GSI Japan - 21st of June 1999 GPS-Positioning using Virtual Reference Stations - Theory, Analysis and Applications Herbert Landau Spectra Precision Terrasat

GSI Japan - 21st of June 1999

GPS-Positioning using Virtual Reference Stations -

Theory, Analysis and Applications

Herbert Landau Herbert Landau Spectra Precision Terrasat GmbHSpectra Precision Terrasat GmbH

Overview

Principle of Virtual Reference Stations Principle of Virtual Reference Stations Modelling of Error Sources Modelling of Error Sources Hardware Setup Hardware Setup Software SetupSoftware Setup Implementations - ReferencesImplementations - References Results of RTK Positioning AnalysisResults of RTK Positioning Analysis

Why Virtual Reference Stations?

Improvement of accuracy versus classical RTKImprovement of accuracy versus classical RTK Reliability improvementReliability improvement Productivity improvement Productivity improvement Local reference station is obsoleteLocal reference station is obsolete Positions are automatically derived in a precise Positions are automatically derived in a precise

geodetic reference station system geodetic reference station system Real-time service for ionospheric disturbances can be Real-time service for ionospheric disturbances can be

provided to the user provided to the user

Concept of Virtual Reference Stations (VRS)

A D

C

B

Rover

Virtual Ref.

Modeling systematic Modeling systematic errorserrors

Elimination of errorsElimination of errors Generation of Generation of

interpolated interpolated observations for observations for virtual station virtual station

Real-Time: RTCMReal-Time: RTCM Post-Mission: Post-Mission:

RINEXRINEX

Data Flow in the Network

Reference Station

Reference Station

Reference Station

Reference Station

Raw Data

GPSNetwork

Router


Reference Station

Reference Station

Reference Station

Reference Station

Raw Data

GPSNetwork

Router

Rover


Reference Station

Reference Station

Reference Station

Reference Station

Raw Data

GPSNetwork

Router

NMEA Position


Reference Station

Reference Station

Reference Station

Reference Station

Raw Data

GPSNetwork

Router

NMEA Position


Reference Station

Reference Station

Reference Station

Reference Station

Raw Data

GPSNetwork

Router

Virtual Ref. Station

RTCM

NMEA Position


Major Error Sources in Differential-GPS

IonosphereIonosphereTroposphereTroposphere Error in Error in

Satellite OrbitSatellite Orbit

Tropospheric Modelling

Modified Hopfield Model Ground meteorological measurements not sufficient Water Vapour Radiometers are too expensive Elimination of tropospheric errors is required for

ambiguity resolution in the network Determination of a model scale factor

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

00:00 05:00 10:00 15:00 20:00 25:00 30:00 35:00 40:00 45:00

Tropospheric Scale Factor Convergence on the Network Stations

Minutes

Sca

le fa

cto

r of

fse

t

Improvement due to Tropospheric Scaling in the Bysat Network

020406080

100120140160180200

Erro

r [m

m]

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Elevation

Max. w/o

Max. with

RMS w/o

RMS with

Ionospheric Modelling

Single layer model Determination from L1/L2 carrier phase data All data of all stations The correction by the model is applied to the

observations This is required for wide lane ambiguity fixing

Average yearly number of sun spots

0

50

100

150

200

1960 1970 1980 1990 2000

RZ

25

20

15

10

5

0

19701960 1980 1990 2000

Year

Ionospheric error in GPS L1

Err

or

[m]

Solar Cycle and Ionosphere

Ionospheric Maximum in 2000/2001

1995 1995,5 1996 1996,5 1997 1997,5 19991998 1999,51998,5 2000 2000,5

5

10

15

20

25

30

35

Time (Years)

Mea

n T

EC

(T

EC

U)

CODE Ionospheric model Station Zimmerwald, CH

CODE Ionospheric model Station Zimmerwald, CH

0.06

0.05

0.04

0.03

0.02

0.01

0.00

-0.01

-0.02

-0.03

-0.04

-0.05

-0.0613:00 13:10 13:20 13:30 13:40 13:50 14:00

Local Time

Err

or in

Met

er

Differential Ionosphere Dec. 1998 on 70 km Baseline

Local Time

Err

or in

Met

er

13:00 13:10 13:20 13:30 13:40 13:50 14:00

0.25

0.20

0.15

0.10

0.05

0.00

-0.05

-0.10

-0.15

-0.20

-0.25

Differential Ionosphere Feb. 1999 on 70 km Baseline

Local Time

Err

or in

Met

er

13:00 13:10 13:20 13:30 13:40 13:50 14:00

0.50

0.40

0.30

0.20

0.10

0.00

-0.10

-0.20

-0.30

-0.40

-0.50

Differential Ionosphere Feb. 2000 on 70 km Baseline

Satellite Orbits

Starting with broadcast ephemeris Additional use of predicted ephemeris Comparison of broadcast and predicted ephemeris Typical differences < 10 m Differences of up to several 100 m can be found

during satellite maneuvers Satellites with large differences are not used Estimation of residual error

Integrity Monitoring

Outlier detection in pseudo-ranges Continuous navigation solution for all stations Continuous DGPS solution for all stations

In case of outliers the epoch will not be used

Network Processes

Geometric correction Correction for tropospheric errors Correction for ionospheric errors Correction for multi-path Ambiguity resolution Consistency check

Derivation of Corrections

Estimation of corrections from residuals in L1 and L2 carrier phase measurements

Correction in North-South and East-West direction for each satellite for geometrical part (troposphere and ephemeris),

typical < 2 ppm ionospheric part, typical < 15 ppm

Computation of corrections performed once per second

Computation of VRS-Data

Starting with the data of the reference station nearest to the rover

Geometric displacement of these data to the virtual position

Applying the corrections for the geometric and the ionospheric parts

Transmission of the VRS data via mobile-phone in RTCM Standard with messages 3, 18, 19

Data Communication

M o d e m

M o d e m

M o d e m

R o u te r

M o d e m

M o d e m

M o d e m

M o d e m

M o d e m

G P S N e tw o rkA c c e ss S e rve r

LA N

LA N

R o ve r

R o ve r

R o ve r

GSM Mobile Phone

R e fe re n zsta tio n e n K o n tro llze n tru m

Software Structure Ref.Stn.Ref.Stn. Ref.Stn. Ref.Stn.

