a model study of the buoyancy driven circulation in the gulf of maine abstract a quasi linear prognostic numerical model has been used to study the three dimensional baroclinic circulation in the gulf of maine region with a focus on the buoyancy driven circulation in the inner basins and near the coast where the influence of relatively fresh water from rivers and the scotian shelf is important the model uses a horizontal grid with 6 km resolution and a terrain following vertical coordinate with ten levels to represent the bathymetry and stratification several model experiments explore the influences of fluxes from the principal rivers tidal mixing and shelf water inflow beginning with a case with no initial horizontal density gradients and ending with cases initialized by temperature and salinity data from cruises in june of 1982 and 1983 augmented by climatological data with composite initialization the model develops a circulation containing realistic elements including an anticlockwise gyre in the interior of the gulf and a prominent southwestward coastal curre

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

the kinematic and hydrographic structure of the gulf of maine coastal current the gulf of maine coastal current gmcc which extends from southern nova scotia to cape cod massachusetts was investigated from 1998 to 2001 by means of extensive hydrographic surveys current meter moorings tracked drifters and satellite derived thermal imagery the study focused on two principal branches of the gmcc the eastern maine coastal current emcc that extends along the eastern coast of maine to penobscot bay and the western maine coastal current wmcc that extends westward from penobscot bay to massachusetts bay results confirm that gmcc is primarily a pressure gradient driven system with both principal branches increasing their transport in the spring and summer due to fresh water inflows and flowing southwestward against the mean wind forcing during this period in the spring and summer the subtidal surface currents in the emcc range from 0 15 to 0 30 ms 1 while subtidal wmcc currents range from 0 05 to 0 15 ms 1 the reduction of southwestward transport near penobscot bay is accomplished via an offshore veering of a variable portion of the emcc some of which recirculates cyclonically within the eastern gulf of maine the degree of summer offshore veering versus leakage into the wmcc varied strongly over the three study years from nearly complete disruption in 1998 to nearly continuous through flow in 2000 observations show strong seasonal and interannual variability in both the strength of the gmcc and the degree of connectivity of its principal branches r 2005 elsevier ltd all rights reserved

independent dem of antarctica using gnss r data from techdemosat 1 the first digital elevation model dem of the antarctic ice sheet derived from global navigation satellite systems reflectometry gnss r data from the uk techdemosat 1 satellite is presented this is obtained using 32 months of data from the mission this opportunistic and inexpensive method is shown to produce encouraging results from the technology demonstration platform of techdemosat 1 with median bias under 18 m and root mean square difference under 91 m when compared to the cryosat 2 1 km dem v1 0 discrepancies between the two data sets are explored along with possible causes of such differences and potential improvements to further optimize this technique for future gnss r missions

estimating the basin scale ocean circulation from satellite altimetry part i straightforward spherical harmonic expansion abstract direct estimation of the absolute dynamic topography from satellite altimetry has been confined to the largest scales basically the basin scale owing to the fact that the signal to noise ratio i more unfavorable everywhere else but even for the largest scales the results are contaminated by the orbit error and geoid uncertainties recently a more accurate earth gravity model gem t1 became available thus giving us the opportunity to examine the whole question of direct estimation under a more critical limelight it is found that our knowledge of the earth s gravity field has indeed improved a great deal however it is not yet possible to claim definitively that our knowledge of the ocean circulation has improved through direct estimation yet the improvement in the gravity model has come to the point that it is no longer possible to attribute the discrepancy at basin scales between altimetric and hydrographic results as mostly due to geoid uncertainties a substantial part of the differen

a refined gravity model from lageos gem l2 a refined gravity field model goddard earth model gem l2 has been derived using the lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion this field also contributes to an improved understanding of long wavelength features such as the sea slope across broad ocean basins through its significant improvement of the long wavelength geoid through degree and order 4 the geoid for these terms has an accuracy estimated at 8 cm in gem l2 gem l2 as in all recent goddard earth models relies heavily on the precise near earth satellite laser ranging data in this case provided by nasa s crustal dynamics program two and a half years of lageos laser data acquired from over 20 well distributed stations were combined with the existing data from the best satellite derived model gem 9 to develop the new lageos model testing shows that the lageos gravity field error at long wavelengths is less than half that for gem 9 independent tests using well determined longitude accelerations of 24 hour satellites have verified the improved accuracy of the new model a comparison of global laser base stations from independent data sets of alternating 15 day data segments over the 2 1 2 years of lageos show total inter station positioning to 1 8 cm when using this new field the same comparison using the 1979 versus the 1980 lageos data yields 5 2 cm this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than 5 msec have been derived in the solution the adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in gem l2

altimetric analysis of the sea surface gps reflected signals we describe the development and implementation of a method for extracting altimetric information using the passive reflectometry and interferometry system paris i e from gps signals after their reflection off the sea surface we have formalized one idea laid out in the description of a bistatic system for ocean altimetry using the gps signal by hajj and zuffada jet propulsion laboratory and have extended it to real situations encountered in paris aircraft experiments second we have developed the corresponding algorithms to produce real time altimetric observables to be used in dedicated digital signal processors finally we have applied this method to estimate sea surface height from one flight experiment in the north sea off the coast of norway

circulation from a joint gravity field solution determination of the general ocean with the development of satellite altimetry it is possible to infer the geostrophic velocity of the surface ocean currents if the geoid and the position of the satellite are known accurately errors in current geoid models and orbit computations both due primarily to errors in the earth s gravity field model have limited the use of altimeter data for this purpose the objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi stationary sea surface topography and the model for the gravity field satellite tracking data from twelve satellites were used along with seasat altimeter data for the solution the estimated model of compares well at long wavelengths with the hydrographic model of covariance analysis indicates that the geoid is separable from up to degree 9 at which point geoid error is comparable to the signal of

