ERS-1.ALT.OPR2
The main difference between ERS-1.ALT.OPR2 and ERS-1.ALT.QLOPR originates from
the retracked altimeter signal in the OPR2. Due to the need for a quick distribution, the
fast delivery data, which is the basis for the QLOPR, includes an altimeter measurement
from a coarse wave forms analysis, thus a smoother behavior between adjacent
measurements is obtained. Also the GFZlD-P AF precise orbit information is merged to
the data, leading to more accurate satellite heights above a reference ellipsoid. Radiometer
and precise meteorological data from French Meteorological office result in better
tropospheric corrections than those included in the QLOPR.
TOPEXGDR
The TOPEX altimeter data merged into MSS95A are from the French TOPEX GDR
(AVISO 1992). The fact that the TOPEX satellite has a much smaller size than ERS-l,
a higher altitude and multiple tracking systems (GPS, DORIS, laser reflectors) results in
a radial accuracy of the satellite ephemerides better than 3 cm. The dual frequency
altimeter reveals an instrumental ionospheric path delay correction that is more accurate
than existing models. An on-board radiometer reduces the uncertainties of the wet
tropospheric correction, which appears, if atmospheric models are used. Besides
Schwiderski's tides, the better Cartright's model (Eanes 1993) is merged to the GDR (up
to now improved tidal solutions are not included in TOPEX GDR). Instrument, telemetry
and quality flags help to eliminate bad data.
PROCESSING
In this chapter a description of MSS95A's processing steps is given. It consists of 5
different steps, namely the preprocessing, crossover generation, radial orbit error
estimation, merging of different altimeter data and gridding. Most of the processing steps
are briefly described, only the aspect of merging different altimeter data resulting in
datum problems is highlighted.
The main objective of data preprocessing is to set up and maintain a mission independent
data base including also a formal quality control. A record structure was designed with
a standard format that is applied to all missions. Records of ascending and descending
tracks are separated and are stored in individual files (HRA files). Ascending and
descending tracks are identified by their equator crossing parameters (longitude of equator
nodes, equator crossing time and an indicator for ascending or descending track). These
parameters, complete with pointers to the HRA files, are saved as ont~ file allowing easy
data identification of geographic regions and limited time periods.
Global orbit error reduction is performed with crossover data, because observed sea
surface height differences (crossover differences) mainly reflect the radial orbit errors. A
210
The main difference between ERS-1.ALT.OPR2 and ERS-1.ALT.QLOPR originates from
the retracked altimeter signal in the OPR2. Due to the need for a quick distribution, the
fast delivery data, which is the basis for the QLOPR, includes an altimeter measurement
from a coarse wave forms analysis, thus a smoother behavior between adjacent
measurements is obtained. Also the GFZlD-P AF precise orbit information is merged to
the data, leading to more accurate satellite heights above a reference ellipsoid. Radiometer
and precise meteorological data from French Meteorological office result in better
tropospheric corrections than those included in the QLOPR.
TOPEXGDR
The TOPEX altimeter data merged into MSS95A are from the French TOPEX GDR
(AVISO 1992). The fact that the TOPEX satellite has a much smaller size than ERS-l,
a higher altitude and multiple tracking systems (GPS, DORIS, laser reflectors) results in
a radial accuracy of the satellite ephemerides better than 3 cm. The dual frequency
altimeter reveals an instrumental ionospheric path delay correction that is more accurate
than existing models. An on-board radiometer reduces the uncertainties of the wet
tropospheric correction, which appears, if atmospheric models are used. Besides
Schwiderski's tides, the better Cartright's model (Eanes 1993) is merged to the GDR (up
to now improved tidal solutions are not included in TOPEX GDR). Instrument, telemetry
and quality flags help to eliminate bad data.
PROCESSING
In this chapter a description of MSS95A's processing steps is given. It consists of 5
different steps, namely the preprocessing, crossover generation, radial orbit error
estimation, merging of different altimeter data and gridding. Most of the processing steps
are briefly described, only the aspect of merging different altimeter data resulting in
datum problems is highlighted.
The main objective of data preprocessing is to set up and maintain a mission independent
data base including also a formal quality control. A record structure was designed with
a standard format that is applied to all missions. Records of ascending and descending
tracks are separated and are stored in individual files (HRA files). Ascending and
descending tracks are identified by their equator crossing parameters (longitude of equator
nodes, equator crossing time and an indicator for ascending or descending track). These
parameters, complete with pointers to the HRA files, are saved as ont~ file allowing easy
data identification of geographic regions and limited time periods.
Global orbit error reduction is performed with crossover data, because observed sea
surface height differences (crossover differences) mainly reflect the radial orbit errors. A
210