/

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 altimeter measures the ‘Altimeter Range’ which is the distance between the center of mass of satellite to the surface of the Earth (see Figure below). This allows computing the ‘Sea Surface Height’ (SSH) which is the height of the sea surface above the reference ellipsoid. The ‘Satellite Altitude’ refers to the distance of the center of mass of the satellite above a reference point. The reference point will usually be either on the reference ellipsoid or the center of the Earth.
SSH = Satellite Altitude - Altimeter Range – Corrections

The ‘Corrections’ due to environmental conditions need to be applied in order to retrieve the correct ‘Sea Surface Height’. 

The Mean Sea Surface (MSS_N) is the temporal mean of the SSH over a period N. It is a mean surface above the ellipsoid of reference and it includes the Geoid.
MSS_N=<SSH>_N

The Sea Level Anomaly (SLA_N) is the anomaly of the signal around the time-mean component. It is deduced from the SSH and MSS_N :
SLA_N = SSH – MSS_N

The Mean Dynamic Topography (MDT_N) is the temporal mean of the SSH above the Geoid over a period N.
MDT_N = MSS_N – Geoid

The Absolute Dynamic Topography (ADT) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond with the ocean surface if ocean was at rest (i.e. with no currents under only the gravity field). Then, when the ocean is also influenced by wind, differential heating and precipitation and other sources of energy, the ocean surface moves from the geoid. Thus, the departure from the geoid (MDT) provides information on the mean effect on SSH from ocean dynamics.
The ADT is the sum of the SLA_N and MDT_N:
ADT= SLA_N + MDT_N = SSH - MSS_N + MDT_N

The reference period N considered can be changed as described in Pujol et al. (2018). Currently it is 1993-2012.

The Mean profile is the mean of 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.

Reference:
Pujol, M.-I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. (2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013503 .

Category: Data and processing questions

Wave L4 gridded products now disseminated by CMEMS

The Copernicus Marine Environment Monitoring Service CMEMS is now in charge of the processing and distribution of the gridded (L4) NRT wave products. Indeed in the frame of the Transition AVISO+ to CMEMS, AVISO+ focuses on the distribution of innovative products whereas CMEMS is dedicated to the operational products and services.
Since July 2019, gridded L4 wave NRT products (called WAVE_GLO_WAV_L4_SWH_NRT_OBSERVATIONS_014_003) have been added to the CMEMS catalogue. 
Therefore the CMEMS catalogue disseminates Near-Real-Time gridded L4 wave products as well as two along-track L3 wave products (altimetry and spectral). 

Compared to AVISO+, the CMEMS gridded L4 wave products on a 2°x2° grid are based on several up-to-date missions and benefit from an improved processing fully described in the CMEMS QUality Information Document: QUID. 
You will find the list of AVISO+ and CMEMS wave products in the document Migration AVISO+ to CMEMS and AVISO+ to C3S (section 8) and a brief overview of the differences between the two L4 NRT wave products in section 8. Note that for the moment on CMEMS, only the NRT products are distributed beginning on July 2019. It is planned to disseminate a Delayed-time product on CMEMS by the beginning of 2021.

Thus, the production of the AVISO+ gridded L4 NRT wave products will stop by December 2019. So the user is invited to register to CMEMS as soon as possible: the useful links to CMEMS are indicated in the  document Migration AVISO+ to CMEMS and AVISO+ to C3S.

Note that for the moment, the gridded L4 NRT wind products are still disseminated by AVISO+.

Category: Data and processing questions

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.

Category: Data and processing questions

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.

Category: Data and processing questions

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 1^st 1950, 0h0min0s for Cnes Julian days, January 1^st 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.

Category: Data and processing questions

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.

Category: Data and processing questions

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).

Category: Data and processing questions

What are Jason1&2&3, 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&3 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.

Category: Data and processing questions

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

Category: Data and processing questions

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).

