Geodetic
Reference Frame
SiReNT infrastructure is developed based on the GPS (Global
Positioning System) technology that provides data for surveying,
positioning and navigation purposes. The SiReNT system supports
GPS users by providing DGPS correction data
to increase the positioning accuracy. With SiReNT a user
needs only one receiver to carry out Differential GPS (DGPS)
positioning. SiReNT supports both real-time and post-processing
DGPS.
In real-time mode, SiReNT supports RTCM 2.1/ 2.3 and CMR
format. Both formats are commonly adopted by all GPS equipment
manufacturers. The real-time DGPS data from SiReNT are accessible
via Internet using the NTRIP standard. In post-processing
mode, SiReNT’s GPS data is distributed via SiReNT
website in RINEX 2.1 (Receiver INdependent EXchange) or
Compact RINEX (HATANAKA compression) version 1.0 format.
SiReNT system adopts the latest technology of Network-DGPS
both for RTK and Code-based DGPS.
For detail information about GPS and GPS positioning techniques,
you may refer to the following websites:
Keywords: GPS, DGPS, RTK, Network-DGPS, Network-RTK, VRS
Differential GPS (DGPS) technique is a positioning method
that enhances the accuracy of stand-alone GPS positioning.
The technique involves the simultaneous operation of two
GPS receivers, one that's stationary with known coordinates
and another that's roving around making position measurements.
The stationary GPS receiver is known as the GPS reference
station, while the user is known as the roving station.
At the reference stations, DGPS corrections data are calculated
and transmitted to the roving user. SiReNT system provides
the GPS reference stations infrastructure to support DGPS
technique. The autonomous GPS positioning accuracy of 5
to 10 metres can be improved to sub-metre using the DGPS
technique. Currently, DGPS is widely used in marine navigation
and land surveying.
Two types of measurements are possible in DGPS, the Code
and Carrier-phase DGPS measurements. The Code-based DGPS
is commonly known as the “DGPS”, while the
Carrier-phase DGPS is known as the “RTK” (Real-Time
Kinematic).
You can refer to this website
for more information about DGPS technique.
Code-based differential techniques use the GPS 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
GPS for code measurements, because it is an absolute measurement.
RTK technique is based on measuring distances to the satellites
with carrier phase. In RTK, one receiver occupies a known
reference station and broadcasts a correction message to
one or more roving receivers. The roving receivers process
the information to solve the WGS-84 vectors by solving the
integers of carrier phase cycles in real-time within the
receiver to produce an accurate position relative to the
reference station. The process of solving the integers is
known as the "Initialisation".
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, troposheric 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-DGPS technique. It
uses the network software, GPSNet from Trimble
Terrasat GmbH at the Data Control Centre which supports
the Virtual Reference Station (VRS) technique. The GPSNet
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 GPSNet will provide all
GPS 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.
You may visit Trimble
website for more information on Trimble VRS.
Traditionally, GPS reference stations are operated on stand-alone
basis and the DGPS corrections generated are based on single
reference station. However, when a few GPS reference stations
are connected in a network, significant advantages are observed.
Some of the advantages of connecting the GPS reference
stations in a network are
• reduce systematic errors
• improve initialisation and accuracy for RTK survey
• increase productivity
• eliminate needs to establish local reference station
• reduce investment for the user as he only needs
one GPS equipment
• provide integrity monitoring
• all users in common, established coordinate reference
frame
• one common access point for the whole network
The concept of VRS is based on a virtual reference station
which is an imaginary, unoccupied and located only a few
meters from the RTK user. Observation data are created for
this virtual reference station from the data of surrounding
reference stations (permanent reference stations) as though
they had been observed on that position by a GPS receiver.
How does the VRS concept work?
1. The data from the SiReNT reference stations are transmitted
to Data Control Centre (DCC) directly via dedicated link.
2. The RTK user sends his approximate position (NMEA string)
3. The software at DCC determines in which triangle the
user is stationed and computes a virtual reference station
based on the transmitted approximate position.
