Jason-2 Design

Jason-2 carries five payload instruments and three 'passenger' instruments.

Image - Satellites - Jason-2 - thumb

The lightweight platform (500 kilos at launch) is known as the 'Platforme Reconfigurable pour l'Observation, les Telecommunications Et les Usages Scientifiques' or PROTEUS. The three-axis stabilisation and nadir pointing is maintained by reaction wheels and magnetic torque rods.

The altimetry technique

See the Animation on Jason-2 instruments

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-2. 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

Poseidon-3 (supplied by CNES) is the mission's main instrument, derived from the Poseidon-2 altimeter on Jason-1. It is a compact, low-power, low-mass instrument offering a high degree of reliability.

Poseidon-3 is a radar altimeter that emits pulses at two frequencies, 13.6 GHz (Ku-band) and 5.3 GHz (C-band), and analyses the return signal reflected by the surface. The signal round-trip time is estimated very precisely, to calculate the range after applying corrections.

The primary goal of the dual-frequency operation is to provide a precise ionospheric correction. Besides a differential ionospheric path delay, Ku- and C-band signals are differentially and significantly affected by geophysical quantities, such as atmospheric precipitation and sea surface roughness.

Advanced Microwave Radiometer (AMR)

Photo of the Advanced Microwave Radiometer (AMR)

The AMR, supplied by NASA, measures radiation from the Earth's surface at three frequencies (18, 21 and 37 GHz). These different measurements are combined to determine atmospheric water vapour and liquid water content. Once the water content is known, it is possible to determine the correction to be applied for radar signal path delays.

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

Photo of Doppler Orbitography and Radio-positioning Integrated by Satellite

DORIS, supplied by CNES, 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 in orbit determination models to keep permanent track of the satellite's precise position (within 3 cm) in its orbit.

Global Positioning System Payload (GPSP)

Photo of Global Positioning System Payload

The NASA-supplied GPSP (Global Positioning System Payload), 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 NASA, 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-2 (Carmen-2) — studies radiation in the satellite environment (supplied by France.
  • Light Particle Telescope (LPT) —  studies radiation in the satellite environment (supplied by Japan).
  • Time Transfer by Laser Link (T2L2) — consists of detectors for ultra-precise time transfer. T2L2 uses a laser link for high accuracy comparison and synchronisation of remote ground clocks (supplied by France) .
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