comparison of ice sheet satellite altimeter retracking algorithms the nasa and esa retracking algorithms are compared with an algorithm based upon a combined surface and volume s v scattering model first the s v nasa and esa algorithms were used to retrack over 1 3 million altimeter return waveforms from the greenland and antarctic ice sheets the surface elevations from the s v algorithm were compared with the elevations produced by the nasa and esa algorithms to determine the relative accuracy of these algorithms when subsurface volume scattering occurs the results show that the esa sub 25 algorithm produced slightly higher surface elevations than the s v algorithm the nasa retracking algorithm produced lower surface elevations than the sn retracking algorithm with average differences ranging from 0 3 to 0 9 m the lower nasa elevations can only account for a portion of previously reported differences between altimeter and geoceiver surface elevations suggesting that the remainder is probably due to orbital differences next by analyzing several thousand satellite crossover points from the greenland and antarctic ice sheets the author estimated the repeatability of the surface elevations derived from the different retracking algorithms the elevations derived from the esa sub 25 and s v algorithm had the smallest standard deviations for the crossover differences for a time period where no significant change in surface elevation should occur the nasa standard deviations were approximately 0 2 m larger than those from the esa sub 25 and s v algorithm which represents an average increase in error of approximately 0 5 m in the datasets since previous ice sheet growth estimates have been based upon the elevations produced by the nasa retracking algorithm further work needs to be conducted to determine if the esa sub 25 or s v retracking algorithms produce growth estimates that are significantly different from the previous estimates

ers 1 altimetry on the greenland ice sheet preliminary investigations of annual variations the existence of an annual signal in the ers 1 35 day repeat altimetry measurements over the greenland ice sheet is investigated in this paper spectral analysis from a collinear set of long tracks indicates that the timevarying part of the elevation signal is rather constant at a level around 20 cm at frequencies higher than 1 cycle per degree at lower frequencies the presence of a stationary signal can be seen this signal can be estimated in a collinear analysis from the entire set of 35 day altimetry data over the ice sheet spanning one and a half years separation of an annual signal revealed a signal highly correlated with topography the mean amplitude is around 40 cm with amplitudes between 10 20 cm around the summit of the ice rising to between 50 80 cm approaching the ablation zone of the ice sheet

exxon global cycle chart an event for every occasion the basic premise of the recent exxon cycle chart that there exists a globally correlatable suite of third order eustatic cycles remains unproven many of the tests of this premise are based on circular reasoning the implied precision of the exxon global cycle chart is not supportable because it is greater than that of the best available chronostratigraphic techniques such as those used to construct the global standard time scale correlations of new stratigraphic sections with the exxon chart will almost always succeed because there are so many exxon sequence boundary events from which to choose this is demonstrated by the use of four synthetic sections constructed from tables of random numbers a minimum of 77 successful correlations of random events with the exxon chart was achieved the existing cycle chart represents an amalgam of regional and local tectonic events and probably also includes unrecognized miscorrelations it is of questionable value as an independent standard of geologic time

surface elevation contours of greenland and antarctic ice sheets surface elevations of the ice sheets are contoured at 50 m intervals for the region of greenland covered by seasat radar altimetry south of 72 n and at 100 m intervals for a region of east antarctica north of 72 s the surface elevations were obtained from computer retracking of the radar altimeter waveforms which were recorded at 0 1 s intervals corresponding to 662 m spacings on the surface the precision of the elevation measurements before adjustment for radial orbit errors is 1 9 m as shown by analysis of elevation differences at orbital crossover points this precision is partly determined by radial errors of approximately 1 0 m in orbit determination and partly by noise due to ice surface irregularities adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small relatively flat region reduces the rms difference to 0 25 m which is indicative of the optimum precision obtainable over the ice sheets however the precision degrades as the slope of the surface or amplitude of the undulations increases yielding an overall precision of 1 6 m the preliminary contour maps are not corrected for slope induced displacements a 2 m contour map in a region of highest data density illustrates the three dimensional characteristics of some surface undulations

