Journal of Global Positioning Systems (2005)
Vol. 4, No. 1-2: 240-244
Benefits of Telecommunications Technology to GPS Users
Thomas Yan
School of Surveying & SIS, U niversity of New South Wales, Sydney NSW 2052, Australia
e-mail: [e-mail] Tel: [phone]; Fax: [phone]
Received: 27 November 2004 / Accepted: 17 October 2005
Abstract. For many years, telecommunications
technology has assisted GPS users in accomplishing their
tasks. Dial-up system over copper phone line enables
users to download data from base station at remote
locations. Radio modem provides wireless
communications link between a base station and a rover
to enable surveyors to carry out RTK-surveys. While
these techniques are still very much in use, developments
in telecommunications technology over the last decade or
so has brought more services providing easier use, faster
speed and wider coverage. Fast spread of Internet has
made TCP/IP protocols ubiquitous resulting in more
devices being IP-enabled and Internet-connected.
Wireless technology such as GPRS and 3G make better
use of bandwidth providing faster speed and better
coverage to mobile users. This paper looks at these new
emerging technologies and how they could have impact
on GPS users. It also discusses recent GPS-related
protocols such as Ntrip and RTCM 3.0 which were
designed in response to these new developments.
Examples will be presented based on local trends, settings
and conditions in Australia.
Key words: telecommunications, network, wireless,
mobile, protocols, GPS
1 Introduction
While Global Positioning System (GPS) is mainly used
for navigation and surveying purposes, many aspects of
the system rely on telecommunications technology. All
three segments that make up the system, the space
segment (a constellation of satellites), the control
segment (ground base stations) and the user segment (the
signal receiver) are essentially common building blocks
to a telecommunications system. Both the space and
control segments of GPS are controlled solely by the U.S.
Department of Defence. The user segment, the receivers,
are designed and produced by various manufacturers and
used widely by public. Naturally, this renders the user
segment to much innovative development and fast
adoption of new technology.
The simplest use of GPS receiver is by using it
autonomously, independent of external feedback. This
technique typically only gives a positioning accuracy of
approximately 15 m. This figure can significantly be
improved by using differential measurement techniques,
making it more useful for many applications. These
techniques require user’s receiver to communicate to one
or more other receivers to produce measurement with
higher accuracy. Traditionally, users would need to setup
their own reference receiver and communication link to
the rover receiver. In some areas, the reference receivers
might be operated as a service by government agencies or
commercial companies. For example, in some countries
beacon transmitters have been established along the
coasts to assist marine crafts in navigation. All these
require dedicated communication link to be established
separately. Within the last decade however, Internet and
mobile networks have grown very rapidly. This provides
users with servicing technology with which they can
employ differential techniques widely.
2 Protocols
A protocol defines the format and the order of messages
exchanged between two or more communicating entities,
as well as the actions taken on the transmission and/or
receipt of a message or other event (Kurose and Ross,
2003). Usage of common protocols means compatibility
and interoperability. This section looks into several
current and new protocols that the author believed will
have significant role to GPS users.

Yan: Benefits of Telecommunications Technology to GPS Users 241
2.1 RTCM 3. 0
Several different protocols exist for exchange of GPS
data but two protocols have become standard, NMEA
0813 and RTCM. As the names suggest, these protocols
were produced by the National Marine Electronics
Association (NMEA) and Radio Technical Commission
for Maritime Services (RTCM). In February 2004,
RTCM released the third version of their recommended
standards for differential GNSS service commonly
referred to as RTCM 3.0.
RTCM 3.0 has been developed as a more efficient
alternative to previous versions. It was developed based
on requests from service providers and vendors for a new
standard that would be more efficient, easy to use and
more easily adaptable to new situations. The main
complaint was that the parity scheme of Version 2 was
wasteful of bandwidth. Another complaint was that the
parity was not independent from word to word.
Furthermore, even with so many bits devoted to parity,
the actual integrity of the message was not as high as it
should be. RTCM 3.0 is intended to correct these
weaknesses (RTCM, 2004).
RTCM 3.0 consists primarily of messages designed to
support real-time kinematic (RTK) operations. The
reason for this emphasis is that RTK operation involves
broadcasting a lot of information, and thus benefits the
most from an efficient data format. RTCM 3.0 provides
messages that support GPS and GLONASS RTK
operations, including code and carrier phase observables,
antenna parameters and ancillary system parameters.
