Technical note
Table of Contents
The proposed activity called
Advanced Concept for RAdar Sounder shall give the opportunity to
assess the feasibility of new measurement concepts for
subsurface sounding
using along and across-track synthetic aperture processing to enhance
the resolution.
The use of satellite-based ground penetrating radar is already a
reality for planetary exploration (with MARSIS: Mars Advance Radar for
Subsurface and Ionospheric Sounding already launched) and is of high
interests for the future with ELRR: Europa Low Resources Radar and for
Earth Observation (e.g. Antarctic ice-sheet sounding).
However there are a number of challenges for achieving an
efficient subsurface imaging, which are due to the mandatory use of a
low radar frequency, resulting in a wide antenna footprint:
(1)
Presence of strong
surface clutter, which masks the radar echo from depth in both
directions (along and across-track).
If not dealt with specific measures, it could mask the useful
signal down to 2 - 3 km depth from the surface. To suppress these
clutter ambiguities, the study proposes new measurement concepts, which
shall be assessed (SAR processing, multiple paths approaches, antenna
size reducing processing, …).
(2)
The ionospheric
fluctuations over Earth and other planets such as Mars introduce phase
variability of the radar signal, limiting the proposed concepts. A
better modeling of the ionosphere, adding real-time corrections using
dual-frequency technique like GPS/Galileo, or auto-focusing techniques
to overcome small-scale variation need to be developed for mitigating
such disturbing effects.
The study intends to find a mean to decrease the Clutter effect on both directions (along track and across track) using the proposed new concepts and to assess the ionospheric effect to recommend a way of correction.
In final, the proposed work shall offer the possibility to design a space sounder mission with a better ratio, performances (achievement of the scientific requirement) versus payload complexity and cost.
For Earth observation, a P-band Antarctic ice-sheet sounder (MIMOSA) was proposed as an Earth Explorer Opportunity mission. Its mission objectives are to:
- Improve the knowledge of the bedrock
topography and ice thickness on whole
- Provide information on the internal stratification by studying internal layering echoes
- Provide information on basal roughness, condition and existence of sub-glacier 'lakes‘
- Improve the ice-sheet thermo-mechanical models through the infusion of Ice Sounder data into these models
- Enhance the understanding of the role of
A first mission assessment and a related CDF study
(internal ESA study achieved in May 2003) were aimed at making an
initial analysis of the feasibility for a simple concept similar to
MARSIS, but at P-band. However the problem of surface ambiguity
suppression was not solved, making the mission difficult to consider
(low performances and high cost). Further internal analyses on clutter
suppression using multiple receivers (antenna size reducing processing
techniques) and on along-track SAR processing have been carried out
showing promising results on simulated data. An ITT has been issued for
the development of an airborne P-band radar for improving our knowledge
on ice sounding.
For planetary explorations, the experience of MARSIS constitutes a good starting point for sounding mission, but analysis of acquired data is necessary to validate the concept.
All these studies increasing our knowledge on the subject have stimulated new ideas, and finally advanced concepts of measurement arose and are proposed in this study.
The expected enhancement of the sounding capability (resolution, contrast, link budget) could promote future missions like Antarctic ice-sheet sounding for Earth observation, or ELRR for planetary explorations.
3 - New concept and known activities worldwide on the subject
The feasibility study is based on the assessment of new measurement concepts, which have never been applied for a space borne sounder except Doppler filtering processing for Marsis. The purpose of these new concepts is to decrease the impact of clutter in both directions. At least the following ways of improvement shall be studied:
- In along track:
- SAR processing applied to sounder for sub-surface imagery. This technique called also Focused SAR processing is based on the classical SAR processing but the algorithm is design to take into account the geometry of the sounder and also the electromagnetic propagation of the wave inside the surface to focus the data under the surface. It has already been used for P-Band airborne sounder over ice.
