−0.2 −0.1 0 0.1 0.20

100

200

300

400

500

600EOF #2

−0.2 −0.1 0 0.1 0.20

100

200

300

400

500

600EOF #3

AJHansen 6/30/98

Energy Spectrum of Electron Density Distribution

0 5 10 15 20 250

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Energy in Ionospheric Spectra (IRI90)

SSN = 150

SSN = 60

BandwidthConstraint

ModelResiduals

AJHansen 6/30/98

National Satellite Test Bed (NSTB)

Network of dual-freq receivers (Trimble, Ashtech, NovAtel)connected in real-time via multicast UDP to central serverplatform (DEC Alpha 4100s).

AJHansen 6/30/98

Software IFB Calibration

� Relative IFBs simple if receivers collocated

� Single network wide bias is tolerable in position domain (�clock)

� Remote IFB calibration necessarily relies on a model ionosphere

� IFB must pass through a non-linearity to be observed

� One such transformation is obliquity factor (2D iono model)

� Using a 3D model, receiver IFB is addressed directly

AJHansen 6/30/98

Interfrequency Bias Calibration

IFB1

Tgd

Ne(r,t)

IFB2

Tgd

IFB

Ne(r,t)

Tgd

IFB

Ne(r,t)

Tgd

IFB

Ne(r,t)

AJHansen 6/30/98

Tomographic IFB Calibration

15 20 25 30 35 40 45 50−20

−15

−10

−5

0

5

10

15

UT (hr)

L 1/L2 IF

B (m

)

Interfrequency Bias Calibration (40 receivers)

AJHansen 6/30/98

Tomographic Tgd Calibration

15 20 25 30 35 40 45 50−5

0

5

10

UT (hr)

T gd (m

)

Group Delay Calibration (27 SVs)

AJHansen 6/30/98

Real-time Tomographic Reconstructions

Electron Density @ 100W Vertical Reduction @ 350km

2530

3540

4550

55

200

300

400

500

6000

0.2

0.4

0.6

0.8

1

x 10−11

N latitude

Ne @ 100W UT 13:49

altitude (km)

Ne

(#/m

^3)

−140−120

−100−80

−60−40

20

30

40

50

600

0.5

1

1.5

2

2.5

3

3.5

4

W longitudeN latitude

vert

ical

del

ay (

m)

AJHansen 6/30/98

Horizontal System Performance (25 March 1998)

0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

25

30

35

40

45

50

σ Hm

ajor

(m

)

8

7

6

5

3

2

1

0

Failures: 0

HPLWAAS

Failures: 0

Integrity

Horizontal Performance at FAA Tech Center − 0x0121 (103466 epochs)

Error (m)

Alarm Epochs: 0

System Unavailable

Failures: 0

HPLWAAS

Normal Operation> 99.999%

HP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

104

AJHansen 6/30/98

Vertical System Performance (25 March 1998)

0 5 10 15 20 250

5

10

15

20

25

σ V (

m)

4

3

2

1

0

Failures: 0

VPLWAAS

Failures: 0

Integrity

Vertical Performance at FAA Tech Center − 0x0121 (103650 epochs)

Error (m)

Alarm Epochs: 3

System Unavailable

Failures: 0

VPLWAAS

99.997%Normal Operation

VP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

AJHansen 6/30/98

Vertical System Performance, Tomo IFBs (25 March 1998)

0 5 10 15 20 250

5

10

15

20

25

σ V (

m)

4

3

2

1

0

Failures: 0

VPLWAAS

Failures: 0

Integrity


Error (m)

Alarm Epochs: 5

System Unavailable

Failures: 0

VPLWAAS


VP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

AJHansen 6/30/98

Vertical System Performance, Dual freq (25 March 1998)

0 5 10 15 20 250

5

10

15

20

25

σ V (

m)

4

3

2

1

0

Failures: 0

VPLWAAS

Failures: 0

Integrity


Error (m)

Alarm Epochs: 16

System Unavailable

Failures: 0

VPLWAAS


VP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

AJHansen 6/30/98

Ionospheric Data Collection (NSTB)

� 40+ calibrated (IFB,SNR) receivers at 29 locations

� Dual-frequency carrier smoothed code (multipath free)

� Multi-resolution (10/300/1800 sec) �lter bank for data reduction

� Receiver archive data format

{ time history per SV pass

{ entries are time PRN El Az IL1 N� � slip

{ 1� 11

2MB per day (� 12GB per year)

{ slip index denotes detected bias break not a receiver ag

� High level (Matlab) archive interrogation

AJHansen 6/30/98

Software Simulator for WAAS

GPS Almanac

* SV Clock Models* Orbit Disturbances

* SV Ephemeris

Σ ∆Φ

Iono Delay

Σ

WAAS Simulator

Tropo Delay

Σ

Multipath

Synthetic Truth Data

WG2 Message

LOS Signal

Propagation

ArcataNominal Models

Bangorpr,

ColumbusSNR, Dopp

Great Falls

��

Greenwood

GenerateWAASCorrection

TMS

* Spatial Dependence

* Anomalous Behavior* Temporal Dependence

Ionospheric Correlation

IonosphericDisturbanceDatabase

��

��

��

��

��

��

��

��

��

��

TRS Observables

AJHansen 6/30/98

Ionospheric Disturbance Scenario

� Nominal models

{ 26 SV Constellation (no GEOs)

{ background iono model based on PIM

{ SNR receiver model with live data, Dai (IEEE PLANS, 1998)

{ tropo model from WAAS MOPS

{ multipath model, Enge, et.al (Proc. IEEE, 1995)

{ 27 reference receivers, 2 monitors

� Injected electron density enhancement

{ Gaussian bubble layer at 300(km)

{ 500 (km) 1-� centered at 35N, 100W

{ amplitude is 25% of nominal

{ �xed in solar-magnetic coordinates

� Truth available in both pseudo-range and position domain

AJHansen 6/30/98

Horizontal System Performance (Simulated Iono)

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

σ Hm

ajor

(m

)

12

11

10

9

8

7

5

4

3

2

1

0

Failures: 0

HPLWAAS

Failures: 0

Integrity

Failures: 0

HPLWAAS

Horizontal Performance at FAA Tech Center − 0x0121 (20989 epochs)

Error (m)

Continuity Threats: 0

System Unavailable


HP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

AJHansen 6/30/98

Vertical System Performance (Simulated Iono)

0 5 10 15 20 250

5

10

15

20

25

σ V (

m)

4

3

2

1

0

Failures: 0

VPLWAAS

Failures: 0

Integrity


Error (m)

Continuity Threats: 0

System Unavailable

Failures: 0

VPLWAAS


VP

L WA

AS (

m)

Num

ber

of P

oint

s pe

r P

ixel

100

101

102

103

AJHansen 6/30/98

Documents

GPS - Stanford University...Energy in Ionospheric Spectra (IRI90) SSN = 150 SSN = 60 Bandwidth Constraint Model Residuals AJHansen 6/30/98. National Satellite T est Bed (NSTB) Net