Real Time Streaming of Differential GPS Corrections over the Internet

Georg Weber

Federal Agency for Cartography and Geodesy, BKG Germany

EUREF Symposium, 5-8 June 2002, Ponta Delgada, Portugal

Abstract

� With the increased capacity of the Internet, applications that transfer continuous data streams by IP-packages, such as Internet Radio, have become well-established services. Compared to these applications, the bandwidth required for the transfer of real-time GPS data is relatively small. As a consequence, EUREF currently considers the use of the open Internet for real-time collection and exchange of GPS data, as well as for broadcasting derived products. Possible fields of geodetic application are the collection of raw or RINEX data from reference stations, and the distribution of RTCM corrections over the Internet for precise positioning and navigation purposes.

� This presentation focuses on various aspects of the implementation of a EUREF-based DGPS/RTK service using the Internet. The current status of a trial system called EUREF-IP is described. It streams RTCM corrections over the Internet and connected GSM/GPRS cellular phone networks. Server and client software characteristics are outlined. Problem areas like latency and IP-Multicast are discussed. The achievable accuracy is demonstrated based on test measurements. Conclusions are drawn concerning the potential of DGPS/RTK over the Internet for establishing location-based services.

DGPS Real-Time Communication

Cellular Networks: GSM

Space based: Communication Satellites

Terrestrial: AM,FM,VHF

Internet GPRS

Reference / Server Rover / Client

Differential GPS Data Rates

� DGPS 50 byte/sec, RTCM 2.0� RTK 500 byte/sec, RTCM 2.1

� Much less demanding than receivingan Internet Radio Station or Teleconferencing withmin. 5…20 Kbit/sec

DGPS & InternetPrincipleGPS

ServerPC

EUREF-IP Station on the Internet

CellularPhoneNetwork

GPRSModem

ClientPDAInternet elsewhere

DGPS

EUREF-IP

Trial-Serversin Europe

GermanyItalySpain Switzerland

Coming soon:NorwayPortugal

GREF-IPNational Real-TimeNetwork, Germany

GPS/GLONASSReference Stationsnetworked throughGPSNet software

GREFGREF--IPIPReference Reference StationStation

VirtualVirtualReference Reference StationStation

GREFGREF--IP IP plannedplannedReference Reference StationStation

Accuracy of DGPS/RTK over Internet & GSM

RTK over Internet & GPRS

Latency of DGPS and RTK Corrections

Latencies „Switzerland – Spain“

Latencies Latencies of DGPS of DGPS Corrections received via Corrections received via InternetInternetInstitutInstitut CartogràficCartogràfic de de CatalunyaCatalunya, , Espanya Espanya

ReferenceReference: Zimmerwald: ZimmerwaldRover: Rover: BarcecolaBarcecola

Late

ncy

(sec

)

.................................. 8th May 2002 ................................

Distributing the Workload, Server Concept

Primary Server Secondary Server

Real-Time Accessto DistributedData

Pocket PC & GPRS Modem

DGPS:0.7 Cent/min

IP Multicast using UDP

Server

Client

Client

Client

Client

Client

Router

Router

Router

Best solution,must be supported by allparticipating sub-networks

Not available at mostEuropean Mobile IP Provider

Sub-network

Sub-network

TCP/IP Unicast Client1

Client2

Client3

Client4

Client5

Clientn

Serv

er

Disadvantage: Workload on server-side,but good experience from Internet Radio

Costs: 20,000 Euro per 1000 DGPS clients and year

HTTP Splitter-Server

� HTTP application layer on top of TCP/IP

� Administration/Client/Sourcecommunication handled through one single port

� Any request/response begins withtype-specificmessage e.g.

.....$DGPS;euref-ip.ifag.de;2101;Frankfurt;D;RTCM 2.0;50.12;8.68;1;GPSNet

RTCM Correction Signal 1

GSM/GPRS/EDGE/UMTS

N TCP/IP Streams

Streaming Application

Internet/ Mobile IP

RS232

Client 1

RS232

. . .

RTCM Correction Signal M

DGPSSplitter Server

. . .

. . .

DGPSSource 1

. . .

DGPSSource M

M TCP/IP Streams

Administration

Client N

Administration

Client N

Source M

Communication Layer Model

Supporting networkapplications

Host-host data transfer

Routing of datagramsFrom source to destination

Data transfer betweenNeigboring network elements

Bits „on the wire“

Messages (e.g. SourceGet DGPS Data, OK,City Table, DataSegment

Datagram

Frame

Protocol Data Unit

Application

Transport

Network

Link

Physical

HTTP

TCP

IP

EthernetPPP

Varying Stream Content, One Single Port

Header RTCM Byte Stream

SinglePort

$DGPS;euref-ip.ifag.de;2101 DGPS/RTK Messages

TCP or UPD Protocol, pros and cons

� TCP causes additional overhead when setting up and tearing down connections, less problems with „long connections“

� UDP needs more sophisticated programming on client side

Security

� Usage of various port numbers for different data types critical with respect to Firewalls

� DGPS Internet applications probably best based like HTTP/FTP etc. andon one single port, e.g. 2101

Requirements for Participationof Service Providers

� GPS receiver

� Capable of generating RTCM

� Next to a PC� On the Internet� Running server software� Placed outside firewall

Conclusion� Streaming Differential GPS corrections over

Internet and cellular phone networks is feasible, yet under development

� No significant lack of performance compared to usage of other transportation media

� Advantages for service providers� Various developments depending on products� Promising solution for DGPS-based GIS

business & LBS� Trail Services within framework of EUREF

References

� DGPS corrections over the Internethttp://www.wsrcc.com/wolfgang/gps/dgps-ip.html

� NMEA Serverhttp://home.hiwaay.net/~taylorc/gps/nmea-Server

� RTCM Decoding Programm, Bäumker, FH-Bochumhttp://www.fh-bochum.de/fb5/baeumker/homemb2.htm

� Real-Time Streaming of DGPS Corrections via Internet, Feasib. StudyH. Gebhard and R. Kays, Informatik Centrum Dortmund,March 2002, unpublished

� WinCE DGPS/RTK Client Programhttp://www.networkgps.it/netison/download/software/idb

� EUREF-IP Trial Service, Server & Client Programshttp://igs.ifag.de/root_ftp/software/euref-ip-dgps.tar.gzhttp://igs.ifag.de/root_ftp/software/euref-ip-rtcm.zip