However, the format is specifically designed to make it
straightforward to accommodate modifications to these
systems (e.g., new L2C and L5 signals) and to new
systems that are under development (e.g. Galileo).
RTCM 3.0 has been designed using a layered approach
adapted from the Open System Interconnection (OSI)
standard reference model. A diagram of the OSI standard
reference model is shown here.
Fig. 1 Seven layers of OSI m od el (D oyle & Zecker, 1996 )
The protocol defines message format on Application,
Presentation and Transport layers. The bulk of the
document is on the Presentation Layer and describes the
message, data elements and data definitions.
Implementation on Data Link and Physical layers are left
to service providers to determine as they see appropriate
to the application.
The higher efficiency of RTCM 3.0 will make it possible
to support RTK services with significantly reduced
bandwidths. This is especially relevant in wireless and
mobile networks where the bandwidth available is much
less than that of wired network. The expected
performance of this protocol will open ways to more
stringent and unique applications of high-accuracy
positioning technique. Bock et al. (2003) presented a
network-based RTK technique in which raw data from
several reference stations were aggregated and delivered
to users via wireless channel. Such application would
benefit tremendously from reduced use of bandwidth.
For wireless link users, who are charged by the amount of
bandwidth used, reduced bandwidth means reduced
operating cost. For CORS network operators with leased
data lines from telecommunications service provider, the
reduced bandwidth allows them to provision link with
lower data rate and naturally, lower charge from the
service provider.
Major GPS manufacturers such as Trimble, Leica and
NovAtel have expressed support for RTCM 3.0 by
providing firmware upgrade to their products and
integrating RTCM 3.0 capability into their current
products.
2.2 TCP/IP
The Internet protocols are the world’s most popular op en -
system protocol suite because they can be used to
communicate across any set of interconnected networks
and are equally well-suited for Local Area Network
(LAN) and Wide Area Network (WAN) communications.
The Internet protocols consist of a suite of
communications protocols, of which the two best known
are the Transmission Control Protocol (TCP) and the
Internet Protocol (IP).
TCP/IP was first developed in the mid-1970s and has
since become the foundation on which the Internet is
based. IP occupies the Network Layer on OSI reference
model while TCP occupies the Transport Layer. TCP/IP
is the foundation on top of which many other Application
level protocols such as HTTP and FTP are built.

242 Journal of Global Positioning Systems
2.3 Ntrip
Ntrip stands for “Networked Transport of RTCM via
Internet Protocol”. It is an Application layer level
protocol which is used to stream Global Navigation
Satellite System (GNSS) data over the Internet. Ntrip is a
generic, stateless protocol based on the Hypertext
Transfer Protocol (HTTP). The HTTP objects are
enhanced to GNSS data streams. Ntrip was built on top of
the TCP/IP foundation. It was developed by the Federal
Agency for Cartography and Geodesy (known as BKG),
Germany.
Ntrip is designed for disseminating differential correction
data (e.g. in the RTCM-104 format) or other kinds of
GNSS streaming data to stationary or mobile users over
the Internet, allowing simultaneous PC, Laptop, PDA or
receiver connections to a broadcasting host. Ntrip
supports wireless Internet access through Mobile IP
networks such as GSM, GPRS, EDGE or UMTS.
Recently, Ntrip has been adopted by RTCM as their
recommen ded s t andard.
The Ntrip system consists of three software components:
NtripClient, NtripServer and NtripCaster. The
NtripCaster is the actual HTTP server program while
NtripClient and NtripServer act as HTTP clients. In the
diagram below, NtripServer receives data from a source
(typically a GPS reference receiver) and forward it to the
NtripCaster. The NtripCaster acts as a ‘switchboard’
which connects NtripClients to their required streams.
Fig. 2 Connection in an Ntrip system (Weber, 2004)
2.4 Ethernet
The term Ethernet refers to the family of Local Area
Network (LAN) products covered by the IEEE 802.3
standard that defines what is commonly known as the
CSMA/CD (Carrier Sense Multiple Access Collision
Detect) protocol. Three data rates are defined in the
standard with 10 Mbps (10Base-T Ethernet) and 100
Mbps (Fast Ethernet) being the most common rates at the
moment (Cisco Systems, 2003).
