DGNSS Standards

Fundamentals
Title	DGNSS Standards
Edited by	GMV
Level	Intermediate
Year of Publication	2011

The classical DGNSS technique technique is an enhancement to a primary GNSS system, that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station.

Introduction

Unlike other GNSS Augmentation systems as SBAS or GBAS, DGNSS is not used for civil aviation, but for urban transportation and in offshore areas. Therefore, the standards applying to DGNSS systems are the ones of data transmission standards defined by the Special Committee 104 on DGNSS of the Radio Technical Commission for Maritime Services (RTCM). Except for RTCM, there exist other proprietary DGPS standards, such as Trimble Compact Measurement Record (CMR).

RTCM Standards

The internationally accepted data transmission standards for DGNSS are defined by RTCM, particularly by its Special Committee SC-104. RTCM SC-104 is a standard that defines the data structure for differential correction information for a variety of differential correction applications. It was developed by the Radio Technical Commission for Maritime Services (RTCM) and has become an industry standard for communication of correction information. Note that RTCM is a binary data protocol.

The applicable documents to DGNSS systems are listed in the following table and constitute the current version of the core set of documents to be used for the development of a new DGNSS system.

Document Item	Reference	Issue	Comments
Recommended Standards for Differential GNSS (Global Navigation Satellite Systems) Service	RTCM 10402.3	2.3	This standard is used around the world for differential satellite navigation systems, both maritime and terrestrial.
Differential GNSS (Global Navigation Satellite Systems) Services	RTCM 10403.1	3.1	A more efficient alternative to RTCM 10402.3
Standard for Networked Transport of RTCM via Internet Protocol (Ntrip)	RTCM 10410.0	1	An application-level protocol that supports streaming Global Navigation Satellite System (GNSS) data over the Internet
Standard for Differential Navstar GPS Reference Stations and Integrity Monitors (RSIM)	RTCM 10401.2	2	A companion to RTCM 10402.3, this standard addresses the performance requirements for the equipment which broadcasts DGNSS corrections

The RTCM 10402.3 standards defined the messages for differential correction information. There are 64 types of messages. The message format is a sequence of 30 bits. The messages 1 to 17 are available in older RTCM versions, while messages 18-21 have been added in version 2.3 to made the standard applicable to RTK corrections.

The RTCM 10402.3 DGNSS message types are summarized in the following table:

Message type identifier	Message Name
1	Differential GPS Corrections
2	Delta Differential GPS Corrections
3	Reference Station Parameters
4	Surveying
5	Constellation Health
6	Null Frame
7	Beacon Almanacs
8	Pseudolite Almanacs
9	Partial Satellite Set Differential Corrections
10	P-Code Differential Corrections (all)
11	C/A-Code L1, L2 Delta Corrections
12	Pseudolite Station Parameters
13	Ground Transmitter Parameters
14	Surveying Auxiliary Message
15	Ionosphere (Troposphere) Message
16	Special Message
17	Ephemeris Almanac
18	Uncorrected Carrier Phase Measurements
19	Uncorrected Pseudorange Measurements
20	RTK Carrier Phase Corrections
21	RTK Pseudorange Corrections
22-58	Undefined
59	Proprietary Message
60-63	geMultipurpose Usa

The release of RTCM version 3.0 is developed to be more efficient in terms of broadcast bandwidth and also provides higher integrity . Note that Version 3 messages are not compatible with Version 2.x. The new release, Version 3.1, now designated as RTCM Standard 10403.1, incorporates GPS Network Corrections, which enable a mobile receiver to obtain accurate RTK information valid over a large area. In addition, new GPS and GLONASS messages provide orbital parameters to assist in rapid acquisition. A Unicode text message is also provided for the transmission of textual data. Finally, a set of messages are reserved for vendors who want to encapsulate proprietary data in their broadcasts. Unlike Version 2.x, this standard does not include tentative messages. The messages in Version 3 have undergone testing for validity and interoperability, and are considered to be permanent. Since many receivers have been designed and programmed for use with Version 2.x messages, RTCM is maintaining both standards 10402.3 and 10403.1 as "current" standards.

Other Standards

The Compact Measurement Record (CMR) was developed by and initially used by Trimble in 1992. The format was developed as a method of transmitting code and carrier phase correction data in a compact format from GPS base stations to GPS rovers for RTK GPS surveying. Dr. Nicholas Talbot of Trimble publicly introduced the CMR format at the 1996 Institute of Navigation (ION) Conference.