EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

1. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS Mr. J. Aponte, Dr. X. Meng, Prof. T. Moore, Dr. C. Hill IESSG, The University of Nottingham, UK Mr. M. Burbidge Leica Geosystems, UK Sponsored by: EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

2. Content • Objectives • Motivation • Network Based RTK (NRTK) GPS • NRTK Service: Leica SmartNet • Assessment Methodology • Availability and Mobility Results • The Accuracy of the Solutions • Conclusions EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

3. 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

4. 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? EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

5. Reference Stations Correction Broadcast Rover Distance between RSs (up to 100 km or more) Baseline Rover RS Baseline length (up to 40 km or more) 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). Conventional RTK Network RTK EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

6. Network RTK Service: Leica’s SmartNet • A NRTK GPS service for Great Britain and Ireland: • 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 Nottingham: Testing area EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

7. Y Z AX1202 Antenna X ΔY ΔZ Leica GX-1200 GPS Receiver ΔX CIMU Honeywell RTCM 3.1 corrections received via GPRS CIMU and GPS Post-processed solutions integrated by a loosely coupled solution Real Time NRTK solution for analysis (NMEA sentences) 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. Data Collection Equipment configuration IESSG’s surveying van EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

8. 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 EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

9. 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 • What causes the low availability? • Broken wireless communication link • Loss of lock to the GPS signals EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

10. Availability and Mobility Results (2) • 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 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%). EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

11. Availability and Mobility Results (3) • 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

12. Availability and Mobility Results (4) • 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

13. Availability and Mobility Results (5) • 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

14. NRTK epochs DGPS epochs Availability and Mobility Results (6) • 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

15. 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

16. IMU/GPS epochs NRTK epochs DGPS epochs The Accuracy of the Solutions (2) Accuracy (Ave.) and precision (Sd) obtained during T2 for the X, Y and Z coordinates (m) • All = All the epochs e.g. Standalone, DGPS and NRTK • NRTK = Only valid NRTK observations EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

17. The Accuracy of the Solutions (3) Time series of errors during T2 • 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

18. 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. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS

19. EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS Mr. J. Aponte, Dr. X. Meng, Prof. T. Moore, Dr. C. Hill IESSG, The University of Nottingham, UK Mr. M. Burbidge Leica Geosystems, UK Sponsored by: EVALUATING THE PERFORMANCE OF NRTK GPS POSITIONING FOR LAND NAVIGATION APPLICATIONS