slope induced errors in radar altimetry over continental ice sheets ranges obtained by radar altimetry from a satellite to a sloping surface on the earth are not measurements of the surface elevation at the subsatellite point instead the reflecting point is displaced upslope from the subsatellite point causing a slope induced error between the true range to the subsatellite point and the indicated range for the seasat 1 altimeter this range error is nearly 80 m for a 0 8 degree slope ice sheet surface slopes are frequently large enough to cause errors of 10 m or more in addition undulations in the surface about a mean slope cause pronounced variations in these errors altimeter measurements of ranges to modeled irregular surfaces are simulated and two correction schemes are used to reconstruct the modeled surfaces from the simulated data the results illustrate the fundamental limitations inherent in single beam radar altimetry for mapping irregular surfaces in the simpler two dimensional case for which cross track slopes are neglected a relocation scheme which constructs a surface consistent with the altimeter ranges removes 85 of the rms error the mean error along profiles of about 75 km or longer is usually reduced more than the rms error an alternate slope correction scheme which uses the local slope to calculate the expected error is less effective in the two dimensional case however over a simulated three dimensional surface where groundtracks are widely spaced and cross track slopes are significant the slope correction method must be used in at least the cross track direction the effectiveness of the three dimensional correction depends on the relative size of the errors caused by the local slope on surface undulations in comparison to the regional slope

growth of greenland ice sheet measurement measurements of ice sheet elevation change by satellite altimetry show that the greenland surface elevation south of 72 degrees 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

monitoring ice sheet behavior from space satellite remote sensing has revolutionized ice sheet research a variety of instruments sensitive to different parts of the electromagnetic spectrum take what the human eye detects as a flat white desert and provide data sets rich in scientific information image based maps of ice sheets are becoming commonplace and have become an integral component of field work more than a pretty picture the digital character of the satellite data from these instruments has become fundamental to the production of elevation motion accumulation and reflectance data sets visible imagery shows the scientist a wealth of features that offer clues to the history and current behavior of the ice sheet radar and microwave imagery provide information from beneath the surface and have been used to estimate snow accumulation rates interferometry principles have recently been applied to measure surface topography and ice motion with unparalleled precision nonimaging instruments also keep a watchful eye monitoring the ice sheet for indications of growth or shrinkage further expansion of the uses of satellite data is anticipated in the future

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

eustatic sea level changes interpreted from seismic stratigraphy a critique of the methodology with particular reference to the north sea jurassic record seismic stratigraphic methods for interpreting eustatic sea level change are based on generalized stratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning onlap and offlap seismic models may not indicate sea level change but flexural subsidence facies shifts or changes in sea level in the opposite direction to that proposed resolution of some of these problems may be achieved by publication of basic data now in confidential industry files the widely acclaimed vail curves should not be used as templates for basin correlation until such data have been provided and such use should be supported by appropriate conventional lithostratigraphic and biostratigraphic documentation from each new basin where such data have been made available as with the north sea jurassic serious questions of interpretation are raised

comparative analysis of methods used to define eustatic variations in outcrop late cambrian interbasinal sequence development interbasinal correlation of late cambrian cyclic carbonates from the appalachian and cordilleran passive margins the texas craton and the southern oklahoma aulacogen defines six major third order depositional sequences graphic correlation of biostratigraphically constrained strata was used to establish equivalency of stratigraphic sequences between the individual sections relatively isochronous biomere boundaries were used as time datums for lithostratigraphic correlation although the individual sections are composed of different types of meter scale cycles and component lithofacies that reflect the various environmental settings of the localities the overall upward shallowing character of individual sequences is evident the sequences are late cedaria mid crepicephalus late crepicephalus aphelaspis to earliest elvinia elvinia to early saukia and saukia to the cambrian ordovician boundary interbasinal correlation of stratigraphic sequences permits an evaluation of quantitative techniques for determining accommodation history correlation of fischer plots of cyclic successions from separate basins supports a eustatic control of late cambrian sequence development r2 r3 curves derived from subsidence analysis of the late cambrian sections provide good resolution of the second and third order scales of accommodation change and interbasinal correlations of r2 r3 curves also support eustatic control on sequence development comparing the accomodation curves and subsidence analysis with paleobathymetric trends of late cambrian cyclic strata suggests that themore curves may approximate the form of the eustatic sealevel signal a composite eustatic sealevel curve for late cambrian time in north america was created by qualitatively combining the accommodation curves defined by the different techniques for each of the four localities 129 refs 16 figs 3 tabs less

