2. Objective: This GPS Correction Message Enhancement System is a sub-system of the IESSG developed RTK (Real Time Kinematic) research software suit. The whole system architecture is shown in Fig. 1.

The objective of this system is to enhance the GPS NRTK correction service by providing detailed information with analysis, and also messages repair if necessary. It has two main functions: one is message analyzing, monitoring and displaying function, which could show the current received message details on screen in real time, and plot the positions of satellites and reference stations; The second function is message checking and repairing, which could check the received message integrity,

removing the duplicated messages and simulating the missed messages when possible.

Summer Placement 2011

By Yiming Quan Supervisor: Dr Lei Yang

GPS Correction Messages Enhancement System

Correction Messages Repair Function: When some of the GPS correction messages are missing due to the transmission reason, the repair function will be activated. It would use the recorded historical messages to predict the missing messages. The simulated prediction message will be encoded and send to the destination client in standard formats. When the enhanced system detected that the scheduled messages are not received, this repair function could be triggered automatically, and last for a short period as predefined. Alternatively, the repair function can also be manually triggered for research purposes.

References: 1. Yang, L., Hill, C. and Moore, T., Implementation of Wide Area Broadcast NRTK on a Communication Satellite Platform, ION GNSS 10,

Portland, USA, 2010 2. Yang L., Hill C., Meng X. and Aponte J., Quality Evaluation of NRTK Correction Transmission, Coordinates, 5(2), pp.14-18, 2009 3. RTCM, Standard for Networked Transport of RTCM via Internet Protocol (Ntrip), Version 1.0 with Amendment 1 (RTCM Paper 200-

2004/SC104-STD), Virginia, US, 2004

Acknowledgement: I would like to thank to Dr. Lei Yang, for his supervision and guidance during the whole project. My appreciation is also due to Xiaobin Ma for his help during the static and kinematic test. Finally, special thanks to Nuffield Foundation for funding this project.

1. Introduction: In the professional surveying area, accuracy of the GPS instruments has been improved to the centimetres level. To achieve this high accuracy, corrections to the raw GPS measurements are essential. In the UK, a network of more than 100 reference stations provides nationwide GPS corrections service. The correction messages are generated in a central processing server, and delivered to the user via different wireless communication links. However, due to the limitation of the message transmission media, the correction data service coverage and quality cannot be guaranteed. During the transmission it often happens that the correction messages are damaged, lost or delayed. Therefore a checking and repairing system is required, which could improve the transmission integrity effectively. Meanwhile the correction messages are designed in compact formats to optimize the transmission efficiency, which means the information they carried is not easily understandable to the user. This poster presents the development of a message enhancement system, which could analysis, display and repair the GPS corrections. It could help user to interpret and understand the live correction information, and it could also check and simulate some missing messages. This message enhancement system has already been used to facilitate researchers in analyzing the GPS correction systems and develop improvements to it.

3. System Architecture:

4. User Interface:

A User Interface(UI) has been designed and developed via Matlab to help user to access the details of the correction messages in real time. It supports user to get feedback from four sections of system (as grouped in orange frames in Fig. 2). The UI also contains the controls for the message communication and repair sections.

By clicking the 'Ephemeris' button‘, user can check the location of all GPS satellites with a polar view. The radial coordinate represents the elevation of the satellites.

In the ‘Data Destination’, User can forward received (or simulated ) messages to a target port via UDP communication

Antenna Interface could show antenna details of each station

The contour information is depending on the individual satellite. User can select different satellite to review the related contour distribution.

By clicking ‘1015’ or ‘1016’ button, the corresponding horizontal contour distribution of the GPS Ionospheric or Geometric Carrier Phase Correction will be displayed.

5. Static and Kinematic Tests:

Objectives: Static test was designed to assess the impact, validity, integrity and repeatability of the developed GPS Correction Messages Enhancement System in a controlled environment, whereas kinematic test was intended to investigate the system practicality in the real scenario.

Results: The system worked properly during both tests. The validity, integrity and repeatability were fully proved in the static test; and kinematic test investigated practicality of system in real scenario. The comparison results show that system has no adverse effects on the message transmission. According to the data log, the repairing function had been activated in the 3-hour static test. Yet it was not triggered in the one hour kinematic test. This is mainly because the GSM wireless link changed the equipment IP at every time when it connected to a different base station. Thus the connection has to be manually re-initiated. This practical issue discovered will be considered in the further improvements..

Equipment Configurations & Theory:

Static test was performed at the IESSG geodesy lab. The data was collected by using two receivers which sharing the same antenna. The equipment configuration for the static tests is shown in Figure 3. The correction messages were sent to the receivers via Ethernet link, which is assumed to be stable and reliable. Correction Messages Enhancement System was only applied to one receiver and the other was used as a reference, so that the impact of the enhanced system can be analyzed via comparing the two receivers.

For the kinematic test (Fig. 4) , the equipments and test method used were set as the same as those in the static test, except for all the equipment was installed in a survey vehicle. The antenna was amounted on the roof of vehicle.

Fig. 4 Van used in the kinematic

Fig. 3 Static test equipment configuration

Correction Messages

from Control Centre Server

GPS Correction Messages

Enhancement System

Fig. 1 Whole System Architecture

As is shown in Fig. 2, the architecture of this system comprises six sections: user action, message communication, message repairing, background raw data processing, data monitoring, and data plotting. Data communication contains receiving and forwarding of the messages. In data processing, all received messages will be decoded and archived, and repaired if necessary. Meanwhile, some selected key information will be displayed in data monitoring section, e.g. message type, observation details, signal to noise ratio, antenna info, and etc. Last, according to the decoded data, all locations of reference stations are plotted in real-time with the updating info, as well as the satellites.

Fig. 2 Algorithm of GPS Correction Messages Enhancement System