4. The DCC transmits the data of the virtual reference
station to the user.
The figures below illustrate the working concept of VRS.
Refer to this website for more information on VRS.
Note:
Graphics are used with permission from Trimble
The SiReNT system consists of one Data Control Centre, six
GPS Reference Stations and one Remote Access Terminal. Currently,
SiReNT’s 6 GPS reference stations are situated at
strategic locations to ensure good coverage of whole Singapore
island. The reference stations are operating continuously
24/7. The map below shows the 10km radius coverage of each
reference station.
Figure: The coverage of SiReNT
The main equipment at the reference station is a set of
dual-frequency geodetic grade GPS receiver and antenna.
The reference station also consists of other items for communications
and monitoring. All the equipments are housed in an equipment
box. The other peripherals installed at the reference station
are as follow:
| Item
|
Description
|
| 1 |
100Ah Sealed lead acid battery |
2 |
BMV 501 Battery Monitor |
3 |
Nport 5210 Terminal server |
4 |
Victron Blue power battery charger |
5 |
Low voltage disconnect |
6 |
APC Telephone line lighting arrestor |
7 |
Ruggedcom RX1000 Industrial Router |
8 |
MOXA EDS-205 Industrial 5 port switch |
9 |
Trimble NetRS |
10 |
Polyphasor Lighting arrestor |
11 |
Choke Ring Geodetic antenna |
12 |
Cooling fan with thermal switch |
Figure: SSEK reference station
Figure: A typical
set up of an equipment box
The GPS raw data observed at each reference station are
transmitted directly to the SiReNT Data Control Centre (DCC)
in real-time via 256Kbps ADSL communications link. A separate
56Kbps dial-up internet access is used the back-up communications.
At the DCC, the data are processed, archived and disseminate
to users in real-time. The Remote Access Terminal (RAT)
which is situated at SLA allows for remote access to the
DCC. The main function of RAT is for the maintenance of
the SiReNT.
The data dissemination of SiReNT is supported by the iGate
(for post-processing applications) and NTRIP (for real-time
applications). Both the iGate and NTRIP are installed at
the DCC. See below for detail of iGate and NTRIP.
| |
iGate |
NTRIP |
| Survey style |
Post-processing |
Real-time |
| Client sends |
iGate commands |
HTTP requests |
| Client receives |
RINEX |
RTCM or CMR |
| Network type |
LAN, Internet |
Wireless Internet |
| Transport protocol |
TCP/IP |
Mobile IP over GSM, GPRS |
| Client software |
Internet Explorer, Trimble Total Control |
Survey Controller v.11.31 |
SiReNT users make use of the SiReNT website to access
the post-process GPS data via iGate. RINEX version 2.1
and Compact RINEX can be generated from the website under
the PP On-demand product. User is able to generate up
to 12 months of data from SiReNT website. Data older then
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 SiReNT server via wireless
Internet. Currently, rover users can subscribe to GPRS
data plan from any mobile provider
SiReNT adopts the NTRIP for the real-time dissemination
of DGPS corrections. NTRIP (Networked Transport of RTCM
via Internet Protocol) 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
GPS receiver connections to a broadcasting host. NTRIP
supports wireless Internet access through Mobile IP Networks
such as GPRS. The use of NTRIP allows rover users to be
controlled using 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)
SiReNT is a nation-wide reference station network infrastructure
used for the purpose of high precision positioning and surveying
with GPS. In the geodetic perspective, SiReNT serves as
the national geodetic reference frame for surveying and
positioning activities. It is regarded as the “Zero”
order geodetic control of Singapore which is the reference
frame for all mapping, positioning and surveying applications.
SiReNT infrastructure is key component in the new SVY21
cadastral survey system, implemented in August 2004 by
the Singapore Land Authority. Under the new system, surveyors
are required to use the SiReNT data for establishment
of the Integrated Survey Network (ISN) control marks as
the survey control in cadastral work. A guideline was
introduced for the utilisation of SiReNT for this purpose.
This ensures a homogeneous reference system for cadastral
survey.