high resolution depth and coastline over major atolls of south china sea from satellite altimetry and imagery abstract current bathymetric models for the south china sea scs are largely based on predicted depths from gravity and sparse single beam echo sounder measurements such models lack high resolution coastlines and shallow water bottom features around atolls and islands this study refines the gravity field of the scs using sea surface heights from measurements of satellite altimeter geosat gm ers 1 gm jason 1 gm and the original cryosat 2 a new one minute gravity anomaly grid is determined the modeled gravity anomalies show a 6 mgal rms discrepancy with shipborne measurements in shallow waters an altimeter only bathymetric model is derived from the new gravity grid by the gravity geological method that uses the latest global and regional models of the ocean depth and marine gravity as a priori knowledge the new model outperforms current scs bathymetric models and is accurate to 100 m based on comparison with multi beam depth measurements optical images from ikonos 2 quickbird 2 geoeye 1 worldview 1 2 and 3 are rectified and digitized to derive the zero coastline and 20 m depth contours reef lines around 44 atolls which are integrated with the altimeter only depths giving significantly improved accuracies and spatial resolutions in modeled depths the improvement percentages of coastlines by the satellite imagery range from 50 to 97 at 41 of the 44 atolls we establish a webpage for free access to the optical and depth images and the depth and gravity grids we will continue to update satellite images altimeter derived gravity grids and bathymetric models over major atolls of the scs

geostatistical evaluation of satellite radar altimetry for high resolution mapping of lambert glacier antarctica the potential of satellite radar altimetry for high resolution mapping of antarctic ice streams is evaluated using retracked and slope corrected data from the lambert glacier and amery ice shelf area east antarctica acquired by geosat during the exact repeat mission erm 1986 89 the map area includes lower lambert glacier north of 72 18 deg s the southern amery ice shelf and the grounded inland ice sheet on both sides the geosat erm altimetry is found to provide substantially more complete coverage than the 1978 seasat altimetry due to improved tracking variogram methods are used to estimate the noise levels in the data as a function of position throughout the map area the spatial structure in the data is quantified by constructing experimental variograms using altimetry from the area of the grounding zone of lambert glacier which is the area chiefly of interest in this topographically complex region kriging is employed to invert the along track height measurements onto a fine scale 3 km grid the unsmoothed along track geosat erm altimetry yields spatially continuous maps showing the main topographic features of lower lambert glacier upper amery ice shelf and the adjacent inland ice sheet the probable position of the grounding line of lambert glacier is identified from a break in slope at the grounded ice floating ice transition the approximate standard error of the kriged map is inferred from the data noise levels

analysis and retracking of continental ice sheet radar altimeter waveforms the seasat i radar altimeter data set acquired over both the antarctic and greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface the radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans causal factors identified in this complicated response include sloping surfaces undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam limited footprint off track reflections and dynamic lag of the altimeter tracking circuit retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters the entire set of seasat i altimetry over the continental ice sheets is being retracked by fitting a multi parameter function to each waveform many waveforms have double ramps indicating near normal reflections from two distinct portions of the ice surface within the altimeter beam two independent range measurements differing by less than 25 m are obtained from retracking the double ramp waveforms

