1/15/13 Version 21.1 gravity is similar to V20.1 but with the following added data. Retracked Cryosat data for 12 months between July 2010 and December 2012 (28 months). This includes all the LRM, SAR, and SARIN data. Retracked Envisat data for cycles 10-42 and cycles 74-113. This includes 56 of the old 35-day repeat cycles and 17 of the new 30-day repeat cycles until the end of the mission. The major improvement comes from Jason-1 data - 1500 days from the 10-day repeat mission and 210 days from the new Geodetic mission. 6/22/12 Version 20.1 gravity is similar to V19.1 but with the following added data. Retracked Cryosat data for 12 months between July 2010 and April 2012. This includes all the LRM, SAR, and SARIN data. Retracked Envisat data for cycles 88-113. This includes 9 of the old 35-day repeat cycles and 17 of the new 30-day repeat cycles until the end of the mission. 2/2/12 Version 19.1 gravity is similar to V18.1 but with the following added data. Retracked Cryosat data for 16 months between July 2010 and December 2011. This includes all the LRM, SAR, and SARIN data. Retracked Envisat data for cycles 90-106. This includes 4 of the old 35-day repeat cycles and 10 of the new 30-day repeat cycles. 6/9/08 Version 18.1 gravity is similar to V16.1 except that the remove/restore was performed with a new global gravity model EGM2008. This model has variable resolution on land up to 5 minutes depending on the availability of source gravity data. The web address and reference for EGM2008 follows. http://users.auth.gr/~kotsaki/IAG_JWG/IAG_JWG or http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/index.html Pavlis, N.K., S.A. Holmes, S.C. Kenyon, and J.K. Factor, An Earth Gravitational Model to Degree 2160: EGM2008, presented at the 2008 General Assembly of the European Geosciences Union, Vienna, Austria, April 13-18, 2008. Over the ocean it has the same 1-minute cell spacing and same resolution as V16.1 gravity. The main differences from V16.1 is be near shorelines where this new gravity model should be more accurate in the 50-200 km wavelength band. 3/19/07 Version 16.1 gravity is similar to V15.1 except that the gravity field runs to a higher latitude. param V15.1 V16.1 ___________________________ nlon 21600 21600 nlat 12672 17280 rlt0 -72.006 -80.738 rltf 72.006 80.738 ___________________________ The projection is the same spherical Mercator used in all of our other gravity grids. Some additional parameters for this model: - low-pass filter has a 0.5 gain at a wavelength of 14 km. - the tension in the bi-harmonic spline fit is 0.25 - there is one spline knot for every other grid cell. V15 had a spline knot for every third grid cell. The file curv.img.15.1 is a file of vertical gravity gradient derived from the curvature of the ocean surface. The units of this file are 0.05 Eotovos units. Note that the EGM96 gravity model was not added back to the grid so the long wavelengths (> 200 km are absent). 5/1/05 Version 15.1 gravity is a major improvement over V11.1 and all previous versions. Here is an evaluation of this model in relation to previous models based on the rms difference (mGal) between the satellite gravity and three shipboard gravity profiles. ship V7.2 V8.1 V9.1 V11.1 V15.1 _____________________________________________________ conrad 2802 4.42 4.17 4.00 3.33 2.63 conrad 2912 5.85 5.66 5.44 5.15 3.99 ewing 921 4.07 3.64 3.56 4.03 2.85 _____________________________________________________ More extensive evaluations including a blind analysis by Marcia Maia at IFREMER can be found at: ftp://topex.ucsd.edu/pub/sandwell/grav_eval Specific improvements: 1) All ERS-1 geodetic mission data were retracked to improve the along- track slope precision from 8.23 microradians to 4.01 microradian. The along-track resolution of repeat profiles also improves from 43 km to 32 km. A more complete discussion of the retracking methods as well as results are provided in [Sandwell and Smith, GJI, 2004] http://topex.ucsd.edu/sandwell/publications/105.pdf 2) The second major improvement is that all Geosat Geodetic Mission data were retracked using the same basic algorithm. These data will be available at the NOAA Laboratory for satellite Altimetry. Please contact John Lillibridge John.Lillibridge@noaa.gov 3) We applied a correction for amplitude and phase shift in Geosat and Topex data (not retracked) caused by the onboard alpha/beta tracker. The phase shift is 1.67 km along-track. This shift caused some blurring of E-W oriented gravity anomalies. 4) Errors due to mesoscale variability and tides were removed from all along- track slope profiles. This was accomplished by low-pass filtering the difference between the profiles and the V11.1 grid using a filter with a 0.5 amplitude gain at 120 km wavelength. This smoothed difference, which absorbs the mesoscale variability and tide error, was then removed from each profile. 