Many scientific centers show interest in satellite altimetry.
For example, by an order from the USA Navy, a new satellite was launched
in May 1998 for a ten-year active service with the GFO
(GEOSAT Follow-On) altimeter on board. This year, the USA and the countries
of Western Europe plan to launch satellites with altimeters TOPEX-2
(JASON-1) and ENVISAT on
board. In this connection, the Integrated Satellite Altimetry Data Base
(ISADB) can serve as a sound foundation for carrying out the monitoring
of the World Ocean during the next 10-15 years and provide unique possibilities
for a wide range of researches in geodesy, geophysics, marine geology,
oceanology, etc.. This is why the Geophysical Center of the O.Yu. Shmidt
Joint Institute of Physics of the Earth, Russian Academy of Siences (GS
RAS), is planning to continue the stocking of the data base with the results
of new measurements, and the study of their interpretation.
The ISADB is stored on optical disks of the CD-ROM type,
and it is accessable through the Internet by the following procedure.
In the on-line regime, the available data
base contains information on the stock, data distribution, and their statistical
estimations, thus allowing to carry out the express analysis of the
available information . After the express analysis, the remote user can
formulate a demand to the complete data base,
which is sent to the administrator and is carried out in the off-line
regime. With the help of the DBMS ARGOS, the administrator
selects the required information, copies it in a single (unified) format
and sends the data to the user by e-mail, or copies it onto carriers. The
procedure of the ISADB formation included a complete recalculation to the
adopted unified data records format
and reference ellipsoid was carried
out for all satellites GEOIK, GEOSAT
and TOPEX/POSEIDON.
It is considered expedient not to include as yet into the ISADB the operative
data from the TOPEX/POSEIDON satellites available in the on-line
regime owing to their lower accuracy as compared to that of the data in
the MGDR-B
format. If necessary, these data can be obtained in the on-line
regime by the FTP protocol from NASA Physical Oceanography Distributed
Active Archive Center (PO.DAAC).
In order to automatize the process of execution of requests
to the ISADB (data selection, installation of the auxiliary information,
etc.), about 20 modules of the programs were elaborated at the GC RAS,
which, for example, include the modules: calculations of tidal corrections,
calculations of the geoid altitude by using the models of the gravity field
of the Earth of high degrees (up to 700), calculations of gravity anomalies
from the altimeter measurements data, calculations of corrections for the
environmental effect, and so on.
The ISADB is represented in the form of three levels:
At the end of 1998, the total volume of the base was about
30 Gbite. The Hypertext base and the file system on a hard disk include
patters of ground tracks with statistical characteristics on the number
of measurements over the sea, over the land, and over the ice; it also
includes bad measurements by months and half-year
periods for GEOIK and cycles for Exact Repeat Mission
(ERM) and Geodetic Mission (GM) GEOSAT programs. For the TOPEX/POSEIDON
satellite only one scheme is given for its 10-days ground tracks owing
to their complete identity.
The structure of
ISADB files
(Data Records
Format)
| Item |
Bytes |
Parameter |
Description |
Units |
| 1 |
4 |
Time_UTC |
Universal Time Coordinated (since the beginning of 1985) |
sec |
| 2 |
2 |
Time_MS |
Additional time |
10-3 sec |
| 3 |
4 |
Lat |
Latitude |
10-6 degree |
| 4 |
4 |
Lon |
Longitude |
10-6 degree |
| 5 |
2 |
SSH |
Sea surface height (SSH) corrected by instrumental, tropospheric
(Wet and "Dry"), ionopsheric and Electromagnetic Bias corrections |
10-2 m |
| 6 |
1 |
Nval_H_A |
Number of valid points for 1 second SSH |
|
| 7 |
2 |
RMS_H |
Root Mean Square (RMS) range SSH |
10-2 m |
| 8 |
2 |
H_Off |
Height offset to be added to all SSH |
m |
| 9 |
2 |
dH_Inv_Bar |
Inverse barometer correction |
10-3 m |
| 10 |
2 |
H_MSS |
Mean sea surface height |
10-2 m |
| 11 |
2 |
H_Geoid |
Geoid height |
10-2 m |
| 12 |
2 |
GA |
Gravity anomaly |
10-4 gal |
| 13 |
2 |
H_EOT |
Elastic ocean tide |
10-3 m |
| 14 |
2 |
H_LT |
Loading tide effect |
10-3 m |
| 15 |
2 |
H_SET |
Solid earth tide |
10-3
m |
| 16 |
1 |
H_Pol |
Geocentric pole tide |
10-3m |
| 17 |
2 |
H_Levitus |
Mean dynamic topography by hydrological measurements |
10-2 m |
| 18 |
2 |
D_Ocean |
Ocean depth |
m |
| 19 |
2 |
SWH |
Significant Wave Height |
10-3 m |
| 20 |
1 |
WS |
Wind speed |
10-2 m/s |
| 21 |
1 |
Flag |
Flag |
|
The corrected sea surface height (SSH) is the elevation of
the sea surface relative to the reference ellipsoid (a = 6378136.3
f = 1/298.257) and is calculated by the corrected altimeter data and the
orbit altitude likewise determined relative to the reference ellipsoid.
