1/19 EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS Mr. J....

EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

1/19

EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION

APPLICATIONS

Mr. J. Aponte, Dr. X. Meng, Prof. T. Moore, Dr. C. HillIESSG, The University of Nottingham, UK

Mr. M. BurbidgeLeica Geosystems, UK

Sponsored by:


2/19

Content

• Objectives

• Motivation

• Network Based RTK (NRTK) GPS

• NRTK Service: Leica SmartNet

• Assessment Methodology

• Availability and Mobility Results

• The Accuracy of the Solutions

• Conclusions


3/19

Objective

• Preliminary analysis of the performance of Network RTK GPS in terms of accuracy, availability and mobility when employed for precise real time vehicle positioning.


4/19

Motivation

• Many Intelligent Transport System (ITS) applications such as precise vehicle navigation, lane based traffic or fleet management, active cruise control, and parking guidance require the provision of high rate and accurate real time positions (at least lane level).

• The use of GPS positioning for land navigation applications has been constrained by the accuracy provided by the PNDs (only a few metres to tens of metres). Such accuracy does not meet the above requirements.

• Inertial Navigation System (INS) can provide the accuracy required for ITS but is too expensive for massive market applications.

Can Network RTK GPS fulfil these requirements?


5/19

Network Based RTK GPS

• Distance dependent errors can be accurately modelled using the measurements of an array of GPS reference stations surrounding a rover receiver.

• Rover users can be located at a distance of 40 km or more from the nearest reference station (RS).

• Reduces the number of RSs needed to cover a region (inter-station distances as long as 100 km or more can be used).

Reference Stations

Rover

Distance between RSs (up to 100 km or more)

Baseline length (up to 40 km or more)

RS RoverBaseline

Correction Broadcast

Conventional RTKNetwork RTK


6/19

Network RTK Service: Leica’s SmartNet

• Partnership: Leica Geosystems, OSGB, OSI, and OSNI

• 153 RSs

• RSs: Leica 500/1200 Receivers and Choke Ring Antennas

• Operational from January 2006 in GB and July 2007 in Ireland

• Leica SpiderNet Software (MAC)

• GPS receivers supporting RTCM formats 2.3/3.1 through GSM/GPRS

• There are other similar commercial services in the UK

A NRTK GPS service for Great Britain and Ireland:

Nottingham: Testing area


7/19

Assessment Methodology (1)

• 2 road tests (T1 and T2).

–T1 evaluates accuracy , availability and mobility against Google Earth.

–T2 evaluates accuracy, availability and mobility using IMU/GPS.

• GPRS service from Vodafone.

• Fixed IP from Wireless Logic.

• 10° Cut off angle.

Equipment configurationIESSG’s surveying van

AX1202 Antenna

CIMU Honeywell

Leica GX-1200 GPS Receiver

CIMU and GPS Post-processed solutions

integrated by a loosely coupled solution

Real Time NRTK solution for analysis (NMEA sentences)

RTCM 3.1 corrections

received via GPRS

ΔZ ΔY

ΔX

X

YZ

Duration

Date Time Date Time hh:mm:ss

T1 20/05/08 13:46:00 20/05/08 15:05:59 01:20:00

T2 08/07/08 12:23:20 08/07/08 12:53:07 00:29:48

Test UTC Start Time UTC Finish Time

Data Collection


8/19

Assessment Methodology (2)

• T1 (about 92 km):

– A return trip from the IESSG car park to Junction 23 on the M1

– Four tracks:

• Tracks 1 and 2 (about 37 km each), performance of NRTK GPS on a busy Motorway: Return route covered from junction 25 to junction 23

• Tracks 3 and 4 (about 9 km each), performance of NRTK GPS on a semi-urban route: from the IESSG car park to Junction 25 (Track 3). Track 4 covered the return trip.

• T2 (roughly 32 km):

– Motorway environment from a car park in Toton Ln (B6003) to junction 23 and back to stop just before junction 25

Tests’ Trajectory


9/19

Availability and Mobility Results (1)

Availability: percentage of observations in which a NRTK solution (integer ambiguities resolved) was achieved.

Detailed percentage of observations performed under the different solution types during tests

Average Availability on Motorway is 45%,and 41.5% for the Semi-Urban area

Test TrackLost Lock

Standalone DGPS Availability

1 16.41 0.48 37.83 45.29

2 9.58 0.00 39.60 50.82

3 29.86 20.86 15.57 33.71

4 12.34 0.00 36.76 50.90

T2 - 7.44 0.00 52.80 39.77

T1

What causes the low availability?• Broken wireless communication link• Loss of lock to the GPS signals


10/19


• Availability (NRTK) requires age of correction (AoC) between 0 and 10 sec

• DGPS requires AoC between 10 and 60 sec

• Standalone Requires AoC > 60 sec or none

• Lost Lock means that no information was collected (blank NMEA message)

Detailed percentage of observations that could be theoretically performed under different solution types

during the tests according to the AoC

Test TrackLost Lock

Standalone DGPS Availability

1 16.41 0.6 1.43 81.56

2 9.58 0.85 3.94 85.63

3 29.86 20.86 2.43 46.85

4 12.34 0.77 3.86 83.03

T2 - 7.44 0.17 6.21 86.18

T1

Apart from T1-3 the messages were in theory capable of providing a NRTK solution over 84% of the time (about twice the actual number of NRTK observations).