Category: Data and processing questions

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 <link en data data-access-services data-extraction-tool index.html>Data access services.
2. Use OPeNDAP tool: refer to <link en data data-access-services opendap index.html>OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Category: Data and processing questions

/

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 altimeter measures the ‘Altimeter Range’ which is the distance between the center of mass of satellite to the surface of the Earth (see Figure below). This allows computing the ‘Sea Surface Height’ (SSH) which is the height of the sea surface above the reference ellipsoid. The ‘Satellite Altitude’ refers to the distance of the center of mass of the satellite above a reference point. The reference point will usually be either on the reference ellipsoid or the center of the Earth.
SSH = Satellite Altitude - Altimeter Range – Corrections

The ‘Corrections’ due to environmental conditions need to be applied in order to retrieve the correct ‘Sea Surface Height’. 

The Mean Sea Surface (MSS_N) is the temporal mean of the SSH over a period N. It is a mean surface above the ellipsoid of reference and it includes the Geoid.
MSS_N=<SSH>_N

The Sea Level Anomaly (SLA_N) is the anomaly of the signal around the time-mean component. It is deduced from the SSH and MSS_N :
SLA_N = SSH – MSS_N

The Mean Dynamic Topography (MDT_N) is the temporal mean of the SSH above the Geoid over a period N.
MDT_N = MSS_N – Geoid

The Absolute Dynamic Topography (ADT) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond with the ocean surface if ocean was at rest (i.e. with no currents under only the gravity field). Then, when the ocean is also influenced by wind, differential heating and precipitation and other sources of energy, the ocean surface moves from the geoid. Thus, the departure from the geoid (MDT) provides information on the mean effect on SSH from ocean dynamics.
The ADT is the sum of the SLA_N and MDT_N:
ADT= SLA_N + MDT_N = SSH - MSS_N + MDT_N

The reference period N considered can be changed as described in Pujol et al. (2018). Currently it is 1993-2012.

The Mean profile is the mean of 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.

Reference:
Pujol, M.-I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. (2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013503 .

Category: Data and processing questions

Wave L4 gridded products now disseminated by CMEMS

The Copernicus Marine Environment Monitoring Service CMEMS is now in charge of the processing and distribution of the gridded (L4) NRT wave products. Indeed in the frame of the Transition AVISO+ to CMEMS, AVISO+ focuses on the distribution of innovative products whereas CMEMS is dedicated to the operational products and services.
Since July 2019, gridded L4 wave NRT products (called WAVE_GLO_WAV_L4_SWH_NRT_OBSERVATIONS_014_003) have been added to the CMEMS catalogue. 
Therefore the CMEMS catalogue disseminates Near-Real-Time gridded L4 wave products as well as two along-track L3 wave products (altimetry and spectral). 

Compared to AVISO+, the CMEMS gridded L4 wave products on a 2°x2° grid are based on several up-to-date missions and benefit from an improved processing fully described in the CMEMS QUality Information Document: QUID. 
You will find the list of AVISO+ and CMEMS wave products in the document Migration AVISO+ to CMEMS and AVISO+ to C3S (section 8) and a brief overview of the differences between the two L4 NRT wave products in section 8. Note that for the moment on CMEMS, only the NRT products are distributed beginning on July 2019. It is planned to disseminate a Delayed-time product on CMEMS by the beginning of 2021.

Thus, the production of the AVISO+ gridded L4 NRT wave products will stop by December 2019. So the user is invited to register to CMEMS as soon as possible: the useful links to CMEMS are indicated in the  document Migration AVISO+ to CMEMS and AVISO+ to C3S.

Note that for the moment, the gridded L4 NRT wind products are still disseminated by AVISO+.

Category: Data and processing questions

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.

Category: Data and processing questions

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.

Category: Data and processing questions

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 1^st 1950, 0h0min0s for Cnes Julian days, January 1^st 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.

Category: Data and processing questions

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.

Category: Data and processing questions

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).

Category: Data and processing questions

What are Jason1&2&3, 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&3 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.

Category: Data and processing questions

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

Category: Data and processing questions

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).

