03 February 2005, minor modifications 11 December 2006 and 16 June 2008

Helpful comments by Dan Norin (SWEPOS, Sweden) are gratefully acknowledged.

Many more aspects need to be considered when selecting a communication technique for the transmission of RTK corrections. 

• Technical aspects: range and coverage, transmission bandwidth, protocol, reliability and error correction. Furthermore, the accuracy of RTK positions decrease with an increasing latency of the reference data. Centimetre-accurate positioning requires data transmission latencies of one second or shorter. Another aspect concerns the size of the communication antenna/receiver which must be small enough to be integratable into the GNSS equipment or at least small enough to be carried by the RTK user.
• Economical aspects: The communication costs often consist of the purchase price of the appropriate transmitters and receivers only. But when using a communication service, as e.g. mobile phone, additional costs, like a monthly charge plus additional charges depending on the communication time or transmitted and received data volume, must be taken into account.
• Administrative aspects: Radio frequency bands cannot be used freely since governmental restrictions in power and frequency exist in most regions of the world. In many countries parts of the UHF frequency band may be used for low power transmission which, however, limits the communication range to just a few kilometres. 

The most common communication method for single-base RTK is to utilize radio transmission in the UHF band or sometimes in the VHF band at data rates up to 9600 bps. The exact choice of frequency depends on the licensing requirements for the specific area. Mainly due to power restrictions the working range is usually limited to a few kilometres in maximum. With a more powerful amplifier the range can be extended to a few tens of kilometre in open areas.

Network RTK is usually offered as a service covering a certain region. Hence, it is convenient to utilize existing communication services, which cover the same region, for the transmission of Network RTK corrections. In recent years mobile phone networks based e.g. on the GSM standard developed to the primary means for Network RTK data transfer. Network RTK service providers usually establish "dial in" access servers providing data communication without any protocol. A further enhancement for data transfer with GSM was introduced under the acronym GPRS. Latest developments include EDGE and the third-generation mobile phone technologies CDMA2000 and UMTS. These newer techniques divided the data stream into packets and thus require a communication protocol. In remote areas where terrestrial cellular service is unavailable satellite communication provided e.g. by Iridium or Globalstar may be an alternative. Examples of other modes of delivery are: FM sub-carrier broadcast using the Data Radio Channel (DARC) protocol (Park et al. 2002),  terrestrial television broadcasting with the data stream being modulated onto the audio sub-carrier (Sasano et al., 2000), and terrestrial digital audio broadcasting (DAB).

An important aspect with respect to delivering Network RTK corrections is whether the communication techniques are able to operate just in simplex or in duplex mode. Mobile phone networks are examples for duplex communication techniques, sub-carrier radio or video broadcast are examples for simplex techniques. Some forms of Network RTK require duplex mode of operation since the user has to send his approximate position to the central computing facility in order to 

• identify the surrounding reference stations and provide him with their observation data streams or
• identify the appropriate sub-network and provide him with network observations on a common ambiguity level or
• identify the closest reference station and provide the user with its observation data plus the coefficients of regional network correction models or
• compute Virtual Reference Station (VRS) observations to be delivered to the user.

Communication channels operating in simplex (broadcast) mode are sufficient if the user

• selects the surrounding reference station himself and establishes communication links to each of these stations or
• selects the appropriate sub-network himself in order to receive network observations on a common ambiguity level or
• selects the closest reference station himself in order to receive its observation data plus the coefficients of regional network correction models or
• selects the appropriate VRS data stream out of many VRS data streams offered for a dense grid of VRS-locations.

Communication channels operating in simplex mode have the advantage that they can serve an infinite number of users. An important advantage of duplex mode operation is the ability to identify each user individually for billing purposes.

In recent years the transfer of real-time data over Internet Protocol (IP) capable communication channels gained importance. The application-level protocol NTRIP was designed to disseminate differential correction data or other kinds of GNSS streaming data to users over the Internet. It allows simultaneous computer or receiver connections to a broadcasting host. NTRIP supports wireless Internet access through mobile IP networks like GSM, GPRS, EDGE, or UMTS (Weber et al. 2003, NTRIP, 2004). It is part of the RTCM standard (RTCM, 2004). Examples of Internet-based data transfer for single-base RTK or Network RTK have been published by Hu et al., 2002, Liu, 2004, Peterzon 2004, and Chen et al., 2004.
 

Chen, R., Li, X., Weber, G. (2004): Test Results of an Internet RTK System Based on the NTRIP Protocol. European GNSS 2004, Rotterdam (PDF file, 325 kB)

Hu, G.R., Khoo, V.H.S., Goh, P.C., Law, C.L. (2002): Internet-based GPS VRS RTK Positioning with a Multiple Reference Station Network. Journal of Global Positioning Systems, Vol. 1, No. 2:113-120 (PDF file, 1203 kB)

Liu, G.C. (2004): GPS RTK positioning via Internet-based 3G CDMA2000/1X wireless technology. GPS Solutions 7:222-229.

NTRIP (2004): Networked Transport of RTCM via Internet Protocol (NTRIP), Version 1.0 (PDF file, 249 kB)

Park, J.U. + 8 co-authors (2002): Multi-Reference GPS Network for the Nationwide RTK Service in Korea. ION GPS 2002, 2334-2341.

Peterzon, M. (2004): Distribution of GPS-data via Internet, LMV-report 2004:01, Lantmäteriet, Gävle, Sweden (PDF file, 1069 kB)

RTCM (2001): RTCM Recommended Standards for Differential GNSS (Global Navigation Satellite Systems), Version 2.3, RTCM Standard 10402.3 (RTCM webpage)

RTCM (2004): RTCM Recommended Standards for Network Transport of RTCM via Internet Protocol (Ntrip), Version 1.0, RTCM Standard 10410.0 (RTCM webpage)

RTCM (2006): RTCM Recommended Standards for Differential GNSS (Global Navigation Satellite Systems), Version 3.1, RTCM Standard 10403.1 (RTCM webpage)

Sasano, K., Petrovski, I., Ishii, M., Torimoto, H., Townsend, B. (2000): Method of Using a TV Sound Multiplexed Sub-Carrier Data Link for a DGPS/RTK-Service. ION GPS 2000, 2418-2423.

Weber, G., Dettmering, D., Gebhard, H. (2003): Networked Transport of RTCM via Internet Protocol (NTRIP), IUGG General Assembly, Sapporo (PDF file, 259 kB)