Testing, testing: CryoSat radar at work in Baltic

CryoSat, ESA’s ice observation mission satellite, will be launched in early October, but its radar system was already being kept busy back in March. The SIRAL radar system was tested during a validation campaign that turned out to be a perfect opportunity for scientists to successfully address critical issues relating to sea ice validation before CryoSat is launched.

Finnish marine research vessel Aranda knows the waters of the Baltic pretty much by heart. The Finnish Institute of Marine Research (FIMR) sends it several times each year to the Baltic Sea – and sometimes on the further trip down to Antarctica – to make different kinds of measurements and observations.

Aranda is the go-to ship for Finnish marine research. It has been through a lot, but its mission last March was different. That mission was to validate a space radar system that has yet to fly.

SIRAL, as CryoSat’s radar altimeter is known, is a new design that will measure small shifts in ice cover down to single-centimetre accuracy. SIRAL exploits sophisticated radar techniques for improved resolution and observing. Its full name – Synthetic Aperture Interferometric Radar Altimeter – encapsulates these extended capabilities, used to measure sea-ice thickness and monitor thickness changes in ice sheets on land, particularly at their fast-evolving edges.  

SIRAL

Several radar altimeters have previously flown in space, but they have differed from CryoSat in not being specifically designed for ice observation. Radar altimeters in general work by sending out a short radar pulse and measure the time that it takes for this signal to travel from the spacecraft to the ground and back.

CryoSat’s altimeter sends out a burst of pulses with shorter time intervals between them. When the returning bursts are correlated, and by treating the whole burst at once, the instrument’s data processor can separate the echo into strips arranged across the satellite track by exploiting the slight frequency shifts – caused by the Doppler Effect – in the forward- and aft-looking parts of the beam. And so the SIRAL system will beat all previous attempts at measuring ice thickness with radar altimetry.

The mission for Aranda was to validate SIRAL’s radar altimetry measurements. A copy of CryoSat’s SIRAL instrument was installed in a German Dornier 228 research aircraft then flown above the ice covering the northernmost Baltic Sea between Sweden and Finland. The task for Aranda and its research personnel was to measure actual ice cover thickness, morphology and thermodynamics at the same time, the airborne SIRAL radar made its measurements over the same areas.

By comparing the information gathered by radar and the field researchers, the radar altimeter’s performance can be validated. Ice morphology (or shape), the thickness of the ice cover and type of sea ice affect the reflected radar signal, as can the ice’s thermodynamic characteristics and possible snow overlying the ice.

Scientists drilling ice cores during CryoVex 2004

Snow cover can present a particular problem: the weight of the snow tends to push the ice floes lower into the water, so CryoSat – which translates the height of the ice surface above the waterline into ice thickness data – may underestimate the true thickness of the ice. Therefore this kind of validation campaign forms a critical component of the whole mission, as they provide a means to assess comprehensively just how accurate CryoSat’s ice thickness measurements actually are.

This was not the first validation campaign for the mission. An earlier CryoSat Validation Experiment (called CryoVex 2004) was focused on land ice and took place in Greenland and the Canadian Arctic. This last campaign in the Baltic was tasked with scrutinising sea ice in particular. The northern Baltic Sea – the Bay of Bothnia to be exact – was selected because of easy accessibility and good ice cover. It provided an excellent field of activity.

RV Aranda was anchored to the ice field on the evening of Wednesday 9 March 2005, about 15 km south of Hailuoto Island. Finnish, Swedish, German and British research groups started their work the next day by lining up a measurement line, which was used to document the properties of the flat ice cover and pack ice. FIMR staff were accompanied by people from Germany’s Alfred Wegner Institute (AWI), the Helsinki University of Technology (HUT), the Scottish Association of Marine Research (SAMS) and the Swedish Institute of Meteorology and Hydrology (SMHI).

The porosity of the pack ice was measured by the German group and HUT researchers analysed the snow cover by measuring the water consistency, density and crystal structure of the snow. The Swedish group placed drift buoys on ice cover to detect its drifting, and the thickness of the ice was measured by helicopter equipped with electromagnetic instruments provided by AWI. The HUT SC7 Skyvan aircraft assisted the measurements with Finnish remote sensing instruments. And the groups drilled hundreds of holes in ice to measure the real, actual ice and snow thickness.

Above it all was the Dornier with its SIRAL radar altimeter – called ASIRAS for the Airborne Synthetic Aperture and Interferometric Radar Altimeter System – making observations just like CryoSat will make under a month.

CryoSat’s radar beam footprint

In total some 1600 kilometres of data were collected from the air over the Bay of Bothnia and from several altitudes along a calibration line established close to the island of Hailuoto. The calibration line covered ice of varying thickness – from as little as 19 centimetres to as much as 20 metres – and also included ice featuring differing topographies, from the smooth and level to very rough ridged forms.

“We were very fortunate in many respects,” said Jari Haapala, research scientist from FIMR. “We managed to reach the targeted region with the Aranda research vessel, although breaking through the thickest ice proved impossible at times. Nevertheless, after three days of crashing through the ice we found an ice researcher’s paradise. During the days we were taking measurements, cold sunny weather prevailed so that the logistics of the campaign and the equipment all worked without any difficulty – and we even had the chance to relax in the sauna every evening!

“We received real-time satellite data, which enabled us to design the optimal measurement paths. Because sea ice drifts as it floats in the sea, one of the most challenging tasks was to collect coincidence aircraft and helicopter measurements. We managed to perform these measurements during very clear calm weather conditions and expect to get 120 kilometres of coincidence data.”

CryoSat

Although the results of the campaign were better than anticipated, the beginning of the measurements was tricky. The weather conditions were hard and the Aranda, well equipped for hard winter conditions, could push only a few kilometres per hour ahead. The biggest measured ice walls were 20 metres thick. Finally during the main measurements on 13 and 14 March, the weather was just perfect.

“We are very pleased with the Bay of Bothnia campaign on two accounts,” said Malcolm Davidson, ESA CryoSat Validation Manager. “Firstly, the data collected during the campaign helps us quantify, and eventually improve the accuracy of the CryoSat sea ice measurements, and secondly, this campaign saw the first successful flights with the ASIRAS sensor at low altitude.

This is a critical issue for ESA, as ASIRAS is our validation workhorse – it will be used extensively at low altitudes during major large-scale campaigns in 2006. The lessons learned from the Bay of Bothnia campaign means that we are now much better prepared and confident for next year.”

The last validation campaigns are to be done in 2006, when CryoSat is already in orbit. The CryoSat validation activities in 2005 were co-funded by ESA together with the National Technology Agency of Finland (TEKES) and other national and European sources.