Abstract: Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often overâestimated, par t icularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted rootâmeansquares of observationâmodel differences: 4.9â5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.
Abstract: The demand for geodetic time series that are accurate and stable is increasing. One factor limiting their
accuracy is troposphere refraction, which is hard to model and compute with sufficient resolution, both in time
and space. We have studied the effect of numerical weather model (NWM)-derived troposphere slant delays and
the most commonly used mapping functions, Niell and Vienna, on Global Positioning System (GPS) processing.
Six months of data were calculated for a regional Finnish network, FinnRef, which consists of 13 stations, using
Bernese v. 5.0 in double difference mode. The results showed that when site-specific troposphere zenith delays
or gradients are not estimated, the use of NWM-based troposphere delays improved the repeatabilities of all three
components of station positions (north, east and up) statistically significantly and up to 60%. The more realistic
troposphere model also reduces the baseline length dependence of the solution. When site-specific troposphere
delays and the horizontal gradients were estimated, there was no statistically significant improvement between
the different solutions.
Abstract: <p>Reaaliaikainen kinemaattinen mittaus (RTK) ja pysyvien GPS-tukiasemien verkottumisen myötä erilaiset verkko-RTK-sovellukset, kuten virtuaalitukiasemakonsepti (VRSâ¢), ovat viime vuosina yleistyneet nopeasti.</p>
<p>Geodeettinen laitos (GL) tutki sekä perinteisen RTK-mittauksen että virtuaalitukiasemakonseptin tarkkuutta vuosina 2003â2004. VRS:ää tutkittiin sekä Geotrim Oy:n GPSNet.fi-verkossa että Tampereen seutukunnan VRS-verkossa. Referenssipisteinä käytettiin EUREF-FIN-järjestelmään luotettavasti
mitattuja kiintopisteitä, ja mittauksia suoritettiin tasaisin välimatkoin tukiasemista. RTK-tutkimusta varten mitattiin uusi testikenttä staattisin GPS-mittauksin.</p>
<p>RTK ja VRS antavat hyvin samankaltaisia taso- ja korkeustarkkuuksia
(noin 3 cm, 1-Ï). Alustusaikojen suhteen VRS on hieman hitaampi kuin RTK. GPS-mittauksen virhe jaetaan yleensä vakiovirheeseen ja mittaus-etäisyydestä riippuvaan osaan. VRS:llä mittausetäisyydestä riippuva osa on pienempi kuin RTK:lla, mutta toisaalta RTK:n vakiovirhe on pienempi. Tutkimustulosten perusteella RTK on siis hieman tarkempi aivan lyhyillä vektoreilla, mutta VRS tarjoaa tasalaatuisempaa tarkkuutta laajoilla alueilla. RTK:n näennäinen paremmuus tukiaseman lähellä johtuu tutkimuksessa
käytettyjen testipisteiden tarkkuudesta eikä niinkään järjestelmien (RTK/VRS) välisestä erosta. RTK:lla testikenttä oli sisäisesti erittäin tarkka, kun taas VRS:n tapauksessa referenssipisteet olivat hierarkisesti eri luokkaa.</p>
<p>RTK tarjoaa tukiaseman koordinaateista riippuvia tuloksia, joten ne sopivat hyvin ympäristön kiintopisteistöön. VRS puolestaan tarjoaa homogeenisia
EUREF-FIN-määritelmän mukaisia koordinaatteja valtakunnallisesti
ilman omaa tukiasemaa.</p>
Abstract: <p>Geodeettinen laitos (GL) on osallistunut Suomen Etelämannertutkimusohjelmaan
vuodesta 1989 alkaen. Tällä hetkellä GL tutkii
maankuoren liikkeitä Kuningatar Maudin maalla. Tätä varten GL rakensi
vuonna 2003 Suomen Etelämannerasemalle, Aboalle, GPS-aseman aiemmin
rakennetun painovoimalaboratorion yhteyteen.</p>
<p>Nykyiset ja muinaiset jäämassat aiheuttavat muodonmuutoksia maankuoressa.
Nykyisten jäämassojen maankuoreen aiheuttaman muodonmuutoksen
sekä muinaisten jäämassojen massakompensaation hitaan muutoksen
yhteisvaikutuksen saamme selville painovoimamittausten avulla. GPSaikasarjoissa
havaitsemme vain maankuoren nykyisen muodonmuutoksen.
Kun yhdistämme GPS- ja painovoima-aikasarjat voimme teoriassa erotella
nykyisten ja muinaisten jäämassojen aiheuttaman muutoksen toisistaan.
