February 24, 2017

Poseidon 3

Poseidon 3 instrument during AIT on 25/04/2007 Poseidon 3 instrument during AIT on 25/04/2007
Poseidon 3 instrument during AIT on 25 April 2007
© CNES - Thales Alenia Space

Poseidon-3 is the main instrument of the mission and a follow-on to the Poseiden-2 altimeter used on Jason-1. Small, lightweight and with low power consumption, it is also extremely reliable. It consists of a radar that transmits waves at two different frequencies (13.6 and 5.3 GHz, to make it possible to identify the atmosphere's electron content) then receives and analyses the signal reflected by the surface. Precise estimates of the round-trip time of the wave make it possible to calculate the distance between the satellite and the surface (after applying a few corrections). Poseidon-3 is coupled with Doris/Diode, with a view to improving measurements obtained near coastal areas.

Coupling Doris/Diode and the Poseidon-3 altimeter

Diode/Poseidon 3 coupling principleAs part of the Jason-2 project, it was requested that Diode provide in real time a "geodetic bulletin" detailing the satellite's position: latitude, longitude and altitude, in particular with respect to geoid. The Poseidon-3 altimeter benefits from this information in two experimental operating modes. Like all radars, the altimeter must first search and engage the target (the acquisition phase), and then follow it (the tracking phase):

  • 1st mode: using the altitude information contained in the geodetic bulletin to the altimeter, the 1st experimental mode will be used to significantly reduce the search range in which the return echo is expected. By reducing the length of the acquisition phase in this way, Poseiden-3 should increase the production of useful data during Earth-sea transitions and improve altimetry in coastal areas. Useful signal tracking is then conducted using traditional methods in the same way as Poseiden-2 (but improved to be more robust over continental waters).

  • 2nd mode: this experimental mode modifies the tracking loop. Indeed, the planned target distance will be calculated directly by the altimeter, by combining the altitude information provided by Doris/Diode, and the altitude of a pseudo digital elevation model (DEM) of the track overflown by Jason-2 (recorded on board in the altimeter's memory). Depending on the quality of this pseudo DEM, and on natural surface variations (tides, etc.), the return echo positioning precision in the altimeter receive window will be to the nearest few metres. The combined use of Diode data and the pseudo DEM altitude will make it possible to directly control the reception window position to ensure tracking of any target, regardless of the return echo format. As a result this mode could prove extremely useful for tracking areas of special interest, such as rivers and lakes (which are not always sufficiently well covered by traditional altimeters).

Tracking of specific interest zones

Studies are currently in progress to develop these two probative modes. We have high hopes for their forthcoming validation, but in the meantime, the operating modes for Poseidon-3 remain those validated for Poseidon-2.

This new interaction function between the Doris instrument and the Poseidon-3 altimeter benefits both the Jason-2 mission and the DORIS instrument line, by extending the altimeter's capabilities in coastal areas and in "continental waters", but also by broadening the spectrum of flight opportunities for DORIS and thus contribute to increasing the number of DORIS instruments flying different orbits. Performance of the DORIS system is indeed improved by increasing the number of DORIS instruments flying in different orbits. In return, this "DORIS constellation" contributes to a better positioning of DORIS beacons, used for a better control of the "Terrestrial Reference System" – an essential part of altimetry missions.

Characteristics

POSEIDON-3 instrument’s main characteristics:

Transmission frequency (GHz)Dual frequency (Ku, C) - 13.575 and 5.3
Pulse repetition frequency (Hz)Interleaved 2,060 {3Ku-1C-3Ku}
Pulse length (microseconds)105
Bandwidth (MHz)320 (Ku and C)
Antenna diameter (m)1.2
Antenna aperture (degrees)1.28 (Ku), 3.4 (C)
Power (W)7 (Ku), 27 (C)
RedundancyYes
Special featureSolid state amplifier.
Dual-frequency for ionospheric corrections,
High resolution in C band (320 MHz)