For T1-3 the lack of RTCM correction message has been proved by the increased number of standalone solutions (over 20%).


11/19


• AoC does not indicate that all the required data is received.

• In GSM/GPRS (TCP) communication the data is CRC (Cyclic Redundancy Check) checked, it is not error checked.

• Data can be received but might be not correct or complete.

• The correction message is formed by pseudoranges and phases. When pseudoranges are used each epoch gives a solution independently of prior or later observations. Phases need more data during a certain period else a main factor for the missing fixed solutions.

Further investigation needs to be carried out in order to check not only the availability but the correctness and completeness of the RTCM message as received at the rover device.


12/19


Many factors caused the lack of availability during the tests (1):

• GPS signals blockage and high level of multipath by terrain and passing lorries (static and dynamic multipath). Including Flyover bridges.

• In the semi-urban tracks the availability was mainly affected by the common factors found in this kind of environments when using GPS. Buildings, trees canopies and other surroundings caused signal blockage and shadowing, and multipath.


13/19


Many factors caused the lack of availability during the tests (2):

• The high percentage of DGPS epochs presented in the solutions, even in occasions when the right conditions were presented in order to have fixed ambiguity solutions (more than five satellites in view and uninterrupted reception of the RTCM message), might suggest some problems in the cycle slips and/or ambiguity resolution algorithms. However, algorithms have always demonstrated high robustness during previous research.

The 1200 GPS receiver used as rover during this research is a geodetic receiver whose algorithms are designed for high precision surveys. Accurate and reliable fixes rather than a fix for each epoch.


14/19


In terms of mobility, SmartNet allowed high rate fixed positions in all the tested environments and at any speed the probe van travelled (between 0 and 70 mph).

Theoretically, centimetric accuracy can be achieved anywhere in the UK where establishment of a GSM/GPRS connection is possible.

However, such mobility is hampered by the actual level of availability.

NRTK epochs

DGPS epochs


15/19

The Accuracy of the Solutions (1)

Accuracy can be defined as how far the coordinates calculated during testing are from the true values.

Precision is the degree of repeatability (or closeness) that repeated observations display. Standard deviation: 1 Sigma.

• The concepts above were not applied for T1 results.

• Accuracy during T1 was studied by plotting the observations in Google Earth.

• During the tests the probe van was driven on the outside lane which can be clearly identified on Google Earth.


16/19


• All = All the epochs e.g. Standalone, DGPS and NRTK

• NRTK = Only valid NRTK observations

Mean Sd +/- Mean Sd +/- Mean Sd +/-

All 1.06 19.66 1.73 36.05 -0.55 14.48

NRTK 0.24 0.80 -0.31 0.68 -0.13 0.59

Z

Coordinates

Ep

och

s

X Y

NRTK epochs DGPS epochs IMU/GPS epochs

Accuracy (Ave.) and precision (Sd) obtained during T2 for the X, Y and Z

coordinates (m)


17/19


• It is clear that 3D errors can be as high as 3.7 metres and that there is an NRTK outage of about 4 minutes.

• Errors are typical of the IMU/GPS integrated solution - accuracy drift from the truth as the GPS solution is not available.

Time series of errors during T2

-400.0

-300.0

-200.0

-100.0

0.0

100.0

200.0

300.0

400.0

1

126

251

376

501

626

751

876

1001

1126

1251

1376

1501

1626

1751

Epochs

Err

ors

(cm

)

X Y Z RMSE GPS Outages


18/19

Conclusions

• SmartNet (NRTK GPS) can offer the lane level positional accuracy that precise ITS applications require.

• The lack of availability of the NRTK observations however, could currently constrain its use to applications where uninterrupted tracking of the vehicles is not paramount or to areas with clear open skies and none or few obstacles (bridges, etc.).

• Further research needs to be carried out in order to find the actual causes of the low level of availability.


19/19

EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION

APPLICATIONS

Mr. J. Aponte, Dr. X. Meng, Prof. T. Moore, Dr. C. HillIESSG, The University of Nottingham, UK

Mr. M. BurbidgeLeica Geosystems, UK

Sponsored by:

1/19 EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS Mr. J....

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