Jason-3 Design

On the Proteus platform, Jason-3 has a payload of several instruments - the Poseidon-3B altimeter, the AMR radiometer, three location systems and two experimental instruments.

Jason-3 showing instruments

The primary instrument on Jason-3 is a radar altimeter. The altimeter measures sea-level variations over the global ocean with very high accuracy (1.3 inches or 3.3 centimeters, with a goal of 1 inch or 2.5 centimeters).

Continual, long-term, reliable data of changes in ocean surface topography are used by scientists and operational agencies for scientific research and operational oceanography for the benefit of society.

The altimetry technique

Further details can be found in the Radar Altimetry Tutorial

The altimetry technique

Satellite radar altimetry measures the time it takes for a radar pulse to travel from the satellite antenna to the surface and back to the satellite receiver. Apart from the surface height, this measurement yields a wealth of other information that can be used for a wide range of applications.

As we know, the sea surface is not smooth and flat, it is a surface that is in constant movement. This moving surface is what we call a dynamic topography. If we want to measure the sea surface height, we must measure it relative to a defined, constant surface. This theoretical surface is called the reference ellipsoid. It is a rough approximation of Earth's surface, a sphere flattened at the poles. Since the sea depth is not known accurately everywhere, this reference is the best way of providing accurate, homogeneous measurements.

The satellite flies in an orbit at a certain altitude S from the theoretical reference ellipsoid. The altimeter on board the satellite emits a radar wave and analyses the return signal that bounces off the surface. The time it takes for the signal to make the trip from the satellite to the surface and back again, defines the satellite-to-surface range R. In other words, the range is the actual distance between the satellite and the moving sea surface. The sea surface height (SSH) at any location or point in time is a deviation from the stable reference ellipsoid. The sea surface height is, thus, defined as the difference between the satellite's position with respect to the reference ellipsoid, and the satellite-to-surface range. That is,SSH = S – R.

Waveforms and Frequencies

Visualisation of DORIS location system operating principleAs well as sea surface height, by looking at the return signal's amplitude and waveform, we can also measure wave height and wind speed over the oceans, and more generally, backscatter coefficient and surface roughness for most surfaces off which the signal is reflected. The Poseidon-3 altimeter on board Jason-2 emits in two frequencies, and by comparing the signals with respect to the frequencies used, interesting information can be extracted, e.g. rain rate over the oceans and detection of crevasses over ice shelves.

Orbital Positioning and Interference

An extremely precise knowledge of the satellite's orbital position is necessary to obtain measurements accurate to within a few centimetres over a range of hundreds of kilometres. Three locating systems are carried onboard Jason-3. Any interference with the radar signal also needs to be taken into account. Water vapour and electrons in the atmosphere, sea state and other parameters, can affect the signal round-trip time, distorting range measurements. With the help of the measurements of the Advanced Microwave Radiometer (AMR) we can correct for these interference effects on the altimeter signal.

Altimetry requires a lot of information to be taken into account before being able to use the data. Data processing is also a major part of altimetry, producing data of different levels, optimised for different uses, at the highest levels. The AVISO site has more information about instruments.

 

Poseidon-3 Altimeter

Photo of the Poseidon-3 Altimeter

The Poseidon-3B (supplied by Europe), is the mission's main instrument, derived from the altimeters on Jason-1 and Jason-2. It allows measurement of the range (the distance from the satellite to the Earth's surface), wave height and wind speed. The altimeter implements a mixed mode allowing on-board automatic transitions between the Diode/DEM mode and the acquisition/tracking mode with respect to the satellite position. Jason-3 autonomously switches between the two altimeter modes, to improve measurements over coastal areas, inland waters and ice.

Advanced Microwave Radiometer (AMR)

Photo of the Advanced Microwave Radiometer (AMR)

The AMR (supplied by the US) measures radiation from the Earth's surface. Each frequency is combined to determine atmospheric water vapour and liquid water content. Once the water content is known, the correction to be applied for radar signal path delays is determined. Following lessons learned from Jason-2, some minor designs changes have been made and improvements made to the instrument thermal control and stability.

Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS)

Photo of Doppler Orbitography and Radio-positioning Integrated by Satellite

DORIS (supplied by Europe) uses a ground network of 60 orbitography beacons around the globe, which send signals, in two frequencies, to a receiver on the satellite.

The relative motion of the satellite generates a shift in the signal's frequency (called the Doppler shift) that is measured to derive the satellite's velocity. These data are then assimilated into an orbit determination models to keep permanent track of the satellite's precise position (to within three centimetres) in its orbit.

Following lessons learned from Jason-2 improvements have be made to albedo and infrared pressure, ITRF 2008, pole prediction, Hill along-track empirical acceleration and on-board USO frequency prediction.

Global Positioning System Payload (GPSP)

Photo of Global Positioning System Payload

The GPSP (Global Positioning System Payload) (supplied by the US) uses the Global Positioning System (GPS) to determine the satellite's position by triangulation.

At least three GPS satellites are needed to establish the satellite's exact position at a given instant. Positional data are then integrated into an orbit determination model to continuously track the satellite's trajectory.

Laser Retroreflector Array (LRA)

The LRA (supplied by the US) is an array of mirrors that provide a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, we can locate where the satellite is in its orbit and calibrate altimetric measurements.

Passenger instruments

Environment Characterization and Modelisation-3 or Carmen-3 (supplied by CNES) — a dosimeter used to improve knowledge of particularly aggressive radiation in Jason's orbit (Doris, electronic risk assessment).

Light Particle Telescope (LPT) — also a dosimeter, provided by JAXA and CNES.

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