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Technology Focus

SiReNT provides accurate and precise multi-GNSS correction services by leveraging on advanced GNSS, communications and computing technology.

SiReNT users need only one receiver to carry out DGPS or RTK positioning in real-time. Real-time SiReNT correction data in RTCM 2.1/2.3/3.1 and CMR formats can be received from the internet using the NTRIP protocol.

For users who prefer to post process their data, SiReNT supports it with reference station data ready for download from our website in RINEX v2.10, RINEX v2.11 or RINEX v3.02 formats.

Differential GNSS Technique

Differential GNSS (DGNSS) technique is a positioning method that enhances the accuracy of stand-alone GNSS positioning. The technique involves the simultaneous operation of two GNSS receivers, one that's stationary with known coordinates and another that's roving around making position measurements. The stationary GNSS receiver is known as the GNSS reference station, while the user is known as the roving station. At the reference stations, DGNSS correction data is calculated and transmitted to the roving user. SiReNT provides the GNSS reference stations infrastructure to support DGNSS technique. The autonomous GNSS positioning accuracy of 5 to 10 metres can be improved to centimetres in real-time using the DGNSS technique. Two types of measurements are possible in DGNSS, the Code and Carrier-phase DGNSS measurements. The Code-based DGNSS is commonly known as “DGNSS”, while the Carrier-phase DGNSS is known as “RTK” (Real-Time Kinematic).

Code-based differential technique (DGNSS)

Code-based differential techniques use the GNSS pseudo-range measurement which is obtained by locking onto the pseudo-random code for a given satellite and measuring the time difference between transmit and receive time to determine satellite range. The code measurement is an absolute range measurement, which provides a value of the true range between the satellite and the receiver, after removal of clock errors and other error sources. It is relatively easy to implement differential GNSS for code measurements, because it is an absolute measurement.

Carrier phase differential technique (RTK)

The RTK technique is based on measuring distances to the satellites with carrier phase. In RTK, one receiver occupies a known reference station and broadcasts correction messages to one or more roving receivers.

Each roving receiver uses its own carrier phase measurements, together with the correction messages received from the reference station, to solve the integers of carrier phase cycles in real-time within the receiver. The process of solving the integers is known as the "initialisation" and the result obtained is an accurate position of the roving receiver relative to the reference station.

Network differential techniques

Over the last 5 years, Network-DGPS concept has been extensively used for high accuracy positioning all over the world. Network-DGPS is an improvement over the conventional DGPS method. The principle of Network-DGPS is that a significant portion of ionospheric, tropospheric and ephemeris errors are estimated over a region and this information is provided to rovers in the field. In carrier phase, the network approach is known as the Network-RTK technique. This technique has become very popular due to the benefits achievable. It is the ultimate technique for positioning and is especially significant for surveying because of the short observation time and high accuracy.

SiReNT system supports the Network-DGNSS technique. It uses the network software, Trimble Pivot Platform at the Data Control Centre which supports the Virtual Reference Station (VRS) technique. The software performs continuous computation of the following parameters by analyzing double difference carrier observations:

  • Ionospheric errors
  • Tropospheric errors
  • Ephemeris errors
  • Carrier phase ambiguities for L1 and L2.

Using these parameters Pivot will provide all GNSS data and interpolate to match the position of the rover, which may be at any location within the reference station network. Matching the rover's position provides a very short baseline, which reduces systematic errors for RTK considerably.

SiReNT reference stations

The SiReNT reference stations operate with weather-proof steel cabinets housing advanced GNSS receivers and environmentally hardened modem/routers and atmospheric monitoring sensors. Each station is also built with power and communications redundancy.

Three SiReNT reference stations, namely SNSC, SSMK and SSTS feature the use of stand-alone 3G communications, with SNSC and SSTS also using solar power for their operations.

SiReNT Reference Stations

SiReNT Equipment Box

SiReNT servers

The SiReNT servers consist of advanced hardware running the latest GNSS server software and applications to provide a multi-purpose capability to the SiReNT infrastructure. These servers which make up the Data Control Centre (DCC) are housed securely in the Government Data Centre (GDC) and makes use of the infrastructure of the GDC to provide a reliable and secure platform on which the SiReNT services are built on.

The GNSS raw data observed at each reference station are transmitted directly to the SiReNT DCC in real-time. At the DCC, the data is processed, archived and disseminated to users in real-time.

DGNSS data dissemination

The data dissemination of SiReNT is supported via SiReNT website (for post-processing applications) and NTRIP (for real-time applications). SiReNT users make use of the SiReNT website to download the post-process GNSS data. Data in RINEX v2.10, RINEX v2.11 or RINEX v3.02 can be generated from the website under the PP On-demand product. User is able to generate up to 12 months of data from the SiReNT website. Data older than 12 months will be archived. To access the old data, users need to put up a request to SLA.

For real-time applications of RTK and DGPS services, users will be connected to the SiReNT server via wireless Internet using 3G, 4G, LTE, WIFI or other wireless internet access methods.

Handshaking protocol

SiReNT adopts NTRIP (Networked Transport of RTCM via Internet Protocol) for the real-time dissemination of RTK and DGPS corrections. NTRIP is an application-level protocol used for streaming Global Navigation Satellite System (GNSS) data over the Internet. NTRIP is a generic, stateless protocol based on the Hypertext Transfer Protocol HTTP/1.1. The standard is meant to be an open none-proprietary protocol. NTRIP is designed for disseminating differential correction data (e.g. in the RTCM or CMR format) or other kinds of GNSS streaming data to stationary or mobile users over the Internet, allowing simultaneous PC, Laptop, PDA, or GNSS receiver connections to a broadcasting host. NTRIP supports wireless Internet access through Mobile IP Networks such as 3G, 4G, LTE, WIFI or other wireless internet access methods. The use of NTRIP allows rover users to be managed through the use of registered usernames and passwords.

More information on NTRIP can be found in Networked Transport of RTCM via Internet Protocol (NTRIP) by German Federal Agency for Cartography and Geodesy (BKG).

Last Update : 21 January 2015

 
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