If you wish to contribute or participate in the discussions about articles you are invited to contact the Editor
DGNSS Standards
| Fundamentals | |
|---|---|
| Title | DGNSS Standards |
| Author(s) | GMV |
| Level | Medium |
| 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 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’s 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 10403.0 | 3 | 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 |
La transmisión de correcciones diferenciales entre el receptor de referencia (BASE) y los receptores remotos está estandarizada según propuesta de la Radio Technical Commission for Maritime Services, Special Committee 104, RTCM-SC104. La versión 2.0 incluye mensajes para hacer posible la corrección a las pseudodistancias y variación con el tiempo a la corrección de éstas. La versión 2.1 contiene además tipos para las correcciones a las medidas de fase.
Hay 64 tipos de mensajes disponibles. El formato de los mensajes consiste en secuencias de palabras de 30 bits. Los últimos 6 bits en cada palabra son los bits de paridad. Cada mensaje comienza con dos palabras cabecera. La primera palabra es fija y contiene lo que se denomina preámbulo (preamble), el tipo de mensaje, y el identificador de la estación de referencia. La segunda palabra contiene la marca de tiempo en forma de cuenta Z, el número de secuencia, la longitud de mensaje y la salud de la estación de referencia.
Los mensajes 1-17 están disponibles en ambas versiones de RTCM, mientras que los mensajes 18-21 contienen cabeceras de 3 palabras y tienen como propósito el posicionamiento RTK (Real Time Kinematic) de aplicación en receptores que admitan esta técnica. La "estación base" debe de estar a una distancia de unas pocas decenas de kilómetros (20-30 Km) para que la correción sea efectiva. Esta técnica permite alcanzar precisiones centimétricas en tiempo real.
Número de mensaje Estado actual Título 1 Fixed Differential GPS Corrections 2 Fixed Delta Differential GPS Corrections 3 Fixed Reference Station Parameters 4 Retired Surveying 5 Fixed Constellation Health 6 Fixed Null Frame 7 Fixed Beacon Almanacs 8 Tentative Pseudolite Almanacs 9 Fixed Partial Satellite Set Differencial Corrections 10 Reserved P-Code Differential Corrections (all) 11 Reserved C/A-Code L1, L2 Delta Corrections 12 Reserved Pseudolite Station Parameters 13 Tentative Ground Transmitter Parameters 14 Reserved Surveying Auxiliary Message 15 Reserved Ionosphere (Troposphere) Message 16 Fixed Special Message 17 Tentative Ephemeris Almanac 18 Tentative Uncorrected Carrier Phase Measurements 19 Tentative Uncorrected Pseudorange Measurements 20 Tentative RTK Carrier Phase Corrections 21 Tentative RTK Pseudorange Corrections 22-58 Undefined 59 Tentative Proprietary Message 60-63 Reserved geMultipurpose Usa
Other Standards
The Compact Measurement Record (CMR) by Trimble.