- Other Doppler filtering. This technique called also Unfocused SAR has already been used for MARSIS. It uses a filtering to conserve lower Doppler (higher frequencies) corresponding to points at shorter distance from the sounder. A derivative of this technique is a coherent addition of the raw data that serves as a low-pass filter on the data. These techniques could be used in complement of focused SAR processing to decrease the data rate and the volume of data for a space borne mission.
- In across track:
- Use of several paths
- with interferometric techniques. The same area is imaged using different point of view that can be used to decrease the clutter impact. The way to use the different paths to decrease the Clutter has to be defined for a sounder. Similar techniques have already been tested for others objectives, with side looking Radar to increase the across track resolution or to recover the altitude information, for altimeters with Cryosat, but never for a space borne sounder.
- Sparse array antenna. The processing correlates multiple flight paths over the same area. A specific space-time filter (maximum likelihood or a minimum mean-squared error space-time filter) is then used to push down the clutter to an acceptable level. This technique has already been used for P-Band airborne sounder over ice.
-
Antenna
size reducing processing techniques. This technique is based on
the decomposition of the antenna in sub-antennas to measure and
subtract the signal containing the Clutter. It was proposed
after an assessment by ESA/ESTEC in the CDF study for P-Band
sounder instrument for
This feasibility study investigates Advanced Concepts for Radar Sounder and will give the opportunity to gather sufficient technical knowledge to estimate the achievability of new sounding missions:
-
On the Earth with spaceborne ice sounding radar to better estimate the ice sheet thickness of the
-
For planetary exploration with sounding radar like MARSIS with a higher resolution in both horizontal directions to increase the knowledge of the underground, define ice layer thickness on outer planet moons and detect subsurface water.
The problem of surface ambiguity suppression for space based low frequency sounding radar has been identified in recent investigations within and outside of ESA. The novel approach proposed to be investigated will, if successful, largely increase the science returns and at the same time decrease the mass, size and cost of such missions.
The proposed study will pave the way to future missions in the area of Earth Observation and Planetary Exploration, in particular for enabling electromagnetic soundings of subsurface structures.
The objective of the study is to assess the feasibility of new measurements concepts for subsurface sounding using along and across track processing and techniques to decrease the impact of clutter and enhance resolution and contrast. To achieve this purpose, the study shall include the following points:
-
Definition of reference missions for Earth and Planetary observation: the objective shall be to have a common database of mission parameters (orbit, carrier frequency, …) for each case and to use it along the study. This definition including the instrument budget versus performances shall also be used at the end of the study to compare after the introduction of the new concepts. Even if these techniques could be used for other application, the reference for the work proposed in this study shall be:
-
For the Earth, a mission based on a P-Band (435 MHz) sounder instrument design for
-
For planetary observation:
-
A mission based on a sounder for Mars surface and sub-surface exploration
-
A mission based on a sounder for Europa
-
-
Bibliography and analysis of the news concepts: the study of the proposed new concepts and the analysis of their impact on the payload, shall allow realising a preliminary trade-off between concepts.
-
Ionosphere: Particularly with the introduction of new concepts the investigation of correction mean for ionosphere becomes a crucial point for the missions’ achievability. The purpose shall be to assess the impact of the ionosphere and to propose mean of correction.
-
Processing: The implementation of each concept (and also of mean of correction for the ionosphere) induces challenged processing which need to be validated to confirm the feasibility.
-
Benefits of the proposed concepts for the reference missions: The objective shall be to define the benefits for the three missions (in term of performances versus payload dimensioning) based on the introductions of the most promising concepts and on the results obtained during the study. A comparison with the reference missions defined in the beginning shall confirm this benefit.
[REF1]
Radio Echo Sounding of
[REF2]
Microwave Payloads for the New Earth Explorer Core Candidates,
ESA Workshop on Advanced RF Sensors for Earth Observation, ESTEC,
Noordwijk, the
[REF3] Perspective for Microwave Instruments in ESA’s Future Earth Observation Missions, Proc. Eur. Microwave Assoc., 2005.