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

an estimate of equatorial gravity from terrestrial and satellite data equatorial gravity has been estimated from terrestrial gravity data and from satellite implied data the terrestrial estimate was based on the analysis of 43271 1 1 mean free air gravity anomalies the value estimated was 978032 35 0 5 mgals this value may be slightly 0 to 1 5 mgals too low because of the neglect of the downward continuation of surface free air anomalies to the geoid an alternate estimate based on the comparison of terrestrial anomalies and altimeter derived anomalies yields 978032 6 mgals equatorial gravity from space data depends on gm from lageos and lunar laser data and on a new estimate 6378136 2 0 5 m of the equatorial radius based on doppler tracking station positions and a new spherical harmonic expansion of the earth s gravity field to degree 360 the resultant equatorial gravity is 978032 78 0 2 mgals the difference between the two terrestrial estimates is 0 4 and 0 2 mgals or 0 4 ppm or 0 2 ppm the agreement is within the accuracy of the two separate estimates these comparisons can put constraints on parameters of non newtonian force models

structure and variability of the western maine coastal current abstract analyses of ctd and moored current meter data from 1998 and 2000 reveal a number of mechanisms influencing the flow along the western coast of maine on occasions the eastern maine coastal current extends into the western gulf of maine where it takes the form of a deep order 100 m deep and broad order 20 km wide southwestward flow with geostrophic velocities exceeding 20 cm s 1 this is not a coastally trapped flow however in fields of geostrophic velocity computed from shipboard ctd data the core of this current is roughly centered at the 100 m isobath and its onshore edge is no closer than 10 km from the coast geostrophic velocity fields also reveal a relatively shallow order 10 m deep baroclinic flow adjacent to the coast this flow is also directed to the southwest and appears to be principally comprised of local river discharge analyses of moored current meter data reveal wind driven modulations of the coastal flow that are consistent with expectations from simple theoretical models however a large fraction of the near shore current variance does not appear to be directly related to wind forcing sea surface temperature imagery combined with analysis of the moored current meter data suggests that eddies and meanders within the coastal flow may at times dominate the near shore current variance

a model study of the seasonal circulation in the gulf of maine the princeton ocean model is used to study the circulation in the gulf of maine and its seasonal transition in response to wind surface heat flux river discharge and the m2 tide the model has an orthogonal curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical it is initialized and forced at the open boundary with model results from the east coast forecast system the first experiment is forced by monthly climatological wind and heat flux from the comprehensive ocean atmosphere data set discharges from the saint john penobscot kennebec and merrimack rivers are added in the second experiment the semidiurnal lunar tide m2 is included as part of the open boundary forcing in the third experiment it is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the gulf of maine the spinup of the cyclonic circulation between april and june is likely caused by the differential heating between the interior gulf and the exterior shelf slope region from june to december the cyclonic circulation continues to strengthen but gradually shrinks in size when winter cooling erodes the stratification the cyclonic circulation penetrates deeper into the water column the circulation quickly spins down from december to february as most of the energy is consumed by bottom friction while inclusion of river discharge changes details of the circulation pattern the annual evolution of the circulation is largely unaffected on the other hand inclusion of the tide results in not only the anticyclonic circulation on georges bank but also modifications to the seasonal circulation

characterization of dry snow sub structure using gnss reflected signals abstract signals of global navigation satellite systems gnss bi statically reflected off pristine dry snow in dome concordia antarctica were collected during an experimental campaign with the aim of investigating the potential capabilities of these signals to remotely sense dry snow areas the data have been used to sense the sub surface structure of the snow by means of a new holographic observable the obtained reflected signals present interferometric patterns consistent with the coherent superposition of signals reflected in different layers of the snow sub structure down to a few hundred meters depth a forward model has been developed to compare the amplitude and phase behavior of the interferences with the data a new radio holographic observable is also defined to help identify the layer at which the internal reflections occur this work represents the first step towards the potential use of a space based gnss r mission for densely monitoring the depths of antarctica s thick dry snow and or complementing l band radiometric observations

a geopotential model from satellite tracking altimeter and surface gravity data gem t3 an improved model of earth s gravitational field gem t3 has been developed from a combination of satellite tracking satellite altimeter and surface gravimetric data gem t3 provides a significant improvement in the modeling of the gravity field at half wavelengths of 400 km and longer this model complete to degree and order 50 yields more accurate satellite orbits and an improved geoid representation than previous goddard earth models gem t3 uses altimeter data from geos 3 1975 1976 seasat 1978 and geosat 1986 1987 tracking information used in the solution includes more than 1300 arcs of data encompassing 31 different satellites the recovery of the long wavelength components of the solution relies mostly on highly precise satellite laser ranging slr data but also includes tranet doppier optical and satellite to satellite tracking acquired between the ats 6 and geos 3 satellites the main advances over gem t2 beyond the inclusion of altimeter and surface gravity information which is essential for the resolution of the shorter wavelength geoid are some improved tracking data analysis approaches and additional slr data although the use of altimeter data has greatly enhanced the modeling of the ocean geoid between 65 n and 60 s latitudes in gem t3 the lack of accurate detailed surface gravimetry leaves poor geoid resolution over many continental regions of great tectonic interest e g himalayas andes estimates of polar motion tracking station coordinates and long wavelength ocean tidal terms were also made accounting for 6330 parameters gem t3 has undergone error calibration using a technique based on subset solutions to produce reliable error estimates the calibration is based on the condition that the expected mean square deviation of a subset gravity solution from the full set values is predicted by the solutions error covariances data weights are iteratively adjusted until this condition for the error calibration is satisfied in addition gravity field tests were performed on strong satellite data sets withheld from the solution thereby ensuring their independence in these tests the performance of the subset models on the withheld observations is compared to error projections based on their calibrated error covariances these results demonstrate that orbit accuracy projections are reliable for new satellites which were not included in gem t3

