S. C. Kenyon

New Gravity Field for the Arctic

The study of the Arctic Ocean has been hampered by incomplete basic knowledge of the basin and its structure. An improved gravity anomaly map (Figure 1) and grid have been developed that complement the new International Chart of the Arctic Ocean [Jakobsson et al., 2008]. This article announces the availability of the Arctic Gravity Project (ArcGP) grid version 2.0 and discusses the genesis of the project.

A Preliminary Gravitational Model to Degree 2160

The National Geospatial-Intelligence Agency (NGA) of the USA has embarked upon the development of a new Earth Gravitational Model (EGM), to support future realizations of NGA’s World Geodetic System. Current plans call for the development of the new EGM (EGM05) by the end of 2005. The new model will be complete to degree and order 2160, and aims at a ±15 cm global Root Mean Square (RMS) geoid undulation error requirement. The new model will combine optimally the gravitational information that is extracted from dedicated geopotential mapping satellite missions (CHAMP, GRACE), with the information contained within a global gravity anomaly database of 5′×5′ resolution. This paper describes the development of a Preliminary Gravitational Model (PGM2004 A). We developed PGM2004A by combining the GRACE-only model GGM02S, with a 5′×5′ global gravity anomaly database compiled by NGA. PGM2004A is complete to degree and order 2160, and is accompanied by propagated error maps at 5′×5′ resolution, accounting for the entire bandwidth of the model (from degree 2 to degree 2160), for various model-derived gravimetric quantities (Δg, N, ξ, η). We have evaluated PGM2004A through comparisons with independent data including GPS/Leveling data, astronomic deflections of the vertical over the conterminous US (CONUS), and altimeter data from TOPEX. The results of these comparisons indicate that the goal set for EGM05 is well within reach. We summarize in this paper our current status and technical accomplishments, and discuss briefly our next steps towards the development of EGM05.

Arctic Gravity Project — a status

An international, cooperative effort is ongoing to compile and evaluate all available gravity data north of 64N, in order to produce a public-domain 5’ free-air anomaly data grid by 2001. The data will, e.g., be useful for limiting the “polar gap” problem of the upcoming gravity field satellite missions. The data contributed so far includes older and recent airborne, surface, marine and submarine gravity data from US, Canadian, German and Scandinavian contributors, as well as limited grid coverage of parts of Russia and data from satellite altimetry. In the paper the current data and compilation status is outlined, along with some comparison results of different data sources in overlapping regions. A preliminary data grid and associated geoid model is presented, highlighting among other the complex tectonic patterns in the Arctic region.

Towards The Next Earth Gravitational Model

The development of a new Earth Gravitational Model (EGM) to degree 2160 is progressing with the availability of improved versions of worldwide 5′×5′ gravity databases and GRACE-derived satellite solutions. Critical to the success of this endeavour is the compilation of a complete and accurate 5′×5′ global gravity anomaly database that takes advantage of all the latest data and modeling for both land and marine areas worldwide. This paper will provide an overview of the data being used in the model, describe the status of the development of the new EGM, show comparisons of preliminary models with independent truth data, and discuss the plans for finalizing the model.

Recent developments in high-resolution global altimetric gravity field modeling

In recent years, dedicated effort has been made to improve high-resolution global marine gravity fields. One new global field is the Danish National Space Center (DNSC) 1-minute grid called DNSC08GRA, released in 2008. DNSC08GRA was derived from double-retracked satellite altimetry, mainly from the ERS-1 geodetic mission data, augmented with new retracked GEOSAT data which have significantly enhanced the range and hence the gravity field accuracy.

Arctic gravity project

The gravity field of the Arctic Ocean region is of prime importance for global gravity field and geoid models, for providing information on the geology and tectonics of the Arctic Basin, and for navigation and orbit determination. Planned satellite gravity field missions such as CHAMP, GRACE and GOCE will all to a varying degree be strongly affected by the gravity field of the polar areas, especially for the satellites launched with a non-polar orbit, where a polar gap will remain in the coverage.

Initial results from retracking and reprocessing the ERS-1 geodetic mission altimetry for gravity field purposes

All present global marine gravity fields are based on the lHz ERS-1 Geodetic Mission (GM) altimeter data combined with other altimetric datasets. Close to the coast (<25 km) this investigation shows that the altimetric gravity field determination degrades due to a combination of several factors, where the main reason is the degradation of the quality of the altimeter data. By starting out from the original waveform data and retracking the entire ERS-1 GM mission using a highly advanced expert based system of multiple retrackers, the return time from both open ocean and coastal sea surface as well as from all ice-covered regions within the coverage of the ERS-1 can be derived with higher accuracy than presently available.

Dimension of the Earth's General Ellipsoid

The problem of specifying the Earths mean (general)ellipsoid is discussed. This problem has been greatly simplified in the era of satellite altimetry, especially thanks to the adopted geoidal geopotential value, W0 = (62 636 856.0 ± 0.5) m2 s-2.Consequently, the semimajor axis a of the Earths mean ellipsoid can be easily derived. However, an a priori condition must be posed first. Two such a priori conditions have been examined, namely an ellipsoid with the corresponding geopotential that fits best W0 in the least squares sense and an ellipsoid that has the global geopotential average equal to W0. It has been demonstrated that both a priori conditions yield ellipsoids of the same dimension, with a–values that are practically identical to the value corresponding to the Pizzetti theory of the level ellipsoid: a = (6 378 136.68 ± 0.06) m.

The DNSC07 high resolution global marine gravity field

1. Summary DNSC07 is a new global ocean wide satellite altimetry derived gravity field computed at the Danish National Space Center (DTUDenmark) with a spatial resolution of 1 arc-minute by 1 arc-minute covering all marine regions of the world including the Arctic Ocean up to the North Pole. For more than a decade satellite altimetry has been used to determine the marine gravity field. All present satellite based global marine gravity fields are based on the GEOSAT and ERS-1 Geodetic Mission (GM) data combined with other satellite derived datasets. The most urgent outstanding problem with most existing marine gravity fields (i.e. KMS02, Sandwell and Smith V12, NTU 01, GSFC00) was to improve the coverage of the data and the quality of the satellite observations in order to gain higher accuracy of the derived marine gravity field in particularly high latitudes and in coastal regions where many sedimentary basins are found. By using a new double-retracking system for the entire ERS-1 GM mission using a highly advanced expert based system of multiple retrackers and subsequently repacking/retracking we have been able to obtain both higher quality data but also many more data than seen before. Especially in coastal and ice-covered regions the new DNSC07 global marine gravity field is superior to other satellite based global marine gravity fields. We will present the high resolution new global marine gravity field and comparison with existing marine gravity surveys in several regions of the world (Indonesia, Florida Keys and East Greenland) and demonstrate how much accuracy have been gained by retracking the satellite observations compared with existing global marine gravity fields.