Receiver Interfaces, RINEX Storage, Antenna Phase Center Correction

Synchronizer, Virtual Ref. Station Processor, Ephemeris, Ionosphere andTroposphere modeling

Interpolator

RTCMGenerator

Communication

Module

Position

Mobile Phone

PositionRTCM

Interpolator

RTCMGenerator

Communication

Module

Interpolator

RTCMGenerator

Communication

Module

Mobile Phone

PositionRTCM

Mobile Phone

PositionRTCM

Raw Data

Neufahrn

The Bysat Network in Germany

Weinstadt

Münsingen

Gerstetten

Untereichen

Augsburg

Höhenkirchen

Mainburg

Mühldorf

Ashtech 9 Stations 50-70 km Telekom Net Router Access Server

Performance Analysis in the Bysat Network

HöhenkirchenHöhenkirchen

MainburgMainburg

70 km

AugsburgAugsburgMühldorfMühldorf

RoverNeufahrn

- Data of February 2000Data of February 2000- 90 hours day/night 90 hours day/night - Rover in Neufahrn, 32 kmRover in Neufahrn, 32 km from Höhenkirchen from Höhenkirchen

Comparison of Standard RTK with VRS-RTK

Recorded data of Dec. 6th, 1999 13:30-15:12 local time on rover station Neufahrn

VRS data generated in real-time was recorded Post-processing with Geotracer RTK-Software on PC Automatic OTF search with intervals of 15 seconds Sequential adding of data until ambiguity resolution is

successful

Improvement in Time to Fix by using VRS

0

10

20

30

40

50

60

70

80

Num

ber o

f Fix

es

0.0 60.0 110.0 160.0 210.0 265.0

Initialisation Time [Sek]

Standard RTK

VRS

Real-Time Test Setup in the Bysat Network

Operation of rover Neufahrn (32 km from the nearest reference station) with Geotracer RTK system

After each fix the Geotracer RTK system outputs position data for 30 seconds

After that the RTK system initializes the ambiguity search again, no data from the past is used

All position output is stored on an extra PC and analyzed statistically

Error in North – 32 km Baseline

0

1000

2000

3000

4000

5000

6000

Num

ber

of P

ostio

ns

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error [mm]

Confidence LevelConfidence Level90 %: < 13 mm90 %: < 13 mm99 %: < 26 mm99 %: < 26 mm


Error in East – 32 km Baseline

0100020003000400050006000700080009000

Num

ber o

f Pos

ition

s

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error [mm]



Error in Height – 32 km Baseline

0

500

1000

1500

2000

2500

3000

Num

ber

of P

ositi

ons

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error [mm]



RTK Initialisation – 32 km Baseline

0

2

4

6

8

10

12

Perc

ent

0 20 40 60 80 100 120 140 160 180 200

Initialisation Time [sec]

PerformancePerformance

50 %: < 40 sec50 %: < 40 sec90 %: < 80 sec90 %: < 80 secaverage: 58 secaverage: 58 sec

PerformancePerformance

50 %: < 40 sec50 %: < 40 sec90 %: < 80 sec90 %: < 80 secaverage: 58 secaverage: 58 sec

Bysat Network – 32 km : 11AM – 4PM

0

50

100

150

200

250

Num

ber

of p

ositi

ons

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error in North [mm]

0

50

100

150

200

250

300

350

400

Num

ber

of p

ositi

ons

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error in East [mm]

0

50

100

150

200

250

Num

ber

of p

ositi

ons

-50 -40 -30 -20 -10 0 10 20 30 40 50

Error in Vertical [mm]

0

2

4

6

8

10

12

14

Perc

ent

0 50 100 150

Initialisation Time [sec]

90 % < 17 mm90 % < 17 mm

99 % < 37 mm99 % < 37 mm

90 % < 17 mm90 % < 17 mm

99 % < 37 mm99 % < 37 mmAverage Average

60 seconds60 seconds

Average Average

60 seconds60 seconds

Conclusion GPS-Network successfully creates improved VRS RTCM GPS-Network successfully creates improved VRS RTCM

corrections in real-timecorrections in real-time

VRS reduces systematic errors substantially, but cannot eliminate VRS reduces systematic errors substantially, but cannot eliminate them completely them completely

VRS allows to do RTK positioning at distances a standard RTK VRS allows to do RTK positioning at distances a standard RTK system never will reachsystem never will reach

Virtual Reference Stations improve:Virtual Reference Stations improve:

AccuracyAccuracy

Productivity via shorter Time to FixProductivity via shorter Time to Fix

Reliability Reliability

Documents

GSI Japan - 21st of June 1999 GPS-Positioning using Virtual Reference Stations - Theory, Analysis and Applications Herbert Landau Spectra Precision Terrasat