gravity model improvement for seasat analysis of the data from the seasat altimeter a 10 cm precision instrument requires accurate knowledge of the radial position of the spacecraft for oceanographic applications such as the study of tides and ocean circulation as well as for geodetic applications initial orbit determinations using the best available geodetic and force models gave radial errors approaching 5 m the dominant source of this radial error was the gravity field model improvements in the gravity modeling were made by successively incorporating the seasat laser and unified s band ground tracking data the geos 3 altimeter data and finally the seasat altimeter data with the most recent model a set of spherical harmonic coefficients complete to degree and order 36 seasat ephemerides have been computed for an orbital arc length of 12 days with an rms radial accuracy of about 70 cm

radial variations of a satellite orbit due to gravitational errors implications for satellite altimetry the linear perturbations of the radius of a satellite orbit due to the geopotential are derived from these estimates are made of the radial orbit error due just to geopotential errors from current models employing only conventional satellite tracking data the estimates show that about an order of magnitude improvement in the satellite geopotential will be necessary to utilize the full accuracy of independent satellite altimeter measurements the linear perturbations are used to show that 1 a substantial improvement in the low degree geopotential is possible from the indirect use of seasat altimetry at track crossovers and 2 a separation of permanent sea topography and geoid may be possible from direct use of the heights along the track

determination of the ocean circulation using geosat altimetry a spherical harmonic model of the sea surface topography complete to degree and order 10 and a model of the earth s geopotential field complete to degree and order 50 have been obtained in a simultaneous solution using geosat altimeter data and tracking data from 14 different satellites the sea surface topography model compares well with oceanographic models computed using hydrographic data and ship drift data currently errors in the estimated gravity field model limit the determination of the spherical harmonic coefficients of the general ocean circulation to degrees 10 and lower corresponding to a minimum wavelength of 4 000 km error analysis indicates that the correlation between the geoid and the sea surface topography model is less than 0 2 indicating good separation of the geoid and the sea surface topography at wavelengths of 4 000 km or longer estimates of the scale factor for the significant wave height h sub 1 3 which is used to compute the electromagnetic bias correction and the bias for the geosat altimeter are obtained the estimate of the h sub 1 3 correction is 3 6 plus minus 1 5 and the height bias estimate is zero

sea surface topography retrieved from gnss reflectometry phase data of the geohalo flight mission sea surface topography observations are deduced from an airborne reflectometry experiment a gnss global navigation satellite system receiver dedicated for reflectometry was set up aboard the german halo high altitude long range research aircraft flights were conducted over the mediterranean sea about 3500 m above sea level a signal path model divided into large and small scale contributions is used for phase altimetry the results depict geoid undulations and resolve anomalies of the sea surface topography for the whole experiment 65 tracks over the mediterranean sea are retrieved and compared with a topography model tracks differ between right handed and left handed circular polarization the difference however is not significant for this study precision and spatial resolution decrease disproportionately at low elevations eight tracks with centimeter precision are obtained between 11 and 33 of elevation at higher elevation angles the number of tracks is significantly reduced due to surface roughness in future such retrievals could contribute to ocean eddy detection

tidal modulation of surface gravity waves in the gulf of maine abstractthis study examines the tidal modulation of surface gravity waves in the gulf of maine gom by using in situ observations and numerical model results analysis of observational data demons

high accuracy high resolution gravity profiles from 2 years of the geosat exact repeat mission satellite altimeter data from the first 44 repeat cycles 2 years of the geosat exact repeat mission geosat erm were averaged to improve accuracy resolution and coverage of the marine gravity field individual 17 day repeat cycles two points per second were first edited and differentiated resulting in alongtrack vertical deflection i e alongtrack gravity disturbance to increase the signal to noise ratio 44 of these cycles were then averaged to form a single highly accurate vertical deflection profile the largest contributions to the vertical deflection error is short wavelength altimeter noise and longer wavelength oceanographic variability the combined noise level is typically 6 rad both types of noise are reduced by averaging many repeat cycles over most ocean areas the uncertainly of the average profile is less than 1 rad 0 206 arcsec which corresponds to 1 mgal of alongtrack gravity disturbance however in areas of seasonal ice coverage its uncertainty can exceed 5 rad to assess the resolution of individual and average geosat gravity profiles the cross spectral analysis technique was applied to repeat profiles individual geosat repeat cycles are coherent 0 5 for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths this emit of resolution is governed by the signal to noise ratio thus when many geosat repeat profiles are averaged together the resolution limit typically improves to about 20 km except in shallow water areas further improvements in resolution will be increasingly difficult to achieve because the short wavelength components are attenuated by upward continuation from the seafloor to the sea surface these results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half wavelength resolution of 12 km by a 4 5 year satellite altimeter mapping mission

seasat altimeter timing bias estimation the calibration of the altimeter observation time tags to the millisecond level of accuracy is fundamental to the processing of the data initial analyses of the seasat altimeter data indicated the presence of a time calibration bias which produced altimeter measurement errors in excess of a meter a technique has been developed for the solution of the time tag bias based upon the analysis of sea surface height discrepancies at ground track intersections this technique has permitted very good separation of the dominant once per revolution ephemeris error which amounts to about 1 5 m rms from the timing error signature furthermore the technique does not depend upon the availability of precise geoid data the application of this technique to a global set of seasat altimeter data covering the time period of july 28 august 9 1978 has resulted in a value of 81 0 2 ms for the time tag bias this value agrees to within 2 9 ms of the value derived at the university of texas from a similar analysis of the altimeter data furthermore these values corroborate the revised value of 79 4 ms derived at nasa wallops flight center and the johns hopkins university applied physics lab from a reexamination of the internal instrument time delays the modeling of oceanic tides and the orbit computations are the major error sources in these analyses

