International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 2

Number 1

March 2008

Possibility of Precise Positioning and Precise

Inshore Navigation Using RTK and Internet

L. Kujawa & J. Rogowski

Warsaw University of Technology, Warsaw, Poland

ABSTRACT: The practical need for GNSS positioning in real time led to the development of a medium for

data transmission. DGPS correction data can be transmitted over an area of several hundred square kilometers

using longwave radio frequencies. The RTK technique needs greater radio line throughput capacity as well as

shorter distances between ground based reference stations. The RTK data from the reference stations can be

transmitted through the DARC system by local stations using UHF channels, but the local stations in Poland

are not interested in the propagation of RTCM data. The authors present test results of RTK and DGPS

measurements using data transmission by Internet and mobile phones. The rover user is equipped with a

GPRS (General Packet Radio System) modem in a GSM phone, which is connected to a laptop computer or

receiver controller with special RTCM Client software that receives the RTCM data stream from the server

via the TCP/IP protocol and transmits it via the serial port to the rover GPS receiver. The new Polish EUPOS–

ASGPL active control network allows the use of the RTK technique with a practical range of ten nautical mile

in Poland’s inshore area.

1 INTRODUCTION

Performed experiments involving the providing of

access to satellite data through the Internet and GSM

fully confirmed the effectiveness and capacity of the

utilized media as specified. Attempts at applying the

Internet and GSM were undertaken in parallel at

several research centers throughout Europe. This

concept garnered the interest of the IAG/EUREF

sub–commission and a resolution was passed at the

Ponta Delgada conference in June of 2002 to create

the infrastructure for a pilot project. Due to the

standardization of data transmission protocols as

well as GPS data formats, this was an important

step. The standards were approved in both Europe

and the United States, which makes possible the

utilization of information derived from many

reference stations using a single equipment and

software set.

An accuracy in terms of centimeters in

determining position in real time creates new

possibilities for such tasks as the maneuvering

efforts of watercrafts as well as the parameterization

of the maneuvering properties of watercrafts, the

specifying of navigation marker positions, and

precision navigation in the inshore zone.

Unfortunately, GSM operators lack information

regarding the actual range of their cellular networks

in the Baltic area. Nevertheless, it may be assumed

that important navigation regions such as port

approaches are within telephony range. The same

question regarding Polish territorial waters and the

Polish economic zone are more difficult to answer.

2 THE EUREF–IP NTRIP STANDARD

The approved data transmission standards have been

named “Networked Transport of RTCM via Internet

Protocol” (Ntrip). Ntrip is a general protocol that is

based on the Hypertext Transport Protocol (HTTP),

version 1.1. It was designed to propagate differential

corrections in the RTCM–104 and RAW formats.

Ntrip makes possible connection with desktop,

laptop, and personal digital assistants (PDA) as well

as certain new field receivers. Ntrip is fully adapted

for wireless access to the Internet using GSM

networks (GPRS, EDGE, and UMTS) because it

uses the TCP/IP protocol. The structure performing

data transmission tasks has been subdivided into

several elements for safety as well as organizational

reasons. In addition to the a receiver generating the

data stream in RTCM format and sending those data

to the RS port, the reference station is equipped in a

data server whose task is to acquire information at

the RS port and transfer that information to the

TCP/IP port addressed to the broadcaster. The

newest GPS transmitters have a built–in network

card, which makes possible direct forwarding of data

to the Internet. The task of the broadcaster is to

amass data from many reference stations and make

them available to users. The introduction of a

broadcaster is not vital, except for the convenience

of users, who can select from among many reference

stations under a single IP address as well as

guaranteeing greater safety of the local reference

station network. The last element of the tele–

information structure is the NtripClient software,

which allows users to conveniently connect with the

broadcaster and download data.

Thanks to NtripCaster it is possible to transmit

the following data streams:

− Data in RTCM format for DGPS and RTK,

− Data in RTCA format for EGNOS,

− RAW observational data as well as in RINEX

format,

− Correction in the VRS system,

− Precise orbit parameters in SP3 format.

Currently, data from all of the stations

collaborating within the framework of the EUREF–

IP project are sent to the broadcaster located at the

GPS/GLONASS Data Center (BKG) in Frankfurt

a/Main. That is also the location of the most

important information relating to the current state

and functioning of the project. The list of Polish

reference stations may be found below.

