Conference Agenda

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Session Overview
Session
Session 3D - CryoSat2-IceSat2 synergy towards future polar missions
Time:
Wednesday, 22/Mar/2017:
2:00pm - 3:40pm

Session Chair: Tommaso Parrinello
Session Chair: Erik De Witte

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Presentations
2:00pm - 2:20pm

ICESat-2, its Retrievals of Ice Sheet Elevation Change and Sea Ice Freeboard, and Potential Synergies with CryoSat-2

Thorsten Markus1, Thomas Neumann1, Benjamin Smith2, Ronald Kwok3

1NASA, United States of America; 2University of Washington, United States of America; 3JPL, United States of America

Understanding the causes and magnitudes of changes in the cryosphere remains a priority for Earth science research. Over the past decade, NASA’s and ESA’s Earth-observing satellites have documented a decrease in both the areal extent and thickness of Arctic sea ice, and an ongoing loss of grounded ice from Greenland and Antarctic ice sheets. Understanding the pace and mechanisms of these changes requires long-term observations of ice-sheet mass, sea-ice thickness, and sea-ice extent.

ICESat-2 has three pairs of beams, each pair separated by about 3 km across-track with a pair spacing of 90 m. The spot size is 17 m with an along-track sampling interval of 0.7 m. This measurement concept is a result of the lessons learned from ICESat. The multi-beam approach is critical for estimating cross-track slope around the margins of Greenland and Antarctica enabling the calculation of elevation change on a seasonal basis. For sea ice, the dense spatial sampling (eliminating along-track gaps) and the small footprint size are especially useful for sea surface height measurements in the, often narrow, leads needed for sea ice freeboard and ice thickness retrievals.

Currently, algorithms are being developed to calculate ice sheet elevation change and sea ice freeboard from ICESat-2 data. The talk will present an overview of algorithm approaches and how ICESat-2 and Cryosat-2 data may augment each other.


2:20pm - 2:40pm

Possible Extensions for the ESA Ice Mission CryoSat-2: Exploiting the CryoSat-2/ICESat-2 Synergies

Javier Sanchez1, Xavier Marc2, Gerald Ziegler3, Miguel Angel Martin Serrano3, David Fornarelli4, Elia Maestroni2

1GMV GmbH at the European Space Operations Centre (ESOC), Germany; 2European Space Agency (ESA), European Space Operations Centre (ESOC), Germany; 3SCYSIS GmbH at the European Space Operations Centre (ESOC), Germany; 4Rhea Group at the European Space Operations Centre (ESOC), Germany

The CryoSat-2 satellite was successfully launched on the 8th of April 2010 on a Dnepr rocket from
the Baikonur Cosmodrome in Kazakhstan. The mission is dedicated to the precise monitoring
of changes in the thickness of marine ice floating in the polar oceans as well as variations in the
thickness of the vast ice sheets that overlie Greenland and Antarctica. The CryoSat-2 satellite is
operated following a near-polar reference orbit, with a 92 degrees mean inclination of the orbital
plane and a repeat cycle of 5344 orbits in 369 nodal days. The spacecraft is equipped with two
redundant pairs of 40 mN cold gas thrusters to execute orbit maintenance manoeuvres.

In nearly 7 years of operations the CryoSat-2 satellite has successfully produced a wealth of data in a
field of growing scientific interest. Now, the excellent state of the platform and the load of remaining
propellant (31.5 kg from its initial 36.7 kg) enable the extension of operations even further. As for
the activities foreseen for the coming years, an opportunity has been identified by acquiring a ground-track pattern close to that of NASA mission ICESat-2. The ICESat-2 spacecraft is set for launch in 2018 and will be operated following a near polar reference orbit. The purpose of this paper is to analyse
the feasibility of a CryoSat-2 re-orbiting and orbit maintenance in order to support combined
operations with the ICESat-2 mission. Likewise, its aim is to trigger the interest among the scientific
community on potential benefits of this type of operations and to stimulate users of both missions
into exploiting the synergies that this possibility can offer.

In the general case, the acquisition of a new reference orbit for CryoSat-2 implies: the adjustment of
the orbital plane inclination, in order to acquire the same areas in the polar regions; the selection of a
new orbital period, which satisfies the needs of the combined ICESat-2/CryoSat-2 operations; the
correction of the eccentricity vector and the acquisition of the right phasing.

