Mean Dynamic Topography, Mean Sea Surface, Mean Sea Level or mean profile??-
There are several means used in altimetry processing or altimetry-based studies.
The MSS (Mean Sea Surface) is a mean of several years of altimetry data, eventually using several satellites). It is a reference surface (thus you have values on a lon/lat grid).
The MDT (Mean Dynamic Topography) is the previous surface minus the geoid. It is the ocean topography due to the mean currents. It is also a reference surface.
The Mean profile is the mean of an altimeter measurements along the satellite's tracks. It can be used as reference for computing SLA for the same satellite. It is an along-track data (not distributed)
The MSL (Mean Sea Level) is a figure, often given with respect to time as a curve.
- What are the conditions to get Aviso data?
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All Aviso data are free of charge for non-commercial use, whether online (FTP, Opendap) or on media (DVD).
The request must be justified by a short text explaining how the user plan to use the data. This helps Aviso in refining the available data to be better tuned to the users' needs.
For a first level of education, the best is to work with online maps (gif format), e.g. the one available through the Argonautica project, or with the Live Access Server.
- How are geostrophic velocities computed at the Equator?
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In the geostrophic velocity data distributed by Aviso today, current velocities at the equator are computed using equatorial geostrophy [Picaut et al., 1989]. At the Equator, (lat = 0) we use second derivative. This method is quite usual. In a 5° band around the Equator, a connection is computed to ensure continuity with classical geostrophy. This continuity is not completly satisfying just now, so a study was conducted to test the connection proposed by Lagerloef et al. [1999]. The improvement thus obtained is significant, and was validated using TAO measurements.
- What are the differences between ADT and SLA products?
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Absolute Dynamic Topography (ADT) and Sea Level Anomaly (SLA) are defined by:
ADT=MDT+SLA
where MDT is Mean Dynamic Topography.
- Could you send me an explanation of the times associated with your gridded data?
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The gridded data are computed for a given day. Each map represents the situation on the day indicated in its name. It is not a mean over the period between two files.
To generate these maps, computing methods based on objective analysis allow us to interpolate data in time and in space. In delayed time, the processing window used is centered, e.g. 6 weeks of data before and after the given date are taken into account. In near-real time, only the 6 weeks of data preceding the given day are considered.
- In the gridded data, to which point lat/lon refer to?
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The lon/lat given is the SW corner of the box, as specified in the metadata of the NetCDF file
- Where can I find tables showing the correspondance date / cycle and pass number?
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On-line tools on this website make the correspondance between hour and calendar date and cycle and pass number.
- How to convert measurement time in local time
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For T/P, 86400xTim_Moy_1 + 10-3xTim_Moy_2 + 10-6xTim_Moy_3 = time (in seconds)
For Jason-1, 86400xTime_day + 10-3xTime_sec + 10-6xTim_microsec = time (in seconds)
Time is given in UTC (Universal Time Coordinate, linked to Greenwich meridian), date in Julian day, which is the elapsed time between a reference epoch (January 1st 1950, 0h0min0s for Cnes Julian days, January 1st 1958, 0h0min0s for Nasa Julian days) and measurement date. To convert UTC time in local time, you have to convert it in date, hours, minutes, seconds, then in solar time and finally in your local time. We haven't got any conversion procedure.
- We haven't receive T/P cycle 118 (or 431, 432), Jason-1 cycle 178
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There were no data processed for Topex/Poseidon cycle 118 (resp. 431, 432) and for Jason-1 cycle 178, due to an incident.
- What are Topex/Poseidon or Jason-1 data resolution?
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Spatially, along the track, data are available every 7 km (measurements averaged on 1 second, or "1 Hz data"). GDR 10 or 20 Hz data also exists (depending on the satellite), but those are more noisy. Between tracks, for Jason-1 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.
Gridded Aviso data are available at 1/3° on a Mercator grid.
- What are the caracteristics of the reference ellipsoid you are using?
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The Reference ellipsoid is the first-order definition of the non-spherical shape of the Earth. Concerning Jason-1 and Topex/Poseidon, we are using the following ellipsoid:
- radius : 6378136.3
- inverse Earth flattening coefficient : 298.257
For Envisat:
- radius : 6378137
- inverse Earth flattening coefficient : 298.257223563
- On which platform/Operating system can I use Aviso data?
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You can use Aviso data on every platform and Operating system, but the reading software may not be available for your precise configuration (see the available software).
Note that T/P GDR-Ms are coded in little endian format and Jason-1 GDRs in big endian format.
- I'd like data over a precise area (data extraction)
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- for gridded NetCDF data, an online extraction tool is available
- for Topex/Poseidon and Jason-1 GDR, Aviso can perform extraction for a short period of time on a small area
For the other dataset, users have to perform their own extraction from the global or regional datasets
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