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Frequently asked questions

Registration/data access

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How can I register?

Please, to register for AVISO+ products, fill the registration form available here.

Please give us some details about your motivation. Insert A-Z letters, numbers, '.', '-', '_' and spaces only!. Note especially that the character ',' (comma) is not allowed for the moment in the motivation field.

Once I have registered, how do I have information about the products?

Once you have filled the form, you have received emails for the connexion. Please note that the AVISO+ service desk will manually validate your registration and then you will access the data (an email is sent for the validation).

With your account identifiers, connect on the MY AVISO+ personnal page

You will find information on the 'Help & Contact' page. The products to which you are subscribed are listed in the 'products' page.

you will have access to the services available for each product

 

Can I reset my password?

You can reset your passord in https://www.aviso.altimetry.fr/en/my-aviso-plus/reset-password.html

Note that your account is your email address

How could I log on with my ftp account?

Symptoms:

  • I could not download the data from the authenticated ftp folders.
  • I don't see any place to enter my username/password.

To access to data from authenticated ftp site, firstly you must have a personal account provided by the Aviso Users Services after you have properly filled the registration form available online.

There are three different accesses for the following services

  • Aviso+ Cnes Data Center
  • Online Data Etraction Service (ODES)
  • ftp/Opendap/Gridded Data Extraction Tool

Once you have your access to the ftp/Opendap/Gridded Data Extraction Tool, you have to connect with your personal account information by proceeding as follows:

1/ On your dedicated My Aviso+ web page (see FAQ "Once I have registered, hos do I have information about the products"

2/ With a web browser:

  • Using Firefox: WRITE ftp://yourLOGIN:yourPASSWORD@ftp-access.aviso.altimetry.fr into the URL bar.
  • OR using Internet Explorer: WRITE ftp://ftp-access.aviso.altimetry.fr, click on Page, choose 'Open FTP Site in Windows Explorer' then enter your login and password.

Then browse 'global' directory ... 

3/ Otherwise, see below the other ways to log in:

  • by using a FTP client (FileZilla for example): enter 'ftp-access.aviso.altimetry.fr' in the hostname, then your login and password in the appropriate boxes.
  • By using Unix commands: write 'ftp ftp-access.aviso.altimetry.fr' (and then enter your login/password). You can also use the command line "wget --no-proxy" with the suitable options (--user=YOUR_LOGIN  --password=YOUR_PASSWORD).

Some parameters settings have maybe to be checked: passive mode, proxy settings...

Access to the ftp/Opendap/Gridded Data Extraction tool

There are three different accesses for the following services

  • Aviso+ Cnes Data Center
  • Online Data Etraction Service (ODES)
  • ftp/Opendap/Gridded Data Extraction Tool

Once you have registered on https://www.aviso.altimetry.fr/en/my-aviso-plus/reset-password.html

the login/password has been sent by email for the access to the ftp/Opendap/Gridded Data Extraction Tool. After validation by the Service Desk, this account gives you the access (see FAQ "Once I have registered, how do I have the information about the products" in the FAQ for the complete access to the products).

Access to the Aviso+ Cnes Data Center

There are three different accesses for the following services

  • Aviso+ Cnes Data Center
  • Online Data Etraction Service (ODES)
  • ftp/Opendap/Gridded Data Extraction Tool

To get the access on the Aviso+ Cnes Data Center, 

log on https://aviso-data-center.cnes.fr/

and register as indicated below

 

Access to the Online Data Extraction Service (ODES)

There are three different accesses for the following services

  • Aviso+ Cnes Data Center
  • Online Data Extraction Service (ODES)
  • ftp/Opendap/Gridded Data Extraction Tool

There is a new access to Online Data Extraction Service (ODES). To get ODES data, there is a unique set of login/password for all AVISO+ users.

To get the new ODES login/password, please contact AVISO+ Service Desk.

 

 

 

How can I request new products?

Once you are registered to a list of products you can ask for a new products:

Go on your dedicated MY AVISO+ web page

Go on your dedicated 'products' page

Go in the section 'Other products you may want to access':

And then select one or more products and click on 'Continue'. PLEASE, SELECT ALL THE PRODUCTS YOU NEED IN ONE STEP, AND NOT ONE BY ONE, OUR SYSTEM IS NOT YET ADAPTED TO SUCH REQUESTS. You will have to sign the licence and submit. The Service Desk will validate your request manually.

