A REAL TIME GPS REFERENCE NETWORK FOR CADASTRAL SURVEYS IN RECIFE, BRAZIL

Abstract

The establishment and maintenance of geodetic reference frames which are an essential foundation for all surveys related to land management, GIS and large scale mapping are very expensive. Large countries like Brazil cannot afford a dense, nation-wide control network. Active GPS reference stations provide an alternative concept for reference frames. Because of the high potential of relative carrier phase-based GPS measurements, it is sufficient to tie GPS surveys to the next reference station with precisely known coordinates. The connection to adjacent control monuments is not required as long as network distortions can be either neglected or modelled. Therefore the effort for classical monumented networks can be reduced drastically. For those applications requiring sub decimetre or even centimetre-level accuracy in real time, the working radius of a field station is limited to less than 10 km from the next reference station, especially in the equatorial region. To overcome this unsatisfactory situation, the spatial and temporal correlations of GPS measurement errors introduced by ionosphere, troposphere, and satellite orbit need to be modelled in a real time multi station solution. A concept for such a local GPS reference network covering only densely populated areas or an important economic region has been developed in a collaborative Brazilian-German research project. In the paper, we discuss the set up of a local GPS reference network that was operated in the urban area of Recife during November 2000, and positioning results for control and cadastral surveys are given. The real time multi station solutions yield more precise, more reliable, and much faster results, but are still limited by strong ionospheric anomalies.