systematic changes in metamorphic styles along the dabie hongseong and himalayan collision belts and their tectonic implications abstract different continental collision belts show contrasting metamorphic trend along their length including the distribution of extreme metamorphism i e ultrahigh pressure 100 km depth and ultrahigh temperature 900 1150 c metamorphisms however no previous study has succeeded in explaining these trends the present study investigates the main factors that control the metamorphic trends along collision belts with reference to the dabie hongseong collision belt between the north and south china blocks and the himalayan collision belt between the indian and asian blocks in the dabie hongseong collision belt collision began in the east before 245 ma and propagated westward until ca 220 ma in the eastern part of the belt the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh pressure metamorphism however ultrahigh pressure metamorphism occurred in the western part of the belt slab break off also migrated from east to west with a westward increase in the depth of break off from ca 10 kbar in the west to ca 35 kbar in the east these lateral trends along the belt resulted in a westward change from ultrahigh temperature 915 1160 c 9 0 10 6 kbar to high pressure 835 860 c 17 0 20 9 kbar and finally ultrahigh pressure metamorphism 680 880 c 30 40 kbar in the himalayan collision belt collision started from the west at 50 ma and propagated eastward the amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh pressure metamorphism in the west but not in the east slab break off started in the west at ca 46 ma and propagated eastward with an eastward decrease in the depth of slab break off from 27 29 to 17 18 kbar consequently the metamorphic trend along the belt changes eastward from ultrahigh pressure 690 750 c 27 29 kbar to high pressure and finally high pressure granulite facies metamorphism 890 c 17 18 kbar the differences in metamorphic trend between the dabie hongseong and himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break off along each belt

mapping of the topography of continental ice by inversion of satellite altimeter data satellite altimeter data over ice sheets provide the best tool for mapping their topography and its possible climatic variations however these data are affected by measurement errors orbit errors and slope errors we develop here a three step inversion technique which accommodates the a priori information on the expected topography and correctly handles and propagates the data errors it estimates first a large scale reference surface then maps the residuals related to undulations and finally iteratively corrects the slope error the method is tested on overlapping small fragments of the antarctic ice sheet using a sub set of seasat data finally a topographic map of terre adelie is produced over areas of small slopes the a posteriori error should be of the order of 0 4 m using ers i data it is therefore expected that climatic variations in the ice sheet topography since the introduction of seasat will be observable

measuring geophysical parameters of the greenland ice sheet using airborne radar altimetry this paper presents radar altimeter scattering models for each of the diagenetic zones of the greenland ice sheet aafe radar altimeter waveforms obtained during the 1991 and 1993 nasa multi sensor airborne altimetry experiments over greenland reveal that the ku band return pulse changes significantly with the different diagenetic zones these changes are due to varying amounts of surface and volume scattering in the return waveform in the ablation and soaked zones where surface scattering dominates the aafe return geophysical parameters such as rms surface height and rms surface slope are obtained by fitting the waveforms to a surface scattering model waveforms from the percolation zone show that the sub surface ice features have a much more significant effect on the return pulse than the surrounding snowpack model percolation waveforms created using a combined surface and volume scattering model and an ice feature distribution obtained during the 1993 field season agree well with actual aafe waveforms taken in the same time period using a combined surface and volume scattering model for the dry snow zone return waveforms the rms surface height and slope and the attenuation coefficient of the snowpack are obtained these scattering models not only allow geophysical parameters of the ice sheet to be measured but also help in the understanding of satellite radar altimeter data

a comparison of the performance of the ice and ocean tracking modes of the ers 1 radar altimeter over non ocean surfaces the european space agency s ers 1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non ocean surfaces as part of the ers 1 commissioning activities we have carried out a study of the tracking performance of this instrument over non ocean surfaces statistics for land ice sea ice arid lands and inland water are presented performance in both operating modes is shown to be better than that of previous missions

research on snow and ice this review unlike its predecessor martinelli et al 1979 is an intentionally selective survey of recent advances in selected areas of snow and ice research i have chosen to concentrate on studies relating to large scale aspects of the cryosphere and to consider their wider significance for other disciplines results obtained by new measurement techniques and the application of remote sensing methods are also stressed at the same time the broad lines of advance in glaciological research as represented in books conference themes and major field or laboratory programs are noted and a detailed bibliography is appended

jakobshavns glacier drainage basin a balance assessment maximum and minimum estimates are made of the drainage basin feeding the jakobshavns glacier by using surface elevation maps derived from seasat altimetry benson s 1962 net balance measurements are used to calculate the balance flow within the basin comparisons of the balance flux at the terminus with estimates of actual flux suggest the basin is in overall equilibrium or slightly thickening this agrees with measurements along the nearby egig traverse balance velocities accelerate rapidly within 100 km of the coast farther upstream balance velocities are consistent with both measured velocities along the egig traverse and calculated deformation velocities it is estimated that jakobshavns glacier discharges between 4 8 and 7 6 of the annual net balance over greenland and drains between 3 7 and 5 8 of the ice sheet area