5) A major difference in this version is that residual along-track slopes were gridded using biharmonic splines in tension. The primary references are: Sandwell, D. T., Biharmonic Spline Interpolation of GEOS-3 and SEASAT Altimeter Data, Geophys. Res. Lett., 14, 139-142, 1987 Wessel, P., and D. Bercovici, Interpolation with Splines in Tension: A Green's Function Approach, Mathematical Geology, V. 30, no. 1,, p.77-93, 1998 The final conversion from north and east vertical deflection to gravity has a wavelength of 16 km. 1/24/04 Version 11.1 is a major improvement over V10.2 because it includes re-tracked ERS-1 data. Retracking reduces the noise level by 40%. V11.1 - low pass cutoff wavelength 14.4km 10/14/02 Version 10.2 is just like version 9.2 but a different low-pass filtered was used. V9.1 - low pass cutoff wavelength 14.4km V10.1 - low pass cutoff wavelength 17.1km 06/04/99 Version 9.1 recalculated from filtered north and east grids. Result is the same but the hit map was added in a different way. Gravity values were forced to be even using nint and then 1 was added if this pixel was hit. 01/06/99 Version 9.1 is similar to version 8.1. The main change is that the along-track data were low-lass filtered with a 0.5 gain at 12 km wavelength. Version 8.1 used a 10 km wavelength. The 2-D filter applied later in the processing still has a 0.5 gain at 18 km so the additional along-track filter has little effect on the final resolution of the grid. Here is a simple evaluation of the grid based on the rms difference (mGal) between the satellite gravity and three shipboard gravity profiles. ship V7.2 V8.1 V9.1 _____________________________________ conrad 2802 4.42 4.17 4.00 conrad 2912 5.85 5.66 5.44 ewing 921 4.07 3.64 3.56 _____________________________________ 12/20/98 Version 8.1 is based on completely new gridding software for computing marine gravity anomaly from along-track sea surface slope profiles [Sandwell & Smith, JGR v102, #B5, 10,039­10,054, 1997]. More efficient numerical code enables us to grid the data at a finer 1 minute resolution. Other improvements include: - increased short wavelength resolution is achieved through less filtering of the vertical deflection grids (0.5 gain at 18 km instead of 24 km); - better matching of gravity anomalies across coastlines is achieved by using EGM96 to degree 360, as opposed to JGM3 to degree 70; - a new iteration scheme enforces a consistency check such that the components of vertical deflection have zero curl; - finally, a global approach to the two-dimensional FFT coupled with a higher degree reference model enables us to construct geoid height in addition to gravity anomaly. The results show better short wavelength gravity resolution, especially at the crests of seamounts. However, the noise level increases in proportion to the change in resolution. Agreement with ship gravity measurements reduces the rms misfit to a typical value of 4.2 mGal from 4.5 mGal. This version includes the following data: all ERS-1 GM data (two 176-day cycles Ocean Product) all Geosat/GM data stack of 62 repeat cycles of Geosat/ERM stack of 42 repeat cycles of ERS 35-day repeat stack of 120 repeat cycles of TOPEX 10-day repeat Please try our web site. http://topex.ucsd.edu/mar_grav.html Version 8.1 gravity file also encodes the locations of the altimeter measurements in the lowest bit. Anomalies are stored as milligal times 10, rounded to the nearest even value. Even numbers signify no altimeter measurement in that cell while odd numbers (10*milligal +1) signify the cell contains a measurement. The programs img2xyz and img2grd have been modified to use this information and thus require more options. The files in this directory contain global marine gravity anomalies gridded on a Mercator projection The grid was derived from the following data sources: Geosat/ERM - Average of 62 Geosat Exact Repeat Mission profiles. Geosat/GM - Recently declassified Geosat Geodetic mission data for for all ocean areas. The original Geosat/GM GDR's can be obtained on CD-ROM from: National Oceanographic Data Center User Services Branch NOAA/NESDIS E/OC21 1825 Connecticut Ave., NW Washington, DC 20235 Tel. 202-606-4549 Internet: services@nodc2.nodc.noaa.gov ERS - ERS OPR GDR's from the first 42 repeat cycles of the 35-day repeat orbit were averaged to improve their accuracy and resolution. ERS-1/GM - ERS-1 OPR GDR's from the entire geodetic mission TOPEX - TOPEX GDR's from the first 120 repeat cycles of the mission. The gridded data are stored in an integer*2 format without any header or record information. grav.8.1.img A 12672 by 21600 grid of 2-byte integers = 547,430,400 bytes. Byte order is big_endian. The gravity anomaly units are 0.1 milligal. An even value signifies the cell does not have an altimeter measurement while an odd value signifies that it does. The Mercator projected image spans longitudes from 0 E to 360 E and latitudes from 72.006 N to -72.006 N. A spherical earth is used for the Mercator projection. The center of the upper left grid cell (i.e. the first integer in the file) is located at 72.0009 N, .00833333 E. Longtiudes increase with a 1/30 degree spacing. The The center of the last integer in the file is located at -72.0009 N, 359.933 E. The files can be accessed either with the program img2xyg or the GMT program img2grd. In addition, it can be used with image processing programs such ER-Mapper or GIPS. img2xyg - A Fortran/C program to extract a sub area from the global gravity grid and write an ascii file of longitude, latitude, gravity anomaly values. This file can be gridded using any standard gridding program such as the GMT programs xyz2grd or surface. Either all of the grid cells can be output or just those cells that contain an altimeter measurement. interp_ship - A Fortran/C program to interpolate the gravity field along a random ship track. img2mercgrd - GMT users can make a .grd file of a sub region with either of these img2latlongrd two programs. These programs should be obtained from the GMT3.0 supplements tar file. grav.8.1.img.ers - A header for ER-Mapper with the proper projection information. gips_grav.head - A GIPS header to attach to the front of world_grav.img.new. (GIPS is an image processing package written by Peter Ford at MIT. It is primarily used for Planetary applications.) 6/3/96 Version 7.2 is described in the following reference: Sandwell, D. T. and W. H. F. Smith, Marine Gravity from Geosat and ERS-1 Altimetry, J. Geophys. Res., 102, 10039-10054, 1997. http://topex.ucsd.edu/sandwell/publications/71.pdf