3 FIELD STATION EQUIPMENT

The experimental work involved the use of various

sets of equipment. Trimble 4700 and 5700 as well as

Javad Legacy receivers made available by the

Institute of Geodesy and Cartography were used in

the measurement experiments linked with RTK

techniques. On the other hand, experiments relating

to DGPS measurements used a Garmin 12 XL

receiver. The receiver sets were equipped in several

alternative sets of devices facilitating the registration

of data as well as communications with the data

broadcaster. The Trimble TSC1 field receivers used

did not have a built–in GSM modem nor any

possibility of adding in–house applications. In their

case it was necessary to use a notebook computer,

and a PocketPC at this time. The use of a PocketPC

equipped in a GSM modem eliminates earlier

inconvenience as well as the need to have a cellular

phone set with a GPRS modem. The above

described inconveniences have been eliminated in

new field receivers such as the Trimble ACU and

TSC2. TSC1 receivers and Siemens ME45 and Sony

Ericsson T68i cellular phones were available during

the conducting of field tests. It has now become

possible to use a PocketPC Fujitsu Siemens Loos

720 computer with a GPRS Pretec modem. The

configurations of available equipment sets are

presented in Figure 1.

Fig. 1. Field set for RTK and DGPS measurements

4 USER SOFTWARE

Free user software is available in the following

versions:

− Windows Client GNSS Internet Radio, Version

1.4.3

− Linux Client Perl NtripClient Program for Linux,

Version 0.5

− Windows CE Client GNSS Internet Radio,

Version 1.4.7 WinCE, PocketPC 2002/3,

− Palm OS Demo NtripClient Program, Version

1.2.1 © by Guenther Thalmann.

Fig. 2. The Windows and Windows CE version of NtripClient

4.1 The Józefosław Astrogeodetic Observatory Data

Access System

A correction transmission system in the RTCM–104

format has been developed by the Institute in

collaboration with the ARMIKRO company. The

system consists of server and DCTSRV broadcasting

software receiving corrections from the GPS receiver

and forwarding them to the Internet as well as

DCTGUI client software receiving corrections from

the Internet. A diagram depicting the operation of

the system is presented in Figure 3. The programs

use their own transmission protocol and can work

with any TCP port. The DCTGUI program for

receiving corrections was written in the FreeBSD

operating system environment using free Open

Source tools so as to make possible its use under the

control of the Windows system as well. DCTSERV

software may work under the control of FreeBSD,

OpenBSD, and Linux operating systems. It

collaborates with the Apache www server providing

access to an administration panel facilitating

supervision over operation and configuration using

the SSL.

Fig. 3. Diagram depicting system operations

Fig. 4. DCTGUI program user’s menu

5 INTERNET AND GSM NETWORK

CORRECTION ACCESS SYSTEM TESTS

A parameter of significance to the user is the delay

of the correction packet. In analyzing the time of

passage of the packets between the reference station,

broadcaster, and user it is necessary to isolate the

part of the delay in the Internet and the part caused

by GPRS transmission. It turns out that regardless of

paths taken by the packets in the Internet, delays are

not large and amount to approximately 100 ms,

where the percentage of data loss does not exceed

1%. The major portion of the delay is caused by

transmission in the GSM network; it amounts to

approximately 700 ms, where the quantity of lost

packets amounts to approximately 6%. Average total

time for the acquisition of packets in the EUREF–IP

amounts to 805 ms with data loss at a level of 7%.

The results of tests of data packet delays conducted

using one–day samples are presented in Figure 5.

Data packet acquisition time was investigated in

the Institute of Geodesy and Geodetic Astronomy

system — i.e. between the DCTSERW server

operating in Józefosław and the rover user using a

GPRS connection in the GSM network. For the most

part, results received do not diverge significantly

from those presented above, because the average

packet acquisition time amounts to 686 ms for the

entire day–long experiment, with a loss of 5% of

packets. Also tested was the age of RTK corrections

as received by the GPS receiver from the EUREF–IP

system. It is to this end that the specified positions

were registered using RTK mode on a computer. The

age of the corrections as received by the receiver are

presented in Figure 6. It was confirmed that the

average age of the RTK correction amounts to 1.5 s,

and that 99% of the results are in the 1.0–2.0 s range.

The age of the received corrections is sufficient to

measurements because it is less than the 4 s (the

border value).

Fig. 5. Packet acquisition time from Józefosław to the EUREF–

IP broadcaster as well as from the broadcaster to the user

Fig. 6. Correction age

In order to examine the availability and

repeatability of RTK measurements using the data

transmission system, the RTK position was

registered with a frequency of 1 Hz over three days.

Figure 7 presents breaks in reception of corrections

as well as their duration.

Breaks in reception of corrections for the duration

of the test were very short, with a maximum of 6.0 s,

where the average duration amounted to 4.1 s. The

reason for the occurrence of breaks was the load on

the Internet. Average correction availability was

defined at a level of 99.85%. Figure 8 presents

differences between registered position in RTK

mode and known coordinates.