Tulokset tulevat osaltaan parantamaan jääkauden aikaisten jäämäärien
vaihteluiden ajoittamista.</p>
<p>Tässä artikkelissa esitellään GPS-aseman rakentaminen ja asemalta
saadut ensimmäiset tulokset. Vuoden aikasarjan mukaan absoluuttinen
maankohoaminen Aboalla on 2,9 mm/a. Dataa tarvitaan kuitenkin vielä
useammalta vuodelta ennen kuin voimme vetää lopullisia johtopäätöksiä
liikkeiden suuruudesta. Esittelemme myös painovoimamittauksiin liittyvät
GPS:llä suoritetut jäätikkömallinnukset. Jäätikön liike- ja lumenkertymätutkimuksien
mukaan jäätikön virtausnopeus oli tutkimusalueella maksimissaan
5 m/a. Vuotuinen lumenkertymä oli jäätiköllä noin 0,6 m, kun
Basenin rinteellä se oli 1,2 m. Osoitimme RTK GPS:llä, että vaikka uutta
lunta kertyy paljon, ei lumimassan topografi assa tapahdu suuria muutoksia.
Uusi lumi pakkaa vanhan lumen tiiviimmäksi säilyttäen alueen massatasapainon.
Lisäksi kerromme Etelämantereen laajuisesta SCAR-mittauskampanjasta
sekä muista pienemmistä GPS-mittauksista, joita GL teki vuosien
2003-2004 aikana Etelämantereella.</p>
Abstract: Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present-day, three-dimensional (3-D) crustal deformation rates in Fennoscandia [ Johansson et al., 2002 ]. A preliminary forward analysis of these estimates [ Milne et al., 2001 ] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3-D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the (assumed constant) upper and lower mantle viscosity (νUM, νLM); specifically, we derive a 95% confidence interval of 5 à 1020 ⤠νUM ⤠1021 Pa s and 5 à 1021 ⤠νLM ⤠5 à 1022 Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the (Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from â¼200 km near the base of the upper mantle to â¼700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.
Abstract: This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations, Sealevel and Tectonics). We study loading effects due to ocean, atmosphere and hydrology and their impact on estimated rate parameters. We regularly find the admittance of a modelled perturbation at less than fifty percent of the full effect. We think that the finding relates to a difficult noise situation at all periods, and that a satisfying model for the dominating noise source has not been found yet. An additional reason for low admittance is found in the mapping process of the no-fiducial network solution into a conventional reference frame.
Abstract: Postglacial rebound is a long-studied phenomenon in Fennoscandia, and the general features of contemporary vertical motion (0â8 mm/year relative to mean sea level) are well known from tide gauges and repeated precise levelling. GPS on permanent stations has proved to be a powerful tool in studies of crustal motion, capable of detecting small trends in a fraction of the time required by the classical methods. We determine vertical velocities from 5 years of data in the permanent Finnish GPS network FinnRef®. We compare them with velocities derived from three precise levellings spanning nearly hundred years, and from tide gauge records. From the comparison, both FinnRef velocities and levelled velocities appear to be accurate to 0.4 mm/year (one-sigma). The isobases (lines of equal velocities) are less elongated towards northeast than in geophysical models of the rebound. However, the processing of nearly the same GPS data in BIFROST using different methods produces velocities that disagree with FinnRef more than levelling does. The levelled velocities are between the two GPS results and do not resolve the conflict.
Abstract: Project BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea-level, and Tectonics) combines networks of continuously operating GPS receivers in Sweden and Finland to measure ongoing crustal deformation due to glacial isostatic adjustment (GIA). We present an analysis of data collected between August 1993 and May 2000. We compare the GPS determinations of three-dimensional crustal motion to predictions calculated using the high-resolution Fennoscandian deglaciation model recently proposed by Lambeck et al. [1998a , 1998b]. We find that the maximum observed uplift rate (â¼10 mm yrâ1) and the maximum predicted uplift rate agree to better than 1 mm yrâ1. The patterns of uplift also agree quite well, although significant systematic differences are evident. The root-mean-square residual rate for a linear error model yields estimates of rate accuracy of 0.4 mm yrâ1 for east, 0.3 mm yrâ1 for north, and 1.3 mm yrâ1 for up; these figures incorporate model errors, however. We have also compared the values for the observed radial deformation rates to those based on sea level rates from Baltic tide gauges. The observational error for the vertical GPS rates required to give a reduced Ï2 of unity is 0.8 mm yrâ1. The time series do exhibit temporal variations at seasonal frequencies, as well as apparent low-frequency noise. An empirical orthogonal function analysis indicates that the temporal variations are highly correlated among the sites. The correlation appears to be regional and falls off only slightly with distance. Some of this correlated noise is associated with snow accumulation on the antennas or, for those antennas with radomes, on the radomes. This problem has caused us to modify the radomes used several times, leading to one of our more significant sources of uncertainty.
Abstract: Analysis of Global Positioning System (GPS) data demonstrates that ongoing three-dimensional crustal deformation in Fennoscandia is dominated by glacial isostatic adjustment. Our comparison of these GPS observations with numerical predictions yields an Earth model that satisfies independent geologic constraints and bounds both the average viscosity in the upper mantle (5 x 10(20) to 1 x 10(21) pascal seconds) and the elastic thickness of the lithosphere (90 to 170 kilometers). We combined GPS-derived radial motions with Fennoscandian tide gauge records to estimate a regional sea surface rise of 2.1 +/- 0.3 mm/year. Furthermore, ongoing horizontal tectonic motions greater than approximately 1 mm/year are ruled out on the basis of the GPS-derived three-dimensional crustal velocity field.
