Robots to improve satellite positioning accuracy in Australia

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AUSGEO NEWS ISSUE 110 Jun 2013in communications, ground infrastructure and the expense of thereceivers have meant that this capability has not transferred to themass-consumer market. But this is set to change as the national GNSSnetwork is supplemented with more sites with better communicationlinks, high quality user receivers become cheaper and analysistechniques become more sophisticated.Figure 1: An ideal antenna, no bias (modifed from Gerry Mader).Figure 2: A typical antenna, with bias (modifed from Gerry Mader).Modelling antennabiasesIn the attempt to further improvepositioning accuracy, the geodeticcommunity have looked to eachand every error source and haveattempted to either eliminate orbetter model that error. GNSSantenna biases are one such errorsource. All GNSS antennas havesmall inconsistencies in theirelectronic components caused bythe manufacturing process thatcurrently limit their accuracy(compare Figure 1 and Figure 2).This is where the robots come in:they provide a means to investigateand develop models of the antennabiases. Essentially, antenna biasesvary depending on the position ofeach satellite as they move acrossthe skyline (that is, the satellite’sazimuth and elevation). By rotatingand tilting the GNSS antennawith a robot as it tracks the GNSSsatellites, a highly accurate model ofthe antenna bias can be determined(Figure 3). By applying the modelsderived from the robotic system,these biases can be removed and thepositional accuracy improved.The GNSS research communityhave up until recently used genericantenna models for antenna types.However, with the ever growingaccuracy requirements of GNSSusers, individual antenna-specificmodelling is necessary to furtherimprove positioning accuracy.Antenna-specific models can onlybe derived using robotic calibrationsystems or prohibitively expensiveanechoic chambers, neither ofwhich was available to researchersin Australia, until now.Robots to improve satellite positioning accuracy in Australia www.ga.gov.au/ausgeonews/ | 2
AUSGEO NEWS ISSUE 110 Jun 2013Figure 3: Antenna calibration solution plot.Our robotsGNSS antenna calibration is a niche and highly specialised activity.Geoscience Australia is the home to one of five GNSS antennacalibration capabilities in the world and the only one in the southernhemisphere. The other four are at the National Geodetic Survey(Virginia, USA), University of Bonn (Bonn, Germany), SenB (Berlin,Germany), Geo++ (Hannover, Germany) and the Institute of Geodesy(IfE) (Hannover, Germany). The smaller of the two GeoscienceAustralia robots was purchased from Germany and its primary rolewill be to calibrate antennas prior to them being used in the field sothe positions they record are free from antenna modelling bias. Thelarger of the two robots is unique to GNSS calibrations globally andwill enable experiments to assess the impact of station and antennadesign on positioning accuracy. Furthermore, it has a lifting capabilitylarge enough to support the calibration of GNSS satellite transmittersprior to launch and the speed and agility to replicate the movement aGNSS station would experience in an earthquake.Improvements for science, industry andthe publicScientific and industrial GNSS users require high accuracy and integrityfor applications to fields such as environmental monitoring, crustalmotion, mining and construction. The improvements provided byabsolute antenna modelling will allow these users to acquire positionsan order of magnitude more accurately than they could of in the past.A new nationalcoordinate systemThe current Australian geodeticdatum, the Geocentric Datumof Australia 1994 (GDA94)was established in the 1990s,has relatively poor internalaccuracy, weak linkages to thelatest global coordinate system,and is ‘frozen’ at an epoch dateof 1994. It will be unable tosupport the next generation ofGNSS services and the manyscientific endeavours that requireaccurate positioning, includingsea level rise studies. The roboticcalibration facility will enable anupgrade of Geoscience Australia’snational GNSS network whichwill underpin an update of theAustralian geodetic datum.An improvedunderstanding ofearthquake hazardEarthquakes in Australiaoccur after the long-termaccumulation of strain onfaults that is subsequentlyreleased. Over the last decadescientists have suggested thatprecise GNSS observations ofcrustal motion might provideinsights into this seismiccycle. Towards this objective,Geoscience Australia, togetherwith our State Governmentcounterpart agencies, haveestablished monitoring networksin the seismically active areas ofAustralia including the southwestseismic zone near Perth inWestern Australia, the FlindersRanges near Adelaide in SouthRobots to improve satellite positioning accuracy in Australia www.ga.gov.au/ausgeonews/ | 3
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