SIGNAL: a Ka-band SAR Mission to Monitor Ice, Glacier and Ocean Dynamics
López-Dekker, Paco; Börner, Thomas; De Zan, Francesco; Younis, Marwan; Hajnsek, Irena; Krieger, Gerhard; Moreira, Alberto
German Aerospace Center (DLR), GERMANY


Recently, a team of scientist lead by the German Aerospace Center (DLR) have proposed a Ka-band single-pass interferometric mission concept named SIGNAL (SAR for Ice, Glacier aNd globAL Dynamics) The mission driver of SIGNAL is to estimate accurately and repeatedly topography and topographic changes variations associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. To achieve the required elevation accuracy, in the order of a few decimeter, a formation flying constellation of two compact satellites has been proposed as the only way to achieve the desired cross-track interferometric performance. In order to meet the mission requirements, the proposed system architecture uses digital beamforming techniques both in elevation, using Scan-on-Receive (SCORE) techniques, and in azimuth, with multiple phase centers (MAPS). The use of a MAPS architecture, intended for high-resoltion wide-swath (HRWS) operations, enables also along-track interferometric operation modes, which would be used to measure ocean surface currents.

Scientific Background and Motivation

The Cryosphere, composed of global snow and ice masses, plays a pivotal role in Earth's climate system through its influence on the surface energy and moisture balance, on gas and particle fluxes, precipitation, hydrology, and atmospheric and oceanic circulation. Frozen water stored in the Cryosphere is a a vital resource for human life, agriculture, and hydropower generation in many parts of the world. From the current understanding of the Cryosphere it is clear that it reacts very sensitively to climate change. However, the feedbacks to the global climate system are not well understood, impairing the prediction of the impact of future climate change. This is to a large degree due to the important observational gaps that prevent an accurate quantification of the main cryospheric processes and an improved representation of the Cryosphere in climate models. Satellite measurements are essential for delivering comprehensive and consistent observations of global ice and snow. The important role of satellite systems for ice and snow observations is well documented in the Cryosphere Theme Report of the Integrated Global Observing Strategy (IGOS), and is also stressed in the 2010-2015 Implementation Plan of the World Climate Research Programme. ESA's Living Planet document "The Changing Earth" identifies the following main Cryosphere related scientific challenges: - Quantification of the mass balance of grounded ice sheets, ice caps, and glaciers. - Understanding the role of snow and glaciers in influencing the global water cycle and regional water resources. - Quantification of current changes taking place in permafrost and frozen-ground regimes. - Quantification of the distribution of sea ice mass and freshwater equivalent. Responding to these challenges, the main objective of SIGNAL is to fill major gaps in the data base on mass balance and dynamics of global glacier ice and to thus advance in the knowledge of the processes governing the response of the ice masses to climate forcing. The mission addresses those components of the ice budget that have been subject to accelerated downwasting during the last decade and for which the knowledge of the present mass balance and temporal trends exhibits large error bars: the mountain glaciers and ice caps, and the outlet glaciers of the boundary zones of the Greenland and Antarctic ice sheets. More specifically, the primary mission objectives of SIGNAL are:

  • Reducing the uncertainty in the mass balance of glaciers and ice caps;
  • and improving the knowledge on mass depletion of outlet glaciers in Greenland and Antarctica. Secundary objectives served by the mission would include:
  • Observation of ocean suface currents, with an emphasis on coastal areas (were altimetry derived geostrophic current estimations are less reliable).
  • Downscaling of altimetric elevation data over ice sheets.
  • Mapping the motion of calving glaciers and ice streams in support of mass balance retrieval.
  • Assessing the new opportunities of high-frequency, high resolution interferometric SAR data for sea ice surface parameter retrieval.

    System and Mission Concept

    In order to achieve the mission objectives, a formation-flying single-pass interferometric Ka-band system concept was chosen. Ka-band was chosen because the very short penetration depth on ice and snow, which allows mapping of surface topography variations (at lower frequencies the phase center is deeper under the surface and this depends strongly on the conditions). In addition, the short wavelength allows a compact system design. Due to the fast temporal decorrelation repeat-pass interferometry (both for DEM generation and DInSAR for deformations) is not an option, thus requiring a single-pass interferometry configuration. The target height accuracies, in the order of a few decimeter with spatial product resolutions of 100x100 m2 or less, require interferometric baselines in the order of 100 m, which can only be achieved with a formation flying configuration similar to the demonstrated in the TanDEM-X mission. An additional advantage of this configuration is that it allows using different baselines for different phases of the mission. In particular, the SIGNAL mission could be organized in three phases:

  • An initial DEM generation phase, to provide a high resolution Ka-band DEM. During this phase moderate baselines would be chosen in order to avoid severe phase unwrapping issues.
  • One or several DEM tracking phases during which larger baselines would be use to refine the initial DEMs and to directly monitor DEM variations.
  • One or several speckle tracking phases, where the orbits would be configured to provide the optimum revisit time for feature or speckle tracking algorithms used to measure the horizontal velocities of moving ice masses.

    Mission Performance

    This paper will present a detailed end-to-end mission performance in terms of final product quality and spatial coverage. With respect to previous analysis, where the focus was on the performance of single acquisitions, a main contribution of this paper will be to evaluate the expected DEM and, in particular, deformation rates accuracies combining the time-series of products acquireing during the mission life time. Current point to point relative errors ranging from 10 to about 50 cm, depending on surface type and product resolution, have been estimated. An important part of this analysis in a correct modeling of the calibration strategy, based on the explotation of the redundancy in overlapping and crossing acquisitions, which results in a temporal correlation of the DEM errors. This correlation has to be well characterized in order to correctly the number of independent samples available for averaging.