Robots to improve satellite positioning accuracy in Australia

AUSGEO NEWS ISSUE 110 Jun 2013Robots to improve satellite positioningaccuracy in AustraliaSub-millimetre accuracy for global positioningNicholas Brown, John Dawson, Michael Moore, Shane Nancarrow and Guorong HuRobots are used in many ways to improve our lives by doing thingsmore precisely, efficiently and quickly. As of May 21st 2013,Geoscience Australia can now claim to have two AuScope AustralianGeophysical Observing System (AGOS) funded robots which willimprove the accuracy of satellite positioning in Australia.“Australia is in the fortunate position to beone of the few regions in the world to seeall the new and emerging Global NavigationSatellite Systems.”IntroductionSatellite positioning is increasingly ubiquitous. Global PositioningSystem (GPS) users can currently achieve 5 metre accuracy with ahand held device (for example, smartphone), but within the decadethe accuracy is anticipated to increase to a few centimetres due toimprovements in computational and modelling techniques, andexploiting Australia’s geographic location. Australia is in the fortunateposition to be one of the few regions in the world to see all the newand emerging Global Navigation Satellite Systems (GNSS), includingGalileo (Europe), GPS III (USA), GLONASS (Russia), COMPASS(China) and other regional systems being developed by Japan andIndia. This will enable new geospatial applications in geoscience,intelligent transport, precision agriculture and industrial automationto emerge as well as opening a world of new possibilities forsmartphone applications. Nevertheless, for this ambition to becomereality, a number of advancements and developments are required togeodetic infrastructure, analysis and modelling.Geoscience Australia’s roleGeoscience Australia has a key role in enabling Australia’s NationalPositioning capability by operating and analysing the data from anational network of permanent GNSS stations that both realise thenational coordinate system (thatis, the set of reference locationsthat define Australia’s latitudeand longitude) and support thedetermination of the preciseorbits of the GNSS satellites, akey requirement of precise userpositioning.Centimetre accuratepositioning with GNSSThe underlying signalstransmitted by the GNSSsatellites can be consideredas repeating sine waves.Measurements of these wavesare referred to as carrier phaseobservations and they differfrom the range-to-satellitemeasurements that the currentgeneration of smartphones use.Carrier phase observations enablethe distance between the orbitingsatellites and a user’s receiver tobe determined with centimetreaccuracy and are subsequentlyused to compute the user’s 3Dposition. The more accurate therange measurements are, themore accurately the user cancompute their position.Carrier phase observationshave been used for two decadesby surveyors to undertakerelative centimetre-accuratepositioning but limitationsRobots to improve satellite positioning accuracy in Australia www.ga.gov.au/ausgeonews/ | 1

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

AUSGEO NEWS ISSUE 110 Jun 2013Australia, and south-east Australia (Gippsland and Otway Basins).Geological evidence suggests that the deformation is likely to besmall, perhaps as little as 0.1 mm/yr, and difficult to measure exceptover very long time spans. However, with improved GNSS antennamodels the possibility of measuring deformation before an earthquakeoccurs may become a reality in time spans of several years. With this,an improved understanding of earthquake hazard in Australia willbe achieved.Figure 4: Robots are used to calibrate GNSS antennas and improve howaccurately we can measure the dynamic Earth.Sensing the atmosphere to improve weatherforecastingThe Earth is surrounded by layers of gases held in place by the Earth’sgravity field. Signals, such as those transmitted by GNSS, propagatedfrom space, are delayed as they pass through the atmosphere. Inthe troposphere, the layer of atmosphere from the Earth’s surface toapproximately 20 kilometres altitude, the delay is proportional totemperature, pressure and humidity. The ionosphere, the layer ofatmosphere from 50–1000 kilometres altitude, causes signal delaysas a function of the frequency of the signal. The composition of boththe troposphere and ionosphere vary both in space and time, and thisvariability currently limits theaccuracy, speed and reliability ofGNSS positioning. But it’s notall bad news, and like a computeraxial tomography (CAT) scanin medical science, the newGNSS signals and satellitescan potentially be combined toprovide a more complete 3Dpicture of the atmospheric delayas a function of time. Modelsthat more completely removethe atmospheric delays will leadto improved accuracy, speed andreliability of positioning, and thisability to sense the atmosphereshould ultimately lead toimproved weather forecastingin Australia.Construction processSite selection and design for theantenna calibration facility beganin the second half of 2011 with anumber of site selection surveysto identify the best location forthe facility on the grounds ofGeoscience Australia. Ultimately,the location on the western sideof the Geoscience Australia’ssupport building was chosenas the preferred site because ofits close connection to a powersupply, good ground conditionsand an unobstructed view tothe north. This is importantfor GNSS users in the southernhemisphere because of GNSSsatellite constellations; themajority of satellites are to thenorth of our location.The construction of thefacility was underway in January2013 with Geoscience Australiamanaging the contractors. TheRobots to improve satellite positioning accuracy in Australia www.ga.gov.au/ausgeonews/ | 4

AUSGEO NEWS ISSUE 110 Jun 2013facility includes the smaller Geo++ robot and larger industrial robot,an air-conditioned hut for the robot controller to operate the robots,a rubidium external frequency (high accuracy) clock, a continuouslyoperating GNSS station, multiple GNSS receivers and web camera tomonitor the calibrations.The state-of-the-art Global Navigation Satellite System roboticantenna calibration facility was officially opened by the Minister forScience and Research, Senator Don Farrell, at Geoscience Australia on20 May, 2013.Related articles andwebsitesVideo footage of the robots and thelaunch by Senator Don Farrellwww.ga.gov.au/about-us/news-media/media/accuracy-for-global-positioning.htmlFor more informationemail: ausgeomail.com.auRobots to improve satellite positioning accuracy in Australia www.ga.gov.au/ausgeonews/ | 5