Abstract:
Global Navigational Satellite Systems (GNNS,) of which the Global Positioning System (GPS) of the United States is the most widely used, is increasingly being used by the South African National Defence Force (SANDF) for navigation and positional data. However, the SANDF can only use civilian type GPS receivers, which make use of the Standard Positioning Serves (SPS) and not the encrypted Precise Positioning Service (PPS), which is only available to the United States military forces and its allies. The aim of this work is to understand the influences that impact on the use of a GPS and specifically the capabilities of civilian type GPS receivers. The first objective will be to propose and motivate the use of a standardised reference frame that can be used by the SANDF for positional data in general and for GPS measurements specifically. In this regard it is proposed that the SANDF standardises on the World Geodetic System 1984 (WGS 84) as the standard ellipsoid and also use it as a universal horizontal datum for mapping projects. For survey tasks WGS 84 can be used in combination with a selected International Terrestrial Reference Frame (ITRF) epoch for reference stations. The International GNNS Service (IGS) stations can be used as such reference stations; the geometric distribution can be improved as the African Reference Frame (AFREF) add more stations to the existing IGS network. In the absence of a common vertical datum it is suggested that the Earth Gravitational Model 2008 (EGM 2008) be used. Secondly, the use of GPS for positional data should be aligned with the required positional accuracy requirements and standards of the SANDF. In this regard it is suggested that international positional accuracy standards are accepted and implemented to ensure interoperability. The third objective is to describe and understand how to mitigate influences that impact on the reliability of GPS. This is specifically important with the use of low accuracy civilian type GPS receivers for navigation and the collection of ground control for mapping projects. The fourth objective will be to establish with practical field trials the effect of these influences on GPS measurements and device appropriate data collection strategies. One serious impact is the susceptibility of civilian GPS receivers to jamming. This is addressed but not sufficiently to formulate policy and would require further investigation. There is a worldwide drive to make GPS reliable for safety of life applications such as air and rail transport which also benefits its use for military applications. It is therefore important for the SANDF users to know and understand these influences on GPS in order to optimise its use for operations. Copyright