the depth of penetration in antarctic firn at 10 ghz measurements taken by an x band pulse radar system in east antarctica in 1987 were analyzed to determine the depth of penetration of radar energy in polar firn a minimum estimate of the penetration depth at 10 ghz for the cold and dry polar plateau region is approximately 4 7 m spatial variations in the amount of signal penetration were observed and are related to latitude surface elevation and mean annual temperature the amount of signal penetration and its spatial variation are important factors that should be considered when processing datasets from microwave remote sensing systems that operate in the polar regions

intensity of satellite radar altimeter return power over continental ice a potential measurement of katabatic wind intensity au dessus de glace continentale antarctique des mesures d altimetrie radar par seasat commande automatique de gain ont ete effectuees les variations observees sont dues essentiellement a l intensite du vent catabatique

star spatio temporal altimeter waveform retracking using sparse representation and conditional random fields abstract satellite radar altimetry is one of the most powerful techniques for measuring sea surface height variations with applications ranging from operational oceanography to climate research over open oceans altimeter return waveforms generally correspond to the brown model and by inversion estimated shape parameters provide mean surface height and wind speed however in coastal areas or over inland waters the waveform shape is often distorted by land influence resulting in peaks or fast decaying trailing edges as a result derived sea surface heights are then less accurate and waveforms need to be reprocessed by sophisticated algorithms to this end this work suggests a novel spatio temporal altimetry retracking star technique we show that star enables the derivation of sea surface heights over the open ocean as well as over coastal regions of at least the same quality as compared to existing retracking methods but for a larger number of cycles and thus retaining more useful data novel elements of our method are a integrating information from spatially and temporally neighboring waveforms through a conditional random field approach b sub waveform detection where relevant sub waveforms are separated from corrupted or non relevant parts through a sparse representation approach and c identifying the final best set of sea surfaces heights from multiple likely heights using dijkstra s algorithm we apply star to data from the jason 1 jason 2 and envisat missions for study sites in the gulf of trieste italy and in the coastal region of the ganges brahmaputra meghna estuary bangladesh we compare to several established and recent retracking methods as well as to tide gauge data our experiments suggest that the obtained sea surface heights are significantly less affected by outliers when compared to results obtained by other approaches

a method for determining the average height of a large topographic ice sheet from observations of the echo received by a satellite altimeter ice sheet surfaces have scales of fluctuation that are similar to the diameter of the area illuminated by a satellite radar altimeter the present theory of altimetry developed to describe scattering from the ocean surface does not deal properly with the geometry of ice sheet surfaces in this paper the theory of altimetry is extended to cover this geometry a general relation for the altimeter echo from a surface of unknown geometry is developed including the effects of the penetration of the surface by the radar waves this expression is linearized using the characteristic operating geometry of satellite altimeters and the gentle nature of ice sheet gradients from this expression an integral equation is derived from whose solution the spatial average of the height of the surface relative to a spherical datum can be determined the integral equation is of a volterra type which permits the uniqueness of the solution for the average height to be investigated simply the method is extended to provide a solution for the spatial average of the height of a local region of the ice sheet provided the region remains large in comparison with the area illuminated by the altimeter and to deal with variations in the antenna boresight alignment the results have a number of implications for the collection and reduction of echoes in an experiment to determine the average height of an ice sheet the unique determination of the average height requires the echo to be known over a time interval that depends on the extrema of the surface which therefore must be known a priori the average height itself can be determined by the operation on the echo of a linear operator whose kernel is derived from the solution of the volterra type equation this marks a change from the procedures currently used in practice to reduce echoes from ice sheets

