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
GREF-IPNational Real-TimeNetwork, Germany
GPS/GLONASSReference Stationsnetworked throughGPSNet software
GREFGREF--IPIPReference Reference StationStation
VirtualVirtualReference Reference StationStation
GREFGREF--IP IP plannedplannedReference Reference StationStation
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 ................................
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
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