Abstract: A constant scale difference between GPS solutions and traceable electronic distance measurement (EDM) results was found during semi-annually repeated campaigns performed in Olkiluoto, Finland. Since EDM results are very accurate and uncertainties are well-defined, this leads to an assumption that the GPS solution is biased.
At the KyviÅ¡kÄs test field in Lithuania, the true lengths with traceable uncertainties between observation pillars were measured using a Kern ME5000 Mekometer as a scale transfer standard. GPS observations were processed using individual and type calibrated antenna tables, a local and global ionosphere model, and three different cut-off elevation angles, and several linear combinations and were then compared with the EDM results. The results show that the ambiguity resolution strategy and antenna calibration model play a significant role compared to the cut-off elevation angle and ionosphere model.
Individual antenna calibration is required for the best metrological accuracy by means of the best agreement with traceable EDM results. The best metrological agreement was obtained with an L1 solution and individually calibrated antennas. The rms and maximum difference to the true (EDM) values were 0.3 and 0.7 mm, respectively. However, a clear distance dependency of 0.5 ppm was also evident. In particular, linear combinations with type calibrated tables caused variations up to 4 mm from the true value, even when high quality choke ring antennas were used. With individually calibrated antennas, all solutions were within ±1 mm of the true value.
Abstract: The IAG Reference Frame Sub-Commission for Europe (EUREF) created the European Terrestrial Reference System 89 (ETRS89) and fixed it to the Eurasian plate in order to avoid time evolution of the coordinates due to plate motions. However, the Fennoscandian area in Northern Europe is affected by postglacial rebound (PGR), causing intraplate deformations with respect to the stable part of the Eurasian tectonic plate.
The Nordic countries created their national ETRS89 realizations in the 1990s and have adopted them as the basis for geospatial data. As the most accurate GNSS processing is done in ITRS realizations, an accurate connection to national ETRS89 realizations is required. If the official EUREF transformation is used, residuals are up to 10 cm in the Nordic countries. Therefore, the Nordic Geodetic Commission (NKG) has created a 3-D intraplate velocity model NKG_RF03vel over Fennoscandia and a new transformation procedure to correct for the deformations caused by PGR.
This paper evaluates the NKG approach and compares it to the current recommendation given by EUREF with a 100-point ETRS89 realization in Finland. The results show that, by using a high-quality intraplate velocity model, the transformation residuals are reduced to the cm-level.
Abstract: Static GPS is still commonly used for establishing and maintaining geodetic reference networks. It is general knowledge that the accuracy of long baselines improves when precise orbits are used. Atmosphere is one of the biggest error sources in GPS measurements and it is
divided into two parts: troposphere and ionosphere. The influence of the troposphere is mostly taken into account with a model and ionospheric effects are expected to vanish by using dual frequency receivers. However, some commercial GPS software packages allow users to include an ionosphere model into their processing. We studied the influence of a global onosphere model in practical static GPS surveying for baselines shorter than 33 km. We used different observing times (10 minâ¦6 hours) and ephemerides (precise and broadcast) with CODE global ionosphere model. The model contains measured information of vertical total electron content (VTEC) in the ionosphere. The use of ionosphere model improves accuracy drastically, especially if observing time is short compared to baseline length. For example, improvement was over 90% with 10 minutes observing time if ionosphere model was used. Also the success rate for solving baselines increases. Influence of orbits was negligible compared to that of ionosphere model.
Abstract: Static GPS measurements have been used for precise surveying over two decades. Nevertheless, consistent information about relation between accuracy, baseline length and observing time has been missing. Surveyors have been dependent on the information from various sources (vendors of the GPS equipments, ambiguous guidelines by different companies and institutions, etc) as well as on their own experiences. The outcome of this study gives a relation between the accuracy, baseline length and observing time for static GPS as an easily readable graph. The chart covers all the conventional baseline lengths and observation times as well as broadcast and precise ephemeredes. This study is a part of an ongoing project that studies the quality of geodetic GPS at the Finnish Geodetic Institute. We used data that covers distances between 0.6 and 1,069 km and observing sessions between 10 min and 24 hours. Over 10,000 baselines were processed with broadcast and precise ephemeredes. The set of data used in the study is a random sample chosen from the data from several GPS campaigns. This way it was to give a realistic picture of accuracy by averaging e.g. the influence of atmosphere and satellite geometry. The accuracy is presented for individual baselines i.e. adjustments were not applied. A surface was fitted over the rms values. Since the data was rather heterogeneous a series of fitting schemes were tested and the one with the best fit was chosen. The oodness of fit (R2) for the best fit was 0.91 for broadcast and 0.87 for precise orbits. As a result we generated a graph that shows 1-5 cm regression lines of accuracy as a function of baseline length and observing time.
Abstract: We summarise geodetic activities at the
Finnish Antarctic research station Aboa since 1989.
In 1989â1992 a regional gravity network was established.
Absolute gravity measurements were performed
in 1994 and 2001. In 2003 a permanent GPS
station was installed. In the future we plan to maintain
the GPS time series, and to perform absolutegravity
measurements at other sites in Queen Maud
Land, too.