controls on ers altimeter measurements over ice sheets footprint scale topography backscatter fluctuations and the dependence of microwave penetration depth on satellite orientation we consider the reliability of radar altimeter measurements of ice sheet elevation and snowpack properties in the presence of surface undulations we demonstrate that over ice sheets the common practice of averaging echoes by aligning the first return from the surface at the origin can result in a redistribution of power to later times in the average echo mimicking the effects of microwave penetration into the snowpack algorithms that assume the topography affects the radar echo shape in the same way that waves affect altimeter echoes over the ocean will therefore lead to biased estimates of elevation this assumption will also cause errors in the retrieval of echo shape parameters intended to quantify the penetration of the microwave pulse into the snowpack using numerical simulations we estimate the errors in retrievals of extinction coefficient surface backscatter and volume backscatter for various undulating topographies in the flatter portions of the antarctic plateau useful estimates of these parameters may be recovered by averaging altimeter echoes recorded by the european remote sensing satellite ers 1 by numerical deconvolution of the average echoes we resolve the depths in the snowpack at which temporal changes and satellite travel direction effects occur both of which have the potential to corrupt measurements of ice sheet elevation change the temporal changes are isolated in the surface backscatter cross section while directional effects are confined to the extinction coefficient and are stable from year to year this allows the removal of the directional effect from measurement of ice sheet elevation change

accuracy of satellite altimeter elevations over the greenland ice sheet the accuracy of geosat satellite altimetry over the greenland ice sheet is evaluated by comparing the measured heights to radar elevations from the airborne greenland aerogeophysics project at the center of the ice sheet where the ice surface is nearly level surface comparisons show a fit at the 1 to 3 m level as expected but even at moderately sloping ice regions 0 3 0 6 satellite altimetry mean errors in the range of 10 to 35 m are observed these errors are found for slope corrected and waveform retracked data so most previous accuracy estimates of current satellite altimetry ice sheet elevations in regions of sloping or undulating ice appear to be too optimistic

oceanographic and geophysical applications of satellite altimetry since the last quadrennial report new satellite altimeter data have not been generally available nevertheless important progress has been made from analyses of geos 3 and seasat data the marine gravity field has been determined to an extraordinary resolution and accuracy and important solid earth geophysical interpretations made in the area of oceanography statistical estimates of the variability of dynamic topography have been published methods for incorporating altimeter data into numerical models have been developed and applied and tentative but interesting determinations of the mean circulation have been made in addition altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields finally important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved

tides on filchner ronne ice shelf from ers radar altimetry 1 we use harmonic analysis of 8 years of ers satellite radar altimeter ra data at orbital crossovers to retrieve complex amplitude amplitude and phase coefficients for several major tidal harmonics over the filchner ronne ice shelf fris antarctica we describe a method for estimating the accuracy of this method which ranges from 2 to 8 cm per harmonic a comparison between m2 complex amplitude from a recent ocean model and from our ers ra analyses identifies two regions of the fris where the ra data are inconsistent with the model in both regions the differences can be attributed to incorrect specification of the grounding line location in the model our study demonstrates the value of ers ra data in antarctic ice shelf tide modeling and the potential for future altimeter satellites with high polar orbits to contribute to the definition of global tide height variations

australian topography from seasat overland altimetry retracking of overland returns from the seasat altimeter using algorithms originally developed for recovering elevations over ice has led to the successful recovery of high quality continental topography over australia and other continents cross over analysis both before and after orbit adjustment shows the altimetric data over land to have a 2 3 m quality direct comparison of gridded seasat data with surface data re averaged in the same way shows excellent agreement except where seasat data are sparse due either to poor track spacing or to dropouts caused by loss of tracker lock over steeply sloping ground these results suggest that useful topographic data can be derived from seasat and the more recent geosat altimeters for parts of the world where surface data are few or of poor quality

ice shelf topography and structure determined using satellite radar altimetry and landsat imagery the glaciological structure and dynamics of west and shackle ton ice shelves east antarctica are qualitatively determined using a combination of satellite remote sensing techniques sketch maps traced from unenhanced imagery show the ice edge grounding lines flow lines and rifts surface elevation profiles and contour maps from radar altimetry provide free board elevations of the floating ice and show flow line undulations and rumples characteristic of grounded ice west and shackleton ice shelves consist of a combination of fast moving ice tongues from outlet glaciers and slow moving parts constrained by islands ice rises and ice rumples

