Land survey / geodetics
Geodetic observing systems provide a significant benefit to society in a wide array of military, research, civil, and commercial areas, including sea level change monitoring, autonomous navigation, tighter low flying routes for strategic aircraft, precision agriculture, land surveying, earthquake monitoring, forest structural mapping and biomass estimation, and improved floodplain mapping.
Recognizing the growing reliance of a wide range of scientific and societal endeavors on infrastructure for precise geodesy, and recognizing geodetic infrastructure as a shared national resource, many organizations have evolved to contribute to a growing body of research and development to support the infrastructure needed to meet the demands for increasingly greater precision. The stability and accuracy of the International Terrestrial Reference Frame (ITRF) over long time periods is a primary limiting factor for understanding sea level change, land subsidence, crustal deformation, and ice sheet dynamics. Of these, a quantifying long-term change in sea level imposes the most stringent observation requirements.
Modern geodetic methods have enabled the positions of geodetic stations to be determined within a well-defined frame of reference; as a result, the long-term movements of the Earth’s surface can now be monitored at a level of approximately one millimeter per year. Dense networks of GNSS/GPS stations are used to map the strain in the Earth’s crust at plate boundaries and to observe plate tectonics. GPS contributes the large number of sites that define the ITRF (contributing to its density) and contributes to precise monitoring of polar motion. GPS, DORIS, and SLR are used to position space-orbiting platforms in the ITRF, and GPS is used to position instruments on the Earth’s land and sea surfaces.
Such geodetic reference networks require extremely precise antennas. Reference networks serve multiple roles, including the performance monitoring of all signals broadcast by each GNSS constellation and producing accurate and precise GNSS measurements. Reference networks, which are used to enable RTK and PPP applications, consist of reference stations that require antennas that have the tight phase center variation and precisely known phase center offset. Geodetic quality antennas require a precise calibration, that estimates the phase centre offset at various frequencies (GPS L1, L2, L5 & L6 are typical). In addition to the phase center offset, the phase center variation as a function of azimuth and elevation angle is also calibrated, enabling the observations to be adjusted correctly. Additionally, to minimize multi-path choke ring antennas may be a required option.