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