Other technologies and protocols have been touted as
likely replacements but the market has spoken. Ethernet
is currently used for approximately 85 percent of the
world’s LAN-connected PCs and workstations. Ethernet
has survived as the major LAN technology because it is
easy to understand, implement, manage and maintain. In
provides extensive topological flexibility for network
installation. Being a de facto standard also implies
guaranteed interconnection and operation with other
products regardless of manufacturer. Most networked
devices have Ethernet port as their standard network
connection, from common PC and laptop to the more
controversial Internet-enabled fridge and air-conditioner.
3 Applications
3.1 Network Appliance GPS Receivers
A GPS reference receiver typically has three to four serial
(RS-232) ports over which it communicates with other
devices. In a reference station setup, this receiver is
usually connected to a computer which logs measurement
data from the receiver which is then distributed to users.
This scheme works well for a single base station but has
its limitation. As the nu mber of base stations increased, it
becomes desirable to control the data centrally. For post-
processing use, it is more manageable to have a central
data repository compared to multiple computers storing
its own set of data. Issues such as user access, fault
management, backup, archive and data distribution are all
easier to handle with a centralised system. Also, different
levels of user needs may mean different type of streams
which is currently li mited to the ph ysical number of serial
ports on the receiver.
For real-time use, network-based solutions such as
Network-RTK has proven to be more reliable and offer
better performance compared to a single station solution.
Network-based solutions require real-time data streams
from multiple stations to be aggregated into a central
processing system.
A network-enabled GPS receiver provides a good
solution to these issues and an elegant way to distribute
data and manage the unit. Integration of Ethernet and IP
protocols into a GPS receiver brings about the concept of
network appliance to GPS receivers. With Internet
Protocol (IP) as the primary communications method,
public domain tools such as web browser and FTP client
can be used to configure receiver and access logged data
files.
D DG GP PS S/ /R RT TK K
r
ro ov ve er r r re ec ce ei iv ve er r 1 1
NTri
p
Client NTri
p
Client
NTripCaster
NTri
p
Server 1 NTri
p
Server
NTri
p
Source NTri
p
Source
HTTP HTTP
HTTP HTTP
...
...
...
D
DG GP PS S/ /R RT TK K
r ro ov ve er r r re ec ce ei iv ve er r n n
COM COM
COM COM

Yan: Benefits of Telecommunications Technology to GPS Users 243
As a network appliance, GPS receiver can provide
services to all users attached to the receiver through the
network. Different streaming services may be configured
on different TCP or UDP ports, for example, with
differing data rates or smoothing configurations. To
obtain a service, the client has only to connect to a
specific port. This allows multiple users to access
different streaming services simultaneously.
A network-enabled GPS receiver also provides better
remote access to operator. With a web browser, operator
can access configurations of the receiver via a webpage
without having to conn ect directly to it. This is especially
critical for operators of CORS network where the
reference stations are spread over a large area.
Previously, to change settings on a GPS reference
receiver, operators need to physically connect the
computer running the control software to the receiver via
serial ports. Obviously, it is not ideal if an operator has to
travel hundreds of kilometres only to modify
observations rate or other minor settings.
In a CORS network, this concept allows the central server
to connect to multiple GPS receivers. With RS-232, the
number of data streams is usually limited to the number
of physical serial ports available on the computer which
is around two to four ports. With Ethernet and IP
protocols, it is possible for the server to connect to tens or
hundreds of data streams. RS-232 is also much slower
compared to Ethernet. RS-232 typically has maximum
speed around 115,200 bps whereas Ethernet’s speed is
typically 10 Mbps – a hundred times faster than RS-232 –
with 100 Mbps connection becoming more and more
common.
Using Ethernet protocol allows for repeatab ility, multiple
connections and co mpatibility with high er layer protocols
such as TCP/IP or UDP/IP. Being the most popular link
layer protocol, Ethernet enables for easy connectivity
with other protocols. For example, in SydNET – a real-
time permanent GPS network in Sydney, Australia – it
allows GPS data to be aggregated to a central server via
fibre-optic network which runs on ATM protocol.
