Revised September 14, 2001 NASA/GSFC Pathfinder Adjusted ERS-1 PhaseC Georeferenced Altimetry Version 1.0 Version 1.0 : > Ocean tide model replaced with GSFC GOT00.2 > Adjustment of ERS-1 Phase C Version 5.1 into TOPEX/POSEIDON reference frame (T/P Pathfinder Version 8.8) employing technique similiar to LeTraon, et al., 1998. Detailed documentation of the adjusted ERS data sets will be made available on the project web site: http://iliad.gsfc.nasa.gov/ocean.html Unadjusted: Version 5.1 Updates : > Ocean tide replaced with GSFC GOT99.3 resulting in additional recovery of inland sea areas. Version 5.0 Updates : > Time tag bias modified from 1.7 ms to 1.5 ms > Significant wave height corrected (P. Queffeulou, Quality of Long-Term Satellite Wind and Wave Measurements, ERS-Envisat Symposium, Oct., 2000)and EM Bias correction re-computed with Gaspar BM3 model. Version 4.0 Updates : > UTCSR Mean Sea Surface replaced with GSFC00.1 MSS > Cross track gradient correction recomputed from GSFC00.1 cross track slopes. > Ocean tide of Schrama,Ray 1996 replaced with GOT99.2. > Inter-mission and inter-version (Cersat OPR) biases have been applied to produce a more homogeneous dataset with respect to ERS-2 and OPR Version 6 data products (IFREMER report CLS/DOS/NT/98.070): ERS-1 Phase C Sea Surface Height Version 4.0 = ERS-1 Phase C Sea Surface Height Version 3.0 - 409 mm (absolute range calibration) + 24 mm (range correction for OPR Version 3) Version 3.0 Version 3.0 Updates : > Altimetric Range Corrections Applied: Ultra Stable Oscillator (USO) Drift Scanning Point Target Response (SPTR) Correction Version 2.0 Version 2.0 Updates : > Major update: Sea surface height is based on Delft DGM-E04 orbits with 1.7 ms time tag bias applied. > Ocean tide flag bit (see flag.doc) Version 1.1 > SSH based on UT CSR TEG-3 orbits Version 1.1 Updates : > SSH corrected for 1.6 ms time tag error.. > Recovery of data previously lost to orbit interpolation error. > Retain SSH residuals > 3.0 m with respect to mean sea surface. This README file describes the Pathfinder ERS-1 PhaseC georeferenced altimeter data set, and auxillary files providing documentation and access to the altimetry. The sea surface height parameter file " ssh.dat " is a direct access integer binary file with a record length of 40 bytes. Each record contains an array of sea surface height residuals for the 18 35 day repeat cycles of mission PhaseC at a single location defined by a revolution number and along track index: Parameter File Record Description Variable Word Size Bytes Along Track Index I*2 2 Revolution Number I*2 2 Sea Surface Residual I*2 36 Height Array (mm) Note that a revolution is defined as the ground track between consecutive ascending equator crossings. A directory file of record numbers (directry.dat) provides direct access to the SSH parameter file. Sea surface height information can be retrieved at specific locations or geographical regions of interest defined by revolution number and along track index. An example of accessing a SSH time series at the Johnston Island tide gauge is provided in software located in opf/pathdbase/source/current that enables access to the ssh parameter file and the following external files: reforb.dat : file containing geodetic latitude and longitude of georeferenced locations mss.dat : CSR mean sea surface (CSRMSS95IB) interpolated to the indexed georeferenced locations (cm) flagword.dat : data quality flagword for each observation documented in " flag.doc " time.dat : observation time at index 1 for each revolution directry.dat : ssh data record directory file The sea surface residual heights are in units of mm and are with respect to the UT CSR95 MSS. The mean sea surface is provided allowing the option to add back the mss thus referencing the sea surface height to the TOPEX standard reference ellipsoid : Equatorial Radius = 6378136.3 m Flattening = 1/298.257 The following ocean dynamic and range delay corrections were applied to the sea surface height : Ocean Tide (Schrama, Ray 1996) Load Tide (Schrama, Ray 1996) IRI95 Ionosphere ECMWF Dry Troposphere ECMWF Inverted Barometer (IB) Wet Troposphere radiometer measurement Solid Earth Tide (TOPEX SWT algorithm) Pole Tide (TOPEX SWT algorithm) EM Bias : 5.5 % SWH (Gaspar, 1994) Cross Track Gradient (wrt CSRMSS95IB) The SSH parameter file 'ssh_noib.dat' is available which does not have the inverted barometer correction applied to the sea surface height. The altimetry and associated data files, documentation and software module can be obtained from the following anonymous ftp node: ftp iliad.gsfc.nasa.gov cd opf/ers1/phasec/current for data files cd opf/pathdbase/source/current for software For questions, please contact Brian Beckley (brianb@magus.stx.com), Richard Ray (ray@nemo.gsfc.nasa.gov), or Chet Koblinsky (chet@neptune.gsfc. nasa.gov).