a new post early cambrian archaeocyath from antarctica a new species of archaeocyath sponge dictyocyathus neptunensis is described from the lower third of the nelson limestone of the neptune range antarctica based on the presence of the trilobites nelsonia schesis and amphoton oatesi these strata are interpreted to be mid to late middle cambrian in age d neptunensis shows a branching low integration modular form and a very simple skeletal type such an organization is predicted from known morphological trends within the early cambrian archaeocyatha specimens are not in life position and probably form part of a storm deposit the absence of dictyocyathus from latest early cambrian toyonian strata worldwide suggests a convergent nature for this highly simplified skeletal type in archaeocyaths

improved gravity anomaly fields from retracked multimission satellite radar altimetry observations over the persian gulf and the caspian sea satellite radar altimetry observations are used to derive short wavelength gravity anomaly fields over the persian gulf and the caspian sea where in situ and ship borne gravity measurements have limited spatial coverage in this study the retracking algorithm extrema retracking extr was employed to improve sea surface height ssh measurements that are highly biased in the study regions due to land contaminations in the footprints of the satellite altimetry observations extr was applied to the waveforms sampled by the five satellite radar altimetry missions topex poseidon jason 1 jason 2 gfo and ers 1 along track slopes have been estimated from the improved ssh measurements and used in an iterative process to estimate deflections of the vertical and subsequently the desired gravity anomalies the main steps of the gravity anomaly computations involve estimating improved ssh using the extr technique computing deflections of the vertical from interpolated sshs on a regular grid using a biharmonic spline interpolation and finally estimating gridded gravity anomalies a remove compute restore algorithm based on the fast fourier transform has been applied to convert deflections of the vertical into gravity anomalies finally spline interpolation has been used to estimate regular gravity anomaly grids over the two study regions results were evaluated by comparing the estimated altimetry derived gravity anomalies with and without implementing the extr algorithm with ship borne free air gravity anomaly observations and free air gravity anomalies from the earth gravitational model 2008 egm2008 the comparison indicates a range of 3 5 mgal in the residuals which were computed by taking the differences between the retracked altimetry derived gravity anomaly and the ship borne data the comparison of retracked data with ship borne data indicates a range in the root mean square error rmse between approximately 1 8 and 4 4 mgal and a bias between 0 4062 and 2 1413 mgal over different areas also a maximum rmse of 4 4069 mgal with a mean value of 0 7615 mgal was obtained in the residuals an average improvement of 5 2746 mgal in the rmse of the altimetry derived gravity anomalies corresponding to 89 9 per cent was obtained after applying the extr post processing

sea level changes and stratigraphy of the nelson limestone middle cambrian neptune range antarctica abstract the first detailed history of relative sea level changes known from lower paleozoic strata in antarctica is documented in this study of the nelson limestone basal beds of the formation constitute a nonmarine terrigenous valley fill succession thickness of these strata varies from 0 1 m to 72 m suggesting that significant topographic relief existed below the sub nelson unconformity locally base level must have risen at least 72 m because following fluvial infilling of the valley a succession of carbonates over 400 m thick was deposited as seas inundated the antarctic margin of gondwana during late middle cambrian time on a global scale the broad relative sea level rise recorded in the nelson limestone is synchronous with previously recognized eustatic events during lejopyge laevigata time in the nelson limestone smaller fluctuations of relative sea level are indicated by three sequences each about 100 m thick the lower sequence is bounded below by the sub nelson unconformity and includes the valley fill succession transgressive sandstone and highstand carbonate deposits the top of a subaerially exposed interval of carbonates in one section is interpreted as a sequence boundary with correlative conformity in the deeper water areas the subaerially exposed interval is roughly correlative with a succession of five or more slump deposits 1 2 m thick in another section subaerial exposure at nearly the same stratigraphic level as the slumps marked a fundamental shift in depositional style following marine flooding an offshore carbonate shoal with an associated restr cted lagoon was established during deposition of the middle sequence the upper sequence records marine flooding above the middle sequence and subsequent reestablishment of an offshore carbonate shoal prior to deformation and volcanism in the urea related to the ross orogeny