detection of arctic ocean tides using interferometric gnss r signals 1 this paper evaluates the usage of reflected gps signals for earth observations to study changes of sea level and sea ice in remote sensing in a coastal setup 670 m above disko bay greenland signals with different carriers l1 and l2 were recorded a method is presented that analyses the interferometric phase between the reflected and the direct signals and derives the height of the reflecting surface the analysis includes a ray tracing and an estimation of signal coherence it is shown that coherent reflections are related to sea ice coverage absolute heights are derived with a time interval of 30 min the altimetric results show semidiurnal tides that are validated using the aodtm 5 tide model the residual height has a mean of 9 7 cm for l1 and 22 9 cm for l2 the dispersion is not significant but a significant tropospheric bias is detected with an error of up to 20 cm

water sources for georges bank abstract the origins of georges bank source waters can be quantified by the oxygen isotope salinity tracer regionally water is tagged by freshwater of different h 2 18 o h 2 16 o ratios and these waters can subsequently be identified after mixing downstream this conservative lagrangian tracer of source waters is useful for tracking passive plankton and larvae that may be important to sustain the georges bank ecosystem the origin of georges bank freshwater sources is primarily in the gulf of st lawrence consisting of labrador shelf water which enters via the strait of belle isle and st lawrence river water these two mix with the underlying slope water in the gulf of st lawrence and along the scotian shelf at the northeast channel the input to the gulf of maine there is a single freshwater endmember consisting of 5 6 st lawrence river water and 94 6 labrador shelf water by volume these proportions remained constant for the years 1995 1997 the water on central georges bank consists of a mixture of water entering the northeast channel with gulf of maine river outflow there is considerable interannual variation in the maine river water proportion ranging from 3 3 in 1982 to 0 23 by volume in 1996 during 1995 1998 there was no correlation between georges bank salinity which decreased monotonically and the proportion of maine river water on the bank the interannual fluctuations of salinity on georges bank are driven by variations in upstream scotian shelf sources and its mixing onto the bank and not by changes in local river outflow

absolute measurement by satellite altimetry of dynamic topography of the pacific ocean satellites are the only foreseeable possibility for obtaining observations of the ocean circulation on the basin and global scales required to understand it such observations are essential for an understanding of the role of the ocean in various important problems for example climate the 3 month seasat mission1 has demonstrated2 3 that altimetry is capable of providing global observations of oceanic variability we show here that the data from this short sub optimum mission are also adequate for a determination of the absolute sea surface topography of the ocean on large scales a much more difficult4 and even more valuable measurement an absolute determination of the subtropical gyre of the north pacific ocean has been obtained we believe this to be the first direct measurement showing the existence of such a feature that does not depend on conventional hydrography and a series of assumptions

sea level monitoring and sea state estimate using a single geodetic receiver gnss reflectometry gnss r altimetry has demonstrated a strong potential for sea level monitoring interference pattern technique ipt based on the analysis of the signal to noise ratio snr estimated by a gnss receiver presents the main advantage of being applicable everywhere by using a single geodetic antenna and receiver transforming them to real tide gauges classical snr analysis method used to estimate the variations of the reflecting surface height h t has a limited domain of validity due to its variation rate dhdtt assumed to be negligible we present here a significant advance in this altimetric methodology using gnss multipath to conjointly estimate h t and dhdtt over areas characterized by high amplitudes of tides and presence of waves it drastically enhances the temporal and spatial monitoring of tides and waves inversion approach is based on a least square method lsm combining simultaneous measurements from different gnss constellations gps glonass our method is validated with snr data acquired on an offshore site of 60 meter height in conditions were assumptions of the classical snr analysis method are not valid i e with a semi diurnal tide amplitude of 4 m vertical velocity of the sea surface due to tide reaching 0 2 mm s and presence of waves with amplitude up to few meters linear correlation between the estimates with our method and tide gauges records are better than 0 97 whereas it only equals 0 82 with the classical method over the whole 3 months of acquisition our dynamic snr method allows a very good estimate of the main tide periods and permits to detect swell and waves with realistic amplitudes and periods which is not the case with tide gauges located in protected areas or classical snr analysis method

simulation of l band bistatic returns from the ocean surface a facet approach with application to ocean gnss reflectometry we present the implementation of a facet based simulator to investigate the forward scattering of l band signals from realistic sea surfaces and its application to spaceborne ocean global navigation satellite system gnss reflectometry this approach provides a new flexible tool to assess the influence of the ocean surface roughness on scattered gnss signals the motivation stems from the study by clarizia which revealed significant differences between delay doppler maps ddms obtained from uk dmc satellite data and ddms simulated with the zavorotny voronovich z v model here the scattered power and polarization ratio pr are computed for explicit 3 d ocean wave fields using a novel implementation of the kirchhoff approximation ka which we call the facet approach fa we find that the fa is consistent with the full ka and the geometrical optics go used in the z v model while being less computationally expensive than the ka and able to represent polarization effects not captured by the go instantaneous maps of the bistatic normalized radar cross section computed with the fa show clear patterns associated with the underlying waves the wave field is particularly visible in the pr indicating that the scattering is generally dominated by the hh component particularly from ocean wave troughs polarization effects show for the first time a strong correlation to the explicit sea surface from which the scattering originated ddms of the scattered power computed with the fa reveal patchy patterns and power distributions that differ from those obtained with z v and show closer similarities with observed ddms from uk dmc

