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Poseidon-3 integrated on Jason-2. (Credits Cnes).

The Poseidon-3 altimeter

The Poseidon-3 altimeter is the main instrument on the Jason-2 mission. Derived from the Poseidon-1 altimeter on Topex/Poseidon and the Poseidon-2 on Jason-1, it measures sea level, wave heights and wind speed. It operates at two frequencies and is also able to estimate atmospheric electron content.

Function

Poseidon-3 measures range (the distance from the satellite to the Earth's surface), wave height and wind speed.

New features wrt Poseidon-2 onboard Jason-1

Poseidon-3 will be coupled with Doris/Diode, in order to improve measurements over coastal areas, inland waters and ice (see Coupling Doris/Diode with the Poseidon-3 altimeter).

Poseidon-3 is equipped with an open-loop tracking technique for which a Digital Elevation Model (DEM) has been developed.  The altimeter's onboard memory contains the elevation values of areas overlayed by the ground tracks. These data, combined with Doris, are used to position the radar echo receiving window in advance, in order to anticipate the contrasts of the topography and to give priority to measurements over water. This technique prevents the altimeter from losing track as sometimes occurs with a conventional tracking loop, and enables measurements to be acquired close to the shoreline.

Principle

The altimeter emits a radar beam that is reflected back to the antenna from the Earth's surface (see how altimetry works for details). Poseidon-3 operates at two frequencies (13.6 GHz in the Ku-band, 5.3 GHz in the C-band) to determine atmospheric electron content, which affects the radar signal path delay. These two frequencies also serve to measure the amount of rain in the atmosphere.

Technical data

Poseidon-3, or SSALT (for Solid State ALTimeter), uses solid-state amplification techniques.

Emitted Frequency (GHz) Dual-frequency (Ku, C) - 13.575 and 5.3
Pulse Repetition Frequency (Hz) 2060 interlaced {3Ku-1C-3Ku}
Pulse duration (microseconds) 105
Bandwidth (MHz) 320 (Ku and C)
Antenna diameter (m) 1.2
Antenna beamwidth (degrees) 1.28 (Ku), 3.4 (C)
Power (W) 7
Redundancy Yes
Specific features Solid-State Power Amplifier.
Dual-frequency for ionospheric correction

Further information:

  • G. Quartly, OST/ST 2008 meeting: Jason-2 rain-flagging, going back to basics, pdf.
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