The execution of inclination changes via Out-Of-Plane manoeuvres presents an exceptional
challenge for the CryoSat-2 mission. This manoeuvre type requires a 90-degree attitude slew in order
to perform a delta-V perpendicular to the orbital plane. In addition to this, the performances of the
cold gas thrusters lead to a relatively low delta-V, which forces the split of the necessary inclination
changes into several manoeuvres. The implementation of inclination control for the CryoSat-2
mission was not foreseen in the original requirements and feasibility analysis and has never been
attempted in flight. The impact on the space segment of this type of operations will be analysed in
depth. This paper details the baseline to acquire the target inclination, as well as its maintenance during the combined operations period. Also, results of the initial operational feasibility assessment will be presented.

Concerning the change in orbital period, two options have been analysed in this study.
The purpose of these examples is to illustrate the performances that can be achieved in terms of
synchronization of the grid of equatorial nodes of both missions. However, different solutions can be
found in this respect depending on the specific requirements for combined operations of both
payloads.


2:40pm - 3:00pm

Continuation of Service for CryoSat using the Sentinel-6 Heritage Platform with an Interferometric SAR Altimeter

Bjoern Barthen1, Klaus Koeble1, Robert Cullen2, Pierrik Vuilleumier2, Daniel Sausen1, Frank Blender1, Inge Vanschoenbeek1, Florian Reuscher1

1Airbus DS GmbH; 2European Space Agency

In view of the international user needs to have a long term climate data record of land and marine ice sheet retrievals we have investigated the constraints to launch a potential follow-up mission to CryoSat-2. Situated in a non-Sun-synchronous, near-polar orbit with an altitude just over 700 km and an inclination of 92 deg, CryoSat-2 successfully concluded its nominal mission in October 2013 and is continuing to provide valuable data post its extended.

At perfect health and more than 6 years in orbit, CryoSat-2 provides unique data of the cryosphere. In order to ensure continuation of this data, one must think, however, beyond CryoSat-2. Building a new satellite takes a long time and bears development risks both jeopardising the successful continuation of the CryoSat mission This article presents the possibility of refitting the Sentinel-6 spacecraft with an interferometric SAR altimeter, minimising development time and risk towards the continuation of CryoSat.

Already prior to the launch of CryoSat-2, ESA initiated a study focusing on the continuation of the the well-recognized series of TOPEX/Poseidon and Jason-1, 2 and Jason-3 while relying on the CryoSat platform concept, which eventually became the Jason-CS/Sentinel-6 mission. The Sentinel-6 platform itself is the result of studying the possibility to re-use CryoSat while making required adaptions. Adaptations were made in order due to cope with a far more severe radiation environment, hosting payloads supporting the new oceanographic mission, ensuring space environment sustainability by performing de-orbiting after the nominal mission and coping with obsolescences of the CryoSat platform.

It is only logical that the Sentinel-6 platform is by itself perfectly suited to host a new SIRAL-3 instrument, requiring only few modifications. This new satellite will meet permit continuing CryoSat with exiting or surpassing performances. Observation of continental ice sheets and ocean can be performed in SAR burst and no longer Low Resolution Mode. Next to DORIS and the laser retro reflector, a GNSS receiver can be used for orbit determination additionally. An increase in lifetime can be expected due to Sentinel-6 being designed for a longer mission in a radiation critical Low Earth Orbit.


3:00pm - 3:20pm

Evolutions of the SIRAL instrument for the Cryosat Follow-On mission

Yves Le Roy1, Eric Caubet1, Pierluigi Silvestrin2, Erik De Witte2, Robert Cullen2

1THALES ALENIA SPACE, France; 2ESA-ESTEC

This paper presents the outputs of recent studies which have been carried out by Thales Alenia Space (TAS) to support ESA in the definition of the SIRAL instrument for the Cryosat Follow-On (FO) mission. The SIRAL-2 instrument of the currently in-orbit Cryosat-2 mission was under TAS responsibility.

