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

Thilantha L. Dammalage
Geoinformatics Center, Asian Institute of Technology
PO Box 4, Klong Luang, Pathumthani, Thailand.
Thilantha.dammalage@ait.ac.th

Panithan Srinuandee
Ultimate Positioning Co.,Ltd
Room 5A Viwatchai Bldg. Phaholyothin Rd. Jomphol Jatujak Bangkok
Thailand 10900
arm@up.co.th

Lal Samarakoon
Geoinformatics Center, Asian Institute of Technology
PO Box 4, Klong Luang, Pathumthani, Thailand.
lal@ait.ac.th

Junichi Susaki
Geoinformatics Center, Asian Institute of Technology
PO Box 4, Klong Luang, Pathumthani, Thailand.
susaki@ait.ac.th

Teerasak Srisahakit
Ultimate Positioning Co.,Ltd
Room 5A Viwatchai Bldg. Phaholyothin Rd. Jomphol Jatujak Bangkok
Thailand 10900
teerasak@up.co.th

Abstract:-
Networked Transport of RTCM via Internet Protocol or NTRIP is a recently developed application-level protocol streaming Global Navigation Satellite System (GNSS) data over the Internet by the Federal Agency for Cartography and Geodesy of Germany. NTRIP enables the streaming of DGPS or RTK correction data for the stationary and mobile users via Internet by using GPRS or other modern communication technologies (EDGE, or UMTS), allowing simultaneous PC, Laptop, PDA, or receiver connections to a broadcasting host. Since NTRIP becomes the world standard protocol for GNSS data streaming in year 2004, Geoinformatics Center at Asian Institute of Technology establishes the first real-time GPS data streaming station using NTRIP in Thailand. This paper examines technical basics of NTRIP and illustrates the potential accuracy and practical benefits of the NTRIP technique over the conventional RTK and DGPS correction methods that is presently used in different GPS surveying Institutions in Thailand. Results illustrated in this paper are based on field observations carried out during March to May 2006 and it was found that NTRIP is a possible solution to replace conventional Differential GPS techniques, being able to use with any type of GPS instrument, such as dual frequency (L1/L2) or single frequency (L1). Also, it was found that it is possible to enhance Low-Cost hand-held GPS receiver observations. This paper demonstrates the achievable accuracy of GPS observations with various distances from GPS base station (Baseline), comparing between NTRIP and conventional methods.

1 INTRODUCTION
At the present, many government organizations and private companies in Thailand have been using Global Positioning System for their field surveying and other applications of precise positioning. Government organizations such as Department of Lands and Department of Town and Country Planning (DTCP) maintains the countries geodetic control network (established using GPS) and carrying out the field GPS observations with the most conventional methods of Differential GPS (DGPS). One of the most common methods that many government organizations draw on is Post-Processing DGPS, but due to the rapid developments of GPS integrated applications, many private companies dealing with GPS now concentrating more on real-time differential GPS than Post-Processing DGPS.

Either post-processing or real-time differential processing GPS techniques allow GPS users to achieve the precision levels required for different applications. Post-Processing DGPS (Differential GPS), Real-Time DGPS / RTK (Real-Time Kinematics), WAAS (Wide Area Augmentation Systems), LAAS (Local Area Augmentation Systems) and VRS (Virtual Reference Station) are some of the existing techniques that give real time correction for GPS observations. These real-time correction methods are based on information communication between the measurement equipment and other devices, such as permanent reference stations. That means, the data distribution from one GPS station to another GPS station is very much essential when real-time corrections are required. Due to the unavailability of the methods like WAAS, LAAS and VRS techniques, the most common method of real-time high accuracy positioning in many countries in this region (Asia, Asia-Pacific and South-Asia) is carrying out DGPS or RTK with Very High Frequency (VHF) radio signal transmitter. Even though, it has been observed that the radio communication technology has improved from time to time; still there are significant numbers of difficulties in using VHF signal in RTK or DGPS. For example, the difficulties of accessing VHF signal in the field without distraction and limitations of baseline distance.

The increase of available bandwidth of Internet enables data streaming applications like Internet-Radio or Internet-TV possible. Researchers are now trying to use Internet as an alternative method for transmitting GPS data for the real-time or near real-time corrections of GPS observations. As a result, a new technique using Internet for streaming RTK/DGPS corrections allowing precise positioning and navigation was announced late 2004, under the name of “Networked Transport of RTCM via Internet Protocol (NTRIP)”. The development of this new technique was carried out by the Federal Agency for Cartography and Geodesy, Germany (BKG). Hence, GPS data distribution through Internet is becoming more and more demanding due to the common availability and easy accesses of Internet facility. Also the development of systems for mobile Internet access through GPRS (General Packet Radio Service)-Internet and GSM (Global System for Mobile Communication)-Internet, provides a fast and reliable method to distribute raw GPS data or re-transmit differential corrections (DGPS/RTK) to a GPS receiver in any area covered by a mobile telephone network.

According to the field tests carried out during this research, it was found that, there are many advantages of using NTRIP as an alternative to the more traditionally accepted methods of obtaining real time RTK/DGPS corrections. NTRIP enables data streams from reference stations (GPS base stations) or databases for GIS applications to be accessed by a variety of clients/users through one defined communication technique. Mobile users such as RTK/DGPS or Surveying/GIS field teams could use their hardware with a mobile GPRS phone to access Internet in the field while at the same time stationary applications in the reference station periphery could be accessing the same data. This is the most important advantage of this technique; it overcomes the single user problem. For an example, the DGPS data streaming operates by Asian Institute of Technology is capable of providing accesses to around hundred DGPS users simultaneously. Next few chapters will discuss these facts in more details including theoretical aspects of NTRIP and field observations.

2 NTRIP PROTOCOL (VERSION 1.0)
NTRIP is a generic, stateless protocol based on the Hypertext Transfer protocol HTTP/1.1. The HTTP objects are extended to GPS data streams. The system is implemented in three applications, named NTRIPServer, NTRIPClient and NTRIPCaster. The NTRIPServer and NTRIPClient are technically functioning as HTTP clients, while the NTRIPCaster is act as true HTTP server. NTRIP is meant to be an open non-proprietary protocol for the real-time streaming of DGPS or RTK corrections to mobile receivers. NTRIP protocol disseminating GPS differential correction data in the RTCM, SC-104 format and this protocol was standardized and publish by the Radio Technical Commission for Maritime Services (RTCM) in November, 2004. (RTCM Paper 234-2004/SC104-PR)

Major characteristics of NTRIP dissemination technique are the following: (NTRIP Documentation, Version 1.0, http://igs.ifag.de/index_NTRIP.htm)

It is based on the popular HTTP streaming standard; it is comparatively easy to implement when limited client and server platform resources are available.
Its application is not limited to one particular plain or coded stream content; it has the ability to distribute any kind of GPS data.
It has the potential to support mass usage; it can disseminate hundreds of streams simultaneously for up to a thousand users when applying modified Internet Radio broadcasting software.
Regarding security needs, stream providers and users are not necessarily in direct contact, and streams are usually not blocked by firewalls or proxy servers protecting Local Area networks.
It enables streaming over any mobile IP network because it uses TCP/IP.