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Figure 1: ESA Altimetry Missions: ERS, Envisat and CryoSat. NASA/CNES Altimetry Missions: GEOSAT, TOPEX-POSEIDON, GFO, Jason-1, Jason-2, Jason-3 and Altika.

 

ESA Altimetry Missions

Since 1991, there has been a continuous series of ESA altimetry missions, starting with ERS-1 (1991) and followed by ERS-2 (1995), Envisat (2002) and CryoSat (2010).

The objective of the Sentinel-3 mission is to provide operational measurements for 20 years using robust and stable altimetry technology already proven in previous ESA missions. The minimum baseline altimeter performance required shall be the same as Envisat and CryoSat altimeters.

The altimetry instrument is similar to that of the CryoSat mission but with the interferometry mode not included. CryoSat is ESA's dedicated mission to monitor changes in the thickness of marine ice floating in the polar oceans, and to monitor variations in the thickness of the ice sheets covering Greenland and Antarctica. The Sentinel-3 mission continues the monitoring of the sea state, wind speed, sea-ice and ice thickness as started by the ERS, Envisat and CryoSat missions.

The operating modes available to the SENTINEL-3 mission are High Resolution Mode (commonly called SAR) and Low Resolution Mode (LRM). High Resolution Mode is set for 100% of the coverage for all types of surfaces. LRM is a back-up mode only. LRM is the operating mode used in ENVISAT. High Resolution Mode was also used by CRYOSAT but only operated over sea-ice areas and over some ocean basins and coastal zones. Continuity of the ENVISAT mission is  guaranteed by the generation of pseudo-LRM data products from High Resolution Mode data products at Level-1 processing.

The Sentinel-3 orbit (altitude and inclination) is almost the same orbit as Envisat, allowing continuation of the ERS/Envisat time series.

The C-band frequency is a standard chosen in almost all altimetry missions. The S-band frequency was chosen for ENVISAT because the SAR instrument (ASAR) was working on C-band frequencies and the radar altimeter instrument could produce interference to the SAR instrument (ASAR).

The revisit time has improved from 35 days (Envisat) to 27 days (Sentinel-3) due to the inclination (98.65° instead of 98.55°) and altitude (815 km instead of 800 km) parameters chosen for the Sentinel-3 mission.

Mission Lifetime Altimetry instrument Altitude Inclination Frequency Revisit time
ERS-1 July 1991 -
June 1996
RA 785 km 98.52° Ku-band 35 days
ERS-2 April 1995 -
June 2003
RA 785 km 98.52° Ku-band 35 days
Envisat March 2002 -
April 2012
RA 800 km 98.55° Ku-band
S-band
35 days
CryoSat April 2010 -
Ongoing
SIRAL 717 km 92° Ku-band 369 days
(30 day sub-cycle)
Sentinel-3 2016 SRAL 814.5 km 98.65° Ku-band
C-band
27 days

Table 1: ESA altimetry missions' main characteristics

 

Other Altimetry Missions

The next ESA altimetry mission is SENTINEL-6 in 2020.

See the Radar Altimeter Tutorial for further information about past, present and future altimetry missions.