/

ABC of Altimetry: SSH, SLA, ADT, MSS, MDT

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: Altimetry

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.

 

Category: Altimetry

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.

Category: Altimetry

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.

Category: Altimetry

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

Category: Altimetry

/

ABC of Altimetry: SSH, SLA, ADT, MSS, MDT

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: Altimetry

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.

 

Category: Altimetry

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.

Category: Altimetry

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.

Category: Altimetry

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

Category: Altimetry

/

ABC of Altimetry: SSH, SLA, ADT, MSS, MDT

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: Altimetry

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.

 

Category: Altimetry

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.

Category: Altimetry

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.

Category: Altimetry

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

Category: Altimetry

/

ABC of Altimetry: SSH, SLA, ADT, MSS, MDT

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: Altimetry

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.

 

Category: Altimetry

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.

Category: Altimetry

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.

Category: Altimetry

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

Category: Altimetry

/

ABC of Altimetry: SSH, SLA, ADT, MSS, MDT

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: Altimetry

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.

 

Category: Altimetry

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.

Category: Altimetry

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.

Category: Altimetry

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

Category: Altimetry