Geosat Chapter 3

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Geosat JGM-3 GDRs

3. GDR Format

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)
or dh(fm) suspect		 dh(swh/att)
or dh(fm) nominal		 See JHU/APL (1985).
3 Any 10/sec height invalid All 10/sec heights valid Invalid heights = 32767.
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 suspect Sea state bias valid SSB suspect if SWH < 0 or > 11 m;
if WS < 1.5 or > 20 m/s;
or if ATT > 1.1 deg.
9-15 All remaining bits set to 0.


For a plain text version of this table, click here .

Next Chapter: 4. How to Construct Corrected Sea Height

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