fibre ocean tide loading - communities@rics

FINDINGS IN BUILT AND RURAL ENVIRONMENTS NOVEMBER 2008

FiBRESERIESAN ASSESSMENT OF OCEAN TIDELOADING MODELS FOR PRECISEGPS POSITIONING IN GREAT BRITAINNigel Penna, Peter Clarke and Stuart EdwardsSchool of Civil Engineering and GeosciencesNewcastle University, UK

Research

2 OCEAN TIDE LOADING

GPS positioning and ocean tide loading

Precise Global Positioning System (GPS) estimatedpositions are degraded by ocean tide induced periodicdisplacements of the land upon which the GPS antenna is sited. This ocean tide loading (OTL) displacementarises from the Earth’s response to the change in massdistribution of the water in the oceans as they moveperiodically due to the gravitational attractions of theMoon and Sun. The (predominantly vertical) OTLdisplacement is spatially very variable and not simply afunction of distance from the coast. In some places,notably Cornwall, Great Britain, the land can move by upto about 14 cm in around 6 hours, which is amongst thelargest in the world. The periodic land displacement canbe represented as a sum of harmonic constituents, thedominant of which around Great Britain have approximatelysemi-diurnal periods, i.e. ‘M2’ (12.42 hours) and ‘S2’ (12hours). Figures 1 and 2 shows the amplitudes of the M2and S2 OTL height displacement across Great Britain,demonstrating that they are largest in south-west England,and that the S2 displacement is about one third the sizeof the M2 displacement.

For the most precise applications, GPS surveyors andgeodesists must mitigate ocean tide loading effects.Whilst relative positioning is usually used to removecommon error sources at reference and user stations,the OTL spatial variability makes it difficult to quantify thismethod’s validity for OTL mitigation. Therefore OTLdisplacement coefficients are needed, requiring a (wellknown) model of the structure of the Earth, but also amuch less well known global model of all the ocean tidesrelative to the observation point, and this limits theaccuracy of the OTL displacement coefficients. BeforeOTL displacement coefficients can be used in GPSpositioning with confidence, the quality of the ocean tidemodels used must be assessed, and this study sought todetermine which ocean tide model(s) should be used forhigh precision GPS positioning in Great Britain. The focuswas on 24 hour GPS positioning and accumulatedcoordinate time series, as typically used for thecoordination of GPS reference stations (e.g. for RTKpositioning), national reference frame definition andgeophysical monitoring.

Figure 1. M2 OTL displacement amplitudes across theBritish Isles for a 0.25 degree grid using the NAO.99bocean tide model. For clarity, the amplitudes have beencapped at 50 mm, which is only exceeded in the ocean SW of Land’s End.

348˚ 350˚ 352˚ 354˚ 356˚ 358˚ 0˚ 2˚62˚

60˚

58˚

56˚

54˚

52˚

50˚

62˚

60˚

58˚

56˚

54˚

52˚

50˚

0 10 20 30 40 50

348˚ 350˚ 352˚ 354˚ 356˚ 358˚ 0˚ 2˚mm

Figure 2. S2 OTL displacement amplitudes across theBritish Isles for a 0.25 degree grid using the NAO.99bocean tide model. For clarity, the amplitudes have beencapped at 15 mm, which is only exceeded in the ocean SW of Land’s End.

348˚ 350˚ 352˚ 354˚ 356˚ 358˚ 0˚ 2˚62˚

60˚

58˚

56˚

54˚

52˚

50˚

62˚

60˚

58˚

56˚

54˚

52˚

50˚

0 3 6 9 12 15

348˚ 350˚ 352˚ 354˚ 356˚ 358˚ 0˚ 2˚mm


Testing ocean tide models using GPS coordinates time series

This study contributed to the first demonstration with realdata that unmodelled semi-diurnal ocean tide loadingdisplacements propagate into spurious long wavelengthsignals in 24 hour GPS coordinate time series. The largestspurious signals have roughly fortnightly and semi-annualperiods, which hence degrade GPS position estimates,and care must be taken to avoid erroneous interpretationof the resulting coordinate time series.

Whilst the ocean tide loading induced spurious signalsare a nuisance for precise positioning, Penna et al. (2007)showed that they can also be used to provide a means oftesting the quality of the ocean tide models. This newGPS approach to the testing of ocean tide model qualitywas used in this study for a range of continuous GPSstations distributed across Great Britain, in order toattempt to evaluate which ocean tide model should beused by the surveyor and geodesist in Great Britain forGPS positioning. The method involved processing manyyears of continuous GPS data per station consideredusing the NASA Jet Propulsion Laboratory’s GIPSYsoftware. The ocean tide loading coefficients applied inthe processing were generated using different ocean tidemodels, namely no model, FES99, NAO.99b and TPXO2,then a set of coordinate time series generated per modelconsidered for each station. By assessing the size of thespurious signals at fortnightly (M2 ocean tide loadinginduced) and semi-annual (S2 ocean tide loading inducedamongst other processes) arising when each of thedifferent models were applied, the quality of the oceantide models was assessed.


