James G. Marsh

Growth of Greenland ice sheet: measurement

Measurements of ice-sheet elevation change by satellite altimetry show that the Greenland surface elevation south of 72� north latitude is increasing. The vertical velocity of the surface is 0.20 � 0.06 meters per year from measured changes in surface elevations at 5906 intersections between Geosat paths in 1985 and Seasat in 1978, and 0.28 � 0.02 meters per year from 256,694 intersections of Geosat paths during a 548-day period of 1985 to 1986.

5’ Detailed gravimetric geoid in the northwestern Atlantic ocean

A 5’ detailed gravimetric geoid has been computed for the northwest Atlantic Ocean as ground truth for the GEOS‐3 satellite altimeter experiment. Comparisons of this geoid with satellite derived geoceiver station heights show an r.m.s. difference of 1.2 m. Initial comparisons with GEOS‐III altimeter derived geoid profiles have indicated a relative agreement of generally better than 2 m.

Global detailed geoid computation and model analysis

This paper presents a survey of recent work on the gravimetric geoid. The gravity models considered are those published in the past few years by the Goddard Space Flight Center (GSFC), the Smithsonian Astrophysical Observatory (SAO) and the Ohio State University (OSU). Comparisons and analyses have been carried out through the ose of detailed gravimetric geoids which we have computed by combining the above-mentioned models with a set of 26 000, 1ox1o mean free air gravity anomalies. The accuracy of the detailed gravimetric geoid computed using the most recent Goddard Earth Model (GEM-6) in conjunction with the set 1ox1o mean free air gravity anomalies is assessed at 2 m on the continents of North America, Europe And Australia, 2 to 5 m in the North-East Pacific and North Atlantic areas and 5 to 10 m in other areas where surface gravity data are sparse. Rms differences between this detailed geoid and the detailed geoids computed using the other satellite gravity fields in conjunction with same set of surface data range from 3 to 7 m. The maximum differences in all cases occurred in the Southern Hemisphere where surface data and satellite observations are sparse. These differences exhibited wavelengths of approximately 30o to 50o in longitude. Detailed geoidal heights were also computed with models truncated to 12th degree and order as well as 8th degree and order. This truncation resulted in a reduction of the rms differences to a maximum of 5 m. Comparisons have been made with the astrogeodetic data of Rice (United States), Bomford (Europe), and Mather (Australia) and also with geoidal heights from satellite solutions for geocentric station coordinates in North America and the Caribbean.

Seasat Altimeter Calibration: Initial Results

Preliminary analysis of radar altimeter data indicates that the instrument has met its specifications for measuring spacecraft height above the ocean surface (� 10 centimeters) and significant wave height (� 0.5 meter). There is ample evidence that the radar altimeter, having undergone development through three earth orbit missions [Skylab, Geodynamics Experimental Ocean Satellite 3 (GEOS-3), and Seasat], has reached a level of precision that now makes possible its use for important quantitative oceanographic investigations and practical applications.

An improved error assessment for the GEM‐T1 Gravitational Model

Several tests have been designed to estimate the correct error variances for the GEM-T1 gravitational solution that was derived exclusively from satellite tracking data. The basic method uses both independent and dependent subset data solutions and produces a coefficient by coefficient estimate of the model uncertainties. The GEM-T1 errors have been further analyzed using a method based on eigenvalue-eigenvector analysis, which calibrates the entire covariance matrix. Dependent satellite data sets and independent altimetric, resonant satellite, and surface gravity data sets all confirm essentially the same error assessment The calibration test results yield very stable calibration factors, which vary only by approximately 10% over the range of tests performed. Based on these calibrated error estimates, GEM-T1 is a significantly improved solution, which to degree and order 8 is twice as accurate as earlier satellite derived models like GEM-L2. Also, by being complete to degree and order 36, GEM-T1 is more complete and has significantly reduced aliasing effects that were present in previous models.

Regional mean sea surfaces based on GEOS‐3 and SEASAT altimeter data

Abstract Altimetric sea surfaces provide a basis for detailed analyses of the earths gravity, crustal structure, and the oceanic surface circulation. We have computed long‐term mean surfaces for the Bering Sea, Northwest Atlantic Ocean, and Gulf of Mexico based on a combination of the entire SEASAT (three‐month) and GEOS‐3 (3.5‐year) altimeter data sets. The number of available passes ranged from 558 in the gulf to 1396 in the Atlantic. The large amount of data in these areas, coupled with the increased constraint provided by the combination of data from two orbital inclinations, has permitted the accurate removal of the effects of radial ephemeris error through crossing arc adjustments. The precision of these regional mean sea surfaces is approximately 15 cm, with horizontal resolutions approaching 25 km.

Mean sea surface computation using GEOS‐3 altimeter data

The mean surfaces of several regions of the worlds oceans have been estimated using GEOS‐3 altimeter data. Included in these regions are the northwest Atlantic, the northeast Pacific off the coast of California, the Indian Ocean, the southwest Pacific, and the Philippine Sea. These surfaces have been oriented with respect to a common earth center‐of‐mass system by constraining the separate solutions to conform to precisely determined laser reference control orbits. The same reference orbits were used for all regions assuring continuity of the separate solutions. Radial accuracies of the control orbits have been demonstrated to be on the order of 1 m. In the computation of these surfaces, the altimeter‐measured sea surface height crossover differences were minimized by the adjustment of tilt and bias parameters for each pass with the exception of laser reference control passes. The tilt and bias adjustments removed long wavelength errors, which were primarily due to orbit error. Ocean tides were modeled w...

Mean elements of GEOS 1 and GEOS 2

A combined analytical-numerical procedure for determining mean orbital elements is presented and applied to the orbits of GEOS 1 and GEOS 2. The precision of the mean semi-major axes of these orbits is a few tens of centimeters when optical flash data are used to determine 2 day orbital arcs. Four day Minitrack orbits give mean semi-major axes of a few meters precision. The mean orientation parameters (i, Ω) determined from the optical data are obtained to a precision of about 0″.1.

Global detailed gravimetric geoid

A 1 ° × 1 ° global detailed gravimetric geoid has been computed, using a combination of the Goddard Space Flight Center (GSFC) GEM‐8 potential field model and a set of 38,406 1° × 1° mean surface free air anomalies. Numerous short wavelength features are shown in the geoid contour map, e.g., the steep gradients associated with oceanic trenches. Comparison of this geoid with geoceiver derived and astrogeodetic geoid heights in the United States resulted in an r.m.s. difference of about 1.7 m. Comparisons with three GEOS‐3 altimeter derived geoidal profiles revealed that for areas with good surface data coverage, the relative agreement is generally better than 5 m.

A global mean sea surface based upon GEOS 3 and Seasat altimeter data

A mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS-9012, has been computed on a grid of 1/8° resolution. Each elevation value was calculated by fitting all data within 111 km to a local biquadratic surface using Bayesian least squares. Individual data points were weighted inversely to the square of the distance to the grid location in the gridding process. The surface covers the global ocean between 70°N and 72°S. In the vicinity of sea ice the altimeter heights have been corrected for the on-board tracker error that occurs over non-Gaussian surfaces. Comparisons are made between MSS-9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data.