Category: Data and processing questions

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 <link en data data-access-services data-extraction-tool index.html>Data access services.
2. Use OPeNDAP tool: refer to <link en data data-access-services opendap index.html>OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Category: Data and processing questions

/

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 altimeter measures the ‘Altimeter Range’ which is the distance between the center of mass of satellite to the surface of the Earth (see Figure below). This allows computing the ‘Sea Surface Height’ (SSH) which is the height of the sea surface above the reference ellipsoid. The ‘Satellite Altitude’ refers to the distance of the center of mass of the satellite above a reference point. The reference point will usually be either on the reference ellipsoid or the center of the Earth.
SSH = Satellite Altitude - Altimeter Range – Corrections

The ‘Corrections’ due to environmental conditions need to be applied in order to retrieve the correct ‘Sea Surface Height’. 

The Mean Sea Surface (MSS_N) is the temporal mean of the SSH over a period N. It is a mean surface above the ellipsoid of reference and it includes the Geoid.
MSS_N=<SSH>_N

The Sea Level Anomaly (SLA_N) is the anomaly of the signal around the time-mean component. It is deduced from the SSH and MSS_N :
SLA_N = SSH – MSS_N

The Mean Dynamic Topography (MDT_N) is the temporal mean of the SSH above the Geoid over a period N.
MDT_N = MSS_N – Geoid

The Absolute Dynamic Topography (ADT) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond with the ocean surface if ocean was at rest (i.e. with no currents under only the gravity field). Then, when the ocean is also influenced by wind, differential heating and precipitation and other sources of energy, the ocean surface moves from the geoid. Thus, the departure from the geoid (MDT) provides information on the mean effect on SSH from ocean dynamics.
The ADT is the sum of the SLA_N and MDT_N:
ADT= SLA_N + MDT_N = SSH - MSS_N + MDT_N

The reference period N considered can be changed as described in Pujol et al. (2018). Currently it is 1993-2012.

The Mean profile is the mean of 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.

Reference:
Pujol, M.-I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. (2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013503 .

Category: Data and processing questions

Wave L4 gridded products now disseminated by CMEMS

The Copernicus Marine Environment Monitoring Service CMEMS is now in charge of the processing and distribution of the gridded (L4) NRT wave products. Indeed in the frame of the Transition AVISO+ to CMEMS, AVISO+ focuses on the distribution of innovative products whereas CMEMS is dedicated to the operational products and services.
Since July 2019, gridded L4 wave NRT products (called WAVE_GLO_WAV_L4_SWH_NRT_OBSERVATIONS_014_003) have been added to the CMEMS catalogue. 
Therefore the CMEMS catalogue disseminates Near-Real-Time gridded L4 wave products as well as two along-track L3 wave products (altimetry and spectral). 

Compared to AVISO+, the CMEMS gridded L4 wave products on a 2°x2° grid are based on several up-to-date missions and benefit from an improved processing fully described in the CMEMS QUality Information Document: QUID. 
You will find the list of AVISO+ and CMEMS wave products in the document Migration AVISO+ to CMEMS and AVISO+ to C3S (section 8) and a brief overview of the differences between the two L4 NRT wave products in section 8. Note that for the moment on CMEMS, only the NRT products are distributed beginning on July 2019. It is planned to disseminate a Delayed-time product on CMEMS by the beginning of 2021.

Thus, the production of the AVISO+ gridded L4 NRT wave products will stop by December 2019. So the user is invited to register to CMEMS as soon as possible: the useful links to CMEMS are indicated in the  document Migration AVISO+ to CMEMS and AVISO+ to C3S.

Note that for the moment, the gridded L4 NRT wind products are still disseminated by AVISO+.

Category: Data and processing questions

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.

Category: Data and processing questions

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.

Category: Data and processing questions

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 1^st 1950, 0h0min0s for Cnes Julian days, January 1^st 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.

Category: Data and processing questions

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.

Category: Data and processing questions

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).

Category: Data and processing questions

What are Jason1&2&3, 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&3 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.

Category: Data and processing questions

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

Category: Data and processing questions

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).

Category: Data and processing questions

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 <link en data data-access-services data-extraction-tool index.html>Data access services.
2. Use OPeNDAP tool: refer to <link en data data-access-services opendap index.html>OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Category: Data and processing questions

/

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 altimeter measures the ‘Altimeter Range’ which is the distance between the center of mass of satellite to the surface of the Earth (see Figure below). This allows computing the ‘Sea Surface Height’ (SSH) which is the height of the sea surface above the reference ellipsoid. The ‘Satellite Altitude’ refers to the distance of the center of mass of the satellite above a reference point. The reference point will usually be either on the reference ellipsoid or the center of the Earth.
SSH = Satellite Altitude - Altimeter Range – Corrections

The ‘Corrections’ due to environmental conditions need to be applied in order to retrieve the correct ‘Sea Surface Height’. 

