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SALP, the CNES ground segment for altimetric missions

With the successful launch of EnviSat and Jason-1 missions, the scientists can have access to data gathered by at most as 5 altimetric missions. To ease data use, to enlarge the user community and to insure the data quality on the long term, a new ground segment (SALP) has been developed. For example, this system is used for the altimetric payload (RA2, MWR, Doris, LRA) onboard ENVISAT.

SALP system is in charge of :

• control of all DORIS instruments• control and maintenance of the DORIS beacon network • control of the Poseidon altimetric payload onboard TOPEX/POSEIDON, Jason-1,

• data processing from level0 to ephemeris files for all Doris data,• data processing from level0 to level2 products for Jason-1 and TOPEX/POSEIDON missions,• RA2 data processing from level1 to level2 products for Envisat mission,• high level product generation to merge all altimetric input data in an unique “easy to use” product,

• in depth control of the processing performed by systematic validation of all product• in depth control of the Doris signal integrity

•Long term archive of all Jason-1, Doris data•Long term archive of all ENVISAT RA2, MWR data (level 1B and higher)•Long term archive of all CryoSat data

• user service for data dissemination (systematic or upon request)• data information, outreaching, …• friendly user interface to order either online or on media all product type

“CNES Toulouse involvement in Cryosat in-orbit mission”Authors: Service d’Altimétrie et de Localisation Précise (SALP), CNES, TOULOUSE

Service d’Altimétrie et de Localisation Précise

CryoSat is the first Opportunity Mission of the ESA’s Earth Observation Envelope Programme. CryoSat main mission objective is to measure the thickness variations of sea-ice as well as land-ice over a period of 3 years. This will be achieved in combining the measurements made by SIRAL, an interferometric radar altimeter, with a very accurate determination of the satellite orbit based on the DORIS system.The DORIS system, developed by CNES together with IGN, is able to provide precise orbit determination down to a few centimetres. DORIS has successfully been flown on various satellites (SPOT, TOPEX, JASON-1, ENVISAT) demonstrating reliable and highly accurate performance.In the frame of the CryoSat development, ESA has acquired the latest model of the DORIS instrument. And for the first time, ESA is using Doris to provide onboard services to the AOCS system (orbit and attitude control).Concerning in-orbit operations, the SALP CNES project has developed a specific centre, SSALTO, and is responsible for the maintenance of the on-board Diode DORIS navigator and the DORIS ground beacon network, specific Diode operations, expertise in support ESOC operation team, as well as the processing of the data generated by the DORIS instruments for precise orbit determination. SSALTO is also providing all the external auxiliary data to the PDS L1b processing. ESA has developed the ground segment of the CryoSat mission based on a close interaction between the CryoSat project team and SALP (Toulouse).Moreover, CNES has developed the ESRIN Cryosat LTA dedicated to long-term archiving for all mission products, off-line reprocessing of SIRAL products and support to ESA user services. At this end, it has to interface with PDS (Payload Data System) for product long term archiving, to report to centralised facilities (OHS, MMC, RPF, MF), and serve users requests. Its components are part of the SALP Altimetry Mission Centre called SSALTO.The DORIS/CRYOSAT and LTA projects team are rooted in the Altimetry Division at CNES Toulouse to guaranty that the development and the in-flight and ground based operations are co-ordinated with the other parts of the so-called « Altimetry Mission Centre » SSALTO already including the Envisat F-PAC. In conclusion, to fulfil needs of the altimetric users community, CNES has set up a system named SALP (for "Service d’Altimétrie et de Localisation Précise") with the following objective : to provide continuous series of altimeter, orbitography and localization products mainly generated with the altimeter payload onboard Jason-1, ENVISAT, T/P, ERS and CryoSat missions. CNES operations for both ENVISAT and CryoSat are included in the SALP operations frame work using SALP common components or dedicated software and hardware developed in the context of the ENVISAT or CryoSAT ground segments and which will be used in order to fulfil ESA requirements.

