Potential Accuracy and Practical Benefits of NTRIP Protocol Over Conventional RTK and DGPS Observation Methods.


Figure 2.1 illustrates the typical NTRIP data streaming flowchart. This illustrates the most general case of NTRIP component setup with M number of NTRIPsources and N number of clients that are accessing the streaming data.

NTRIPSources: provides continuous GPS data (e.g. RTCM 2.0, RTCM 3.0, RAW) as streaming data. A single source represents a GPS reference station data referring from a specific location. Each GPS data sources is identified with a unique ID call mount-point.

NTRIPServer: is used to transfer GPS data of an NTRIPSource to the NTRIPCaster.

NTRIPCaster: is basically an HTTP server supporting a subset of HTTP request/response messages and adjusted to low-bandwidth streaming data (from 50 up to 500 Bytes/sec, per-stream). The NTRIPCaster administrator organizes all available NTRIPSources and defines all source IDs (Mount-points).

NTRIP Client: will be accepted by and receive data from an NTRIPCaster, if the NTRIP Client sends the correct request message (TCP connection to the specified NTRIPCaster IP and communicating Port). With respect to the message format and status code, the NTRIPClient-NTRIPCaster communication is fully compatible to HTTP 1.1. NTRIPClients choose an NTRIPSource (mount-point) with reference to the source table information (Identifier) provide by NTRIP Caster.


Fig. 2.1 NTRIP Components (Source RTCM Documentation Version 1.0)


3 REAL-TIME GPS DATA STREAMING AT ASIAN INSTITUTE OF TECHNOLOGY
Geoinformatics Center (GIC), Asian Institute of Technology distributing DGPS and RAW GPS data for public use since the beginning of year 2006 using NTRIP and it has become the first real-time GPS data distribution station using NTRIP in Thailand. This GPS base station operates with two NTRIP servers, and it is uploading RAW and DGPS data to the NTRIP caster (IP: 203.159.29.16 :80) operates at Geoinformatics Center, Asian Institute of Technology itself. This NTRIPCaster (Standard NTRIPCaster Version 1.0) or the GPS data broadcaster provides possibility to connect about 100 users simultaneously to accesses real-time streaming GPS data. In order to access these GPS data from the NTRIP caster, users have to register through GIC, GPS home page ( http://www.geoinfo.ait.ac.th/gps/index.html ), also users have to state the purpose of data access. Once the user successfully submitted the registration form and get the approval, they will get data streaming details including a user name and a password for real-time data access. The field experiments that will be discussed in this paper was carried out with the use of Geoinformatics Center base station streaming data. In order to get RTK streaming for this field test, the base station was temporally updated with a dual (L1/L2) receiver with the contribution of Ultimate Positioning Co.Ltd.

4 FIELD TEST OF INTERNET DGPS/RTK STREAMING.
Field test were carried out from March to May, 2006 in order to check the possibility and the achievable accuracy with NTRIP in real-time DGPS and RTK field observations. These field test were carried out with the collaboration of Geoinformatics Center and Ultimate Positioning Co.,Ltd. Field tests had been carrying out with SOKKIA GSR2600 dual frequency (L1/L2) receiver, Trimble ProXR single frequency (L1) receiver and Garmin eTrex handheld GPS receivers in order to check the possible accuracy that can achieve and potential use of Internet DGPS/RTK streaming corrections, using existing GPS data collection software. Since NTRIP becomes the world standard protocol for internet DGPS and RTK streaming, many commercial software products also updated with the capability of handling NTRIP technology. Table 4.1 shows some of the software available commercially that supports NTRIP protocol. This information is extracted from the BKG-NTRIP home page (http://igs.ifag.de).

Table 4.1 Some Commercial software supporting NTRIP protocol
ArcNTRIP Galileo Sistemi, ArcPad GIS Data Collection Software, NTRIP Client
TerraSync Trimble GIS Data Collection and Data Maintenance Software, NTRIP Client
SurveyController Trimble Rover Control Software, NTRIPClient
GX1200 Leica GPS Rover, NTRIPClient & Server
GSR2700 IS Sokkia GPS Rover, NTRIPClient
TopSURV Topcon Rover Control Software, NTRIPClient
EuroNet EuroNav DGPS Network Processing Software, NTRIPClient & Server
EuroRef EuroNav Reference Station Software, NTRIPClient


Figure 4.2 illustrates the system component setup for both high accuracy GPS receivers (Trimble ProXR GPS receiver and SOKKIA GSR2600 receiver) and also for low accuracy handheld (Garmin eTrex) GPS receiver observations. In all cases, the custom software has to be used in order to connect to the NTRIPcaster. Sometimes this custom software are capable of being NTRIPClient software and GPS receiver operation software at the same time. In the case of Trimble ProXR receiver, the custom software used is TerraSync and it does both connecting to the NTRIP caster through Internet and handling receiver operations. Similarly, SDR+ software is used in connecting to the NTRIPcaster and receiver operations for the system setup with SOKKIA GSR2600 dual frequency receiver. When the system was setup with a handheld GPS receiver, NTRIPClient is the custom software and it is only used to get the DGPS correction from NTRIP Caster connecting through GPRS. Receiver operations and handling was performed using the receiver inbuilt software. The handheld (Garmin eTrex) GPS receiver that used in this field test is compatible to handle the RTCM 2.0 (DGPS correction) input messages. NTRIPClient software was used to get the DGPS streaming by connecting to the NTRIP Caster and to upload the DGPS correction to the Garmin eTrex GPS receiver. A Pocket PC with GPRS connection was used in order to access the Internet in the field as well as to operate NTRIP Client software.


Fig. 4.2 System component setup for real-time DGPS correction


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