Experiments and results of GPS testing of ocean tide models

Six continuous GPS stations evenly distributed acrossGreat Britain were selected to assess the size of thefortnightly signals arising in the GPS height time seriesand hence test the quality of the different ocean tidemodels available across the country. Contour maps ofthe root mean square differences between the M2 oceantide loading displacement amplitudes from the differentocean tide models were computed to assess where theexpected biggest ocean tide model errors were expectedto aid in the station selection. The largest discrepancieswere found around the Bristol Channel, but no GPSstations with sufficiently long data records were availablein that region (to confidently discriminate via spectralanalysis the size of the spurious signals arising on applyingthe different ocean tide models long time series of at leastseveral years were needed). Therefore as a compromisebetween data record length, even geographical coveragedistribution and closeness to areas of inter-modeldiscrepancies, the stations selected were Newlyn (50.10°N, 5.54° W), Aberystwyth (52.42° N, 4.00° W), Lowestoft(52.47° N, 1.75° E), Newcastle (54.98° N, 1.62° W),Glasgow (55.85° N, 4.30° W) and Lerwick (60.14° N,1.18° W). Since the M2 ocean tide loading displacementcontour maps had suggested that the five modern oceantide models considered (CSR4.0, FES99, GOT00.2,NAO.99b, TPXO6.1) were in close agreement at thesestations, two were arbitrarily selected as a check, namelyFES99 and NAO.99b, together with the older TPXO2 andno ocean tide loading being modelled at all.

Clear fortnightly signals of several millimetres in amplitudewere seen in the GPS height time series at all six stationswhen no ocean tide loading was modelled, which wereconsiderably reduced (almost to the noise level) wheneither the FES99 or NAO.99b ocean tide model was used.This suggests that either of these models may be usedfor precise GPS positioning at these stations acrossGreat Britain when using 24 hour data batches. Since thevalues of the other three modern ocean tide modelsconsidered (i.e. CSR4.0, GOT00.2 and TPXO6.1) werevery similar to the FES99 and NAO.99b values, these canimplicitly be considered suitable also. When using theolder TPXO2 model however, fortnightly signals withamplitudes above the time series noise level remained,suggesting that this should not be used for precise GPSanalysis at these stations.


Conclusion and Outlook

This study helped to demonstrate that GPS height timeseries formed from 24 hour data batches contain spuriousfortnightly and semi-annual signals due to unmodelledsemi-diurnal ocean tide loading displacement, and thatthese signals can be used to test the quality of ocean tide models.

For the six GPS stations considered that were distributedacross Great Britain, any of the five modern ocean tidemodels CSR4.0, FES99, GOT00.2, NAO.99b and TPXO6.1can be considered equivalent for mitigating ocean tideloading effects when processing GPS data in 24 hourbatches, as used for very precise applications. It wasshown that the older TPXO2 model should not be used.

The ocean tide model testing method demonstrated canbe further used in the future across Great Britain (andelsewhere) as continuous GPS data records continue togrow both temporally and spatially, to enable denser andmore localized evaluations and recommendations ofwhich ocean tide model should be used by surveyors.This is likely to assume further relevance if commercialGPS softwares incorporate the facility to model oceantide loading displacement.


About the study

This study was supported with funding from the RICSEducation Trust and contributed to the findingspublished in Penna N.T., M.A. King and M.P. Stewart(2007) GPS height time series: Short-period origins ofspurious long-period signals, Journal of GeophysicalResearch – Solid Earth, 112, B02402,doi:10.1029/2005JB004047.

The study was made possible through the free provisionof GPS data from the NERC BIGF Facility(www.bigf.ac.uk) and the International GNSS Service(http://igscb.jpl.nasa.gov). The GIPSY v4.0 software andprecise orbits and clocks were provided by the NASA JetPropulsion Laboratory; the SPOTL software of DuncanAgnew was used, as was the Ocean Tide Loadingprovider web servicehttp://www.oso.chalmers.se/~loading/ of Machiel Bosand Hans-Georg Scherneck. The providers of thedifferent ocean tide models also made this studypossible.

Advancing standards in land, property and construction.

RICS is the world’s leading qualification when it comes toprofessional standards in land, property and construction.

In a world where more and more people, governments, banks andcommercial organisations demand greater certainty of professionalstandards and ethics, attaining RICS status is the recognisedmark of property professionalism.

Over 100 000 property professionals working in the major establishedand emerging economies of the world have already recognised theimportance of securing RICS status by becoming members.

RICS is an independent professional body originally establishedin the UK by Royal Charter. Since 1868, RICS has been committedto setting and upholding the highest standards of excellence andintegrity – providing impartial, authoritative advice on key issuesaffecting businesses and society.

RICS has a worldwide network. For further information simply contactthe relevant RICS office or our Contact Centre.

AsiaRoom 1804Hopewell Centre183 Queen’s Road EastWanchaiHong Kong

T +852 2537 7117F +852 2537 [email protected]

United Kingdom12 Great George StreetParliament SquareLondon SW1P 3ADUnited Kingdom

T +44 (0)870 333 1600F +44 (0)20 7334 [email protected]

AmericasThe Lincoln Building60 East 42nd StreetSuite 2918New York, NY 10165USA

T +1 212 847 7400F +1 212 847 [email protected]

OceaniaSuite 2, Level 161 Castlereagh StreetSydneyNSW 2000Australia


Rest of EuropeRue Ducale 671000 BrusselsBelgium

T +32 2 733 10 19F +32 2 742 97 [email protected]

AfricaPO Box 3400Witkoppen 2068South Africa

T +27 11 467 2857F +27 86 514 0655 [email protected]

RICS HQ

12 Great George StreetParliament SquareLondon SW1P 3ADUnited Kingdom

Worldwide mediaenquiries:E [email protected]

Contact Centre: E [email protected] +44 (0)870 333 1600F +44 (0)20 7334 3811

rics.org

Middle EastOffice F07, Block 11Dubai KnowledgeVillageDubaiUnited Arab Emirates


Europe

fibre ocean tide loading - communities@rics

Documents