In order to formate the ISADB, usually the height of the satellite orbit
is applied, calculated by NASA,
CNES, or the Russian
Space Agency (RKA), correspondingly. The corrected altimeter indications
(H_Alt) are the indications of the altimeter averaged by time ~1 sec and
corrected by instrumental correction, which includes calibration and a
number of apparatus corrections.
The accuracy of calculation of the satellite altitude with
respect to the sea surface depends on the lagging time of the useful sign
recovery on board caused by the atmospheric and ionospheric effects and
by the state of the underlying surface. The calculation of this effect
was carried out by means of corresponding corrections: for wet and "dry"
atmosphere, for ionosphere, and for the state of the sea surface (Electromagnetic
Bias). When forming the ISADB data, a correction was introduced for humidity
calculated from the data of microwave radiometer on board. If humidity
is absent, then the model correction is introduced, interpolated by the
moment of measurement. The correction for "dry" atmosphere is calculated
by the field of pressure and, by analogy with the previous correction,
is interpolated for the corresponding time value of satellite measurements.
The correction for the ionopshere is calculated by tradition using
the data on electron content in the ionosphere and the IRI-95 model. The
correction for the state of the underlying surface is assumed, if it is
present, from the calculations by the Gaspar formular on the basis of data
about the near-water wind and the height of waves for the working frequency
13.6 GHz (Ku range).
The residual height of the sea surface or dynemic topography
are determined as the deviations of the corrected sea surface height (SSH)
relative to the geoid with due regard for tidal effects and with correction
of the inverse barometer (dH_Inv_Bar). The residual height allows to determine
the following geophysical parameters:
-
the real topography of the ocean surface formed by oceanic
currents;
-
the dynamic effect of the inverse barometer;
-
the difference between the real tides and the tide models;
-
synoptic changes in dynamic topography.
Depending on the problem under solution, the topography can
be determined relative to the mean sea surface height (H_MSS), or relative
to the geoid height (H_Geoid). Both surfaces are determined relative to
one and the same reference ellipsoid
as the sea surface height (SSH). They are interpolated at the points of
subsatellite measurements only for the aquatory of the ocean. The difference
between the mean sea surface height and the geoid height produces the mean
dynamic topography, which allows to estimate the averaged dynamic pattern
without consideration of the synoptic changeability. The availability of
an average dynamic topography (H_Levitus) designed on average hydrological
fields of temperature and salinity, in turn allows to investigate synoptic
changes in dynamic topography.
The combined tidal effect is obtained from the sum of the
elastic sea tide (dH_EOT), the crustal tide (H_SET), and the pole tide
(H_Pol) values. All these corrections, if available, are preserved in ISADB
data, for which the elastic sea tide value is obtained according to the
model of the Texas University
(CSR 3.0). The values of the earth and the pole tides are given in
the original files.
The correction of the inverse barometer (dH_Inv_Bar) takes
into account the changes on the sea surface height from the basic effect
of the atmosphere and is detemined by atmospheric pressure (in millibars),
which can be obtained by applying the "dry" atmosphere correction.
Supplementary information includes the depth of the
ocean, the height of waves, and wind velocity. The depth at the point of
the sub-satellite track is determined by the 5-min field of the relief
of the World Ocean ETOPO-5 of the USA National Geophysical Data Center
(NGDC). For the height of waves
and wind velocity (whenever present) the values are presented as given
in the original files (if available).
The TOPEX/POSEIDON data were obtained from the NASA Physical
Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory
/ California Institute of Technology (PO.DAAC).
© Geophysical Center RAS, 1998
Last upgrade 25 September 1999 |