What are the conditions to get Aviso data?

Please refer to the Licence agreement for the conditions of use of Aviso+ products. 

When a product is requested, please, justify by a short text explaining how you 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.

I'd like data over a precise area (data extraction)

- for some gridded NetCDF data, an online extraction tool is available, please refer to your dedicated products page
- for the other dataset, users have to perform their own extraction from the global or regional datasets

Data and processing questions

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Why latest Pistach data are from May 2017 ?

Following Jason2 satellite Safe Hold Mode in May 2017, Pistach data processing has been stopped. Last data are from May 17th with few passes between July 11th and 13th for cycle 500 when the satellite was on a new orbit. 

Pistach Coastal and Hydro products remain available from cycle 001 to cycle 327 pass 111.

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 trend (a number of mm/year), often given with respect to time as a curve.


Could you send me an explanation of the times associated with your gridded data?

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 a period, definitely not a "weekly mean" (as often written, erroneously deduced from the fact that at some point the data were computed only for every 7 days).
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?

Each value has been computed exactly on the Lat/Lon values
defined by LatLonMin and LatLonStep , as specified in the metadata of the NetCDF file.

Explanation is given is section 4.3 of the handbook "Ssalto/Duacs User Handbook: (M)SLA and (M)ADT Near-Real Time and Delayed Time products" that you can find here

Where can I find tables showing the correspondance date / cycle and pass number?

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 ?
  • For T/P, 86400xTim_Moy_1 + 10-3xTim_Moy_2 + 10-6xTim_Moy_3 = time (in seconds)
  • For Jason-1&2, 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.

What is the impact of missing data in the Duacs products?

When one satellite is missing for a while, due to an incident example, the impact on the products is twofold:

  • the along-track files are missing if the incident lasts for more than one day
  • the impact on the generation of the map depends on the gap in the data: 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 preeceding the given day are considered. The maps of formal errors (in the directory "err") will give an overview of the error induced by the missing data in the generation of the maps.
We haven't receive T/P cycles 118/431/432, Jason-1 cycles 178 and 375 to 500

There were no data processed for Topex/Poseidon cycles 118, 431, 432 and for Jason-1 cycle 178, due to incidents.

The cycles 375 to 500 for Jason-1 are missing because the mission ended the repeat orbit phase (until 374) and began geodetic phase from cycle 500 (the cycles have been recomputed).

What are Jason1&2, Topex/Poseidon data resolution?

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&2 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 characteristics of the reference ellipsoid you are using?

The Aviso products are referenced to the Topex/Poseidon ellipsoid (see the references below). So the Sea Surface Height of the MSS, CorSSH products are referenced to T/P ellipsoid, even for ERS-1, ERS-2, Envisat and Cryosat products. The SLA and MSLA products are not concerned since they are anomalies.

The reference ellipsoid is the first-order definition of the non-spherical shape of the Earth. For the GDRs, concerning Jason-1&2 GFO and Topex/Poseidon, the reference ellipsoid is:

  • radius : 6378136.3
  • inverse Earth flattening coefficient : 298.257

For ERS-1, ERS-2, Envisat, Cryosat-2:

  • radius : 6378137
  • inverse Earth flattening coefficient : 298.257223563
On which platform/Operating system can I use Aviso data?

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.

How get/exploit Aviso products available in NetCDF format (.nc)?

Firstly, to get Aviso data in NetCDF format, download the corresponding files via FTP client (for example FileZilla the Open Source FTP client filezilla-project.org). After downloading, in order to get the corresponding NetCDF files ('.nc'), extract the files with a 'gunzip' command if extension is '.gz' , use the 'tar -xzvf' commande for extensions '.tar.gz'

Aviso proposes two ways to directly work with Aviso NetCDF data :

  1. Use Aviso data extraction tool: see Data access services.
  2. Use OPeNDAP tool: refer to OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Altimetry questions

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Does sea state influence the measurement of the satellite-sea surface range?