motion of major ice shelf fronts in antarctica from slant range analysis of radar altimeter data 1978 98 slant rangeanalysis of radar altimeter data fromthe seasat geosat and european remote sensing satellite ers 1and 2 databases is used to determine barrier location at particular times and estimate barrier motion kma 1 for majorantarctic ice shelves the analysis covers various multi year intervals from1978 to 1998 supplemented bybarrierlocationmapsproducedelsewherefor1977 and1986 barriermotion isestimated as the ratio between mean annual ice shelf area change for a particular interval and the length of the discharge periphery this value is positive if the barrier location progresses seaward ornegative if thebarrierlocation regresses break back either positive ornega tive values are lower limit estimates because the method does not detect relatively small area changes due to calving or surge events the findings are discussed in the context of the three ice shelves that lie in large embayments the filchner ronne amery and ross ice shelves and marginal ice shelves characterized by relatively short distancesbetween main segments of grounding line and barrier those in the dronning maud land sector between 010 1 wand 032 5 e and thewest and shackleton ice shelves the ice shelves included inthestudyaccountforapproximately three quartersofthetotal ice shelf area of antarctica anddischarge approximately two thirds of thetotal grounded ice area

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

comparison of retracking algorithms using airborne radar and laser altimeter measurements of the greenland ice sheet compares four continental ice sheet radar altimeter retracking algorithms using airborne radar and laser altimeter data taken over the greenland ice sheet in 1991 the refurbished advanced application flight experiment aafe airborne radar altimeter has a large range window and stores the entire return waveform during flight once the return waveforms are retracked or post processed to obtain the most accurate altitude measurement possible they are compared with the high precision airborne oceanographic lidar aol altimeter measurements the aafe waveforms show evidence of varying degrees of both surface and volume scattering from different regions of the greenland ice sheet the aol laser altimeter however obtains a return only from the surface of the ice sheet retracking altimeter waveforms with a surface scattering model results in a good correlation with the laser measurements in the wet and dry snow zones but in the percolation region of the ice sheet the deviation between the two data sets is large due to the effects of subsurface and volume scattering the martin et al model results in a lower bias than the surface scattering model but still shows an increase in the noise level in the percolation zone using an offset center of gravity algorithm to retrack altimeter waveforms results in measurements that are only slightly affected by subsurface and volume scattering and despite a higher bias this algorithm works well in all regions of the ice sheet a cubic spline provides retracked altitudes that agree with aol measurements over all regions of greenland this method is not sensitive to changes in the scattering mechanisms of the ice sheet and it has the lowest noise level and bias of all the retracking methods presented

surface topography of the greenland ice sheet from satellite radar altimetry surface elevation maps of the southern half of the greenland subcontinent are produced from radar altimeter data acquired by the seasat satellite a summary of the processing procedure and examples of return waveform data are given the elevation data are used to generate a regular grid which is then computer contoured to provide an elevation contour map ancillary maps show the statistical quality of the elevation data and various characteristics of the surface the elevation map is used to define ice flow directions and delineate the major drainage basins regular maps of the jakobshavns glacier drainage basin and the ice divide in the vicinity of crete station are presented altimeter derived elevations are compared with elevations measured both by satellite geoceivers and optical surveying

a coastal retracking system for satellite radar altimeter waveforms application to ers 2 around australia 1 satellite radar altimeter derived sea surface heights ssh are in error in coastal regions due in part to the complex nature of echoes returned from rapidly varying land and sea surfaces this paper presents improved altimeter derived ssh results in australian coastal regions using the waveform retracking technique which reprocesses the waveform data through a coastal retracking system the system based upon a systematic analysis of satellite radar altimeter waveforms around australia improves ssh data from several retrackers depending on the waveforms characteristics central to the system is the use of two techniques the least squares fitting and the threshold retracking algorithms to overcome the problem of fading noise the fitting algorithm has been developed to include a weighted iterative scheme the retrackers include five fitting models and the threshold method with varying threshold levels a waveform classification procedure has also been developed which enables the waveforms to be sorted and then retracked by an appropriate retracker two cycles of 20 hz waveform data from ers 2 have been reprocessed using this system to obtain the improved ssh estimates using the ausgeoid98 gravimetric geoid model as a quasi independent reference the system improves ssh estimates from beyond 22 km to beyond 5 km from the coastline