The Mean Sea Surface (MSS_N) is the temporal mean of the SSH over a period N. It is a mean surface above the ellipsoid of reference and it includes the Geoid.
MSS_N=<SSH>_N

The Sea Level Anomaly (SLA_N) is the anomaly of the signal around the time-mean component. It is deduced from the SSH and MSS_N :
SLA_N = SSH – MSS_N

The Mean Dynamic Topography (MDT_N) is the temporal mean of the SSH above the Geoid over a period N.
MDT_N = MSS_N – Geoid

The Absolute Dynamic Topography (ADT) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond with the ocean surface if ocean was at rest (i.e. with no currents under only the gravity field). Then, when the ocean is also influenced by wind, differential heating and precipitation and other sources of energy, the ocean surface moves from the geoid. Thus, the departure from the geoid (MDT) provides information on the mean effect on SSH from ocean dynamics.
The ADT is the sum of the SLA_N and MDT_N:
ADT= SLA_N + MDT_N = SSH - MSS_N + MDT_N

The reference period N considered can be changed as described in Pujol et al. (2018). Currently it is 1993-2012.

The Mean profile is the mean of 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.

Reference:
Pujol, M.-I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. (2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013503 .

Category: Data and processing questions

Wave L4 gridded products now disseminated by CMEMS

The Copernicus Marine Environment Monitoring Service CMEMS is now in charge of the processing and distribution of the gridded (L4) NRT wave products. Indeed in the frame of the Transition AVISO+ to CMEMS, AVISO+ focuses on the distribution of innovative products whereas CMEMS is dedicated to the operational products and services.
Since July 2019, gridded L4 wave NRT products (called WAVE_GLO_WAV_L4_SWH_NRT_OBSERVATIONS_014_003) have been added to the CMEMS catalogue. 
Therefore the CMEMS catalogue disseminates Near-Real-Time gridded L4 wave products as well as two along-track L3 wave products (altimetry and spectral). 

Compared to AVISO+, the CMEMS gridded L4 wave products on a 2°x2° grid are based on several up-to-date missions and benefit from an improved processing fully described in the CMEMS QUality Information Document: QUID. 
You will find the list of AVISO+ and CMEMS wave products in the document Migration AVISO+ to CMEMS and AVISO+ to C3S (section 8) and a brief overview of the differences between the two L4 NRT wave products in section 8. Note that for the moment on CMEMS, only the NRT products are distributed beginning on July 2019. It is planned to disseminate a Delayed-time product on CMEMS by the beginning of 2021.

Thus, the production of the AVISO+ gridded L4 NRT wave products will stop by December 2019. So the user is invited to register to CMEMS as soon as possible: the useful links to CMEMS are indicated in the  document Migration AVISO+ to CMEMS and AVISO+ to C3S.

Note that for the moment, the gridded L4 NRT wind products are still disseminated by AVISO+.

Category: Data and processing questions

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.

Category: Data and processing questions

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.

Category: Data and processing questions

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 1^st 1950, 0h0min0s for Cnes Julian days, January 1^st 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.

Category: Data and processing questions

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.

Category: Data and processing questions

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).

Category: Data and processing questions

What are Jason1&2&3, 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&3 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.

Category: Data and processing questions

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

Category: Data and processing questions

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).

Category: Data and processing questions

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 <link en data data-access-services data-extraction-tool index.html>Data access services.
2. Use OPeNDAP tool: refer to <link en data data-access-services opendap index.html>OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Category: Data and processing questions

/

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 altimeter measures the ‘Altimeter Range’ which is the distance between the center of mass of satellite to the surface of the Earth (see Figure below). This allows computing the ‘Sea Surface Height’ (SSH) which is the height of the sea surface above the reference ellipsoid. The ‘Satellite Altitude’ refers to the distance of the center of mass of the satellite above a reference point. The reference point will usually be either on the reference ellipsoid or the center of the Earth.
SSH = Satellite Altitude - Altimeter Range – Corrections

The ‘Corrections’ due to environmental conditions need to be applied in order to retrieve the correct ‘Sea Surface Height’. 