The SIRAL-2 instrument is a nadir-looking Ku-Band radar altimeter with interferometric capability. The radar supports three different modes which are switched according to the surface type, the conventional Low Resolution mode (LRM) as well as the SAR and SAR Interferometric (SARIn) modes, these two modes being operated in closed-burst fashion. The first objective of the SIRAL Follow-On radar is to meet, as a minimum, the existing Cryosat-2 requirements. For this, the SAR and SARIn modes providing the same performances as SIRAL-2 are proposed as a baseline in order to ensure continuity in the SIRAL-2 measurements. The SARIn interleaved mode operated with a PRF of 18 kHz is proposed as an enhanced mode in order to improve the azimuth resolution over sea ice surfaces and provide better detection of sea ice freeboards. The instrument architecture which responds to these requirements is based on the Jason-CS/Poseidon-4 architecture whose development is in progress as well as on the SIRAL-2 heritage. This leads to a cost-efficient flexible solution including state of the art technologies while ensuring optimum continuity in the altimeter product line.

Snow loading on sea ice has proved to alter the accuracy of sea ice thickness retrieval in Ku-Band. The addition of a Ka-band nadir channel would improve the retrieval of ice elevation and snow thickness by taking advantage of deterministic snow penetration rates in Ku- and Ka-Band. This is supported by the comparisons made by scientists between the SIRAL measurements in Ku-Band and the Alti-Ka measurements of the SARAL mission. Furthermore, ionospheric corrections can also be performed by using the dual Ku/Ka measurements in order to improve the Sea Surface Height accuracy. The Ka-Band functionality, which is built on top of the Ku-Band without modifying its original performances, allows simultaneous acquisitions in the dual band, with reduced impacts on the instrument architecture and a reasonable increase of mass and power consumption budgets.

The architecture and the performances of the radar in the baseline configuration and with the enhanced functionalities (SARIn interleaved mode in Ku-Band, addition of Ka-Band) will be presented and discussed.


3:20pm - 3:40pm

AltiCryo: A CNES Altimetry Concept Study for Cryosphere Monitoring

Amandine Guillot1, Frédérique Rémy2, François Boy1, Jean-Luc Courrière1, Anne Lifermann1

1CNES, France; 2LEGOS, France

The cryosphere monitoring is crucial for environment and climate studies. Alongside Cryosat-2, another altimetry mission has contributed to advances in cryosphere studies: SARAL/AltiKa, developed by CNES and ISRO mission, and launched in 2013. First, SARAL/AltiKa flew on the orbit as Envisat until last July, providing continuity of topography measurement on the ground track monitored by ERS1,2 and ENVISAT since 1992. Second, the AltiKa instrument provides active (altimeter) and passive (radiometer) measurements in Ka-band, which is valuable to understand the ice and snow properties (Frédérique Rémy, Thomas Flament, Aurélie Michel & Denis Blumstein (2015) Envisat and SARAL/AltiKa Observations of the Antarctic Ice Sheet: A Comparison Between the Ku-band and Ka-band, Marine Geodesy, 38:sup1).

Based on the SARAL/AltiKa feedback over cryosphere, the French space agency (CNES) has initiated a study to propose an altimetry concept optimized for the cryosphere scientific needs. This concept will be derived from AltiKa and has to be compact and cost-effective. This paper will describe the objectives and the organization of this study.

The first step of the study is to document user needs for cryosphere, based on the actual knowledge acquired with current missions (both SARAL and Cryosat) and physical measurement skills. The orbit choice will also be discussed.

The second step consists in the definition of an instrumental configuration which will satisfy the user needs. A first concept to be studied is a dual frequency Ka/Ku altimeter, operating in LRM or SAR mode. Based on the SARAL/AltiKa experience, some instrument parameters will be tuned to optimize the cryosphere observation. Moreover, the AltiKa radiometer function could be maintained, to help characterizing surface properties over ice, as well as providing wet tropospheric correction over ocean.

Once the instrumental configuration will be set up, the associated end-to-end performance will be assessed , including ground processing up to the product level. In particular, the impact of Ku-Ka band combination will be studied. The advantages and drawbacks of this configuration will be also compared with the Cryosat-FO concept.

Lastly, the impacts on the platform will be listed.

The AltiCryo study should run until July 2017.



 
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