Laurent Rey is the Project Manager for CryoSat's primary instrument: SIRAL. Laurent explains how this new radar altimeter heralds a new generation of space altimetry systems.
Laurent Rey, a French national, took up the challenge of designing and validating a new altimeter concept in 2000. The resulting SIRAL instrument will measure changes in the thickness of continental ice and sea ice with unprecedented accuracy.
Before working on SIRAL, Laurent was responsible for the Poseidon-2 altimeter on the Jason-1 oceanography mission. Laurent graduated from the École Nationale Supérieure des Télécommunications de Paris, France, and joined Thales Alenia Space in 1992.
ESA: SIRAL will determine the change in thickness of land ice and sea ice; how has this been achieved?
The development of SIRAL was based on experience gained from the altimeters on the series of joint NASA/CNES Jason oceanography missions, where an accuracy of around 1 cm was needed.
For CryoSat, the resolution was enhanced further by introducing the Synthetic Aperture Radar (SAR) mode and increasing the ground swath to a width of 250 m. This high-resolution mode is crucial for detecting ice floating in the oceans.
The surface of the edges of the ice sheets on land is completely different to the topography of the smooth ocean surface. To observe this 'chaotic' surface, the instrument uses an interferometric mode, which is basically where the two radars, or 'eyes', look at the same target. From the difference in the signal returning to the satellite we can derive the position of the ice surface.
The challenge did not lie in designing SIRAL, but more in understanding the behaviour of the signals that return to the instrument. Tests on the ground have exceeded our expectations, so we expect excellent results in orbit.
ESA: SIRAL measures the distance from the satellite to the ice surface; how is the satellite's position known?
Measuring the distance to the surface of Earth works only if you know the exact position of the satellite. A small mistake in understanding CryoSat's position in orbit could mean that Mont Blanc no longer sits on the border between France and Italy, but is, say, in Italy! To ensure we know where CryoSat is, it carries the Doppler Orbit and Radio Positioning Integration by Satellite (DORIS) instrument. This works like GPS, but instead of linking to a satellite to determine your position on the ground, the satellite receives signals from a global network of tracking stations. This system is used successfully by the majority of altimetry missions.
However, when SIRAL is in its interferometric mode, this is not enough because we have to know the precise orientation of the baseline of the two radar antennas. CryoSat measures this baseline orientation, with startrackers, that use the position of the stars in the sky.
ESA: What were the biggest technical challenges developing SIRAL?
As I have mentioned, through experience we were well-prepared to deal with measuring with centimetre accuracy with an 'autonomous' instrument that could be 700 km above Earth, or twice that high.
The challenge we had was going from a o-e dimensional measurement, the altitude, to a three-dimensional measurement: altitude, along-track and across-track. The first issue was that we did not want to degrade the altimetry measurement by adding new functions.
The second and biggest challenge was working out how to track the phase difference accurately in the returning signals for the interferometric mode. To obtain the signal's angle of arrival we had to know the exact phase difference between the radar chains: less than 0.01° for the electronics and less than 15 arcseconds for the antennas. One arcsecond represents the angle from the centre of Earth when you walk 30 m on the surface – tiny!
The solutions were as simple as possible, such as using an ultra-stable base plate to mount the twin antennas and reducing their relative pointing difference. The design period was fairly intense, as it was for the other aspects of the mission.
Rebuilding SIRAL after the loss of CryoSat in 2005 posed yet another challenge but we are well-rewarded with the realisation of a high-performance instrument.
ESA: Will the technology on CryoSat benefit future Earth observation missions?
Tests have shown that the technology works well on the ground, so we have high expectations for its operation in orbit. We have always considered that the SIRAL technology will be used for future missions. In fact, some parts are already being used, such as for Poseidon-3 on Jason-2 and for upcoming missions like AltiKa and ESA's Sentinel-3.
The benefit is not only a technical one, but also knowing how to address instrument performance. We built up a lot of experience, and based on this, we have some new ideas for future generations of instruments.
ESA: What has been the most rewarding aspect of working on the CryoSat mission?
Working with the team! CryoSat is a great mission and the team spirit has been excellent. ESA's Earth Explorers fall into two categories related to how the mission is implemented: core and opportunity. CryoSat is an ‘opportunity’ mission and this term has been appropriate throughout.
The opportunity scientists and engineers from industry and ESA have had to share and achieve the same goal; the opportunity to build the mission together with open minds and the opportunity to make friends.
ESA: Will you still be involved with the mission when it is delivering data?
I will be keeping an eye on the results for the first six months in orbit and I will be participating in the commissioning review before data start to be delivered to the user community. We 'technical people' like to follow the results produced by the scientists.
SIRAL will help us learn more about Earth's ice and also, through its operation in orbit, we will learn more about how we can make technical improvements for the future.
ESA: Where will you be for launch?
I will be with my friends from the CryoSat team at ESA's European Space Operations in Darmstadt, Germany but I will also be thinking of the SIRAL team at Thales Alenia Space.
This is one in a series of interviews with a few of the key people that are involved in the CryoSat mission. Please check back as the list will be added to over the coming weeks.