Sea state influences the measurement of the satellite-sea surface range because the altimeter is sensitive to sea surface elements perpendicular to the target line. These elements are more frequently in the wave trough than in the crest, so the mean height of these elements doesn't match the geometric mean height of all sea surface elements that make the mean sea level (electromagnetic bias). In this way, the altimetric-measured mean is shifted toward wave trough, and moreover if waves are high.

 

About the radiometer, how brightness temperature is defined, and why 3 frequencies are used?

The brigthness temperature of a surface is equal to the product of the emissivity of this surface by its physical temperature.
The radiation measured by the radiometer depends on the ocean surface emissivity, its physical temperature and water vapour and cloud absorption in the atmosphere. If you want to know precisely the atmospheric water vapour contents, you have to substract surface and cloud contribution from the signal received by the radiometer. That's why several frequencies (3 in most cases) are used, each one being more sensitive than the other to one of these contributions. By combining measurements done at each frequencies, you can extract the water vapor signal.

What are the differences between ADT and SLA products?

Absolute Dynamic Topography (ADT) and Sea Level Anomaly (SLA) are defined by:
ADT=MDT+SLA
where MDT is Mean Dynamic Topography.

What is the difference between SSH and ADT?

SSH is what the altimeter system measures, and the quantity from which everything else is deduced (with also some in situ data for the MDT).
SSH = (satellite altitude with respect to reference ellipsoid) - (satellite-to-surface distance)
(see altimetry pages)
Thus, in altimetry (it's not the case in all oceanography), SSH can be defined as the sea surface height with respect to the reference ellipsoid, which means that, compared to ADT, there's also the geoid height in it.
You can say that : SSH = SLA + MDT + geoid = ADT + geoid
with SLA computed from SSH and a mean sea surface over several years

Applications questions

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Is it possible to estimate geostrophic currents from altimetry data?

Satellite altimetry allows to measure the Sea Surface Height (SSH) with a few centimeters precision. Appart from the equatorial band (ie, 5°N,5°S), the geostrophy can be assumed with a good certainty: it means that the slope of the sea surface measured by the altimeter along the satellite track is assumed to be DYNAMIC TOPOGRAPHY. In other word, it means that this slope is directly related to the pressure gradient at the sea surface, and thus, to a GEOSTROPHIC VELOCITY at the surface.

I'd like to have some information about ocean bottom cartography.

You can have a look at our "Geophysics" section (in the "Applications"). Here are some websites with more information on this subject :

 

I would like some information about El Niño.

On this subject, you can consult our web pages (including an extensive list of websites)

I would like some information about tide time-tables.

We suggest you to consult these websites:
Tides and Altimetry
http://tbone.biol.sc.edu/tide/sitesel.html

Where can I find information about sea waves (satellite imagery and also lectures)?

On this website
  - Wind and wave maps
  - Applications: Wind and waves: marine meteorology 

 

I'm doing a report about currents (physical aspect, modelisation). Could you give me some adresses?

You can look at the Ocean application section

ocean circulation model :
- Southampton Oceanography Centre
- Ocean Surface Currents

Can you give me anything about troposphere?

We do not study troposphere as such but only since troposphere crossing disrupt the radar wave of the altimeter
We rather suggest you to contact meteorological centers (ex. CNRM, UK met. office, ECMWF), atmospheric studies centers (ex. CETP), or dedicated satellite missions (Nasa's TES).

Why is the sky blue ? Is it because the reflection of the sea ?

The blue of the sky is due to the way the oxygene molecule absorbs and diffuses light. The blue of the sea comes from another diffusion process, in the water. For more information on these subjects:
http://www.waterw.com/~science/blue.html , http://www.exploratorium.edu/snacks/blue_sky.html or http://world.std.com/~mmcirvin/bluesky.html (blue of the sky)

Doris questions

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How the Doppler effect with Doris can give the distance between the satellite and the Earth?

Doris can give the position of a satellite because it compares a model of orbit (giving position and velocity) with its measurements:
Measurement data for permanent network beacons are processed to yield the satellite's precise trajectory. This processing includes modelling of forces acting on the satellite (terrestrial gravitation, etc.) in order to determine its precise trajectory.

Figures marked "Aviso", are copyright Cnes/CLS, but please feel free to use them, conditional on the figures not being altered, and their source being acknowledged, and with a link to this site where possible.
All other figures are copyrighted. Please do not copy without the owner's permission.

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