[REF4]
Improvement of Radar Ice Thickness Measurements of Greenland
Outlet Glaciers Using SAR Processing
D. Braaten+,
S. Gogineni, J.M. Stiles, D. Tammana‡,
S.K. Namburi, J.D. Paden, and K. Gurumoorthy, Radar Systems and Remote
Sensing Laboratory and Dept. Physics and Astronomy, The University of
Kansas, Lawrence, KS 66045 U.S.A.
[REF5]
A. Hérique, W. Kofman, P. Baüer, F. Remy and L. Phalippou, “A
Spaceborne Ground Penetrating Radar: MIMOSA,” Proc. CEOS Workshop on SAR
Calibration and Validation, 26-29 Oct. 1999,
[REF6] Performance and surface scattering models for the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) G. Picardi; D. Biccari, R. Seu, L. Marinangeli, W.T.K. Johnson, R.L. Jordan, J. Plaut, A. Safaenili, D.A. Gurnett, G.G. Orib, R. Orosei, D. Calabrese, E. Zampolini
[REF7] Focused Synthetic Aperture Radar Processing of Ice Sounder Data collected over the Greenland Ice Sheet. Justin J. Legarsky, Sivaprasad P. Gogineni,and Torry L. Akins IEEE transactions on geoscience and remote sensing, vol. 39, No 10, October 2001
[REF8]
Coherent cancellation of surface Clutter for Radar Sounding G.
Picardi,
[REF9]
Simulations of Ionospheric Effects on SAR at P-Band Jun Liu Yasuo
Kuga and Akira Ishimaru (
[REF10]
Ionospheric Effects on SAR Imaging: A Numerical Study
, Jun Liu,
Yasuo Kuga, Akira Ishimaru, Xiaoqing Pi, and Anthony Freeman, IEEE
TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 41, NO. 5, MAY 2003
[REF11]
Ionospheric Effects on SAR at 100MHz to 2 GHz Akira Ishimaru, Jun
Liu, Yasuo Kuga, Yunjin Kim and Tony Freeman IEEE 1998
[REF12]
Analysis techniques for coherent airborne radar sounding:
Application to West Antarctic ice streams Matthew E. Peters, Donald D.
Blankenship, and David L. Morse Institute for Geophysics,
[REF13]
B. Rommen, B. Ramírez Velado, C.C. Lin, J. Guijarro, “Scientific
rationale for a spaceborne P-band ice-sounder”,Proc.XI SCALOP Symposium,
June 2004, Bremen, pp.164-176.
[REF14]
Sar processing of Radar Echo Sounder Data C. Leuschen, S.
Gogineni, D. Tammana The University of Kansas IEEE 2000.
[REF15] Armand and Smirnov, “Distortion of radar pulses by the Martian ionosphere”, Radio Science, vol 38, no 5, 2003.
[REF16] Steinshleiger et al., “On overcoming the destructive effect of the ionosphere on the resolution of VHF trasnionospheric radar for remote sensing of the Earth”, Physics Uspekhi, 41, (12), pp1249-1251, 1998.
|
ACRAS |
Advanced
Concept for RAdar Sounder |
|
BAS |
British
Antarctic Survey |
|
CDF |
Concurrent
Facility Design |
|
ELRR |
Europa Low Resources Radar |
|
ESA |
European
Space Agency |
|
ESTEC |
|
|
GPS |
Global
Positioning System |
|
KO |
Kick off |
|
MARSIS |
Mars Advance
Radar for Subsurface and Ionospheric Sounding |
|
MIMOSA |
Mapping of
Antarctic Ice and Monitoring of Subsurface |
|
MTR |
Mid Term
Review |
|
PM |
Progress
Meeting |
|
SAR |
Synthetic Aperture Radar |
|
SIRAL |
Synthetic
Aperture Radar Interferometric Radar Altimeter |
|
TBC |
To Be
Confirmed |
|
TBD |
To Be
Defined |