KIRUNAPDS

CNES Toulouse - SALPSOD, Long Term Archive

ESRIN FrascatiMulti mission catalog (OHS, MMC)

ESOC DarmstadFlight Operations Segment

From PDS : Doris telemetry files to SSALTOall products and aux data to LTA

From SSALTO :Doris orbit products and SALP auxiliary files

From ESRIN : User orders

From LTA :Catalog inventory updatesOrders follow-up

From ESOC : WebTDRS, WebRMRecorded HK Telemetry, Attitude data

From SALP :Command requestsSoftware upload files

Control

Processing

UserService

SALP is involved in one way or the other in all current altimetric missions. One key part of those missions is the Doris system, onboard Jason-1, T/P EnviSat and CryoSat. This system allows to determine with a high accuracy the real time position of the satellite. Processed off line, the Doris measurements permit to compute the satellite position with an accuracy of 2 to 3 centimeters.

Data

preservation

Expertise

Ssalto/Duacs Gridded Sea Level Anomalies

Ssalto/Duacs Along-Track Sea Level Anomalies

NRT-SLA, DT-SLAMedia: FTP, DVD RomDelay: near-real time & delayed timeSatellites:T/P, Jason-1, Envisat, GFO, ERS

Geophysical Data RecordsIGDR, GDRMedia: FTP, DVD RomDelay: near-real time & delayed timeSatellites:T/P, Jason-1, Envisat

SALP Altimetric Products

Along-Track Wind / Wave data

OSDR, RA2 WWV, Wind/Wave Media: FTPDelay: real time, delayed-timeSatellites:Jason-1, T/P, Envisat

Gridded Wind / Wave data

NRT-MWind and NRT-MSWHMedia: Live Access Server, OpendapDelay: near-real timeSatellites:T/P, Jason-1, Envisat, GFO

Ssalto/Duacs Gridded Absolute Dynamic Topography

NRT-MADTMedia: FTP, Live Access Server,

Opendap & ImagesDelay: near-real time Satellites:merged T/P, Jason-1, Envisat, GFO

Ssalto/Duacs Along-Track Absolute Dynamic Topography

NRT-ADTMedia: FTPDelay: near-real timeSatellites:T/P, Jason-1, Envisat, GFO

SALP User Service: aviso@cls.fr; www.aviso.oceanobs.com

SGDR, WaveformsMedia: DVD RomDelay: delayed timeSatellites:T/P, Jason-1, Envisat

NRT-MSLA, DT-MSLAMedia: FTP, DVD Rom, Live Access Server,

Opendap & ImagesDelay: near-real time & delayed timeSatellites:T/P, Jason-1, Envisat, GFO, ERS

independently and merged

Geophysical sensor data

Diode, a real time localisation service

Doris system• Unlike the Global Positioning System (GPS), which beams signals from a constellation of satellites to Earth (or to other satellites in lower

orbits), Doris uplinks signals to satellites. Collection of data on the satellite and at the SSALTO multi mission control and processing center makes the system particularly robust and is a definite advantage for distributing products in near-real time. It also facilitates the use of a single timing scale. An uplink system also allows the station network to be monitored remotely, thus significantly improving the quality.

• The Doris instrument is part of the satellite payload. It comprises an MVR receiver that measures radial velocity, an ultra stable oscillator which insure a datation accuracy of 1 to 2 micro seconds, and an antenna. Every 10 seconds, it measures the Doppler shift at two frequencies : 400 MHz and 2 GHz (400 MHz channel being used to correct for the ionospheric effects).

• On ground, a network of 60 beacons emits signals.

Service d’Altimétrie et de Localisation Précise

• Since the launch of the Spot 4 satellite in January 1998, the Diode navigator has added real-time measurement processing capability for satellite navigation. Every 10 seconds, the onboard software determines the satellite position. Onboard Jason-1 and Envisat satellites, the position of the satellite is routinely determine within 50 centimeters in 3D (in normal conditions) with an accuracy of about 10 to 15 centimeters on the radial axis. This allows real time or near real time delivery of precize altimetric data to the user community.

• Moreover, onboard CryoSat, ESA is using DORIS/Diode capabilities to provide onboard services to the AOCS system (orbit and attitude control)

LTA system

The LTA system, part of CryoSat ground segment, will insure : long term archiving of all CryoSat data upon ESA request, reprocessing of CryoSat data (for example at the end of the Kiruna operations). On demand user requests (the PDS being the nominal data distribution site during the CryoSat lifetime)

This system has been designed using existing facilities already used for ENVISAT operations and part of the ground segment set up in place in Kiruna. Inside SALP system, this will allow in the future to develop higher level products as the one depicted below.