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Establishment & Testing of Dubai Virtual Reference System (DVRS) National
GPS-RTK Network
Mr. Y. Al Marzooqi
Head of Mapping & Control Unit,
Dr. H. Fashir
Senior Geodetic Specialist,
Mr. Tarig Babiker
Geodetic Quality Controller,
P.O.Box 67
Survey Section
Planning and Survey Department
Dubai Municipality
Dubai, United Arab Emirates
Tel. +97142063555
Fax + 97142217871
Email: geodesy@dm.gov.ae
Web site: http://www.dm.gov.ae
SUMMARY
The Survey Section of the Planning and Surveying Department has now implemented the latest technology for professionals engaged in the fields of Surveying and Construction Engineering. Recent technological advances, both in GPS Survey & digital data, have made it possible to obtain accurate positioning and design data in real time. These changes introduce many new opportunities and exciting challenges to the surveying discipline within today's construction engineering sector. The Survey Section has reinforced its objectives in technology development by establishing the "Dubai Virtual Reference System" (DVRS). This represents the latest concept in the field of Global Positioning Systems (GPS).
DVRS consists of five continuously operating Base Stations provided by Swiss instrument manufacturer LEICA, and are located in different sectors of the Dubai Emirate. The Control Room, with a central Server, is situated in the Survey Section offices within the main Dubai Municipality building in Deira. All five stations are continuously receiving GPS data and are linked to the Central Server by dedicated telephone lines. These lines continuously transmit the data to the control unit (Control Central Server), where data is processed, and corrections transmitted to the end-users, as they require.
All GPS continuous operating Base Stations in the network send on-line raw GPS data via permanent connection (Modem lines) to the Control Central Server. Here, data is processed by a software system called GNSMART, produced by Geo++, of Germany. GNSMART performs all quality checks, computes ionospheric, tropospheric and satellite orbit errors, and delivers the corrections to the end-users. Employing a GPS 'Rover' unit, users can directly start survey work anywhere in the Dubai Emirate. When the GPS receiver is operated, it calculates its position to within a few meters and transfers this information to the Control Reference Station via GSM (Mobile) in NMEA format. The Control Central Server sends back valid correctional data to the user in RTCM format. This defines the user's actual position. This complete task is effectively achieved at the 'press of a button' in the field. Such real-time kinematic GPS technology enables work to be carried out within the entire network coverage area, with homogeneous absolute position to centimetre accuracy.
A multiple GPS reference station approach is superior to a conventional RTK single baseline approach, as it allows for "network-based" homogeneous positioning solutions with centimeter accuracy. The DVR System has already been subject to stringent testing, which found that the expected accuracy to be in the order of 2-3 cm in planimetry, and 3-5 cm in altimetry.
1. Classical GPS
Global Positioning System (GPS) technology is a fast and accurate method of determining the location of any point of interest anywhere on earth at any time during the day or night. The technology collects and processes signals from GPS satellites in orbit around the earth to determine the location of points of interest on the ground.
Surveying with single frequency measurement is called "static" mode GPS surveying. Some single frequency systems can collect data in static or Kinematics mode, but the rate of data collection cannot be compared to that produced by dual frequency systems. If the single-frequency unit loses its signal when operating in kinematic mode, it takes some time to re-initialize before survey work can begin again.
GPS Dual frequency system requires post-processing when operating in static or "fast static" mode. In Real Time Kinematics (RTK) GPS, the positional data are displayed and recorded immediately. Sub-centimeters to millimeter level accuracies (both horizontal and vertical) are obtainable with both single and dual frequency technologies. To obtain latitude, longitude, and elevation for new point, both systems need to occupy a number of existing established points. Considerable skill, training, and expertise are required to operate either type of system effectively.
The idea GPS all-differential positioning techniques are to correct bias errors at one location with estimated bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal for all satellites in view. GPS receivers require software that can apply individual pseudo-range corrections for each Space Vehicle (SV) prior to computing a position solution.
1.1 Literature Review
Real-Time Kinematics (RTK) GPS is now widely used for surveying and other precise positioning applications. The classical RTK technique requires that GPS data be transmitted from a single reference receiver to one or more roving units.
Algorithms in the mobile unit combine the reference station data with measurements from the roving receiver to resolve the integer ambiguity required to calculate precise ranges from the GPS carrier phase measurements. The process of ambiguity resolution is often referred to as "initialization".
RTK can provide centimeter position accuracy, but the accuracy and reliability of the standard RTK solution decreases with increasing distance from the reference station. This limitation on the distance between the roving GPS receiver and the RTK base station is due to the systematic effects of ephemeris, tropospheric and ionospheric errors (Wübbena, et al.,1996).
These systematic errors result in reduced accuracy and increasing initialization time as the distance between base and rover increases. This phenomenon is becoming increasingly evident as we approach a maximum in the cycle of solar activity.
1.2 Limitations of Classical RTK Surveying
The restriction in range of classical RTK is due not only to the systematic errors described above but also, in many cases, to the range of available radio telemetry solutions. In practice this means that a temporary RTK base station must be established close to the work area, often at a location that does not provide any physical security or continuous power supply. Each time such a temporary reference station is established there is an opportunity to introduce an error in the reference station co-ordinates that will be transferred into the position calculated by the rover RTK receiver. Such an error can easily go undetected when using a single base station. In addition to the potential for introducing errors, productivity of the surveyor is lost each time the base station has to be set up at different reference station.
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