global gravity field recovery from the aristoteles satellite mission one of the primary objectives of the future aristoteles satellite mission is to map earth s gravity field with high resolution and accuracy in order to achieve this objective the aristoteles satellite will be equipped with a gravity gradiometer and a global positioning system gps receiver global gravity field error analyses have been performed for several combinations of gradiometer and gps observations these analyses indicated that the bandwidth limitation of the gradiometer prevents a stable high accuracy high resolution gravity solution if no additional information is available however with the addition of high accuracy gps observations a stable gravity field solution can be obtained a combination of the measurements acquired by the high quality gps receiver and the bandwidth limited gradiometer on board aristoteles will yield a global gravity field model with a resolution of less than 100 km and with an accuracy of better than 5 mgal for gravity anomalies and 10 cm for geoid undulations

simultaneous estimation of the gravity field and sea surface topography from satellite altimeter data by least squares collocation summary the signal content of seasat altimeter data is evaluated with respect to magnitude and wavelength based on this information covariance functions associated with the different signals errors have been determined the covariance function associated with the sea surface heights contains covariances associated with the geoid the stationary sea surface topography and the time varying sea surface topography these three components have been estimated from seasat altimeter data in the faeroe islands region using least squares collocation in the solution error covariances associated with errors in the orbits corrections for instrumental atmospherical and tidal effects and sea state related bias are taken into account the accuracies of the solutions are evaluated by computing error estimates and error correlations a special effort has been made to evaluate the correlations between the estimated geoid and the other signals errors the results show that geoid undulations may be estimated with an accuracy of 0 13 0 16m approximately 50 per cent of the error variance has a long wavelength signature which is mainly correlated with errors in the estimation of the permanent sea surface topography the short wavelength part of the error is mainly correlated with errors due to liquid water in rain and clouds free air gravity anomalies may be estimated with an accuracy of 8 8 11 7 times 10 5 ms 2 this error has a short wavelength signature which is mainly correlated with errors due to liquid water in rain and clouds however a comparison with gravity observations resulted in an agreement of 7 8 times 10 5 ms 2 both the permanent and the time varying sea surface topography may be estimated with accuracies of about 0 21 m

tidal and geodetic observations for the seasat altimeter calibration experiment a calibration experiment for the seasat radar altimeter was conducted in the bermuda calibration area during september and october of 1978 by a group of federal government agencies universities and research organizations as part of the seasat calibration activities a tide gage was installed by the national ocean survey at an open coastal location on bermuda to provide a determination of the instantaneous sea surface height during the seasat overflights of the island the tide gage was geodetically tied to the laser tracking station on bermuda so that seasat s position relative to the sea surface could be determined independently and compared with the value provided by the altimeter measurements the root sum square error in the determination of the vertical position of the laser relative to the sea surface has been estimated to be 4 0 cm exclusive of possible errors arising from the present lack of precise information on the elevation of the geoid at bermuda

an error covariance model for sea surface topography and velocity derived from topex poseidon altimetry in order to facilitate the use of satellite altimeter derived sea surface topography and velocity in oceanographic models a methodology is presented for deriving the total error covariance and its geographic distribution from topex poseidon measurements the model is formulated using a parametric model fit to the altimeter range observations the topography and velocity are modeled with spherical harmonic expansions whose coefficients are found through optimal adjustment to the altimeter range residuals using bayesian statistics all other parameters including the orbit geoid surface models and range corrections are provided as unadjusted parameters the maximum likelihood estimates and errors are derived from the probability density function of the altimeter range residuals conditioned with a priori information estimates of model errors for the unadjusted parameters are obtained from the topex poseidon postlaunch verification results and the error covariances for the orbit and the geoid except for the ocean tides the error in the ocean tides is modeled first as the difference between two global tide models and second as the correction to the present tide model the correction derived from topex poseidon data a formal error covariance propagation scheme is used to derive the total error our global total error estimate for the topex poseidon topography relative to the geoid for one 10 day period is found to be 11 cm rms when the error in the geoid is removed thereby providing an estimate of the time dependent error the uncertainty in the topography is 3 5 cm rms this level of accuracy is consistent with direct comparisons of topex poseidon altimeter heights with tide gauge measurements at 28 stations in addition the error correlation length scales are derived globally in both east west and north south directions which should prove useful for data assimilation the largest error correlation length scales are found in the tropics errors in the velocity field are smallest in midlatitude regions they are less than 2 cm s over most of the southern ocean for both variables the largest errors are caused by uncertainty in the geoid more accurate representations of the geoid await a dedicated geopotential satellite mission substantial improvements in the accuracy of ocean tide models are expected in the very near future from research with topex poseidon data consequently estimates of the time dependent sea surface topography from topex poseidon could achieve a total uncertainty of less than 3 cm rms this level of accuracy from a spaceborne sensor represents a remarkable engineering achievement and a major contribution to ocean science

the eddy experiment accurate gnss r ocean altimetry from low altitude aircraft 1 during the eddy experiment two synchronous gps receivers were flown at 1 km altitude to collect l1 signals and their reflections from the sea surface for assessment of altimetric precision and accuracy wind speed u10 was around 10 m s and swh up to 2 m a geophysical parametric waveform model was used for retracking and estimation of the lapse between the direct and reflected signals with a 1 second precision of 3 m the lapse was used to estimate the ssh along the track using a differential model the rms error of the 20 km averaged gnss r absolute altimetric solution with respect to jason 1 ssh and a gps buoy measurement was of 10 cm with a 2 cm mean difference multipath and retracking parameter sensitivity due to the low altitude are suspected to have degraded accuracy this result provides an important milestone on the road to a gnss r mesoscale altimetry space mission

