Coupling Doris/Diode with the Poseidon-3 altimeter:
a team effort in orbit
Christian Jayles, Guy Carayon (Cnes), Patrick Vincent (Ifremer), Philippe Escudier (CLS)
Diode (Détermination Immédiate d'Orbite par Doris Embarqué, or Immediate Onboard
Orbit Determination by Doris) is a suite of real-time onboard orbit determination
programs which processes Doppler measurements acquired by a Doris receiver on a
satellite.
Since its integration with Doris (on Spot4), Diode has been used with increasing regularity:
- by the Doris receiver, in particular to provide data which enable it to locate the receiver's
tracking loop more efficiently,
- by external users (such as the satellite's AOCS, other payload instruments and their
related ground segments), to determine the host satellite's precise orbit and provide time-
tagging data.
Each flight opportunity requires a new Expression of Needs to be defined by the relevant
project teams: for Jason-2 an innovative coupling with the Poseidon-3 altimeter was
conceived.
In the framework of the Jason-2 project, Diode was commissioned to provide users with a
'Geodetic Bulletin', containing up-to-date information on the satellite's position: latitude,
longitude and altitude, particularly in relation to the geoid. The Poseidon-3 altimeter will be
able to use this information in two experimental operating modes, which each modify one of
the altimeter's main operating phases. Like all radars, the altimeter must first find and lock
onto its target (this is the Acquisition phase), so that it can then follow it (the Tracking
phase).
First mode: Using altitude data from the Diode geodetic bulletin which is transmitted to the
altimeter, this experimental mode should result in the predicted search range of the return
echo being significantly narrowed. Accelerating the Acquisition phase in this way should
enable Poseidon-3 to generate a larger quantity of useful data during Earth/Sea transitions,
thus improving altimetry in coastal areas. In this mode, the usefu