25 years of altimeter developments at Alcatel Alenia Space
Laurent Phalippou(1) , Eric Thouvenot(2) , Philippe Escudier(3) , and Constantin Mavrocordatos(4)
ALCATEL ALENIA SPACE,
26 Avenue Champollion,
31037 Toulouse, BP 1187, Cedex 1,
(2) CNES, 18 Av. Belin, 31401 Toulouse Cedex 4, France
(3) CLS, 3 rue HERMES, 31526 Ramonville ST Agne, France
(4) ESA/ESTEC, PO box299, 2200 AG Noordwijk, Netherlands
Alcatel Alenia Space is involved in altimeter design and manufacturing for more than 25 years since the early definition phase of the Poseidon altimeter for the reference Topex-Poseidon (T/P) mission. Alcatel Alenia Space is developing state of the art altimeters presented in this paper.
T/P is a joint NASA/CNES mission, launched in 1992 and still in orbit after 13 years of operation, which paved the way for high accuracy sea level measurements. The Ku-band Poseidon altimeter on T/P was embarked as an experimental payload to demonstrate in flight the new technology used. It has provided excellent measurements which were found to agree with the measurements delivered by the US operational altimeter of the mission.
The T/P mission has been pursued by the Jason 1 mission launched in 2001, embarking a dual frequency (Ku and C band) altimeter Poseidon 2, as the single operational altimeter of the mission. The satellite and Poseidon 2 were manufactured by Alcatel Alenia Space under CNES contracts. Poseidon 2 is providing excellent measurements (see dedicated presentation in this conference). The range measurements are provided by the Ku band channel while the C band is used for correcting the excess path length due to the ionosphere. Poseidon 2 inherited from Poseidon-1 state of the art technology in order to have excellent performances (impulse response and calibration) while minimising as much as possible the demand towards the satellite in terms of mass, volume and power. For instance, Solid State Power Amplifiers technology has been used in Ku and C band for the Poseidon series.
In the mean time ESA launched the ERS-1 (1991), ERS-2 (1995), and ENVISAT (2002) multi-applications satellites on sun-synchronous ~800 km orbits. The Radar Altimeter (RA) on board ERS satellites is a single Ku-band altimeter with two bandwidth (330 MHz and 82 MHz) allowing operation over the ocean and over ice-surfaces. The repeat period of ERS 1-2 and Envisat orbits is 35-day allowing inter-track sampling of the T/P and Jason 10 day repeat period. The Radar Altimeter of Envisat (RA2) is a dual frequency altimeter (Ku and S bands), with bandwidth agility (320 MHz, 80 MHz and 20MHz) for measuring the ocean and the ice surfaces. The pulse width of RA and RA2 is 20 µs (against 100µs for Poseidon) which requires a larger RF transmitted power (delivered by a Travelling Wave Tube). The RA and RA2 demand (mass, power) towards the satellite is therefore larger than for the Poseidon family, but the level of end product performances falls in the same category, i.e. around 2 cm for the Sea Surface Height measurements.
In 1997, Alcatel Alenia Space started a concept study with MSSL under ESA contract for a high resolution altimeter with the objective of monitoring the land-ice mass variations over Antarctica and Greenland and sea-ice thickness variations. This concept has matured to give birth to SIRAL (Sar Interferometric Radar Altimeter) embarqued on the CryoSat satellite which had an unsuccessful launch in October 2005. SIRAL operates in the conventional pulse-limited altimeter mode over the flat areas, and in two advanced modes, the SAR mode over sea-ice and the SAR interferometric (SARIn) mode over the sloppy areas (ice sheet margins and glaciers). SIRAL will be the first SAR interferometric radar to fly on a satellite. The required interferometric phase accuracy of the instrument is less than 1.5 degree, a challenging value requiring extreme care in the design of the instrument and of the internal calibration, and for the stability of the instrument.
The Jason 2 mission will be a follow-on of Jason 1. The Jason2 satellite will embark the Poseidon-3 Ku/C band nadir altimeter. The performances of Poseidon-3 will be similar to those of Poseidon-2 in the operational mode over ocean surfaces. Improvements are implemented by Alcatel Alenia Space under CNES contract to improve the performances of non ocean surface tracking. They include new tracking algorithms (derived from SIRAL) and the possibility to directly control the altimeter altitude tracked using the satellite position information derived from the real time on-board navigator of Doris (Diode) and the local elevation provided by an on-board DEM (Digital Elevation Map) file. This new feature will improve dramatically the tracking of inland water while preserving high resolution range measurement. Technologically, the Digital Processing Unit is derived from SIRAL in order to ensure a product line synergy and to cope with the obsolescence of electronics components. Phase C/D of Poseidon-3 started in spring 04.
The Alti-Ka concept proposed by CNES in 1998 is a single-frequency Ka band (35 GHz) pulse limited radar altimeter for ocean and ice surfaces. The main technological motivation for Alti-Ka is to have a compact instrument compatible with the resources available on micro-satellites (or as an opportunity passenger). Scientifically, thanks to the larger allocated bandwidth (500 MHz) and larger pulse repetition frequency the accuracy of the measurement of sea surface elevation is improved by a factor of ~2 compared with Ku/C bands altimeters, as in Ka band the radar vertical resolution is 30 cm. For a given antenna size the spatial resolution of the altimeter is also improved in Ka band compared to Ku band. The Alti-Ka altimeter has a built-in dual frequency microwave radiometer for wet path delay corrections. An high data rate mode (1 second of complex data) will be included, to analyse speckle and echo characteristics in coastal region. The open loop range tracking mode (Diode+DEM Mode) defined for Poseidon 3 will also be adapted and implemented in Alti-Ka. The altimeter has been studied by Alcatel Alenia Space under CNES contracts (Phases A and B) and technological developments have been performed on the chirp generation and on the SSPA. Alti-Ka phase C/D will start by the end of 2005, for a possible launch on the Indian OceanSat 3 ISRO satellite.
The paper will present the Alcatel Alenia Space altimeter product line with a status on current developments and also future studies for advanced interferometric oceanic altimeters.