Radar altimeters on board satellites
Satellite observations are not included in the mandate of the National reference body for sea level observation. However, for more than twenty years, radar altimeters on board satellites have been able to provide an excellent determination of the level of the ocean with sub-10 centimetre accuracy.
The vertical distance from the satellite to the surface of the sea is measured as follows:
It consists of measuring the vertical round trip travel time Δt of an electromagnetic pulse emitted by the on-board radar and reflected by the sea (the reflecting surface is of the order of 10 km² to 20 km²). Due to the distance travelled by the wave (around 2000 km) in an environment that is not homogeneous since it crosses the ionosphere and troposphere, its speed c is not exactly the speed of light in vacuum. Corrections are made for the ionospheric and tropospheric effects (water vapour) on the propagation of the electromagnetic pulse.
The vertical distance ζ of the satellite above the sea surface is:
From the ITRS to the chart datum
Monitoring the satellite's orbit provides its altitude ζe in a certain reference point related to the centre of the Earth, the height of the ocean surface he in the same reference point is the difference between two values:
The precise positioning of the satellite is carried out by laser telemetry, GPS or the DORIS system, from stations located along the track of the satellite. The positions of these stations are established in an ITRS (International Terrestrial Reference System) coherent geodetic reference frame with 1-centimetre accuracy. As a result, the sea level is determined with respect to the same reference frame.
The correspondence of the altimetric sea level, compared to the usual references (land levelling, average level, chart datum), raises the problem of the relative position of these references to one another. This is why orbital tracking stations, especially GPS and Doris, are placed near coastal tide observatories.
Value of tide gauges for calibrating altimeters
Calibrating an altimeter consists of comparing the satellite measurement of the height of the sea to the measurement of the tide gauge in the same reference system. However, measurements from satellite altimetry are usable only at a certain distance from the coast, about 10 to 20 km, for at least two reasons.
- The first is technical; the reflection of the altimetric signal is disrupted by the simultaneous presence of land and water,
- The second relates to the rapid variation of the shape of the geoid as it travels from the sea to the land.
It is not east to interpret the difference between the coastal tide gauge measurement and the first usable satellite measurement without prior knowledge of the shape of local geoid.
Altimeters on-board satellites measure the height of the sea at a given point a few days apart. The orbital tracks around the globe cut the equator at an angle of about 60 ° and form a regular grid whose density, the order of a few hundred kilometres, varies depending on the latitude. Thus the data at a point are only available at much longer time intervals than the fundamental tidal periods. This raise many analysis and data processing problems, including the problem of multiple tidal aliasing. Using appropriate methods, the lack of sufficient time sampling at the fixed point can be addressed in part by taking into account the continuous spatial sampling all along the orbit and measurements at track intersection points.
Despite these difficulties, this spatial technique, which has accuracy of a few centimetres, provides valuable information for geophysical and meteocean studies across the globe. The information collected with the radar altimeter can be used to determine the marine geoid, wave height and wind speed as well as the difference between the sea surface and the geoid.
The topography of the ocean surface (with respect to the geoid) is especially useful in physical oceanography for determining surface currents, but is accessible only after correcting for the tide, which contributes significantly to altimetric variability. Satellite altimetry is generating renewed interest in the scientific community for tide gauging across the globe.
To find out more:
- Calibration of satellite altimetry
- Past altimetric missions
- Current altimetric missions
- Future altimetric missions
Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.
Last updated: 12/12/2012