seasonal and interannual variability in gulf of maine hydrodynamics 2002 2011 abstract in situ observations including long term moored meteorological and oceanographic measurements and multi year gulf wide ship survey data are used to quantify interannual variability of surface wind river runoff and hydrographic conditions in the gulf of maine during summers 2002 2011 the cumulative upwelling index shows that upwelling downwelling favorable wind conditions were most persistent in 2010 2005 over the 10 year study period river discharge was highest in 2005 peak runoff occurred in early april in 2010 as opposed to late april to middle may in other years moored time series show that coastal water temperature was 0 5 2 c warmer than average in summer 2010 and about 2 c colder than average in 2004 coastal salinity in april 2010 was the lowest in the 10 year study period both moored acoustic doppler current profiler adcp current measurements and dynamic height geostrophic velocity calculations based on gulf wide ship survey data show may june 2010 had one of the weakest alongshore transports in the western gulf of maine during the 10 year study period likely associated with intrusions of warm slope water and fresher than usual scotian shelf water comparisons of coastal currents to the paralytic shellfish poisoning psp closure maps resulting from alexandrium fundyense blooms suggest a linkage between alongshore transport and the downstream extent of toxicity

dynamic sea surface topography gravity and improved orbit accuracies from the direct evaluation of seasat altimeter data a gravitational model incorporating seasat altimetry surface gravimetry and satellite tracking data has been determined in terms of global spherical harmonics complete to degree and order 50 this model pgs 3337 uses altimeter data as a dynamic observation of the satellite s height above the sea surface a solution for the ocean s dynamic topography is recovered simultaneously with the orbit parameters gravity and ocean tidal terms the recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 2 000 km and is very similar to the mean upper ocean dynamic height derived from historical ship observations the pgs 3337 geoid has an uncertainty of 60 cm rms globally but only 25 cm rms over the ocean because of the altimeter measurements seasat orbits determined in this solution have an estimated accuracy for the radial position of 20 cm rms the difference between the altimeter observed sea height and the geoid plus dynamic topography model is 30 cm rms contained in these residuals are the sea height variability as well as errors from the geoid orbits tidal models and altimeter range measurements this performance level is 2 to 3 times better than that achieved with previous goddard gravitational models

geodetic and oceanographic results from the analysis of 1 year of geosat data the first year of the geosat exact repeat mission has been analyzed to obtain improved orbits gravity model and representation of sea surface topography the starting point for this study were the original geophysical data records merged with orbits by haines et al 1990 computed with the gem t1 gravity model a radial orbit error model following the development of engelis 1987a was modified to account for resonant effects above degree 36 twenty four 17 day arcs were analyzed separate 17 day and a single 1 year solution for a gravity model and a spherical harmonic representation of the sea surface topography to degree 10 were found the root mean square crossover discrepancy with the a priori i e gem t1 model was 1 1 m which was reduced to 0 22 m with the 1 year solution comparison of overlaps in the arcs showed discrepancies of 1 3 m with the a priori field which were reduced to 0 07 m with the 1 year solution the determination of the degree 1 0 term in the sea surface topography was not possible in this analysis because of the high correlation of this term with an orbit error term in addition the 1 1 terms showed serious incompatibility with that implied by historical oceanographic information the final solution used here incorporated the degree 1 coefficients from oceanographic data the problem with the 1 1 coefficients may imply a small center of mass offset in the tracking station positions used in the orbit calculations or may result from the poor distribution of the available tracking stations error analysis indicated a 10 cm sea surface topography commission error to degree 10 the geoid commission error to degree 36 was 9 cm in the ocean areas the new potential coefficient model was compared with terrestrial data in a manner to that done by engelis and knudsen 1989 to show the improvement of the new model

the eddy experiment gnss r speculometry for directional sea roughness retrieval from low altitude aircraft we report the retrieval of sea surface directional mean square slope from global navigation satellite system reflections gnss r delay doppler map ddm data collected during an experimental flight at 1 km altitude we emphasize the utilization of the entire ddm to more precisely infer ocean roughness directional parameters and argue that the ddm exhibits the impact of both roughness and scatterer velocity obtained estimates are analyzed and compared with jason 1 measurements ecmwf numerical weather model outputs and optical data

improved gnss r ocean surface altimetry with cygnss in the seas of indonesia ocean surface altimetry with gnss reflectometry gnss r has the potential to improve the observation and retrieval of mesoscale ocean current flows by increasing the spatial density of ocean surface observations the leading challenges in current gnss r altimetry investigations include precise delay retracking correction of ionospheric effects and spacecraft receiver positioning here we present improved methods to account for these challenges a reflection model based approach is presented for delay retracking that uses simulated delay doppler maps ddms to retrieve the specular delay from measured ddms global ionosphere maps are used to estimate the group delay effect along the direct and reflection paths precise orbit determination techniques are used to improve estimates of the receiving spacecraft position and reduce systematic intersatellite biases we analyze altimetric height retrievals from two cases strongly coherent or diffuse reflections we study the residual systematic error signals with a focus on understanding the errors identified above the strong coherent observations have narrow symmetric delay waveforms and provide higher than average signal to noise ratio the ddms of diffuse observations originate from relatively rough ocean surfaces and have the characteristic horse shoe shape only measurements from the ocean and seas surrounding indonesia are considered as this area provides a significant number of strongly coherent reflections more than 50 000 single point observations and week long averaged results are presented to quantify the noise and systematic characteristics of the retrieved surface heights

