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.

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

5 ACCURACY COMPARISON.
The required accuracy for land survey purposes is generally accepted to be at the centimeter level. Sometimes for very precise measurements it is required to have below centimeter level accuracy. Reaching this accuracy using GPS observations in differential mode is only possible through evaluating carrier phase using dual frequency GPS receivers. Depending on the accuracy requirements, differential correction of the single frequency GPS receivers and handheld low accuracy GPS receivers also provides improved accuracy level than the uncorrected observations. Field tests were carried out to check the achievable accuracy of the observations with NTRIP technique. Table 5.1 illustrates the observed accuracy with L1 receiver, L1/L2 receiver and Handheld receiver with respect to different baseline distance. Also it compares the accuracy of differential corrected observations using conventional (post-processing and radio-RTK) and new technique NTRIP with the uncorrected observations according to the baseline distance. Furthermore the results show that, with the Internet RTCM stream, all three different receivers show enhanced observation value than RAW observations and shows similar accuracy of observation with the conventional DGPS and RTK techniques.

Table 5.1 comparison of the observed accuracy according to the base-line distance.

RAW; Uncorrected, Int-DGPS; DGPS using NTRIP, PP-DGPS; DGPS using Post-Processing, Int-RTK; RTK using NTRIP, Radio-RTK; RTK using Radio communication.

Due to the different accuracy level of the three receivers that were used in this field test, the detailed accuracy analysis for each receiver was carried out individually. The following two figures illustrate the observed accuracy by using single frequency Trimble ProXR GPS receiver.

Fig. 5.1.A Comparison of L1 Receiver Observations Accuracy with Base-Line Distance (Error in meters) Fig. 5.1.B Comparison of L1 Receiver Observations Shift from Original Position. (Observations in 5Seconds intervals)

Figure 5.1.A illustrates the Accuracy of uncorrected, Internet DGPS and post-processing DGPS observations with respect to the baseline distance. It is evident that the accuracy difference of the Internet DGPS and post-processing DGPS observations is not deviate significantly from the accuracy level of 0.5 meters with the increase of baseline distance up to 60 kilometers. Therefore, according to this field test, it is possible to achieve a positional accuracy of up to 0.5m-1m (sometimes even better) with using Internet DGPS streaming data (using NTRIP) for the baselines of up to 60Km. Furthermore figure 5.1.B illustrates the observations at 30Km from the base station and it compares the shift from the original position of observation for each observation method. Observations were carried out for about 0.5 hours with 5 seconds interval, so there are more than 300 observations for each method. The shift in the RAW (RAW_Shift) observations always shows higher value than both differential corrected methods, while both Internet DGPS shift (DGPS_Shift) and post-processing shift (Post_Pros_Shift) maintains almost the same accuracy level. As a result, the standard deviation of the uncorrected RAW GPS observation shift and the shift for Real-Time DGPS corrected observations with using NTRIP and Post-Processing DGPS are 1.926 Meters, 0.34 Meters and 0.416 Meters respectively.

In the case of dual frequency (L1/L2) GPS receiver, both methods have shown the same accuracy level of observations. Nevertheless, it has been observed that the maximum base-line distance is 30 Kilometers for both RTK and DGPS stream is possible within 30 kilometers. In addition, sometimes it takes more than 30 minutes for the initialization when the baseline length is around 25Km to 30Km and more; the same applies to both Internet and radio RTK. This creates a problem of carrying out RTK around 30Km and more. One of the clear advantages of Internet RTK over radio RTK is that the easy accessibility of the transmitting (or streaming) correction signal without any disturbances from the surroundings.

The other important field test was the accuracy enhancement of handheld GPS receiver with Internet DGPS streaming base on NTRIP. Generally the achievable accuracy of any handheld GPS receiver is around 5m-10m. It is also observed that depending on the satellite geometry the maximum accuracy of 2 meters with uncorrected observations is also possible. During the field test of Garmin eTrex GPS receiver, it has been observed that it is possible to enhance the observation accuracy up to 1m-3m while the GPS receiver connect to the Internet DGPS streaming. Even during poor satellite geometry condition, this accuracy level (1m-3m) is still possible with DGPS corrected observations.