As of the time of writing, the author is aware of two
network-enabled GPS receivers in the market which are
equipped with built-in Ethernet and IP capability.
Trimble produces a model called the NetRS while Thales
Navigation also produces a model called the iCGRS.
3.2 Built-in Ntrip
Most parts of Ntrip implementation from BKG have been
released under GNU General Public License which
means it is open source. Th is makes it possible for service
providers and vendors to incorporate an Ntrip
implementation into their products. As of September
2004, implementations of NtripClient are available for
PC, Pocket PC PDA and Symbian mobile phone. Some
GPS receiver manufacturers such as Trimble and Leica
have also added NtripClient and NtripServer
implementations into their receiver software.
A list of hardware and software supporting Ntrip is
available from BKG’s website
(http://igs.ifag.de/ntrip_down.htm).
3.3 Using Mobile IP Networks & Wireless Broadband
While RTCM 3.0 addresses bandwidth issue by reducing
the message size, mobile networks have also developed in
terms of coverage and bandwidth. For example, GSM
technology – which gained popularity in Australia and
Europe – originally has data capability of 9600 bps. This
is only about one-fifth of a 56k dial-up modem.
Currently, all GSM networks in Australia have been
upgraded with GPRS technology which provides higher
data rate and IP network connectivity.
General Packet Radio Service (GPRS) is an upgrade to
GSM, providing packet-switching technology and
bridging the mobile network to IP network. Unlike GSM,
GPRS is an always-on connection where user is charged
not by the length of connection but by the amount of data
traffic. GPRS provides substantial improvement to data
speed with a theoretical limit of up to 170 kbps. GPRS
also allows mobile users to access the Internet directly.
All GSM networks in Australia, which include Telstra,
Optus and Vodafone, support and offer GPRS.
CDMA2000 1X is another technology offered in
Australia by Telstra. It offers data rate of around 144
kbps and connectivity to Internet. As with GPRS, users
have a choice of PCMCIA access card or a PDA with
built-in CDMA capability.
3G technology is another evolution from CDMA with
even higher data rate. It claims to provide up to 384 kbps
of data rate under ideal conditio n. So far, only PCMCIA-
type card is available to access the service. 3G network is
offered in Australia by Hutchison 3G Australia Pty Ltd.
While GPRS, CDMA2000 1X and 3G have all evolved
from mobile voice networks, wireless broadband started
as a service providing broadband connection via radio.
Unlike voice networks, this technology is pure IP and is
dedicated to data service. Wireless broadband has
superior data rate compared to the three technologies
mentioned previously. Its maximum speed is at around 1
Mbps which is almost three times that of 3G.
The main disadvantage of wireless broadband network is
because they’re relatively new their network coverage is
nowhere that of voice networks such as GSM and
CDMA. The iBurst network for example, is only
available in metropolitan area of New South Wales,

244 Journal of Global Positioning Systems
Queensland and Victoria. Another network, Unwired, so
far is only available in Sydney. However, it is expected
that as this technology gains popularity, their network
coverage will expand and cover more areas.
All the technologies mentioned here offer a new and
better ways to GPS users to obtain correction service in
the field. GSM or CDMA networks, being more
ubiquitous in terms of coverage, maybe the choice to
many users while those who demand even higher data
rate can choose to use personal broadband technology
such as iBurst or Unwired where available.
4 Conclusion s
This paper introduced several new technologies which
may have significant impact and use to GPS users in the
very near future. A new and improved protocol from
RTCM allows for reduced bandwidth which means users
with slow or limited data link could now exchange data in
a standard protoco l. Additionally, users now may be able
to provision several data streams usin g the same data link
due to the reduced bandwidth. From business point of
view, reduced band wi dth means reduced operat i ng cost.
Widespread of Internet and the protocols behind it means
a common set of protocols are being adopted. Ethernet
and IP protocols have gained wide popularity with many
devices adopting it as their standard communication
protocols. The concept of GPS receiver as network
appliance offer many benefits over the current serial
device implementation.
Finally, these protocols allow GPS users to utilise
telecommunications networks such as mobile networks to
disseminate correction service without having to build
new infrastructure. The advent of wireless broadband
service with its substantially higher data rate could en able
new techniques not possible previously to be used by
GPS users.
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Instantaneous Network RTK: Positioning and
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