------------------------------------------------------------------------------- Geosat JGM-3 GDRs ------------------------------------------------------------------------------- Item Parameter Units Bytes Description ------------------------------------------------------------------------------- 1 UTC sec 4 UTC time since 01/01/85. Includes +5 msec timing bias correction. 2 UTC (cont'd) microsec 4 UTC time, microseconds part. 3 LAT microdeg 4 Latitude, microdegrees N. 4 LON microdeg 4 Longitude, microdegrees E. 5 ORB mm 4 JGM-3 orbit relative to reference ellipsoid: ae = 6378136.3 m; 1/f = 298.257. 6 H cm 2 1-second average sea height relative to reference ellipsoid. 7 SIG_H cm 2 Standard deviation of the 10/sec values about the 1-second H. 8 MSSH cm 2 Mean sea surface height from the Ohio State MSS95 model. 9-18 H1-H10 cm 10*2 10/sec sea height values. To derive time tags, see footnote #1. 19 SWH cm 2 Significant wave height (see footnote #2). 20 WS cm/sec 2 Wind speed at 10 m height, from Freilich and Challenor (1994) model. 21 SIG_0 0.01 dB 2 Sigma naught, radar backscatter coefficient (see footnote #3). 22 SSB mm 2 Sea state bias derived by Gaspar, Ogor, and Hamdaoui (1996). 23 L_TID mm 2 Load tide from CSR 3.0 model. 24 FLAGS - 2 See table below for flag bit definitions. 25 H_OFF m 2 H offset to be added to all heights over land (flag bit 0 = 0). 26 S_TID mm 2 Solid tide from T/P "TIDPOT" algorithm, based on Cartwright & Edden (1973). 27 O_TID mm 2 Ocean tide from CSR 3.0 model. 28 WET_NCEP mm 2 Wet troposphere correction from NCEP/NCAR reanalysis model. 29 WET_NVAP mm 2 Wet troposphere correction from NASA NVAP climatology. 30 DRY_NCEP mm 2 Dry troposphere correction from NCEP/NCAR reanalysis model. 31 IONO mm 2 Ionosphere correction from IRI95 model. 32 WET_T/S mm 2 Wet troposphere correction from TOVS/SSMI observations. 33 DRY_ECMWF mm 2 Dry troposphere correction from ECMWF model. 34 ATT 0.01 deg 2 Attitude (Spacecraft off-nadir orientation). ------------------------------------------------------------------------------- Total 78 Bytes ------------------------------------------------------------------------------- Footnote #1: Time tags for the H1-H10 heights can be computed with the formula: t(i) = UTC(sec) + UTC(microsec) + 0.98 * (i/10.0 - 0.55); i=1-10 Footnote #2: SWH has been increased by 13%; see Carter, Challenor, and Srokosz (1992). Footnote #3: Sigma-0 values in the GM have been modified by several tenths of a db (Ella Dobson, JHU/APL, personal communication). ------------------------------------------------------------------------------- Definition of Bits in Flag Word (Item #24) ------------------------------------------------------------------------------- Bit On (Bit = 1) Off (Bit = 0) Notes ------------------------------------------------------------------------------- 0 Ocean location Land location Based on 5-minute CSR land mask. 1 Ocean depth > 2250 m Ocean depth < 2250 m Based on 1-degree shallow sea mask. 2 dh(swh/att) dh(swh/att) See JHU/APL (1985). or dh(fm) suspect or dh(fm) nominal 3 Any 10/sec height All 10/sec heights Invalid heights = 32767. invalid valid 4-6 ATT flags suspect ATT flags nominal See JHU/APL (1985). 7 Wind speed suspect Wind speed valid Suspect if WS < 1.5 or > 20 m/s. 8 Sea state bias Sea state bias SSB suspect if SWH < 0 or suspect valid > 11m; if WS < 1.5 or > 20 m/s; or if ATT > 1.1 deg. 9-15 All remaining bits set to 0. -------------------------------------------------------------------------------