The Mean Sea Surface (MSS_N) is the temporal mean of the SSH over a period N. It is a mean surface above the ellipsoid of reference and it includes the Geoid.
MSS_N=<SSH>_N

The Sea Level Anomaly (SLA_N) is the anomaly of the signal around the time-mean component. It is deduced from the SSH and MSS_N :
SLA_N = SSH – MSS_N

The Mean Dynamic Topography (MDT_N) is the temporal mean of the SSH above the Geoid over a period N.
MDT_N = MSS_N – Geoid

The Absolute Dynamic Topography (ADT) is the instantaneous height above the Geoid. The geoid is a gravity equipotential surface that would correspond with the ocean surface if ocean was at rest (i.e. with no currents under only the gravity field). Then, when the ocean is also influenced by wind, differential heating and precipitation and other sources of energy, the ocean surface moves from the geoid. Thus, the departure from the geoid (MDT) provides information on the mean effect on SSH from ocean dynamics.
The ADT is the sum of the SLA_N and MDT_N:
ADT= SLA_N + MDT_N = SSH - MSS_N + MDT_N

The reference period N considered can be changed as described in Pujol et al. (2018). Currently it is 1993-2012.

The Mean profile is the mean of 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.

Reference:
Pujol, M.-I., Schaeffer, P., Faugère, Y., Raynal, M., Dibarboure, G., & Picot, N. (2018). Gauging the improvement of recent mean sea surface models: A new approach for identifying and quantifying their errors. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013503 .

Category: Data and processing questions

Wave L4 gridded products now disseminated by CMEMS

The Copernicus Marine Environment Monitoring Service CMEMS is now in charge of the processing and distribution of the gridded (L4) NRT wave products. Indeed in the frame of the Transition AVISO+ to CMEMS, AVISO+ focuses on the distribution of innovative products whereas CMEMS is dedicated to the operational products and services.
Since July 2019, gridded L4 wave NRT products (called WAVE_GLO_WAV_L4_SWH_NRT_OBSERVATIONS_014_003) have been added to the CMEMS catalogue. 
Therefore the CMEMS catalogue disseminates Near-Real-Time gridded L4 wave products as well as two along-track L3 wave products (altimetry and spectral). 

Compared to AVISO+, the CMEMS gridded L4 wave products on a 2°x2° grid are based on several up-to-date missions and benefit from an improved processing fully described in the CMEMS QUality Information Document: QUID. 
You will find the list of AVISO+ and CMEMS wave products in the document Migration AVISO+ to CMEMS and AVISO+ to C3S (section 8) and a brief overview of the differences between the two L4 NRT wave products in section 8. Note that for the moment on CMEMS, only the NRT products are distributed beginning on July 2019. It is planned to disseminate a Delayed-time product on CMEMS by the beginning of 2021.

Thus, the production of the AVISO+ gridded L4 NRT wave products will stop by December 2019. So the user is invited to register to CMEMS as soon as possible: the useful links to CMEMS are indicated in the  document Migration AVISO+ to CMEMS and AVISO+ to C3S.

Note that for the moment, the gridded L4 NRT wind products are still disseminated by AVISO+.

Category: Data and processing questions

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.

Category: Data and processing questions

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.

Category: Data and processing questions

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 1^st 1950, 0h0min0s for Cnes Julian days, January 1^st 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.

Category: Data and processing questions

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.

Category: Data and processing questions

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).

Category: Data and processing questions

What are Jason1&2&3, 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&3 and Topex/Poseidon, the distance is up to 315 km at the Equator; for ERS and Envisat, it is about 80 km.

Category: Data and processing questions

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

Category: Data and processing questions

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).

Category: Data and processing questions

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 <link en data data-access-services data-extraction-tool index.html>Data access services.
2. Use OPeNDAP tool: refer to <link en data data-access-services opendap index.html>OPeNDAP details, in particular go to the OPeNDAP website or OPeNDAP support.

Category: Data and processing questions