Figure 5.2 illustrates the positional shift of the observations from the original coordinate when it is uncorrected (RAW_Shift) and corrected with NTRIP DGPS source (DGPS_Shift). In order to compare the accuracy both observations were carried out at the same time. This is to normalize the satellite geometric and temporal changes of the surroundings and the atmospheric conditions for both of the observations. In this case the observations were carried out for about 0.5 hours with 5 seconds interval. So there are more than 300 observations for each observation mode that are available for the analysis. Standard deviation of the uncorrected observation was 3.96 meters and after Real-Time DGPS corrected it was 2.20 meters. According to Figure 5.2, the DGPS corrected observation accuracy is not very stable, In order to examine the reasons for these behaviors more field tests will be carry out during future field works.

Fig. 5.2 Comparison of Hand-Held Receiver observations Shift from Original Position. (Observations in 5Second intervals)

6 BENEFITS OF NTRIP.
In the introduction to this paper it was mentioned that NTRIP is becoming a practical level alternative for traditional methods of differential GPS. Since NTRIP is providing a platform to carry out RTK and DGPS observations, so it is clear that NTRIP is a practical tool for any type of conventional applications of RTK and DGPS. According to the accuracy results that were discussed in the previous chapter NTRIP leads to a new generation of differential GPS with providing number of benefits while maintaining the same accuracy level of observations as conventional RTK and DGPS techniques. Such as, it reduces the cost of maintaining additional GPS instrument as a Base station. Also NTRIP provides more reliable and safer method of RTCM correction streaming than transmitting it with radio signals.

Fig. 6.1 NTRIP Real-Time GNSS data Streaming Stations, (March 2006)

In addition, NTRIP provides new concepts of GNSS data sharing technique, leading to many different new GPS applications, such as concept of “Global Real-Time Network of GNSS Reference Stations”. This is a network of GNSS reference stations and it has the possibility to provide real-time GNSS data streaming, which provide access from anywhere in the world from any base station data in the network. This is a fast growing network all over the world and figure 6.1 illustrates the map of the present (March 2006) network of real-time GNSS reference stations. This global network leads to many research activities globally as the coverage of real-time GNSS reference stations data all over the world is available. NTRIP provides potentially great amount of benefits, not only in Global applications but also regional, local area or Individual applications. For example derivation of real-time satellite orbital parameters and satellite clock errors, local and global real-time ionosphere modeling and real-time disaster predictions, such as Earthquake, Tsunami are some of the research level advantages that NTRIP is capable. Not only for these advance research level applications but also in many other RTK or DGPS applications, such as accurate field surveying (Cadastre Mapping), GIS data collection tasks, mobile mapping, Navigation and many more. The possibility of distributing or sharing GNSS data between many users in the same time regardless of the distance from the GNSS reference station is the very important advantage of NTRIP over the conventional means of RTCM data distribution systems.

7 CONCLUSIONS
Using networked transport of RTCM via internet protocol (NTRIP) with any compatible commercial software or NTRIP client (GNSS Internet Radio) provides significant benefits for the GPS users to get RTK/DGPS accuracy observations as discuss in the previous chapter. One of the most positive aspects of NTRIP is the cost factor and the longer data distribution distance in DGPS (or the baseline). Furthermore NTRIP overcomes the single user problem. NTRIP Caster or the GNSS broadcaster has the possibility to stream RTK/DGPS corrections up to 100 users simultaneously. On the other hand, this method has some drawbacks such as sudden disconnections from the server during the survey and high Latency (delay time), especially in RTK surveys. Since this paper is discussing about the RTK/DGPS correction base on GPRS Internet and it has been observed that the latency time of 2 seconds to 4 seconds was good enough for static observations. However the development of advance mobile communication technologies, such as EDGE and UMTS will surely eliminate the delay time of RTK message and this will reduce the communications costs also. In summary, the NTRIP protocol will be the future of RTK applications and it will provide the most cost effective, secure and fastest means of obtaining higher accuracy level of observations.

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