5.3 World Geodetic System of 1984 (WGS 84).
The struggle to tie different regional datums and the advent of Satellite-based positioning systems assert the need for a global geodetic reference system.
World Geodetic System 84(WGS84) in the determination of true shape of the earth through mapping as the regional datums and the advent of Satellite-based positioning Systems, asserts the need for a global geodetic reference system.
The reference ellipsoid of WGS84 is essentially that of the International Union of Geodesy and Geophysics (IUGG) Geodetic Reference System 1980 (GRS80) with some minor changes. The International geodetic community at the IUGG's 17th quadrennial meeting, held in Canberra, Australia in 1979, as best representing the size and shape of the earth adopted this ellipsoid.
World Geodetic System has a long history starting in the early 1960s, when the United States Department of Defense introduced the WGS60. It was created from a global Database of conventional geodetic measurements, satellite observations, data from High-Precision Short-Range Navigation (HIRAN), Airborne trilateration, or range measuring systems developed during and after World War II. In the years following the introduction of WGS60, the accuracy and number of satellite observations greatly increased and led to the development of WGS66 and subsequently WGS72.
WGS72 was initially adopted as the Conventional Terrestrial System (CTS) for describing the orbits of the Global Positioning System (GPS) Satellites in their navigation messages (signals). But as with WGS60 and WGS66, the accuracy of WGS72 eventually was found wanting. WGS72 was superseded by WGS84 and has been used for GPS navigation messages since January 1987.
GPS Nomina Constellation
24 Sattellites in 6 Orbital Planes
4 satellites in each Plane
20,200 km altitudes,55 Degree Inclination
Figure 1. Space Segment (Satellite Constellation by Dana)
[X, Y, Z & T]
Figure 2: Measurements of code phase arrival times from at least four satellites are used to estimate four unknown: position in three dimensions (X, Y, Z) and GPS time (T).
By V.B.Mugemuzi
5.4 GPS Technology in Tanzania
The first time Tanzania introduced GPS receivers, was in 1992 under the Urban Sector Engineering Project (USEP) part of which covered the topographical mapping of nine Cities in Tanzania. The said project was executed jointly by three Companies Viak AS, Blom AS and Norconsult AS all of them from Norway with the collaboration to Surveys and Mapping Division (SMD) of the Ministry of Lands and Human Settlements Development. Since then many projects have been done to increase controls in WGS84 using GPS receivers available at SMD. The long-term aim is to cover the whole country. The recent ICAO project, to establish WGS84 Controls at the airports in Tanzania, aims to provide a very precise geometrical reference for Tanzania majors Airports. This task will serve as a backbone for future surveys and the future GPS-based navigation infrastructure. The actual survey consisted of GPS measurements at 8 main airports in Tanzania; two of them are first order points located at Dar es Salaam and Mbeya airports. The remaining six are located at Kilimanjaro, Zanzibar, Mtwara, Dodoma, Mwanza and Kigoma airports. The distribution intends to provide precise co-ordinates in the international reference frame (ITRF/WGS84). The points provide important navaid positions as well as the position and orientation of the runways. NeSA B.V. from the Netherlands did the fieldwork in co-operation with the Tanzania Directorate of Civil Aviation and the full participation of Tanzanian Surveys and Mapping Division.
6.0 The Immediate Need of Tanzania
As mentioned above, Tanzania is already close to WGS84 through the ICAO project. The main thrust now is to use this basic framework to densify GPS Control networks. In order to improve on this framework, some primary levelling and gravimetric observations are urgently need to complete basic parameters for computing the reference spheroid. Local resources can not accommodate this work which rightly puts Tanzania on the Global System now termed the Global Village Reference Frame. The intended goals will be achieved through joint global efforts upon addressing such existing pressure, where moral and material support are required. The scientist congregation at Taipei, not only discussing modern technology among of members attending the conference, but further to open the eyes for investors to collaborate on projects like control densification and chances of education to accelerate surveys and mapping activities in Tanzania.
Thank you for paying attention.
References
- Alfred Kleusberg, 1992. Precise Differential Positioning and Surveying, Department of Surveying Engineering, University of New Brunswick
- B. Hofmann-Wellenhof, H. Lichtenegger and J. Collins, 1992. GPS theory and Practice
- Jan Van Sickle, 1996. Global Positioning System (GPS) for Land Surveyors.
- Venant B. Mugemuzi, 1991. The question of preserving Survey marks in Tanzania. A paper presented on the technical committee of former Ardhi Institute then University College of Lands and Architectural Studies - Dar es Salaam Tanzania.
- Venant B. Mugemuzi, 1999. Accelerating Cadastral Survey Processes to Improve Up-to-date Digital Parcel Information in Tanzania. Professional Masters Degree report in GIS for Cadastral Applications. ITC, Enschede, The Netherlands.
- Venant B. Mugemuzi, 2000. Interpretation of WGS84 and Advantages. A paper presented on the Tanzania Aeronautical Information Service Officer's Association (TAISOA) being the 2nd Annual General Meeting (AGM) held on 14 April 2000 in Dar Es Salaam.