5 cm precision aircraft ocean altimetry using gps reflections 1 we present the first two aircraft global positioning system gps reflection altimetry measurements the most precise gps ocean altimetry measurement and demonstrate the altimetric precision and spatial resolution necessary to map mesoscale eddies our first experiment demonstrated a 14 cm precision single satellite ocean altimetry measurement while our more recent experiment demonstrates 5 cm altimetric precision with 5 km spatial resolution the new results show significant improvement over our previous effort due to improved modeling greater aircraft altitudes and velocities improved receiver positioning and better experimental control plans to further reduce speckle and refine models to obtain 5 cm altimetric accuracy are presented

global scale sea surface variability from combined altimetric and tide gauge measurements by correcting geosat altimetric data for a set of line frequencies and then doing a heavy average in space and time a set of global maps of sea surface variability is produced over time intervals ranging from three months to two years these maps are demonstrated to be consistent with the global network of tide gauges and sets of maps incorporating those data into the altimetric estimates are also produced tide gauge records too fragmentary for computation of a two year mean are employed by time differencing them an estimate of the global difference of sea level change over one year on very large space scales shows the effects of the 1987 el nino the spherical harmonic coefficients employed in the computations are then used to estimate the full three dimensional frequency wavenumber spectrum of surface circulation variability

calibration validation for the geos 3 altimeter the absolute bias calibration for the geos 3 intensive mode altimeter has been measured by using two satellite passes whose ground tracks were within 1 km of the bermuda laser station the bermuda laser tracked on the two passes rev 4553 on february 25 1976 and rev 5471 on april 30 1976 and was supported by two other nasa lasers on one pass and by the nasa spacecraft tracking and data network on the other pass for each pass the altimeter data around bermuda was smoothed and extrapolated to the point closest to overhead at the laser site this point was used for calibration eliminating almost entirely the effects of geoid model error on the resulting altimeter bias estimate after correcting for tide heights and sea state effects the two passes give calibration biases which are in agreement to within 3 cm and have a weighted mean of 5 61 0 16 m for correcting altimeter measurements to the center of mass of the spacecraft i e including the antenna tracking point correction for data correction this bias must be subtracted from the measured values since a sea state bias correction has been used in the bias estimation a different bias is more appropriate for data users not employing a sea state bias correction for such users a bias of 5 51 m appropriate for moderate seas h 2 m is recommended it was found impossible to reconcile the two calibration passes as well as a set of altimeter crossovers in the middle of the geos 3 calibration area without allowing for a data time tag error on the basis of a selected set of four crossovers and an assessment of probable sources of timing error it was concluded that one interpulse period 10 24 ms should be added to the data time tags this time tag correction should be used with the above bias value

scattering of gps signals from the ocean with wind remote sensing application a theoretical model that describes the power of a scattered global positioning system gps signal as a function of geometrical and environmental parameters has been developed this model is based on a bistatic radar equation derived using the geometric optics limit of the kirchhoff approximation the waveform i e the time delayed power obtained in the delay mapping technique depends on a wave slope probability density function which in turn depends on wind waveforms obtained for aircraft altitudes and velocities indicate that altitudes within the interval 5 15 km are the best for inferring wind speed in some regimes an analytical solution for the bistatic radar equation is possible this solution allows converting trailing edges of waveforms into a set of straight lines which could be convenient for wind retrieval a transition to satellite altitudes together with satellite velocities makes the peak power reduction and the doppler spreading effect a significant problem for wind retrieval based on the delay mapping technique at the same time different time delays and different doppler shifts of the scattered gps signal could form relatively small spatial cells on sea surface suggesting mapping of the wave slope probability distribution in a synthetic aperture radar sar fashion this may allow more accurate measurements of wind velocity and wind direction

evaluation of the seasat altimeter time tag bias because of the variable effect on the altimeter height processing produced by a constant error in the altimeter time tag the accuracy of the time tag is an important factor in the applications of the 10 cm seasat altimeter data an assessment of the internal delays inherent in the altimeter microprocessor suggested that 79 4 ms should be added to the time tags transmitted to the ground receiving stations to ensure the validity of this value analyses were undertaken with the altimeter data two methods were used in these analyses which benefited from a global data distribution in the oceanic areas these methods were 1 the crossover method which used altimeter data differenced at points where the seasat ground track intersected with itself and 2 the direct use of the altimeter data the former method is independent of errors in the geoid model hence it is considered to produce the most reliable results using the crossover method and several different ephemerides various values of the time tag bias were estimated for all crossover results the adopted value of 79 4 ms is within the bounds of the standard deviation associated with the estimates in particular the estimate of 78 1 2 0 ms is considered to best represent the set of estimated values

geophysical applications of satellite altimetry publications related to geophysical applications of seasat and geosat altimetry are reviewed for the period 1987 1990 problems discussed include geoid and gravity errors regional geoid heights and gravity anomalies local gravity field flexure plate tectonics and gridded geoid heights gravity anomalies 99 refs

an estimate of global absolute dynamic topography abstract we estimate the absolute dynamic topography of the world ocean from the largest scales to a short wavelength cutoff of about 6700 km for the period july through september 1978 the data base consisted of the time averaged sea surface topography determined by seasat and geoid estimates made at the goddard space flight center the issues are those of accuracy and resolution use of the altimetric surface as a geoid estimate beyond the short wavelength cutoff reduces the spectral 1eakage in the estimated dynamic topography from erroneous small scale geoid estimates without contaminating the low wavenumbers comparison of the result with a similarly filtered version of levitus historical average dynamic topography shows good qualitative agreement there is quantitative disagreement but it is within the estimated errors of both methods of calculation