Position availability in the RTK–Fixed mode for

the duration of the three–day experiment amounted

to 99.42% on average.

6 TEST TRAVERSE MEASUREMENTS

A test traverse consisting of points set up from 5 km

to 35 km at mutual distances of 5 km was built in

order to study the accuracy of RTK measurements as

a function of the distance to the reference station.

Careful measurements were conducted using static

techniques at the test point as was a series of RTK

measurements encompassing the registration of 200

positions at each point. Differences received in the

RTK and static modes did not exceed 1 cm at

distances of 5, 10, and 15 km from the reference

station. Figure 9 presents the differences for a

distance of 35 km.

Fig. 7. Breaks in reception of corrections over successive days

Fig. 8. Changes in coordinates

Fig. 9. Differences in coordinates for Point No. 1 at a distance of 35 km

Fig. 10. Standard deviation from the RTK measurements at the

test traverse points

Analysis of results as received demonstrates that

measurements using the RTK technique utilizing the

Internet and GPRS for the transmission of

corrections provide a highly accurate solution at

distances of up to 30 km from the reference station.

Receiver initialization time grows longer at distances

in excess of 30 km.

7 SUMMARY

We have confirmed, on the basis of experimental

measurements aimed at examining the usefulness

and effectiveness of the application of the Internet

and GSM in DGPS and RTK measurements, that the

presented solutions form an effective medium for the

transmission of corrections in the RTCM formats. It

may be predicted that transmission using the RTCM

v. 3.0 format, will be equally effective.

The proposed tele–transmission solutions do not

require investment in special infrastructure, they

have a very large range, and they demonstrate no

significant dependence on obstruction in the field

and electromagnetic interference. The proposed

solution only necessitates the startup of the data

server program at the reference stations. On the

user’s side, in work in the field, what is required is

connection to the Internet, which can be achieved

through an inexpensive GPRS connection in the

GSM networks. What is needed for currently used

receivers is an additional module for the reception of

corrections that can be implemented through the use

of personal computers of the PDA type or GSM

telephones servicing the JAVA or SYMBIAN

languages. The newest GPS and GPS/GLONASS

receivers are equipped in software facilitating direct

reception of corrections through the Internet and the

GSM network.

After conducting the experiments, it was

confirmed that delays in transmission through the

Internet and GPRS are insignificant and do not have

an impact on the effectiveness of the applied

technology. After conducting the measurements on

the test base it was confirmed that the effective range

of RTK measurements using the Internet and GSM

telephony amounts to approximately mainly 30 km

from the reference station and is determined by

limitations in RTK techniques. Together with an

increase in distance from the reference station,

initialization time of he measurement grows in

length and amounts to two minutes at a distance of

5 km, increasing to 10 minutes at distances of over

30 km. Measurement using RTK techniques is an

effective technology providing results of significant

accuracy that can be applied in most high–precision

surveying and navigation.

The experience gained thanks to the series of

measurement experiments demonstrates that

measurements using the RTK technique with the

greatest of precision should be conducted twice at

the same point with different satellite configurations

in order to eliminate errors that may occur.

The availability of RTK and DGPS measurements

utilizing corrections in the RTCM format transmitted

through the Internet and GPRS amounts to over

99%. Breaks in reception of corrections are very

short and have a duration of not more than

4 seconds.

The proposed solution may find broad application

in the transmission of corrections generated by the

reference stations of the fixed network. It is also

possible to transmit other data in real time, including

orbit parameters, coefficients of the current models

of the ionosphere and troposphere, observation data

in the receiver’s format (RAW), and others.

REFERENCES

Kujawa L., Leszczyński M., Rogowski J., Accessibility and

reliability of RTK measurements by Internet. Materials of

NTRIP Seminar & Woprkshops „Streaming GNNS Data via

Internet” Frankfurt a Man 2006.

Rogowski J.B., Kujawa L., Leszczyński M., Studies on

accessibility and reliability of RTK measurements by

internet. Procedings of Euref Symposium , Viena 2005.

Kujawa L., Leszczyński M., Rogowski J., Precyzyjna nawiga-

cja z wykorzystaniem Internetu i telefonii. Sesja naukowa z

okazji 85-lecia Wydziału Geodezji i Kartografii Poli-

techniki Warszawskiej, 2006.

Kujawa L., Leszczyński M., Rogowski J., Studies on accessibility

and realiabity of RTK measurements by Internet. The

EUREF Symphosium of the IAG Commision. Riga 2006,