the sea state correction for geos 3 and seasat satellite altimeter data troughs of the ocean surface reflect more electromagnetic radiation to a satellite altimeter than crests causing an apparent lengthening of the altimeter path this effect called the em bias has been estimated to be 2 10 of significant wave height swh by various authors from geos 3 and aircraft experiments or theoretical analyses for the case of the seasat altimeter born et al 1982 at jet propulsion laboratory jpl made an empirical estimate that gave the total correction to the altimeter path length as 7 of the swh a large value was expected for seasat because an instrumental effect in addition to the em bias is known to exist waveform analyses performed at nasa wallops flight center hayne and hancock 1982 give a correction for the instrument effect but according to the authors their results cannot be compared with results obtained elsewhere for several reasons including the large uncertainty 30 of the jpl result in this paper we resolve the situation for both geos and seasat altimeters by reinterpreting the results of hayne and hancock and making empirical studies using very large data sets to estimate the correction required for the effect of sea state empirical estimates of the correction were made by finding a value that minimized in the sense of least squares the differences of many about 300 repeated pairs of altimetric profiles for each satellite the total correction obtained is 1 9 of significant wave height for geos 3 and 6 4 for seasat the latter result consistent with the analysis done at jpl of a smaller data set and our interpretation of the hayne and hancock waveform analysis

drivers of spring and summer variability in the coastal ocean offshore of cape cod ma author posting american geophysical union 2016 this article is posted here by permission of publisher for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 121 2016 1789 1805 doi 10 1002 2015jc011252

tropical atlantic sea level variability from geosat 1985 1989 geosat altimeter data between april 1985 and september 1989 are analyzed in the tropical atlantic ocean first improvements due to the use of new corrections and orbit computations are found to be effective especially in the gulf of guinea where part of the previously missing signal is recovered then the variability of the ocean is examined using empirical orthogonal functions eofs only three eofs are needed to describe 80 of the seasonal variance the first one describes the meridional tilting of the tropical atlantic along the mean location of the intertropical convergence zone with an annual period the second describes a mass redistribution due to the equatorial upwelling peaking in june july the third function presents a clear semiannual signal looking at interannual variability the first eof reveals a mass redistribution between the equatorial region 10 n to 10 s and the northern and southern ones 10 n to 30 n 10 s to 30 s in the equatorial region the upper layer volume increases from about 1 5 to 1 5 x 1014 m3 between 1987 and 1989 occurring 1 year after the pacific el nino this phenomenon recalls the 1984 anomaly observed during the programme francais occan et climat en atlantique tropicavseasonal equatorial atlantic experiments

seasat altimeter height calibration the seasat altimeter has been calibrated for height bias using four overflight passes of bermuda that were supported by the bermuda laser the altimeter data was corrected for tides by using recorded tide gage data for propagation effects by using meteorological data taken around the time of each pass for acceleration lag and for sea state bias including both surface effects and instrumental effects altimeter data for each of the four passes was smoothed and extrapolated across the island interpolation between passes then produced an equivalent altimeter measurement to the geoid at the laser site so that the altimeter bias could be estimated without the use of a geoid model the estimated height bias was 0 0 0 07 m

the time mean circulation in the agulhas region determined with the ensemble smoother the time mean circulation in the agulhas retroflection area is determined by combining topex poseidon data and a two layer quasi geostrophic model using the ensemble smoother by taking the time mean circulation as the unknown in the data assimilation procedure the time varying altimeter signal is used to constrain the time mean field the quasi geostrophic model is applied as a strong constraint with only the time mean circulation containing errors inspection of the posterior penalty function showed that the inversion was successful the errors in the time mean sea surface topography reduced from about 10 to about 3 cm a cyclonic recirculation cell over the agulhas plateau was found related to the northward meander of the agulhas return current another cyclonic recirculation cell was found west of africa probably related to the passage of anticyclonic agulhas rings south of it the new field is compared with advanced very high resolution radiometer infrared satellite data confirming the northward meander of the agulhas return current

application of the gem t2 gravity field to altimetric satellite orbit computation as part of a continuing effort to provide improved orbits for use with existing altimeter data we have recomputed ephemerides for both the seasat and geosat exact repeat altimeter missions the orbits were computed in a consistent fashion using the goddard earth model t2 gem t2 gravity field along with available ground based tracking data such an approach allows direct comparisons of sea level between the two altimeter systems evaluation of the resulting ephemerides indicates that root mean square accuracies of 30 50 cm have been achieved for the radial component of the orbits for both satellites an exception occurs for the last year of the geosat exact repeat mission when the rms radial orbit accuracy degrades to the 1 m level at times owing to the inability to adequately model the drag force arising from the increased solar activity

the calculation of the dynamic sea surface topography and the associated flow field from altimetry data a characteristic function method the quasi stationary sea surface topography qsst and associated oceanic circulation is determined by means of a characteristic function technique the method was originally implemented in an ideal simplified case the present application involves a 4 83 4 8grid in spherical coordinates approximating the boundaries of the main ocean basins the data field is provided by the first year of altimetric data from the topex poseidon mission the method requires the numerical determination of the eigenfunctions spanning the streamfunction field and the associated characteristic functions from the balance equation the former yields the flow field and the latter the surface height distribution or qsst these functions are determined by the geometry and topography of the ocean basins and satisfy the linear steady state dynamical equations they are defined within the basins only and avoid the problems encountered when using functions defined over the entire sphere the velocity field can be computed over the entire ocean area including the equatorial regions the coefficients of the height functions have been determined by fitting the surface height field provided by topex altimetry seasonal variations have been computed by subtracting a 32 cycle solution from each of the four 8 cycle seasonal solutions

sea surface state measured using gps reflected signals we discuss an airborne experiment aimed to establish the potential of the paris concept passive reflectometry and interferometry system to retrieve small features in the sea surface topography the date and location were chosen to coincide with a topex poseidon t p overflight the signals of the global positioning system gps reflected off the sea surface are tracked and compared to the directly received ones to compute the relative delays the features detected in the peak tracking are likely caused by topographic and sea roughness variations while very promising these results open the challenge to use additional information to appropriately separate both contributions